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IN THIS VOLUME


Tribology

2006;():1-7. doi:10.1115/IMECE2006-13492.

In the present work, is developed the tribologic characterization of steels AISI H-13 and D-2, submitted to nitruration ionic process to determine wear resistance in aqueous conditions. Wear test are realized with an abrasion wear tool in an aqueous environment, designed and constructed by SEPI, ESIME, IPN, according to the norm ASTM G105-89. The aim of this investigation is to use a new material at lower prize which has an excellent wear resistance properties for high abrasion in aqueous environments, as occurs in several cases as mining industry equipments.

Topics: Tool steel , Abrasion
Commentary by Dr. Valentin Fuster
2006;():9-21. doi:10.1115/IMECE2006-14412.

Ultra-thin tetrahedral amorphous carbon (ta-C) films were deposited by the filtered cathodic vacuum arc (FCVA) technique. The thickness, structure, and topography of the films were studied by various analysis methods, such as auger electron spectroscopy (AES) depth profile, high resolution transmission electron microscope (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). A tribometer was used to investigate the tribological properties of the ta-C films. The results indicate that ta