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Tribomaterials

2008;():1-3. doi:10.1115/IJTC2008-71079.

The Entropy Generation Theorem recently formulated by Bryant, Khonsari and Ling [1] relates any form of degradation to the irreversible dissipative processes germane to the degradation mechanism. The theorem was formulated and proved [1] based on the first and second laws of thermodynamics In this article, dry sliding wear will be related to dry sliding friction through application of the theorem. The result reproduces the Holm-Archard wear law, but with a wear coefficient based on measurement of temperatures and wear volume lost.

Commentary by Dr. Valentin Fuster
2008;():5-7. doi:10.1115/IJTC2008-71131.

Frictional characteristics of PTFE films deposited by HFCVD method on glass substrates were determined using ball-on-flat and ball-on-disk configurations. Optical microscopy was used to examine the surface profile of the PTFE films left by the UMT after the tests. In general, the COF of the HFCVD-PTFE thin films on glass decreases at low sliding speeds and high loads. Limited number of durability tests show that the COF stabilizes to a fixed value different than the initial one. Based on the results of the tests conducted in this work, it is concluded that the PTFE films deposited by HFCVD display the similar properties to bulk PTFE.

Commentary by Dr. Valentin Fuster
2008;():9-10. doi:10.1115/IJTC2008-71198.

Recent improvements in growth methodologies have decreased the grain sizes and thicknesses of polycrystalline diamond films to the nanometer range, while also increasing the film uniformity and growth rate and preserving the outstanding mechanical properties of diamond. This is rendering such films more technologically and commercially viable. Ultrananocrystalline diamond (UNCD) are the thinnest (<200 nm) and smoothest (Rq < 10 nm) diamond films available.[1] These films demonstrated self-mated friction coefficients as low as near frictionless carbon (μ < 0.007) in environments with sufficient humidity, and the corresponding wear rates could not be measured using scanning white-light interferometry. However, their response to environmental conditions (e.g. relative humidity, ambient species, velocity, and temperature) had not been systematically explored in the past. This study focused on identifying conditions that contribute to favorable tribological performance. We find low friction performance at humidity levels below 1.5% in both nitrogen and argon environments.

Topics: Diamond films
Commentary by Dr. Valentin Fuster
2008;():11-13. doi:10.1115/IJTC2008-71210.

As the first step to understand how hydrogen influences the sliding properties of metallic materials, nine self-mated pairs of metallic elements were tested using a pin-on-disk apparatus. The results of friction force, wear amount, and observations of wear debris showed that the elements could be roughly categorized into two groups; transition and non-transition elements. Chemisorption of hydrogen on the sliding surfaces was thought to be predominant of the tribological properties in the first group of elements, while chemisorption did not take place on the sliding surfaces of the latter group. The effect of hydrogen on sliding appeared similar to that of oxygen in terms of chemisorption.

Topics: Metals , Hydrogen
Commentary by Dr. Valentin Fuster
2008;():15-17. doi:10.1115/IJTC2008-71225.

Wear behavior of polymeric sealing material sliding against austenitic stainless steel was evaluated within gaseous hydrogen atmosphere to ensure the durability and longevity of polymeric seals used in fuel cell vehicles and related hydrogen infrastructures. In this study, unfilled polytetrafluoroethylene (PTFE) was considered as a representative polymeric material for seals and its wear behavior was evaluated by using 3pin-on-disk wear tester coupled with the environmental test chamber. Results indicated that the specific wear rate of unfilled PTFE became significantly smaller in gaseous hydrogen compared with that in air. However, the specific wear rate further decreased in argon gas. Optical microscopy and XPS analysis of the disk specimen surface indicated that the wear behavior of PTFE highly depended on the transfer film formation on the sliding counterface. Notable influences of gaseous hydrogen on the formation process of PTFE transfer film and subsequent wear behavior could be postulated from XPS spectra.

Topics: Wear , Sealants , Hydrogen
Commentary by Dr. Valentin Fuster
2008;():19-25. doi:10.1115/IJTC2008-71259.

A power plant with high-temperature helium-cooled reactors (GT-MHR) (Figure 1) for electricity generation includes a turbomachine (TM) that provides helium circulation in the primary circuit and converts the coolant thermal power into electricity in the direct gas turbine cycle. Operability of the TM for the GT-MHR reactor plant (Figure 2) mainly depends on reliability of the electromagnetic suspension system for the vertical flexible rotor with a total weight of 67.7 tons and length of 29 meters. The electromagnetic suspension system for the TM rotor consists of axial and radial electromagnetic bearings (EMB), EMB control system, and catcher bearings (CB).

Commentary by Dr. Valentin Fuster

Nanotribology

2008;():27-29. doi:10.1115/IJTC2008-71055.

A numerical simulation is presented for several loading-unloading cycles of an adhesive contact between an elastic-plastic sphere and a rigid flat. The main goal of the simulation is to study the plastic deformation evolution in a contact bump material — the microscopic electrode found in a MEMS micro-switch for providing a good electric contact. This bump is subjected to a cyclic contact interaction with a harder substrate and cyclic plasticity of the bump material can lead to its wear and as result to a failure of the whole MEMS device.

Topics: Adhesives
Commentary by Dr. Valentin Fuster
2008;():31-32. doi:10.1115/IJTC2008-71057.

Nanomaterials play important roles in tribolgy and manufacturing. This lecture provides an opportunity for us to review and learn the knowledge and techniques that have been involved in nanomaterials and nanotribology areas. Targeting tribological applications, a brief history of nanoparticulates, dated back more than 2500 years, will be given briefly. The classification and synthesis of nanoparticles will be followed by nanomaterils and nanocomposites, their characterization techniques, and mechanical, tribological, chemical, and physical properties. Discussion ends with summary of applications.

Commentary by Dr. Valentin Fuster
2008;():33-35. doi:10.1115/IJTC2008-71127.

A new model named as the coupled-oscillator model, is proposed to study the atomic-scale static friction. The Maugis-Dugdal model is used to approximately substitute the Lennard-Jones potential of the interfacial friction in new model. Then, the formulas for static friction force and coefficient calculation are deduced. A comparison between the theoretical result and the experimental value obtained by an atomic force microscope is presented to show the model and the formulas practically feasible.

Topics: Stiction , Mechanisms
Commentary by Dr. Valentin Fuster
2008;():37. doi:10.1115/IJTC2008-71135.
FREE TO VIEW

Wear at the nanoscale is a key limitation of conventional silicon and silicon nitride atomic force microscope (AFM) probe tips. Tip degradation and contamination induced by tip-sample interactions can result in decreased resolution and uncertainty in AFM measurements. Prediction and control of the wear behavior is challenging since there is no rigorous theory for the wear of a <100 nm asperity. However, ultrananocrystalline diamond (UNCD) and diamond-like carbon (DLC) are potentially ideal materials for AFM probe applications because of their high stiffness and hardness, low surface roughness, low macroscale friction coefficient and wear, and chemical inertness. The nanoscale adhesion and wear behavior of UNCD, DLC, silicon, and silicon nitride AFM probes have been characterized through systematic AFM wear tests and characterization of the corresponding nanoscale modification of the tips through transmission electron microscopy (TEM) imaging, AFM-based adhesion measurements, and AFM-based blind reconstruction of the tip shape. Our results demonstrate that significant reductions in the nanoscale wear can be achieved through the use of these carbon-based materials. We will discuss how the nanoscale wear behavior of the tips can be linked to their intrinsic materials properties through consideration of the mechanics and physics of nanoscale contacts.

Commentary by Dr. Valentin Fuster
2008;():39-41. doi:10.1115/IJTC2008-71145.

This study aims to contribute to better understand the antiwear action of zinc dialkyldithiophosphate (ZDTP) additives used in car engine lubrication. The antiwear action of ZDTP is associated to the formation of a protective tribofilm onto the rubbing surface. On a mechanical point of view, the efficiency of ZDTP tribofilms results from equilibrium between film formation and wear rates, associated with appropriate rheological properties. In this work, the mechanical properties of a ZDTP tribofilm have been measured by nanoindentation in different test conditions in order to investigate the effect of temperature and strain rate. A Nanoindenter XP® entirely set into a climatic chamber was used to perform the nanoindentation tests. For all tests, an increase of the elastic modulus was observed from a threshold contact pressure value. This effect is similar to the anvil effect observed on polymers: in confined geometry, the elastic modulus increases versus hydrostatic pressure. For the tribofilm, in the studied range, this effect is enhanced at high temperature and low strain rate. Furthermore, when the temperature increases, a change in the rheological behavior of the tribofilm is observed. Up to about 50°C, the tribofilm exhibits viscoplastic behavior — the hardness increases versus strain rate — and above 50°C, the hardness decreases versus strain rate (“shear thinning-like” behavior).

Commentary by Dr. Valentin Fuster
2008;():43-45. doi:10.1115/IJTC2008-71162.

We propose a new approach to the study of friction in molecularly thin films. The instantaneous occurrence of both momentum and thermal kinetic energy diffusions in the film requires a sophisticated energetic approach. This can be done by identifying the energetic terms which are due to thermal agitation or shearing rather than globally thermostating the system. With this approach, it becomes possible to quantify the energy loss due to friction and to describe the local mechanisms of its dissipation inside the liquid and through the interfaces. Implemented, the method can give more accurate results in molecular dynamics simulations of confined films.

Commentary by Dr. Valentin Fuster
2008;():47-48. doi:10.1115/IJTC2008-71164.

We have measured normal and lateral (friction) forces across films of saturated and unsaturated fatty acids with the Surface Forces Apparatus (SFA). When adsorbed onto mica from n-hexadecane, linoleic acid forms a dimer layer between monolayer-covered surfaces. This dimer layer, which is not observed in stearic or oleic acid, is removed at low loads. The remaining linoleic acid monolayers are thinner than the ones formed by stearic and oleic acid, and show a more complex friction response with two regimes of linear friction, higher stability at very high loads, and a dependence of the friction on sliding speed and adsorption time.

Commentary by Dr. Valentin Fuster
2008;():49-51. doi:10.1115/IJTC2008-71178.

Experimental evaluation of hardness, adhesion and Young’s modulus has been performed on polyimide polymeric coatings used in LCD displays and on composite polymer-based materials used in automobiles and aircraft. A novel Universal Nano+Micro Tester UNMT-1 with a nano-analyzer module NA-2 has been utilized. It measures scratch-hardness of coatings and thin films, utilizing the same nano-tip for both scratching and nano-imaging under the constant load. It measures scratch-adhesion with the same diamond tip for both scratching and nano-imaging under the continuously-increasing load. It evaluates homogeneity of films and composite materials by simultaneous Young’s modulus and topography nano-mapping, with a diamond nano-tip in a tapping mode, while frequency and phase of its vibrations are analyzed. The Young’s modulus maps allowed us to evaluate the distribution of SiO/Si02 particles embedded in araldite, with varying SiO/Si02 concentration. While the topography images could not distinguish between the particles and polymeric matrix, the nano-mechanical maps revealed the effects of particle concentration and agglomeration on the local modulus of the material and the relationship between the SiO/Si02 uniformity and uniformity in modulus. The nano-scratches of 60-nm polyimide coatings at progressively increasing and constant loads generated adhesion and scratch-hardness data, respectively. Within the applied loads of 20 to 100 μN, we observed and determined both the critical load, at which the coating was delaminated from the glass substrate, and a corresponding lateral delamination force. The mutually complimentary nano-images and force graphs coincided nicely.

Commentary by Dr. Valentin Fuster
2008;():53-55. doi:10.1115/IJTC2008-71181.

In about a decade, since interest about boundary lubrication properties of diamond-like carbon (DLC) coatings exists, mainly chemical aspects of the DLC lubrication were investigated, i.e. focusing on interactions between various additives and coatings, while physical aspects of coating-oil interactions were not discussed in a greater extent. To elucidate some of these effects, we have analyzed the friction behaviour of DLC coatings and steel under different boundary lubrication conditions by using polyalphaolefin (PAO) base oils having different viscosity grades. Based on this, we have identified several tribo-physical effects in these contacts and for the purposes of this paper we briefly discuss the physical adsorption between the oils and the DLC surfaces, clearly indicating existence of rather strong physically adsorbed layers, which were able to resist high shear stresses under severe boundary lubrication.

Topics: Viscosity
Commentary by Dr. Valentin Fuster
2008;():57-59. doi:10.1115/IJTC2008-71189.

A MEMS device with a configuration similar to that of a micro-bearing was developed to study the friction behavior of the curved sidewall surfaces. This friction testing device consists of two sets of actuators for normal motion and rotation, respectively. Friction measurements were performed at the curved sidewall surfaces of single-crystal silicon. A general model was developed for the equivalent tangential stiffness of the bush-flexure assembly at the contact point by reducing a matrix equation to a one-dimensional formulation for the purpose of using a recently developed quasi-static stick-slip model. The measurement results show that the coefficient of static friction exhibits a nonlinear dependence on the normal load. The true coefficient of static friction was determined by fitting the experimental friction curve.

Commentary by Dr. Valentin Fuster
2008;():61-63. doi:10.1115/IJTC2008-71194.

The unique capabilities resulting from combining a scanning tunneling microscope (STM) and a quartz crystal microbalance (QCM) have been used to characterize the heating and wear at the interface of a tungsten tip and Indium substrate with a change in the contact characteristics of the interface occurring for sufficient sliding speeds. The advantage of this system is the ability to probe heat rise from an asperity contact which will aide in developing a more complete understanding of the complex issue of heat generated via friction.

Commentary by Dr. Valentin Fuster
2008;():65-67. doi:10.1115/IJTC2008-71196.

We report a joint theoretical and experimental study of the tribological properties of gold-yttrium stabilized zirconia (YSZ) based nanocomposite coatings, with a focus on the role of nanocrystalline grain size. Nanocomposites hold great promise for space and ambient applications, on account of their ability to adapt to and exhibit low friction and wear rates in constantly varying environmental conditions. Their internal structure has been the topic of prior literature, but the impact of grain size on tribological performance has heretofore not been considered, and the surface topology has not been reported. As such, we have performed both experimental and theoretical studies, to model the impact of grain size on film stress and wear attributes, and to document surface region grain size distributions through scanning tunneling microscopy (STM) measurements of self-affine fractal scaling properties. Nanocrystalline gold crystal sizes, as determined from STM and x-ray diffraction (XRD) data are consistent with those inferred from high resolution transmission electron microscopy (HRTEM) measurements. Our modeling results associate smaller grain sizes with lower wear rates, consistent with experiments. The findings show promise for nanoscale customization of coatings so as to tailor them at the nanoscale in an application specific manner.

Commentary by Dr. Valentin Fuster
2008;():69. doi:10.1115/IJTC2008-71200.
FREE TO VIEW

In this tutorial, I will review the molecular origin of some of the forces that may be observed between interacting bodies, and the effects of such forces on adhesion and friction of surfaces and thin films at the atomic and nanoscopic scale.

Topics: Force , Friction
Commentary by Dr. Valentin Fuster
2008;():71-72. doi:10.1115/IJTC2008-71201.

Molecular dynamics is the simulation method that is most amenable to the length and time scales of nanotribological experiments. The ability to track the individual motion of every atom in simulations has led to a detailed understanding of the underlying physics that is difficult to extract from experiment. While significant progress has been made in simulations over the past two decades, computational issues still limit the types of problems that can be approached, and the detailed understanding that results. Here we discuss recent advances in molecular dynamics simulations that push the bounds of simulation size, velocity, and chemistry. These state of the art simulation techniques have made great strides in allowing detailed comparisons to experimental results. These advances will be placed in context by addressing the barriers that remain and where future progress lies.

Commentary by Dr. Valentin Fuster
2008;():73-74. doi:10.1115/IJTC2008-71202.

The use of molecular dynamics (MD) simulations in nanotribology has made many advances in the past twenty years. While early simulations were limited to hundreds or thousands of atoms undergoing shear at 100 m/s, the current state of the art simulations in nanotribology approach millions of atoms with shear rates that can, in certain cases, match experiment. However, many of the exciting current simulations involve techniques that can be difficult for non-experts to understand. In this tutorial we hope to alleviate some of this confusion by reviewing the basic concepts that form the foundations of molecular dynamics (MD) simulations. This will include both a discussion of the method in general, was well as a focus on the use of MD in nanotribological simulations.

Commentary by Dr. Valentin Fuster
2008;():75-77. doi:10.1115/IJTC2008-71208.

This paper reports a study on the contact and friction between a gold surface and a rigid tip with a two-dimensional (2D) classical Molecular Dynamics (MD) simulation method. A multiscale method is used in the MD simulation. The MD simulation results are compared with the Hertzian cylindrical contact results. A criterion for the transition from surface sliding to surface scratching is also discussed with the assistance of stress analyses.

Commentary by Dr. Valentin Fuster
2008;():79-81. doi:10.1115/IJTC2008-71215.

A multi-scale mathematical model is used to study the effect of surface roughness on the adhesion and friction of microfibers engaged in side contact. Results are compared to closed-form analytic approximations derived from linear elastic contact mechanics.

Commentary by Dr. Valentin Fuster
2008;():83-85. doi:10.1115/IJTC2008-71220.

The paper investigates the forces that can contribute to the attachment and detachment of the tip of a synthetic fiber to a substrate in normal atmosphere. The contact is subjected to a number of competing forces, which together with the localized contact deflection can promote equilibrium. The pull-off force is predicted as a function of the angle of approach of the fiber toward a substrate and an initial fiber preload.

Commentary by Dr. Valentin Fuster
2008;():87-89. doi:10.1115/IJTC2008-71247.

As the size of the contact region between two bodies decreases to the micro- and nano-scales, the effect of adhesion becomes increasingly important. In this tutorial, we review fundamental concepts of the mechanics of adhesion. Attention is placed on the contact of elastic bodies in which the shapes of the contacting bodies are locally spherical. We also discuss the use of spherical contact theory to model the adhesive contact of an asperity with a flat as part of a multi-asperity contact model.

Topics: Adhesives , Shapes
Commentary by Dr. Valentin Fuster
2008;():91-93. doi:10.1115/IJTC2008-71248.

As the size of the contact region between two bodies decreases to the micro- and nano-scale, the effect of adhesion becomes increasingly important. As introductory remarks to a panel discussion on this topic, we briefly review recent research in the mechanics of adhesion and discuss future research needs. Attention is focused on adhesion with plastic deformation, molecular dynamics simulations, and multi-scale effects.

Commentary by Dr. Valentin Fuster
2008;():95-97. doi:10.1115/IJTC2008-71249.

Nanotribology has been in existence as a recognized discipline for roughly 20 years, with the appreciation of the importance of atomistic mechanisms of tribology existing long before. In this paper, we briefly review why nanotribology is important for advancing the science of tribology in general, and we also highlight emerging applications where nanotribological research is critical.

Topics: Nanotribology
Commentary by Dr. Valentin Fuster
2008;():99-101. doi:10.1115/IJTC2008-71253.

In order to alleviate or eliminate the occurrence of stiction during the actuation of microstructures, the real contact area available for contact must be reduced. Au nanoparticles were intentionally deposited using gas-expanded liquids onto polysilicon cantilever beam arrays to increase surface roughness. The nanoparticle-coated beams were subjected to an actuation voltage of 120 V. Following actuation, the adhesion of beams was quantified by estimating the apparent work of adhesion. Au nanoparticles deposited onto these microstructures were shown to drastically reduce the effects of in-use stiction. Capillary adhesion due to condensation of ambient moisture was effectively eliminated.

Topics: Nanoparticles
Commentary by Dr. Valentin Fuster
2008;():103-105. doi:10.1115/IJTC2008-71282.

The nanoscale structure of single-walled carbon nanotubes (SWCNTs) has unique properties. These nanostructured additives can induce unusual characteristic in many polymer matrix. In one of our recent experiments, it was found that when adding SWCNTs into a polyimide (PI) matrix, friction becomes a function of the concentration of the additive. In this research, we analyze the behavior of the SWCNTs-PI nano-composite using an approximation approach. We report that the frictional behavior of the nanocomposite is dominated by the elastic and plastic deformation through randomly dispersed SWCNTs under different loading conditions. At low concentration of SWCNTs, its elasticity dominates the properties of composite while at higher concentration, plastic behavior of tubes plays a major role in describing the properties of composite.

Commentary by Dr. Valentin Fuster
2008;():107-109. doi:10.1115/IJTC2008-71285.

Surface asperities can range widely in size. Therefore it is important to characterize the effect of size and scale on the contact mechanics. This work presents a molecular model of asperity contact in order to characterize small scale asperity contact. The model is also compared to existing continuum mechanics based models developed originally by Hertz for elastic contact and later expanded by others to include plasticity. It appears that the predictions can be related to each other and that the continuum material properties can be related to the properties describing the molecular forces.

Commentary by Dr. Valentin Fuster
2008;():111-113. doi:10.1115/IJTC2008-71287.

A molecular dynamics model of a nanoindentation experiment was simulated in order to calculate the elastic modulus of several different Lennard-Jones (LJ) solids. It was found that the elastic modulus increased significantly as the depth of the potential well that describes the interactions between the atoms in the sample was increased.

Commentary by Dr. Valentin Fuster
2008;():115-117. doi:10.1115/IJTC2008-71289.

Topographical modification of silicon (100) wafers was performed by the creation of nano-structures that consists of pillars with different pitch. The modified surfaces were investigated for their tribological characteristics at nano-scale in comparison with those of the bare silicon surface. Results showed that the modified surfaces have superior nano-tribological properties, as they reduce adhesion and friction forces significantly when compared to the bare silicon surface. Among the nano-structures, adhesion and friction forces reduce with the pitch. The topographical modification of silicon surfaces provides a promising solution to improve the tribological properties of miniaturized devices such as microelectromechanical systems (MEMS) in which silicon is a typically used material.

Commentary by Dr. Valentin Fuster

Biotribology

2008;():119-121. doi:10.1115/IJTC2008-71032.

The present paper describes wear tests performed using human cartilage on cartilage under various working conditions. Wear was assessed by determining the concentration of proteoglycans (PG) and hydroxyproline in the lubricating solution.

Topics: Wear , Cartilage
Commentary by Dr. Valentin Fuster
2008;():123-125. doi:10.1115/IJTC2008-71037.

In this paper lubricant film thickness for bovine serum (BS) was measured in a ball-on-disc optical device under steady-state rolling and sliding. Tests were carried out for a range of BS concentrations and substrate materials (M52100 steel and chromium coatings) in both low (MPa) and high-pressure (GPa) configurations. The results show that BS forms films 2–50 nm thick over the speed range although this depends on the contact pressure. However there was significant scatter in these results, possibly due to the inherent nature of the fluid, which is an inhomogeneous biological sample. Clearly this will contribute to scatter in wear results. In some cases thick (up to 100nm) films were formed at low speeds under both sliding and rolling conditions, this behaviour was considered representative of high-viscosity surface layers rather than solid films. However residual films of 13–17nm were also measured under static loading. These are attributed to the adsorption of protein molecules and will provide surface protection during stance or on initiation of gait. A small number of results at under low pressure sliding conditions indicated that much thicker films were formed than at high pressures. One interesting aspect of the results is that they are not representative of a simple Newtonian fluid and thus have considerable implications for the development of predictive film thickness models.

Commentary by Dr. Valentin Fuster
2008;():127-129. doi:10.1115/IJTC2008-71155.

In this study the tribological analysis of Al2 O3 nanocomposites/Al2 O3 pair; proposed as a candidate material to fabricate hip prostheses was carried out. Nanopowders of Al2 O3 (AKP 50, 300 nm), TiO2 (PS-25, 50 nm) and Co metallic powder (Nilaco, 28 nm) were mixed and hot pressed. Wear test was carried out in a pin-on-plate tribometer, with a frequency of 1 Hz, a load of 49 N, for 4h; the counterface used was Al2 O3 . Mechanical properties as Vickers hardness, fracture toughness and Young’s modulus were estimated using the indentation method. Distilled water and fetal bovine serum solution (FBSS) were used as environment. It was found that the specific wear rate of Al2 O3 nanocomposites was about 10−8 mm3 /N*m and the coefficients of friction were around 0.3–0.5 for FBSS. Worn surfaces were observed using SEM.

Commentary by Dr. Valentin Fuster
2008;():131-133. doi:10.1115/IJTC2008-71158.

The paratenon is a soft tissue structure that surrounds some tendons, acting to reduce friction during sliding on muscular contraction. Such sliding can, however, cause inflammation of the tissue, leading to paratenonitis. It has previously been reported that paratenonitis incidence was greater in colder temperatures, possibly due to an increase in synovial fluid viscosity and thus friction. This study examined the friction and lubrication between two surfaces approximating the in vivo conjunction, at two different temperatures. The results suggest that an increase in temperature serves to increase friction, in contrast to the previous hypothesis. These results imply that physical activities encourage paratenonitis.

Commentary by Dr. Valentin Fuster
2008;():135-137. doi:10.1115/IJTC2008-71175.

Highly crystalline ultra-high molecular weight polyethylene (UHMWPE) has historically experienced little success as a bearing material in total joint arthroplasty due to oxidation related fatigue failures. Recent sterilization and treatment advances have reduced or eliminated the risk of oxidation, allowing for a renewed interest in this particular material. The current study investigates the wear resistance of a highly crystalline UHMWPE in comparison to clinically relevant control materials. The highly crystalline material exhibits a wear rate superior to never irradiated material, and similar to irradiated material. The wear rate reduction is attributed to the larger crystallites’ ability to slow subsurface crack growth.

Commentary by Dr. Valentin Fuster
2008;():139-141. doi:10.1115/IJTC2008-71224.

Fluorocarbon (FC) films were grafted on low-density polyethylene (LDPE) by a plasma polymerization and deposition process. Nanoscale indentation and friction tests performed with a surface force microscope provided insight into the nanomechanical properties of the grafted FC films. The time-dependent deformation behavior of the FC films is examined in the context of nanoindentation responses. It is shown that plasma treatment modified the surface properties of LDPE significantly.

Commentary by Dr. Valentin Fuster
2008;():143-145. doi:10.1115/IJTC2008-71244.

Despite the serious health concern associated with slip and fall accidents and the numerous slip testing devices that have been developed, few studies have attempted to tribologically model the shoe-floor interface. To this end, a mixed-lubrication model for shoe-floor interfaces is proposed. The model is applied to a pin-on-disk apparatus and uses contact mechanics and hydrodynamic lubrication modeling with iterative methods to solve the mixed lubrication problem. Outputs of the model are load supported by the fluid and load supported by the contacting asperities. Measurable parameters input to the model are: curvature of the shoe material (pin), material properties of the shoe material, roughness of the shoe and floor material, viscosity of the fluid, sliding speed and normal force. COF estimates are generated as a function of the proportional load borne by the fluid and the contacting asperities. The model COF values replicate the experimental data for the two different shoe materials tested. The peak hydrodynamic pressure was found just outside the contact region and peak contact pressure was at the center of the pin. The model represents a first step towards developing a mixed-lubrication model for an entire shoe-floor surface.

Topics: Friction , Lubrication , Disks
Commentary by Dr. Valentin Fuster
2008;():147-149. doi:10.1115/IJTC2008-71308.

A significant component of our understanding of cartilage mechanical behaviour is the ability to model its response to various types of mechanical loading, for which we require detailed knowledge of cartilage material properties. The Finite Element Analysis software ABAQUS is renowned for the ability to model poroelastic materials using the soil consolidation theory. In this research, ABAQUS has been used to model and investigate the mechanical behaviour of articular cartilage, mainly using indentation and unconfined compression techniques. A biphasic model of articular cartilage was first created and subsequently modified to incorporate more detailed material descriptions. Various material constitutive laws (and mechanical properties), accounting for the strain dependent permeability of the porous matrix, solid viscoelasticity and transverse isotropy, have been adopted to produce increasingly sophisticated models. The presence of collagen fibril networks embedded in the solid has been also considered and Fibril Reinforced Elastic and Viscoelastic models produced. A salient feature of these models is their ability to simulate fibril stiffening by replicating the nonlinear fibrillar response. In this paper, we provide an overview of the state-of-art modelling techniques adopted to simulate cartilage behaviour. The comparative study performed by the authors provides a critical assessment of the effectiveness of such techniques.

Topics: Modeling , Cartilage
Commentary by Dr. Valentin Fuster

Emerging Technologies

2008;():151-153. doi:10.1115/IJTC2008-71067.

This paper reviews the application to tribology research of the techniques of high-vacuum surface science, in particular their integration into high-vacuum tribometers. The state of the art, the varied research interests, and the rationale for requiring such research instruments are discussed. The authors have developed a new type of high-vacuum tribometer that is aimed to the detection and characterization of charged-particle triboemission. This tribometer has been successfully used to count triboemitted charges as channel-electron-multiplier pulses and their energy distribution from different material pairs (i.e., ceramics and semiconductors when scratched by a diamond pin, and ceramics-on-same-ceramics). Instrument enhancement is ongoing to include simultaneous under-vacuum photon counting and surface work-function measurement by Kelvin-probe. The enhanced facility will enable one-of-a-kind experiments by characterizations and correlations of electron and photon emission, and of surface property evolution dynamics during sliding contact.

Topics: Vacuum
Commentary by Dr. Valentin Fuster
2008;():155-157. doi:10.1115/IJTC2008-71184.

The degree of acidification is considered a crucial property of lubricating oils for some applications, e.g. gas engines, as corrosive wear of machine components may be caused by acidic contaminations. For this reason, a sensor concept using the effect of material loss of a thin metal film due to corrosion is proposed. The corrosion of these films prepared from lead has been monitored electrically. In laboratory tests, the sensors were immersed in a series of oils characterized by different types and amounts of acidic components. The usefulness of the proposed sensor concept is also illustrated by corrosion tests with used oil samples taken from engines. The experiments clearly showed a correlation of corrosion rate with acidification expressed as TAN.

Commentary by Dr. Valentin Fuster
2008;():159-161. doi:10.1115/IJTC2008-71237.

Palm-sized mesoscale microturbomachinery (100∼200W) have broad potential applications in micro power generation areas, such as air/hydrogen management system for low temperature fuel cells, palm-sized micro gas turbines for unmanned air vehicles, robots, very small solid oxide fuel cells, micro power generations for space micro/nano satellites, etc. This paper reports recent progress on design, manufacturing of mesoscale (bearing diameter of 5mm) foil gas bearings applicable to the mesoscale microturbomachinery. With self-generated preload of 0.4N to the bearing, the mesoscale foil gas bearings were predicted to be stable up to the maximum simulated speed of 600,000 rpm. Manufacturing processes involved UV and X-ray lithography, electroplating, and precision forming. Test rigs were designed and constructed for performance tests of the manufactured mesoscale gas bearings.

Topics: Gas bearings
Commentary by Dr. Valentin Fuster
2008;():163-164. doi:10.1115/IJTC2008-71299.

Granular flow behavior is of fundamental interest to the engineering and scientific community because of the prevalence of these flows in the pharmaceutical, agricultural, food service, and powder manufacturing industries. When granular media come under external load, inter-particle forces in the granular media form an inhomogeneous distribution. In this study, a simulation of the contact forces in granular media was carried out using an explicit finite element method on a biaxial cell of approximately 2500 particles. Some of the key results included the capturing of normal force and tangential force distributions within a granular shear cell.

Commentary by Dr. Valentin Fuster

Special Symposium on Technology-Focused Magnetic Storage Tribology

2008;():165-167. doi:10.1115/IJTC2008-71010.

This work carries on a numerical simulation of the touchdown/takeoff (TD/TO) hysterisis of the spherical pad slider. It numerically studies the meniscus bridge’s formation and meniscus force interaction between the spherical pad and lubricant over the disk surface. It proposes a geometry model for the lubricant bridge, and correspondingly, a force model for the meniscus force acting on the spherical pad slider due to the lubricant bridge. By solving the liquid balance state at the meniscus boundary, it obtains the geometry of the liquid bridge. A parametric study is done to study the effects of the geometry of spherical pad, Hamaker constant of lubricant-disk, and surface energy of lubricant on the formation of the liquid bridge. The overflow phenomenon is analyzed to find out the acceptable dimension of the spherical pad design. Moreover, a three-dimensional (3D) model of spherical pad slider/disk interface is built to study the steady-state flying of the spherical pad slider. The different parameters are analyzed to study their effects on the TD/TO hysteresis.

Commentary by Dr. Valentin Fuster
2008;():169-171. doi:10.1115/IJTC2008-71242.

A probabilistic model of third-body particle embedment was derived for surfaces separated by a hydrodynamic slurry film. The analysis yields estimates of the density of embedded particles in terms of surface topography parameters, particle size distribution, and mean surface gap determined by the applied load, macroscopic geometry of the moving surfaces, and fluid properties. The model has direct application to chemical-mechanical polishing and fabrication of lapping plates for polishing magnetic recording heads.

Commentary by Dr. Valentin Fuster
2008;():173-174. doi:10.1115/IJTC2008-71303.

Spin-off experiments for PFPE Z25 were conducted by using a 3.5 inch disk at a constant speed of 10,000 rpm. The initial film thickness was set to about 2.4 nm for all the experiments while temperature was changed in a wide range. It is found that the rheological behavior of the film is quite different from that expected from the continuum. A novel model based on Fick’s law of diffusion was proposed, which is called the layer molecular dynamic diffusion (LMDD) model. The theoretical prediction was compared with the experimental results and a good agreement was obtained, verifying the effectiveness of the model.

Commentary by Dr. Valentin Fuster
2008;():175-177. doi:10.1115/IJTC2008-71309.

Air bearing vibrations at nanometer clearance levels are a major impediment to reducing head media spacing. Modern air bearings typically exhibit debilitating self-excited vibrations at fly heights of about 1–2 nm. At this point, peak head media spacing and contact pressures will increase, leading to performance and reliability issues. There has been considerable work performed to understand and improve air bearing stability at and around the point of contact, resulting in numerous experimental methods and design concepts in the literature. An area where additional research is worthwhile is the statistical behavior of contact vibrations. This paper explores the time dependence of fly height by using a simplified 2D air bearing with a mechanics based contact simulation. A comparison with measurements of fly height vibration demonstrates the utility of the simplified model and underscores the importance of proper time scale considerations.

Commentary by Dr. Valentin Fuster
2008;():179-180. doi:10.1115/IJTC2008-71311.

In a probe-based ferroelectric recording system a protective coating of lubricant provides low friction and a low wear rate as well as allows the continuous contact of the probe tip with the storage medium. SO5, a member of the commercially available electric connector lubricants polyphenyl ether (PPE), was experimentally proven effective as a lubricant in supporting this probe/media interface and has enabled the demonstration of the probe-based ferroelectric recording technology. The unique stability of SO5 lubricant in an electric field and in the presence of electrons makes this liquid particularly suitable for ferroelectric recording storage devices. A broad range of analytical techniques were used to identify this lubricant as a mixture of alkylated diphenyl ethers having varying molecular weight. A thin layer of this lubricant was uniformly coated on the probe media surface through a dip-lubing procedure. Its thermal stability and wettability was investigated in terms of the requirements for the successful application of this unique ferroelectric recording technology. The impact of this lubricant film on the read/write capability, durability and tribological performance was also explored.

Topics: Lubricants , Probes
Commentary by Dr. Valentin Fuster

Lubricants and Additives

2008;():181-183. doi:10.1115/IJTC2008-71004.

Anomalous low friction of hydrogen-free tetrahedral hybridized carbon (ta-C) coated surfaces lubricated by pure glycerol was observed at 80°C. In the presence of glycerol, the friction coefficient is below 0.01 at steady state, corresponding to so-called superlubricity regime. This new mechanism of superlow friction is attributed to easy glide on tribo-formed OH-terminated surfaces. In addition to the formation of OH-terminated surfaces but at a lower temperature, we show here some evidence that superlow friction of polyhydric alcohols could also be associated with tribo-induced degradation of glycerol, producing a nanometer-thick film containing organic acids and water. Second, we show novel outstanding superlubricity of steel surfaces directly lubricated by a solution of myo-inositol in glycerol at ambient temperature (25°C). For the first time, under boundary lubrication at high contact pressure, friction of steel is below 0.01 in the absence of any long chain polar molecules. Mechanism is still unknown but could be associated with friction-induced dissociation of glycerol and interaction with steel surface.

Topics: Steel
Commentary by Dr. Valentin Fuster
2008;():185-187. doi:10.1115/IJTC2008-71013.

The effectiveness of blends consisting of base oil, some secondary zinc dialkyl dithiophosphate (ZDDP), and different detergents to form antiwear tribofilms on steel surfaces sliding in the boundary lubrication regime was investigated in the temperature range of 105–125°C. The efficacy of the tribofilms formed from these blends was evaluated in terms of contact voltage and wear rate measurements. The best antiwear performance was demonstrated by the tribofilm formed from the blend containing sulphonate detergent. The results of this study provide insight into competing effects between ZDDP and different detergents that affect significantly the antiwear performance of the formed tribofilms.

Topics: Wear , Detergents
Commentary by Dr. Valentin Fuster
2008;():189-191. doi:10.1115/IJTC2008-71016.

A four-ball tester was used to evaluate the tribological performance of bismuth diamyl-dithiocarbamate in mineral oil, and compared with same types of metal additives. The results show that it exhibits better load-carrying capacities than said organic metal additives. The surface analytical tools such as X-ray photoelectron spectrometer (XPS) and Scanning electron microscopy (SEM) were used to investigate the topography, the compositions contents and the depth profile of some typical elements on the rubbing surface of worn scar. Smooth topography of worn scar further confirms that the additive showed good antiwear capacities, the results of XPS indicated that tribochemical mixed protective films consists of bismuth compounds, sulfides, sulphates and metal oxides, which contribute to improve the tribological properties of lubricants. Particularly, a larger number of bismuth containing compounds play an important role in improving extreme pressure properties of oils.

Topics: Tribology , Mechanisms
Commentary by Dr. Valentin Fuster
2008;():193-195. doi:10.1115/IJTC2008-71028.

Viscosity and compressibility have a major impact upon the efficiency and dynamic response of fluid power systems. The viscosity and compressibility of five hydraulic fluids have been measured for temperatures to 150 C and pressures to 350 MPa. A new correlation of viscosity with temperature and pressure based on the thermodynamic scaling rule of Roland et al is offered. This correlation provides a means to extend the accuracy of fluid power system models to higher pressures.

Commentary by Dr. Valentin Fuster
2008;():197-199. doi:10.1115/IJTC2008-71033.

The antioxidation properties of molybdenum dialkyldithiocarbamate (M 807)- and p, p-dioctyldiphenylalmine (V 81)- or mixed octylated and butylated diphenylalmines (V 961)-containing poly-α-olefin (PAO)-derived lubricants were evaluated by differential scanning calorimetry (DSC) and modified penn state micro-oxidation test (PMOT). DSC test measures incipient oxidation temperature (OT) and oxidation induction time (IT) of the lubricant at high temperatures and the oxidation stability of oil weight loss is measured by PMOT test. DSC test shows that OT and IT of V 81-or V 961-containing PAO were improved significantly by M 807 addition. PMOT test indicates that when combining with V 81 antioxidants, M 807 can also effectively reduce the increase in weight loss of PAO and deposits formed in oils. These results suggest that the M 807 shows a good antioxidative synergism with alkylated diphenylamine antioxidants. In addition, FTIR results from PMOT test confirm that addition of M 807 can significantly enhance the oxidation induction time of oils containing V 81 and inhibit formation of oxidation products including carbonyl bonds or hydroxyl group.

Topics: molybdenum
Commentary by Dr. Valentin Fuster
2008;():201-203. doi:10.1115/IJTC2008-71044.

Ionic liquids are salts, many of which are typically viscous fluids at room temperature. The fluids are characterized by negligible vapor pressures under ambient conditions. These properties have led us to study the effectiveness of ionic liquids containing both organic cations and anions for use as space lubricants. In the previous paper we have measured the vapor pressure and some tribological properties of two distinct ionic liquids under simulated space conditions. In this paper we will present vapor pressure measurements for two new ionic liquids and friction coefficient data for boundary lubrication conditions in a spiral orbit tribometer using stainless steel tribocouples. In addition we present the first tribological data on mixed ionic liquids and an ionic liquid additive. Post mortem infrared and Raman analysis of the balls and races indicates the major degradation pathway for these two organic ionic liquids is similar to those of other carbon based lubricants, i.e. deterioration of the organic structure into amorphous graphitic carbon. The coefficients of friction and lifetimes of these lubricants are comparable to or exceed these properties for several commonly used space oils.

Commentary by Dr. Valentin Fuster
2008;():205-207. doi:10.1115/IJTC2008-71058.

Nanofluids, a term proposed by Choi in 1995 [1], are composites consisting of solid nanoparticles with sizes varying generally from 1 to 100 nm dispersed in a liquid. Numerous nanoparticles used as oil additives have been investigated in recent years [2–7]. Results show that they deposit on the rubbing surface and improve the tribological properties of the base oil, displaying good friction and wear reduction characteristics even at concentrations below 2%wt.

Commentary by Dr. Valentin Fuster
2008;():209-211. doi:10.1115/IJTC2008-71088.

Electrical motor bearings for use in automotive control units are sometimes exposed to high-temperature operating conditions. Therefore, these bearings are sometimes packed with fluorine grease in order to meet the demand for long service life performance. In recent years, there is growing demand for lower initial torque in these same bearings without sacrificing service life performance. This paper describes the effects of fluorine grease containing silica nanoparticles on initial bearing torque of electrical motor bearings. Silica nanoparticles are effective in helping to reduce amounts of initial bearing torque and help stabilize bearing torque due to improved channeling characteristics.

Topics: Nanoparticles
Commentary by Dr. Valentin Fuster
2008;():213-215. doi:10.1115/IJTC2008-71090.

Leaf-related low adhesion problems have been present in many railway networks all over the world in the last decades. One of the counter measures adopted by many railway networks is the use of friction modifiers. However, the effectiveness of these friction modifiers is not well proven yet due to the lack of research in controlled conditions. In this paper, a laboratory study of a widely used friction modifier is presented together with another adhesion enhancer designed for wet wheel-rail contacts due to rainfall. A twin-disk roller rig has been used to investigate their performance in leaf contaminated contacts. The adhesion characteristics of both friction modifiers were examined for different slip ratios. The constituents of the friction modifiers are identified and their suitability for leafy contacts is evaluated.

Topics: Friction , Rails , Wheels
Commentary by Dr. Valentin Fuster
2008;():217-219. doi:10.1115/IJTC2008-71095.

The friction and wear behavior of ZP additive is investigated as an alternative to ZDDP (poisonous for catalyst-based emission control systems). The effect of temperature (25°C and 100°C), of load (200 N and 320 N) and the wear kinetics (5 min and 60 min experiments) were studied. Physico-chemical characterization of the wear tracks were performed by XPS. A protection against wear even at ambient temperature was found. This feature was attributed to the presence of zinc polyphosphate in the tribofilm.

Commentary by Dr. Valentin Fuster
2008;():221-223. doi:10.1115/IJTC2008-71111.

OEMs strive to obtain greater mechanical efficiency from their transmissions. It is widely accepted that this can be achieved by reducing the bulk viscosity of the transmission fluid, which reduces the churning losses responsible for a considerable amount of the inefficiency. There is ever-increasing demand on the transmission fluid to maintain or even increase the durability of the transmission components. However, the resulting loss of film thickness due to the lower viscosity fluid produces ever more arduous operating conditions for the transmission components, which rely on strong multifunctional additive packages not only to maintain durability but also to control the contact mechanics. This paper describes the development of a novel bench-top, friction screening test used to gain valuable fundamental knowledge and guide the formulation of manual transmission fluids (MTF) with tailored and controlled frictional performance.

Commentary by Dr. Valentin Fuster
2008;():225-227. doi:10.1115/IJTC2008-71190.

The acid number (AN) is considered an important indicator of oil condition, especially in terms of defining oil oxidation and the extent of acidification of used oils collected from two engines, each running with a different biogas. The results of chemometrical models comprising principal component analysis (PCA) for qualitative evaluation and partial least squares regression (PLS) for quantitative evaluation of used gas engine oils, respectively, are discussed. The investigations are based on infrared spectrometry and acid number measurements of monograde mineral-oil-based gas engine oils SAE 40. It is reported how IR spectrometry and chemometrics can be used to reveal the influence of the gas fuel type on oil aging (PCA) and for the indirect determination of the acid number (PLS). In contrast to the conventional time-consuming AN determination according to ASTM D 664, the joint use of IR spectrometry and chemometrics offers results within a few minutes. The chemometrical “measurement error” is in the range of precision and bias of the standard method.

Commentary by Dr. Valentin Fuster
2008;():229-231. doi:10.1115/IJTC2008-71211.

The interaction between lubricant molecules and the solid surface to be lubricated depends upon the surface forces which can be attractive, and repulsive. It thus forms an interactive zone above the solid surface having a band width and height of surface potential and is considered as ‘Zone of Influence’-(ZOI). Its value will vary with the nature of surface finish, distribution of alloying constituents on surface matrix and its size which play very important role in prediction of stability and failure of boundary lubrication friction including absorption and desorption of lubricant molecules. A theoretical model for the formation of boundary lubrication is proposed by combining Lennard Jones (6–12) potential to incorporate for estimating the critical temperature of boundary lubricant, friction coefficient and variation of ZOI for a given condition. Experimental values using EN 31 Ball sliding against the aluminum surface with 0.4% stearic acid as lubricant data agrees well with theoretical values.

Commentary by Dr. Valentin Fuster
2008;():233-235. doi:10.1115/IJTC2008-71221.

This study explored the effect of oil dilution with biodiesel and ultra low sulfur diesel fuel on acidity and corrosion in two lubricants with API service ratings of CJ-4 and CI-4+. The oils were diluted to levels of 0%, 5% and 10% with ultra low sulfur diesel fuel (ULSD), 20% biodiesel (B-20) and 100% biodiesel (B-100). For each case, the mixture of fuel and oil was subjected to oxidation tests using a procedure similar to ASTM D6594. The oxidation resistance of the CJ-4 and CI-4+ lubricants was similar in each case. The advanced formulation of the CJ-4 lubricant gave comparable overall performance to the CI-4+ lubricant, despite reduced levels of metallic additive compounds. The highest levels of lubricant degradation occurred when the lubricants were diluted with B-100. A substantial increase in acidity was observed when the oils were diluted with B-100. Moderate levels of copper and lead corrosion were observed in the tests with no fuel dilution, and dilution with ULSD. An order magnitude increase in corrosive wear was seen in the tests with B-100. Lower corrosion occurred with the CJ-4 lubricant.

Commentary by Dr. Valentin Fuster
2008;():237-239. doi:10.1115/IJTC2008-71261.

In this investigation, the friction and wear performance of sub-micron scale boric acid powder lubricant additives were studied during extended duration pin-on-disk experiments. The sub-micron (600 nm) and micro (4 μm) powder additives were created from 250 micron sized crystals using an 1800D SPEX Mill/Mixer. Lubricant combinations were prepared by homogeneously mixing the additives with canola oil in a vortex generator. Three different boric acid additives were investigated by combining 5.0 wt. % of 4 μm boric acid particles, 5.0 wt % of 600 nm sized boric acid particles, and a 5.0 wt % mixture of the 4 μm (2.5 wt. %) and 600 nm (2.5 wt. %) boric acid particles. A fourth additive, 5.0 wt. % of 0.5 – 5μm MoS2 powder, was also purchased and mixed with the canola oil to form a basis for comparison. It was determined that the oil mixed with a combination of micro and sub-micron scale boric acid powder additives exhibited better frictional performance than the oil mixed with micro or sub-micron boric acid additives.

Commentary by Dr. Valentin Fuster
2008;():241-243. doi:10.1115/IJTC2008-71280.

This paper describes a capacitance sensing method to detect oil debris contents. Experimental investigations are conducted for both static and dynamic measurements. Static measurements are performed by inserting small metal wires into the oil sample between the two plate electrodes of a capacitor. With a gap of 7mm between the electrodes, it was found that the capacitance measurement can detect metal wires as small as 4 mils (101 μm) in diameter. While measurements of ferrous and the non-ferrous wires show certain measurable differences, a systematic difference between the two sets of measurements cannot be established. The dynamic measurements were conducted by measuring the changes in capacitance between two plate electrodes when a small particle was dropped into the oil between the two electrodes. The passage of a metal particle can be dynamically detected. It is expected that smaller metal debris particles can be detected by reducing the spacing between the two electrode plates in microchannels.

Commentary by Dr. Valentin Fuster
2008;():245-247. doi:10.1115/IJTC2008-71292.

The frictional behaviors of soybean oil and heat modified soybean oils with different Gardner scale viscosities as additives in hexadecane have been examined in a boundary lubrication test regime (steel contacts) using Langmuir adsorption model. The free energy of adsorption (ΔGads ) of various heat modified soybean oils were compared with unmodified soybean oil. It was observed, that relative distribution of unsaturation in the molecule, average molecular weight and thus viscosity can affect adsorption on the metal surface. Heat modified soybean oils consistently have a lower ΔGads values compared to unmodified soybean oil. Lower ΔGads corresponds to stronger adsorption on the metal surface. The heat modification thus provides oils with stronger adsorption and higher viscosities, good for use in industrial lubricant formulations. This information can be used to design suitable lubricant molecules that will have optimum structure for effective metal adsorption as well as exhibit excellent boundary lubrication properties.

Commentary by Dr. Valentin Fuster
2008;():249-251. doi:10.1115/IJTC2008-71305.

Fluid properties have a major influence upon the energy efficiency of hydraulic equipment. The relationship between hydraulic fluid viscosity, shear stability and system fuel economy has been studied. New viscosity selection guidelines for hydraulic pumps and motors are proposed. These guidelines provide a means for improving the efficiency of fluid power systems.

Commentary by Dr. Valentin Fuster

Elastohydrodynamic Lubrication

2008;():253-255. doi:10.1115/IJTC2008-71022.

The full thermal EHD lubrication analysis is applied, therefore, the oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. By using the corresponding computer program, the influence of oil viscosity, pressure-viscosity and temperature-viscosity exponents, supplied oil temperature, speed and minimum oil film thickness on maximum oil film pressure and temperature, EHD load carrying capacity, and on power losses in the oil film is investigated. Small part of the obtained results is presented and discussed.

Commentary by Dr. Valentin Fuster
2008;():257-259. doi:10.1115/IJTC2008-71036.

To improve calculation of the oil supply to the main and conrod bearings in the combustion engine, it is possible to simulate plain bearings on a coupled elasto-hydrodynamic basis while allowing for partial-fill states. In an effort to advance the extended simulation technique, software has been developed by the Institute for Machine Components and Design (IMK) at the University of Kassel and validated in experiments. This paper describes the simulation software that has been developed, including the resultant findings on the lubricating oil requirement.

Topics: Engines , Bearings
Commentary by Dr. Valentin Fuster
2008;():261-263. doi:10.1115/IJTC2008-71038.

The lubrication of rough surfaces has been a focus of researchers in the field for many years now. A good deal of work has been carried out either on the experimental or on the theoretical sides of the research. From the experimental point of view it is obviously more convenient to study the lubrication of artificial roughness features such as ridges, bumps or dents rather than real, random roughness. The advantage of model roughness features is that they are well individualized, located, and characterized, thus a comparison of the surfaces geometry inside and outside an elastohydrodynamic contact can be made. The studies carried out so far have focused on the effect of the geometry of the features and that of the entrainment speed. No detailed experimental work on the effect of the lubricant properties on the behavior of the EHD films has been performed, to the authors’ knowledge. The present study uses the optical interferometry method to measure the EHD film thickness between a flat disc and a ball on which artificial ridges have been sputtered. Two lubricants, with different viscosity and pressure/viscosity coefficient are used in a range of pressures and entrainment speeds.

Commentary by Dr. Valentin Fuster
2008;():265-267. doi:10.1115/IJTC2008-71082.

An effective means of studying lubricant film rheology within EHL contacts is by detailed mapping of the temperature of the fluid and the bounding surfaces within the lubricated contact area. This provides a way of directly measuring the rheology of lubricant films under true EHL conditions. Furthermore, temperature measurement itself provides a very effective means of testing and validating computer simulations. In the current work, the experimental approach initially developed by Sanborn and Winer [1] and then by Spikes and co-workers [2], has been advanced to include a high specification infrared (IR) camera and microscope. This is a similar approach to that taken by Yagi and Kyogoku [3]. As well as the instantaneous capture of full field measurements, this has the advantage of increased sensitivity and higher spatial resolution than previous systems used. The increased sensitivity enables a much larger range of testable operating conditions; namely lower loads, speeds and reduced sliding. In addition, the range of test lubricants can be extended beyond high shearing traction fluids. One additional advantage of instantaneous full field measurements is that the weak infrared optical interference caused by the film can be observed and can used to exactly locate the centre of the contact in the resulting temperature maps. These new possibilities have been used to investigate and compare the rheological properties and compression cooling effects exhibited by a PAO, a group II mineral oil, and a traction fluid.

Topics: Temperature
Commentary by Dr. Valentin Fuster
2008;():269-273. doi:10.1115/IJTC2008-71106.

The paper is aimed at considering the classic isothermal plane EHL problem for heavy-loaded contacts lubricated by Newtonian and non-Newtonian fluids. The analytical analysis of the problem is based on matched asymptotic expansions. For pre-critical lubrication regimes asymptotic equations for pressure and gap are derived and solved numerically in the inlet and exit zones. The number of input parameters in asymptotic equations is reduced. The numerical solutions are stable. Formulas for lubrication film thickness for Newtonian and non-Newtonian fluids for pre- and over-critical regimes are derived based on the asymptotic analysis.

Commentary by Dr. Valentin Fuster
2008;():275-279. doi:10.1115/IJTC2008-71107.

The prediction of elastohydrodynamic lubrication (EHL) film thickness requires knowledge of the lubricant properties. Today, in many instances, the properties have been obtained from a measurement of the central film thickness in an optical EHL point contact simulator and the assumption of a classical Newtonian film thickness formula. This technique has the practical advantage of using an effective pressure-viscosity coefficient which compensates for shear-thinning. We have shown by a perturbation analysis and by a full EHL numerical solution that the practice of extrapolating from a laboratory scale measurement of film thickness to the film thickness of an operating contact within a real machine may substantially overestimate the film thickness in the real machine if the machine scale is smaller and the lubricant is shear-thinning in the inlet zone.

Commentary by Dr. Valentin Fuster
2008;():281-282. doi:10.1115/IJTC2008-71120.

A generalized thermal elastohydrodynamic lubrication (TEHL) model for point contact problems is developed based on an isothermal generalized Newtonian elastohydrodynamic (EHL) model recently developed. The thermal model couples FDM for lubricant energy equation and the DC-FFT method for surface temperature integration. A generalized Reynolds equation is derived considering the change of viscosity with respect to temperature, pressure and shear in three dimensions. Numerical cases are conducted to verify the model.

Commentary by Dr. Valentin Fuster
2008;():283-285. doi:10.1115/IJTC2008-71165.

This paper presents a mode-based elastohydrodynamic formulation employing low-modulus hyperelastic material models which exhibit strongly non-linear load-deflection characteristics. The mode-based method is applied to a reciprocating liquid O-ring seal, where it is found under steady axial motion that a relatively small set of modes captures the essential features of seal deformation when compared with that obtained with standard node-based methods.

Commentary by Dr. Valentin Fuster
2008;():287-289. doi:10.1115/IJTC2008-71235.

A dimensionless theoretical model of Soft Elasto Hydrodynamic Lubrication (SEHL) between partially textured elastomer and rigid counterpart is developed. The model consists of a soft elastomer with partial Laser Surface Texturing (LST) and an absolutely rigid and smooth counterpart moving relatively to each other in the presence of viscous lubricant. The elastomer surface is partially textured at its leading edge in the form of spherical micro-dimples. The pressure distribution in the fluid film and the elastic deformations of the elastomer are obtained from a simultaneous solution of the Reynolds equation and the equation of linear elasticity, respectively. Friction force and load carrying capacity are evaluated by integration of the shear stress and pressure fields in the viscous fluid film, respectively. The main goal of the present work is to study the potential of the elastomer partial LST in SEHL to friction reduction.

Commentary by Dr. Valentin Fuster
2008;():291-293. doi:10.1115/IJTC2008-71278.

At very high speeds, elastohydrodynamic (EHD) films may be considerably thinner than is predicted by classical isothermal regression equations such as that due to Hamrock and Dowson. This may arise because of viscous dissipation, frictional heating or starvation. In this paper, the contact between a steel ball and a glass disc was studied. The disc was driven at speeds of up to 20 ms−1 , and the ball was driven by tractive rolling against the disc, its speed being determined using a magnetic method. It is shown that the results, which fall well below classical predictions, are consistent with inlet shear heating at the observed sliding speeds.

Commentary by Dr. Valentin Fuster

Hydrodynamic Lubrication and Fluid Film Bearings

2008;():295-297. doi:10.1115/IJTC2008-71023.

This paper analyses the thermal and roughness effects on different characteristics of an infinitely long tilted pad slider bearing considering heat conduction through both the pad and slider. The roughness is assumed to be stochastic, Gaussian randomly distributed. Density and viscosity are assumed to be temperature dependent. The irregular domain of the fluid due to roughness is mapped to a regular domain so that the numerical method can be easily applied. The modified Reynolds equation, momentum equation, continuity equation, energy equation and the heat conduction equations on the pad and slider are coupled and solved using finite difference method to yield various bearing characteristics. The solutions with respect to different pad and slider boundary conditions are elaborated through tables and figures.

Commentary by Dr. Valentin Fuster
2008;():299-302. doi:10.1115/IJTC2008-71024.

Foil gas bearings are in widespread commercial use in air cycle machines, turbocompressors and microturbine generators and are emerging in more challenging applications such as turbochargers, auxiliary power units and propulsion gas turbines. Though not well known, foil bearing technology is well over fifty years old. Recent technological developments indicate that their full potential has yet to be realized. This paper investigates the key technological developments that have characterized foil bearing advances. It is expected that a better understanding of foil gas bearing development path will aid in future development and progress towards more advanced applications.

Topics: Gas bearings
Commentary by Dr. Valentin Fuster
2008;():303-305. doi:10.1115/IJTC2008-71041.

The model of a lubrication problem involving a Newtonian fluid with the contaminant particle smaller than the minimum film thickness is developed. The interaction of fluid and particle is considered in the model. The behavior of a particle in the lube oil is also studied. The lube oil is regarded as the continuum phase, and the lubrication problem is solved by the modified Reynolds equation to determine the film pressure and velocity. The dynamics of a particle in the lubricant are studied using Newton’s second law to determine the particle velocity, angular velocity and displacement. The effects of the particle motion including translation and rotation on lubrication characteristics are analyzed. The effect of relative velocity between particle and oil on the pressure is also discussed. The results indicate that the particle motion has a significant effect on the film pressure distribution. When the particle velocity is lower than the film velocity, the motion of particle causes a significant pressure increase. This high pressure only lasts a short time if the particle rotation is neglected. However, when considering the particle rotation, the high pressure will last a much longer time.

Commentary by Dr. Valentin Fuster
2008;():307-309. doi:10.1115/IJTC2008-71065.

This work is intended to evaluate a cavitation model based on the complete Rayleigh Plesset (RP) equation for use in squeeze film damper (SFD) calculations. The RP equation governs the variation of the radius of the cavitation bubbles at rest, surrounded by an infinite incompressible fluid and subject to an external pressure. This equation is obtained from the momentum equation and it takes into account the ensemble of the phenomena related to the dynamics of the bubbles (surface tension, damping, inertia). All the terms in the RP equation will be taken into account in the present work. Numerical results will be compared with experimental data obtained by Adiletta and Pietra [1]. The results then underline the influence of the effects contained in the RP equation on the pressure field.

Topics: Cavitation , Dampers
Commentary by Dr. Valentin Fuster
2008;():311-313. doi:10.1115/IJTC2008-71068.

Lubrication theory has been one of the most successful and widely used theories in all of engineering and applied science. Therefore, researchers from tribology’s ‘traditional’ wing would be surprised to know that lubrication theory is not at all treated as a given by large group of nontraditional users. In a wide range of conditions applicable to many researchers in microelectromechanical systems (MEMS), experimental results seem to indicate that forces separating surfaces vary according to film thickness to the power minus one, rather than minus three, as lubrication theory requires — a large fundamental discrepancy indeed. Hydrodynamic forces are not generally used to separate surfaces, but arise incidentally, and are usually the largest source of parasitic losses. Clearly, they must be accounted for in the design process.

Commentary by Dr. Valentin Fuster
2008;():315-317. doi:10.1115/IJTC2008-71073.

In the designs and analysis of engine bearings for automobiles, the precise prediction of the lubrication condition in severe condition is important. In the mixed-elasto-hydrodynamic lubrication analysis, the contact between the projections of surface roughness distributed stochastically is usually considered. This paper describes a theoretical model under the mixed lubrication in the microgrooved bearing. In this modeling, it is assumed that the section shape of microgrooved bearing alloy takes the circular arc form. In the part where contact is caused, the contact pressure is calculated by the Hertzian equation. The elastic deformation of the bearing by the mixed pressure with which oil film pressure and contact pressure are mixed by each allotment ratio is considered. Moreover, the balance requirement between the sum total of mixed pressure on bearing surface and the journal load is met. Under such an assumption, the numerical calculation model is newly obtained to predict the bearing performance in the mixed lubrication of microgrooved bearing. The numeric solutions of EHL based on the mixed lubrication are compared with EHL based on the fluid lubrication. The predicted oil film thickness at the center of bearing by the mixed lubrication model is remarkably thin compared with that by the fluid lubrication model. This shows that the load ability of the oil film thickness decreases by generating contact.

Topics: Lubrication , Bearings
Commentary by Dr. Valentin Fuster
2008;():319-321. doi:10.1115/IJTC2008-71075.

In this paper a study is presented towards the effect of elastic deformation of the bearing surface of axial externally pressurized centrally pivoted modular air bearings. To this purpose shallow pocket, tapered and grooved air bearings are selected, which look the most promising to apply in small dimensions and with very small film thickness. The elastic deformation is calculated numerically by simultaneously solving the 2D-Reynolds equation for compressible fluids and the 3D-elasticity equation. The effect of the elastic distortion on the bearing performance is visualized in graphs presenting the load capacity and flow as a function of the film thickness. The results obtained from numerical calculation are verified experimentally.

Topics: Deformation , Bearings
Commentary by Dr. Valentin Fuster
2008;():323-325. doi:10.1115/IJTC2008-71078.

In the present paper the influence of wall slip on the EHD lubrication performance is studed. Steady state and dynamically loaded bearings are investigated. The results show that well-chosen slip/no-slip regions can increase the film thickness and considerably decrease the power loss.

Commentary by Dr. Valentin Fuster
2008;():327-329. doi:10.1115/IJTC2008-71093.

Grooved thrust air bearings are widely used to support high-speed, low-loaded shafts in many rotating systems because of their low friction, noiseless operation, and simple structure. Several types of groove geometries, such as straight line, spiral, and herringbone, are commonly used in actual applications. Among them the spiral groove is mainly used. However, as far as the authors know, there is no theoretical evidence that the spiral groove is an optimized groove geometry in all possible groove geometries. This paper describes the optimum design for the groove geometry of thrust air bearings according to various objective functions, such as air film thickness, bearing torque, dynamic stiffness of air film, and combinations of same. In an optimum design, groove geometries are expressed by the third degrees of spline function, and sequential quadratic programming is used as the optimization method. We found that groove geometry optimizing air film thickness or friction torque takes the form of a spiral groove. The geometry optimizing the dynamic stiffness is the modified spiral groove. Some numerical results are compared with the measured data, and good agreements can be seen between them.

Commentary by Dr. Valentin Fuster
2008;():331-333. doi:10.1115/IJTC2008-71096.

Water lubricated bearings are often used to support propeller shaft of modern ships. These environmentally friendly bearings are less expensive than those oil lubricated because of simplicity of the bearing unit and lower material costs [1].

Topics: Measurement , Bearings , Water
Commentary by Dr. Valentin Fuster
2008;():335-337. doi:10.1115/IJTC2008-71097.

The static and dynamic properties of tilting-pad journal bearings with controllable radial oil injection are investigated. The tilting pads are modelled as flexible structures and their dynamics are described using a three dimensional finite element framework and linear elasticity. The oil film pressure and flow are considered to follow the modified Reynolds equation, which includes the contribution from controllable radial oil injection. The Reynolds equation is solved using a two dimensional finite element mesh. The rotor is considered to be rigid. The servo-valve flow is governed by a second order ordinary differential equation, where the right hand side is controlled by an electronic input signal. The constitutive flow pressure relationship of the injection nozzles is that of a fully developed laminar velocity profile and the servo-valve is introduced into the system of equations by a volume conservation consideration. The Reynolds equation is linearized with respect to displacements and velocities of the nodal degrees of freedom. When all nodal points satisfy the static equilibrium condition, the system of equations is dynamically perturbed and subsequently condensed to a 2 by 2 system, keeping only the lateral motion of the rotor. As expected, rotor stability is heavily influenced by the control parameters.

Commentary by Dr. Valentin Fuster
2008;():339-341. doi:10.1115/IJTC2008-71099.

The start-up friction coefficient is a parameter which is very useful for engineers who design hydrodynamic bearings. It is most often issued from an approximation. Several studies can be found in the literature on this topic but most of them are concerned with air bearings or are only numerical. Some studies are more general and deal with the transient thermal behaviour in journal bearings as well as in thrust bearings. Other studies deal with the friction coefficient during running, at a fixed rotational speed. The aim of this study is to provide experimental measurements of the bush torque during start-up of plain journal bearings, varying the specific pressure. Thus, the friction coefficient at start-up (deduced from measurements) is obtained for four different bearings varying feeding conditions, radial clearance and length.

Commentary by Dr. Valentin Fuster
2008;():343-345. doi:10.1115/IJTC2008-71100.

There has recently been much interest in the micro-texturing of slider bearings, where many tiny micro-pockets are incorporated into one of the bearing surfaces. Proponents of this technology have reported that the application of micro-texture to sliding bearing surfaces can result in up to 40% reduction in friction, when the two bearing surfaces have low or zero convergence ratio, i.e. are near parallel [1]. The authors have shown previously, through a series of one dimensional analyses, that the ability of such bearings to generate load support can be explained by a lubrication mechanism termed “Inlet Suction” [2, 3]. In the current paper, these one dimensional analyses are extended to two dimensions with the inclusion of a mass-conserving cavitation algorithm based on Elrod [4]. This allows for a more complete understanding of “Inlet Suction” in real bearing geometries.

Commentary by Dr. Valentin Fuster
2008;():347-349. doi:10.1115/IJTC2008-71112.

The present work deals with the application of the Lattice Boltzmann Method (LBM), which is a relatively new Computational Fluid Dynamics approach, to fluid film lubrication. LBM accounts for the inertia forces, while being easier to implement than Navier-Stokes solvers for complex geometric configurations. The LBM solution for a classic case is presented in comparison with the analytic Reynolds solution and the numeric solution obtained with Navier-Stokes solvers.

Commentary by Dr. Valentin Fuster
2008;():351-353. doi:10.1115/IJTC2008-71124.

A compliance operator, Eji , is often utilized to evaluate elastic deformation in the simulation of transient and steady state elastohydrodynamic lubrication of conformal-contact systems. The values of the compliance operator represent the elastic responses of all nodes when only one node is under a unit load. The accuracy of compliance operator values, computational cost, and storage size are important issues. Based on our previous study on steady-state conformal-contact elastohydrodynamic lubrication systems, an advanced method of selective-fine-mesh with selective storage is suggested, and a special technique of combined selective-fine-mesh with selective storage mapping is proposed. These two techniques enable an efficient elasticity procedure for the simulation of transient conformal-contact elastohydrodynamic lubrication systems.

Commentary by Dr. Valentin Fuster
2008;():355-357. doi:10.1115/IJTC2008-71153.

In this paper a new contactless transport system for thin, flat products, such as glass substrates and silicon wafers, is introduced. The transport function is realized using viscous traction on the product surface. After an explanation of the operating principles, a mathematical model is presented. Preliminary computations indicate that the generated acceleration is approximately 2.5 m/s2 for a glass substrate with a surface area of 2 m2 and a thickness of 0.7 mm. Furthermore, the vertical bearing stiffness for the considered actuator geometry of the system is 4.2·106 N/m, depending on the properties of the inflow and outflow points of the actuator cells.

Topics: Traction
Commentary by Dr. Valentin Fuster
2008;():359-361. doi:10.1115/IJTC2008-71170.

In this paper, the influence of circumferential scratches on the thermohydrodynamic performance of a partial (lobe) journal bearing is studied. The bearing damage is characterized by four factors: the area of the scratched region, the density of the scratches within the affected area, the relative position of the scratched region and the relative depth of the wear defects. The bearing performance is characterized by minimum film thickness, average oil temperature, maximum pressure, friction torque etc., at imposed magnitude and direction of the load. A numerical hydrodynamic model with global thermal effects is used for studying the influence of the different wear related parameters on the bearing performance. The results permit to predict the overall performance loss due to the circumferential wear marks, for different wear profiles. The types of wear profiles that can lead to the bearing destruction (characterized by a critical minimum film thickness) are also investigated.

Commentary by Dr. Valentin Fuster
2008;():363-365. doi:10.1115/IJTC2008-71172.

The present work presents a bulk flow based numerical analysis of squeeze film dampers (SFD) provided with circumferential grooves, feeding orifices and piston ring sealing devices. This means that the bulk flow model for inertia dominated flow regimes (high Re) was adapted for taking into account thin film discontinuities, mass sources located in the thin film zone (for modelling feeding orifices) or on the boundaries (piston ring seals with open slots). In the present work the rotor is whirling on a centered whirl and the bulk flow equation are solved by following a time step integration.

Commentary by Dr. Valentin Fuster
2008;():367-369. doi:10.1115/IJTC2008-71177.

This work describes in details the steps involved within the mathematical modelling of multibody systems (rigid and flexible) interconnected via controllable thin fluid films. The dynamics of the mechanical components are described with help of multibody dynamics and finite element method. In this paper, the methodology is applied to reciprocating machines such as hermetic reciprocating compressors and internal combustion engines. In previous studies [1], it has been shown that for a light duty vehicle, the friction losses may reach until 48% of the total energy consumption of an engine and from that, almost 30% are coming from bearings and crankshaft. Therefore, considering that the dynamics of the fluid films in the journal bearings can be actively controlled by means of different types of actuators, allowing significant reduction of wear and vibrations, one of the aims of this paper is to study the feasibility of applying active lubrication to the main journal bearings of reciprocating machines. In this framework the paper gives a theoretical contribution to the combined fields of fluid-structure interaction and active vibration control. The hydrodynamic pressure distribution for an active lubricated finite journal bearing dynamically loaded can be calculated by numerically solving the modified Reynold’s equation [2], by means of finite-difference method and integrated over the pressure area in order to obtain the dynamic reaction forces among components. These forces are strongly nonlinear and dependent on the relative kinematics of the system. From the point of view of active lubrication and specifically considered the case of a dynamically loaded journal bearing, the injection pressure should be controlled in the time domain. However, taking into account that the pressures and reaction forces in a reciprocating machine have a cyclic behaviour, the fluid film thickness of the main bearings may be modified by controlling the oil pressure injection, depending on the crank angle and the load bearing condition. It can be mentioned that the pressure and flow may be controlled by mechanical cam systems, piezoelectric nozzles [3] [4] or servovalves [5] [6], therefore, an adequate control strategy has to be defined. The fluid film forces are coupled to the set of nonlinear equations that describes the dynamics of the mechanical system. Such a set of equations is numerically solved giving some insights into the following parameters: a) maximum fluid film pressure, b) minimum fluid film thickness, c) maximum vibration levels and d) viscous frictional forces. The behaviour of such parameters is investigated when the system operate with conventional hydro-dynamic lubrication, passive hybrid lubrication and controlled hybrid lubrication.

Commentary by Dr. Valentin Fuster
2008;():371-373. doi:10.1115/IJTC2008-71183.

This work is a thermal simulation extension to the modeling of the fluid flow inside a reservoir-extended porous slider bearing [1]. The bearing system consists of a lubricating film, porous medium, and a reservoir. The velocity flow field generated by the modeling of the fluid flow is used as an input to solve the heat transfer problem. The small parameter used in the asymptotic expansion is the ratio of the reservoir depth to the length of the slider. The thermal effects upon the operation of this system are discussed.

Commentary by Dr. Valentin Fuster
2008;():375-377. doi:10.1115/IJTC2008-71187.

We present a new method to determine the film height distribution of the load-optimal hydrodynamic slider for multiple operating conditions. The use of the method is demonstrated in two examples, the second of which is based on the load conditions in an artificial hip-joint during a standard walking cycle. The results of this optimization may lead to an improved design of the replacement hip-joint, resulting in less wear and an improved lifespan.

Topics: Optimization
Commentary by Dr. Valentin Fuster
2008;():379-381. doi:10.1115/IJTC2008-71195.

Gas foil bearings (GFBs) enable efficient, reliable and maintenance free operation of high-power-density microturbomachinery (< 200 kW). High speed rotors supported on bump-type GFBs, however, are prone to show large-amplitude subsynchronous motions albeit reaching limit cycle performance. Presently, commercial GFBs are simply modified to introduce a mechanical preload that induces a hydrodynamic wedge to generate more load support and direct stiffnesses. Three metal shims inserted under the bump strip layers and in contact with the bearing housing create a multiple lobe clearance profile at a very low cost. Shaft speed coastdown measurements reveal the rotordynamic performance of a rotor supported on original GBFs and (modified) shimmed GFBs. The later GFBs determine a raise in the rotor-bearing system natural frequency, as expected, and also act to delay the onset speed of large-amplitude subsynchronous motions. Predictions of imbalance response implementing linearized bearing force coefficients are in good agreement with measured amplitudes of synchronous response for both GFB configurations, original and modified.

Topics: Bearings
Commentary by Dr. Valentin Fuster
2008;():383-385. doi:10.1115/IJTC2008-71213.

Rigid rotor models are widely used for the rotor-bearing stability analysis. This paper presents parametric studies for a squeeze film damper (SFD) using as parameters pressurization, the retainer spring support, and viscosity under the 2π- and π-film assumptions. The results are then validated by direct numerical simulation of the journal orbit.

Topics: Dampers
Commentary by Dr. Valentin Fuster
2008;():387-389. doi:10.1115/IJTC2008-71222.

This paper presents a numerical investigation into the dynamics and pressure distribution of a full hydrostatic bearing with six pockets. The pockets are rectangular and the effects of their depth are investigated by choosing a deep pocket type (0.25in) and a shallow one (0.01in), respectively. Parametric studies are partitioned in operational and geometric. The operational ones include the effects of bearing eccentricity and shaft rotation speeds. The geometric ones are the concentric clearance and the pocket depth. Whether deep or shallow the pockets have the same footprint projected base. It was found that the pocket’s depth causes the pressure to be either uniform across the pocket (deep pocket) or vary (shallow) in a ramp increasing in the direction of rotation. In both cases a Raleigh step effect is observed at the downstream edge of the pocket. More interestingly, in the convergent region the hydrodynamic effect on the lands creates pressures that are significantly higher than those in the hydrostatic pockets when the rotational speeds are high. It was found that for narrow clearance the effect of pocket depth on cross-coupled and direct stiffness is negligible, while for the large clearance the stiffness decreases with the increase in the pocket depth. For either type of pocket the stiffness increases with eccentricity and rotation whether the concentric clearance is large or small.

Commentary by Dr. Valentin Fuster
2008;():391-393. doi:10.1115/IJTC2008-71231.

This paper presents the behavior of a porous cylindrical bearing that has a self-contained system that excludes the action of an external circulating pump for the lubricating fluid. The bearing contains a stationary porous bushing whose inner surface faces the active space of the bearing while the outer one faces a wrap-around reservoir. Under the cam-like effect of the eccentric shaft the fluid circulates in and out of the bearing clearance and reservoir respectively, without the help of an outside pump. The circulating mechanism is described and numerically simulated. The corresponding pressure profiles and flow patterns are also shown and discussed. The physical implementation of such a bearing system would lead to complete elimination of significant portions of the bearings lubricating circuit.

Commentary by Dr. Valentin Fuster
2008;():395-397. doi:10.1115/IJTC2008-71243.

This paper presents a numerical investigation of the coefficients used in the transition Reynolds equation model [1], which is applicable to the transition flow of thin films in long journal bearings. Calculations show that the overall coefficient A1 (Reh ) increases with the increase of Reynolds number, while the other overall coefficient A2 (Reh ) decreases with the increase of Reynolds number. A1 (Reh ) is always positive and A2 (Reh ) is always negative. The magnitude of both A1 (Reh ) and A2 (Reh ) is larger for the larger clearance ratio C/R at the same Reynolds number. The curves of A1 (Reh ) or A2 (Reh ) for different clearance ratios tend to collapse to one when Taylor number is used as the abscissa, particularly when Taylor number is less than 70. With these coefficients determined the model presented in [1] is fully usable.

Topics: Equations
Commentary by Dr. Valentin Fuster
2008;():399-401. doi:10.1115/IJTC2008-71245.

Traditional tribology references typically provide the cylindrical (or polar) Reynolds equation, which may not be applicable when entrainment velocities vary with radius and/or angle. However, entrainment velocities are known to vary with angle for some cases of pin-on-disk contact and chemical mechanical polishing (CMP). A form of Reynolds equation is derived in this manuscript from the Navier-Stokes equations without entrainment velocity assumptions. Two case studies, related to pin-on-disk and CMP, are presented and results from the derived form of Reynolds equation are compared with results from the traditionally used form. Pressure distributions obtained from the two forms of Reynolds equation varied greatly in magnitude and in pressure shape. Therefore, a new form of the cylindrical Reynolds equation derived in this manuscript is used when entrainment velocities are known to vary with radius or angle.

Topics: Disks , Equations
Commentary by Dr. Valentin Fuster
2008;():403-405. doi:10.1115/IJTC2008-71274.

Foil bearings offer several advantages over traditional oil-lubricated bearings in closed Brayton Cycle (CBC) systems, such as those proposed for long-term space power generation. Proposed CBCs require foil bearings to use an inert gas lubricant at pressures as high as 3.0 MPa as the bearing lubricant. The High Pressure Rig (HPR) at the NASA Glenn Research Center is used to measure foil bearing power loss using potential CBC working fluids at and beyond proposed CBC peak pressures. In the current study foil journal bearing power loss is measured in helium, nitrogen, and carbon dioxide from atmospheric pressure to 4.83 MPa and at shaft speeds from 10 krpm to 42 krpm. Bearings operating in helium show no increase in power loss with increasing pressure for the conditions tested. Bearings operating in nitrogen show increases in power loss with increasing pressure at speeds above 19 krpm, while increases in bearing power loss during carbon dioxide testing were seen at 15 krpm. At speeds above these thresholds, power loss is shown to increase more rapidly in carbon dioxide than in nitrogen. Results suggest that bearing power loss performance is dependent on both gas density and shaft speed.

Commentary by Dr. Valentin Fuster
2008;():407-409. doi:10.1115/IJTC2008-71283.

Advanced closed loop power generation cycles are under consideration for a variety of terrestrial and aerospace power systems [1]. High pressure closed brayton cycles (CBC) and supercritical cycles (SCS) offer an advantage where the cycle working fluid can also be used as the lubricant for the fluid film bearings that support the high speed turbomachinery. Unfortunately the use of supercritical carbon dioxide as a lubricant is not well understood. In the supercritical condition fluids that are typically thought of as ideal gases take on a significantly different characteristic. While these fluids typically maintain gas-like absolute viscosities, their densities are liquid like. The combination of these effects leads to the emergence of inertial effects in fluid film bearings. In addition to the inertial effects that are brought on by the high fluid density, the temperature of the lubricant cannot be controlled independently of the thermodynamic process. This situation leads to technical challenges in maintaining dimensional stability and clearance control between the rotating and stationary surfaces of the bearings.

Commentary by Dr. Valentin Fuster
2008;():411-413. doi:10.1115/IJTC2008-71290.

The effect of the wave amplitude on the dynamic behavior of a three-wave journal bearing is analyzed. A transient method was used to predict the wave bearing behavior after Fractional Whirl Frequency (FFW) occurs. Dynamic trajectories, Poincaré maps, and FFT analyses are used to study the dynamic behavior of the journal bearing. It was found that the threshold of stability is strongly influenced by the wave amplitude. However, even when FFW occurs, the journal maintains its trajectory inside the bearing clearance. The predicted data were found to be in good agreement with the experimental results obtained at the NASA GRC.

Commentary by Dr. Valentin Fuster
2008;():415-417. doi:10.1115/IJTC2008-71295.

An oil lubricated wave bearing has been tested on a new rig for fluid film journal bearings at NASA Glenn Research Center in Cleveland, Ohio, USA. The tests are intended to evaluate the rig possibilities of running without misalignment. Data recorded under steady-state conditions included oil flow rate, input, output and oil supply temperatures, and shaft position. Two sets of data were collected: i) by varying the load to 9000 N at a constant shaft rotation speed of 8,000 RPM and ii) by varying the speed from 9,000 to 12,000 RPM at constant load of 6,700 N. The temperatures of the metal sleeves were also recorded. Vibration levels at four locations were displayed and recorded. These measurements indicated that damping from the wave bearings provides significant attenuation of the vibration generated by the air turbine that drives the rig.

Commentary by Dr. Valentin Fuster
2008;():419-421. doi:10.1115/IJTC2008-71297.

Past studies have shown that oil lubricants with boric acid powder additives have the ability to both lubricate and support large amounts of applied load. In order to develop the predictive capability for a journal bearing that is lubricated with an oil/boric acid mixture, a multi-phase simulation was created which used particle dynamics and computational fluid dynamics (CFD) modeling.

Commentary by Dr. Valentin Fuster

Rolling Element Bearings

2008;():423-426. doi:10.1115/IJTC2008-71009.

Since being originally introduced, cylindrical rolling element bearings have been significantly improved, in terms of their performance and working life. A major objective has been to decrease the Hertz contact stresses at the roller–raceway interfaces, because these are the most heavily stressed areas in a bearing. It has been shown that bearing life is inversely proportional to the stress raised to the ninth power (even higher). Investigators have proposed that under large normal loads a hollow element with a sufficiently thin wall thickness will deflect appreciably more than a solid element of the same size. An improvement in load distribution and thus load capacity may be realized, as well as contact stress is also reduced considerably by using a bearing with hollow rolling elements. Since for hollow rolling element no method is available for the calculation of contact stresses and deformation. The contact stresses in hollow members are often calculated by using the same equations and procedures as for solid specimens. This approach seems to be incorrect. Recently, the Finite Element Analysis (FEA) has been successfully used to evaluate contact problems for the roller bearings. Investigations have been made for hollow rollers in pure normal loading. Different hollowness percentages ranging from 0% to 90% have been analysed in FEA software to find the optimum percentage hollowness which gives minimum stress and finally longest fatigue life.

Commentary by Dr. Valentin Fuster
2008;():427-431. doi:10.1115/IJTC2008-71147.

Modern Wind Turbines adjust their blades orientation at different wind speeds for power control and optimum energy production. A big slewing ring about 2 metres diameter placed at each blade root, allows the blade orientation withstanding highly variable heavy loads, vibrations, continuous rotating oscillations and severe ambient conditions. The blade pitch system design and control strategy in a WTG is strongly conditioned by the load dependant friction of the bearing that shall be accurately defined for cost-effective designs. The pitch system is also the main brake of the rotor requiring high reliability for their components under fatigue loads, and in particular for the slewing rings due to its inherent difficulties for maintenance or replacement. The present methodology allows the fatigue and friction estimation of slewing rings, based on rolling bearing models and classical theories like Hertz, Lundberg-Palmgren, and Miner fatigue cumulative damage. This approach simulates the stress supported by each ball in the contact with the raceways, estimates the bearing friction due to these contact stresses, and the fatigue life of the overall bearing.

Commentary by Dr. Valentin Fuster
2008;():433-435. doi:10.1115/IJTC2008-71154.

A computer program for dynamic analysis of ball bearings, in which a simple mixed lubrication model for the retainer/race and retainer/ball contact is incorporated, is developed and retainer instability in reaction wheel bearings is investigated. Results show that an increase in the amount of oil promotes the instability and that a retainer with rectangular pockets is more stable than one with circular pockets.

Commentary by Dr. Valentin Fuster
2008;():437-439. doi:10.1115/IJTC2008-71254.

This paper reports on a theoretical study on the fluid-assisted surface cracking in lubricated rolling-sliding contact. An analytical 2D model, validated by the CFD simulations, is developed to describe the transient process of lubricant seepage into a surface-breaking crack during the contact loading. The fluid-crack interaction is studied by FE calculations.

Commentary by Dr. Valentin Fuster
2008;():441-443. doi:10.1115/IJTC2008-71269.

In machines with linear-guideway type recirculating linear ball bearings (linear bearings), to prevent shutdown of machines caused by unexpected-failures of linear bearings, there has been an increasing demand for the development of defect (especially flaking) detection techniques of linear bearings by using vibration or acoustic emission (AE) measurements. Although studies [1–3] on vibrations of linear bearings without defects have been carried out, work on vibrations and AE of defective linear bearings are few [4, 5].

Commentary by Dr. Valentin Fuster
2008;():445-448. doi:10.1115/IJTC2008-71296.

An explicit dynamic model of a deep groove ball bearing under a radial load is proposed. All components are treated as rigid bodies whereas the bearing outer ring flexibility is taken into account using fixed interface component mode synthesis (CMS). The classical lubricated Hertzian contact theory is used to calculate elastic deflections and non-linear contact forces. The dynamic loading of the outer ring interface nodes is ensured using C2 -continuous rational cubic splines. A Runge-Kutta-Felhberg 4th/5th order integration scheme is used to solve the dynamic equilibrium of all components. Time and frequency domain analyses are then carried out to investigate the dynamic behaviour of the ball bearing. The accuracy of these works is validated by comparison with the results of an analytical model and a model based on finite elements proposed in prior researchs.

Commentary by Dr. Valentin Fuster

Machine Components Tribology

2008;():449-451. doi:10.1115/IJTC2008-71043.

A numerical soft EHL (elastohydrodynamic lubrication) model of a reciprocating hydraulic step seal has been used to analyze seal performance. The model consists of coupled steady state fluid mechanics, deformation mechanics, contact mechanics and thermal analyses, with an iterative computational procedure. Results for a typical step seal are compared with those of a double lip U-cup seal.

Commentary by Dr. Valentin Fuster
2008;():453-455. doi:10.1115/IJTC2008-71060.

Steel wires are critical load-bearing components in a wide range of applications such as elevator, cranes, mine haulage etc. The traction machine of elevator which transmits power to wire rope causes micro-slip between wire rope and sheave during reciprocating action. The lubrication condition of wire rope is also changed due to the lack of grease. This study focuses on the wear behavior of steel wire and effect of both dry and grease conditions by using the rolling/sliding contact wear tester done under various slip ratios and rolling speeds. The experimental results of the wear volume curve against the number of revolutions under the grease condition are compared with the results under dry condition. The worn surface of steel wire and the size of wear particles were observed by SEM. In order to quantify the wear amount of steel wire we established an equation and finally obtained the wear coefficient.

Commentary by Dr. Valentin Fuster
2008;():457-459. doi:10.1115/IJTC2008-71061.

In this paper frictional heating effect on the propagation of surface crack under Hertzian contact loading based on the fracture mechanics is investigated. For theoretically analyzing this effect, we estimated stress intensity factor of surface crack-tip in shear mode under Hertzian contact sliding friction. Theoretical results showed that thermal load (Th), Peclet number (Pe) and crack angle (β) are very important factors on the propagation of surface crack under Hertzian contact loading. When thermal load (Th) and Peclet number (Pe) are constant, maximum variable range of stress intensity factor, ΔK II, is located in the range of surface crack angle 130°∼150°.

Commentary by Dr. Valentin Fuster
2008;():461-463. doi:10.1115/IJTC2008-71069.

Many researches are conducted on the effect of particles on lubricated friction and wear of contact surfaces, which shows that particles between two contact surfaces increase lubricated friction and do a great effect to the wear of two sliding surfaces. Similar phenomena also occur when a hard containment particle is being entrained into the lubricated contact. This paper presents a simple two dimensional model which describes the entrainment of particles in a gap. Considering elasticity of the sliding surfaces, the model treats conditions under which a spherical particle of size d can be entrained into a gap between a sliding surface and a fixed plane surface. These conditions are determined by size of the particles, d, and friction coefficients between the particle and the sliding surface, and the particle and the plane, denoted by μs and μp respectively. For the particles to be entrained into the contact, this model predicts the maximum of particles size and the relationship of μs and μp .

Commentary by Dr. Valentin Fuster
2008;():465-467. doi:10.1115/IJTC2008-71074.

This paper describes different passive magnetic bearing configurations and associated pros and cons. It details the techniques used by different inventors to overcome the limitations of passive magnetic bearing, such as low damping, low stiffness and its inherent instability. To overcome these limitations different hybridization techniques used by different researchers have been illustrated. Finally, a new hybrid bearing configuration invented by the authors has been described.

Commentary by Dr. Valentin Fuster
2008;():469-471. doi:10.1115/IJTC2008-71081.

Twin disc tests are commonly used to study wear in railway materials. In this work the implementation of a numerical model of the twin disc arrangement is given, which reproduces the distribution of tangential forces over the contact patch between the two discs. Wear is subsequently calculated by relating the forces and creepage between the two discs using three different wear functions found in the literature. The resulting wear rates are compared with experimental data found at the University of Sheffield for discs made of common railway wheel and rail steels. This allows a comparison and assessment of the validity of the different wear algorithms considered.

Commentary by Dr. Valentin Fuster
2008;():473-475. doi:10.1115/IJTC2008-71094.

More than ever, the development process in the field of automotive engineering requires shorter development cycles for a higher range of vehicles. In order to meet the ambitious goals of the automotive industry regarding CO2 reduction and at the same time increasing the vehicle requirements in view of comfort and dynamics, it is necessary to decrease the drivetrain weight. Parallel, research and development in the field of automotive engineering is e.g. characterized by power upgraded combustion engines and by higher transmission ratios in the drive train. As a result, the rising of the weight specific power should not involve a reduction of the drivetrain reliability and the related increase of the failure probability. These demands lead to the conclusion that lightweight and robust designs have to be applied for all drivetrain components. All these factors affect the dimensioning of today’s clutch systems and step up the demands for future clutch systems. Thus, organic friction facings are getting closer to their limits due to temperature resistance. Within the frame work of the Centre of Excellence in Research CER 483 “High performance sliding and friction systems based on advanced ceramics” one approach is to apply advanced ceramics as friction material for e.g. a dry running motor vehicle clutch using the ceramic specific benefits as wear and temperature resistance combined with lightweight design to fulfill today’s demands.

Commentary by Dr. Valentin Fuster
2008;():477-479. doi:10.1115/IJTC2008-71098.

This paper presents a numerical model of mechanical seal operating in mixed lubrication. It is based on a deterministic approach using realistic roughness description. This model is able to simulate the transition from mixed to hydrodynamic lubrication regimes corresponding to a minimum of the friction coefficient. Moreover it is able to work with non Gaussian surfaces and oriented surfaces.

Topics: Lubrication , Modeling
Commentary by Dr. Valentin Fuster
2008;():481-483. doi:10.1115/IJTC2008-71101.

In the field of mechanical engineering there is a constant demand of shorter development cycles combined with increasing requirements concerning product quality and costs. To meet these demands, which can be seen as a main success factor of industrial enterprises, suitable development processes are needed. The main goal of the IPEK is to provide tools, methods and processes to support the development of complex technical systems. Representative for high loaded frictional systems with low pressure and high sliding speed, a lubricated multi-disc clutch is used as a demonstrating system. With this demonstrating system the functionality of the developed methods is validated. For this reason a prototype of a lubricated multi-disc clutch system with ceramic friction materials has been developed and built up. Experimental investigations confirmed the potential concerning power density resulting from the use of ceramic friction materials in combination with a ceramic suitable systems design.

Topics: Ceramics , Disks
Commentary by Dr. Valentin Fuster
2008;():485-487. doi:10.1115/IJTC2008-71121.

A novel piston pin design is proposed for diesel engines having higher load capacity than current pin designs. This shaped piston pin incorporates a stress-reducing profile with lubricant film thickness enhancing features on the surface. Three dimensional FEA models were built and are used to model the dynamic contact between the piston and connecting rod interfaces with a standard piston pin and the new piston pin design.

Commentary by Dr. Valentin Fuster
2008;():489-491. doi:10.1115/IJTC2008-71152.

This paper illustrates the effect of misalignment on performance of cement kiln partial journal bearing. The performance parameters include minimum film thickness, load carrying capacity, power loss and dynamic coefficients such as elastic coefficients and damping coefficients. The bearing analyses have been carried out using a commercial rotor dynamics software package.

Commentary by Dr. Valentin Fuster
2008;():493-497. doi:10.1115/IJTC2008-71160.

A very important condition for describing the frictional behavior of a piston ring correctly is knowledge about the amount of lubricant present. It is often assumed that piston rings operate under fully flooded conditions, but this is not the case in real life operation. In large two-stroke engines the cylinder oil is supplied periodically to the bearing at discrete locations on the cylinder liner. The shifting in lubrication regimes and the non-uniform oil distribution opens for the possibility of starved conditions for the piston ring bearing. Therefore it is important to measure the oil distribution on the liner as a function of the operating conditions. The amount of lubricant available is reflected in the friction absorbed in the bearing. The paper describes an investigation of the tribological condition between a piston ring and cylinder. A test apparatus is used to study the interaction between a piston ring and a cylinder liner.

Commentary by Dr. Valentin Fuster
2008;():499-501. doi:10.1115/IJTC2008-71163.

In this paper an innovative air driven spindle for micro cutting applications is presented. The spindle uses a viscous traction concept which has the advantage that the viscous traction forces can act directly on the cylindrical part of the tool, which makes the tool-holder redundant. Furthermore, the tool can be actuated in the axial direction within the housing. In this paper the concept of the viscous turbine, a design of a prototype spindle along with the traction and load-capacity of the spindle are discussed.

Commentary by Dr. Valentin Fuster
2008;():503-505. doi:10.1115/IJTC2008-71219.

Traditional engine wear models link wear rates to detailed material and contact conditions. Effects of lubricant and additive characteristics are often described by and lumped in empirical parameters such as wear coefficients. With increased environmental concern that lubricant and additives impact emissions and exhaust treatment catalysts, there is increasing interest in optimizing the lubricant and additive combination while providing adequate protection against engine wear. While the mechanisms of anti-wear film formation have been studied in great detail, engineering models of component wear incorporating lubricant chemistry are just beginning to emerge. These models build on the understanding of anti-wear film mechanisms developed and published over the years on the one hand, and on classical wear models for rough surfaces on the other. One such model has been applied to study the sensitivity of the following effects on component wear: Kinetics of ZDDP, which vary according to type and formulation, surface characteristics, contact loads and temperatures, as well as the replenishment and depletion rates of lubricant-additive to the surface-lubricant interfaces. The calculations are done for several engine components under real-world operating conditions.

Topics: Wear , Engines , Lubricants
Commentary by Dr. Valentin Fuster
2008;():507-509. doi:10.1115/IJTC2008-71256.

Using a nonlinear model of a two disk brake system, coupled equations of motion are found for their frictional interaction. The mathematical formulation relates the tribological events at micron scale and the macroscopic scale vibration response of a two-disk brake system. This is accomplished by a viscoelastic account of interaction at the micron scale, its statistical quantification through the approximate analytical representation of the statistical expectation of contact force and the introduction of the contact force into the macro-scale dynamics of the two-disk system. Steady-state analysis of the system establishes the relation between friction torque and speed and supports observed behavior of many mechanical systems with friction. It is shown that, as a result of coupling of the macro-system’s dynamics and contact, there are combinations of parameters at the micro- and macro-scale that yield negative slope in friction torque/sliding speed relation, a well known source of dynamic instability. This results in an effective negative damping that tends to decrease with decrease in the normal load and/or increase in structural damping of the system.

Topics: Friction , Vibration
Commentary by Dr. Valentin Fuster
2008;():511-512. doi:10.1115/IJTC2008-71258.

In this paper, a new kind of instrument used specially for testing the worn surface of casing was introduced, its structure and testing principle was further elaborated. Meanwhile, the testing data were also used for the 3D surface reconstruction of the worn area of casing. The comparative study with SEM pictures could tell that the instrument performed very well and is quite helpful in the observation of worn rejoin of casing; the conclusions drawn from the study could be applied in the research of wear mechanism.

Commentary by Dr. Valentin Fuster
2008;():513-515. doi:10.1115/IJTC2008-71260.

The design of a turbocharger test stand using high pressure compressed air is discussed in this paper. The turbocharger test stand will be used mainly for bearing testing and turbocharger shaft vibration measurement. The purpose of these measurements is to discover a bearing design that will increase the vibration stability of the turbocharger and reduce the amount of oil leakage past the shaft seals. This will ultimately reduce emissions and upkeep cost. This paper summarizes the details of the design and describes the calculations performed that assure the test stand will successfully power the turbo. The initial operation of the test stand indicates the desired operating speed can be achieved for future vibration testing.

Commentary by Dr. Valentin Fuster

Contact Mechanics

2008;():517-519. doi:10.1115/IJTC2008-71003.

Elastic-plastic interaction of a block of rough surface with a smooth plane is considered in this paper. The nonlinear normal vibration response of the block is examined when subject to an external compressive load. Free vibration response of the block is studied. The vibration response corresponds to the application of a constant compressive external load and the study yields closed-form equations for the contact damping rate and contact natural frequency. It is shown that vibration decay rate is constant as opposed to the exponential decay rate for the linear vibrating systems. Closed form equations relating contact damping rate and contact natural frequency to the surface parameters are given.

Topics: Damping
Commentary by Dr. Valentin Fuster
2008;():521-523. doi:10.1115/IJTC2008-71006.

The objective of this paper is to highlight the influence of the rheology of a third body in dry sliding contact conditions. It has been shown that the local cohesion of the third body can create an asymmetric dissipative field through its thickness. The present study puts forward the consequences from a thermal point of view, overcoming the inherent experimental difficulties at this microscopic scale.

Topics: Rheology , Thickness
Commentary by Dr. Valentin Fuster
2008;():525-527. doi:10.1115/IJTC2008-71030.

The creep of a polymer sphere contacting a rigid flat under a constant normal load is investigated experimentally. An optical technique is used to observe in situ the evolution of the contact area while the increase of contact interference is measured simultaneously. The experimental results obtained with PTFE spheres of different diameters pressed against a sapphire flat under different normal loads are presented and the effect of sphere diameter and normal load on the creep of the contact parameters is discussed. The present experimental study validate recently published theoretical results of a contact model for a creeping sphere and a rigid flat showing that a universal similarity solution exists for the dimensionless creep interference and dimensionless creep contact area as functions of the dimensionless time.

Topics: Polymers
Commentary by Dr. Valentin Fuster
2008;():529-531. doi:10.1115/IJTC2008-71063.

A generalized adhesive wear model was derived for three-dimensional fractal surfaces in normal contact. A criterion for wear particle formation was derived based on the critical asperity contact area for fully plastic asperity deformation, taking into account the contribution of the adhesion force to the total normal load applied at the contact interface. The analysis yields a relationship of the adhesive wear coefficient in terms of total normal load (global interference), fractal parameters, elastic-plastic material properties, surface energies, material compatibility, and interfacial adhesion characteristics of the contacting rough surfaces. Numerical results of the wear coefficient of representative engineering material systems illustrate the roles of global interference and interfacial adhesion conditions (lubrication effect) in adhesive wear of surfaces in normal contact.

Topics: Wear , Adhesives , Fractals
Commentary by Dr. Valentin Fuster
2008;():533-535. doi:10.1115/IJTC2008-71064.

This paper presents an experimental study of the friction noise, between two rough and dry surfaces. Two different geometries of contact have been studied: flat-flat contact and sphere-flat contact. The domain of interest is the dry contact under light pressure where the roughness is the dominant cause of noise. In agreement with some other studies, it has been observed that the sound pressure level Lp (dB) is a logarithmic function of the surface roughness and the sliding speed. However, the exponent of the logarithmic law between sound pressure level and surface roughness largely depends on the topography of the sliding surfaces. On the other hand, the exponent of the law between sound pressure level and sliding speed depends on the contact geometry.

Commentary by Dr. Valentin Fuster
2008;():537-539. doi:10.1115/IJTC2008-71070.

The incorporation of silver into the diamond-like carbon (DLC) coatings has shown excellent potential in various applications; therefore the surface and tribological properties of silver-containing DLC thin films deserve to be investigated. In this study we have deposited silver-containing hydrogenated and hydrogen-free DLC coatings by plasma immersion ion implantation and deposition (PIII-D) methods. Atomic force microscopy (AFM) and nano-scratch tests were used to study the surface and tribological properties. The silver incorporation had only slight effects on hydrogenated DLC coatings. However, the incorporation of silver has significant effect on hydrogen-free DLC of smoothing the surface and increasing the surface energy. Those effects have been illustrated and explained in the context of experimental results.

Topics: Tribology , Silver
Commentary by Dr. Valentin Fuster
2008;():541-543. doi:10.1115/IJTC2008-71077.

Measurement of wheel rail contact has been carried out using three techniques in a lab based experiment. The simplest is with the use of commercially available pressure sensitive film. This has been compared to two methods based on reflected ultrasound. The proportion of an ultrasonic wave reflected from a contact depends on the degree of conformity between the surfaces. Maps of reflection have been produced by scanning a transducer back and forth over the region of contact. However, this scanning process is time consuming, and so would be unsuitable for measuring real wheel rail contacts travelling atfull train speed. For this reason a third approach has been used, whereby an array transducer performs a line scan of the contact. The motion of the wheel is then simulated to record several parallel scans to create a contact map.

Topics: Rails , Wheels
Commentary by Dr. Valentin Fuster
2008;():545-547. doi:10.1115/IJTC2008-71085.

The elastic-plastic contact between a deformable sphere and a rigid flat during pre-sliding is studied experimentally. Measurements of friction force and contact area are done in real time along with an accurate identification of the instant of sliding inception. The static friction force and relative tangential displacement are investigated over a wide range of normal preloads for several sphere materials and diameters. It is found that at low normal loads the static friction coefficient depends on the normal load in breach of the classical laws of friction. The pre-sliding displacement is found to be less than 5 percent of the contact diameter, and the interface mean shear stress at sliding inception is found to be slightly below the shear strength of the sphere material. Good correlation is found between the present experimental results and a recent theoretical model in the elastic-plastic regime of deformation.

Commentary by Dr. Valentin Fuster
2008;():549-551. doi:10.1115/IJTC2008-71086.

The cross sections formed by the contact asperities of two rough surfaces at an interference are island-shaped, rather than having the commonly assumed circular contour. These island-shaped contact surface contours show fractal behavior with a profile fractal dimension Ds . The surface fractal dimension for the asperity heights is defined as D and the topothesy is defined as G. In the study of Mandelbrot, the relationship between D and Ds was given as D = Ds +1 if these two fractal dimensions are obtained before contact deformation. In the present study, D, G, and Ds are considered to be varying with the mean separation (or the interference at the rough surface) between two contact surfaces. The D-Ds relationships for the contacts at the elastic, elastoplastic, and fully plastic deformations are derived and the inceptions of the elastoplastic deformation regime and the fully plastic deformation regime are redefined using the equality of two expressions established in two different ways for the number of contact spots (N).

Topics: Dimensions , Fractals
Commentary by Dr. Valentin Fuster
2008;():553-555. doi:10.1115/IJTC2008-71091.

A full-scale 3-D dynamic finite element (FE) model is created to solve the elastic-plastic wheel-rail rolling contact over rough surfaces under different friction forces. Both normal and tangential loads are applied properly. A bi-linear plastic material model is introduced and the real wheel and rail head geometries are simulated. The rolling of a drive wheel with full friction exploitation over a rough contact surface is analyzed in this paper. The stress distributions at the zone with rough surface are derived. From the results at a selected instant, it is found that roughness significantly increases the stress level of the surface layer. Furthermore, plasticity can greatly reduce stress peaks and change stress distributions. The maximum shear stress distribution at the rough surface is also analyzed to assess effects of roughness on fatigue crack initiation.

Topics: Surface roughness ,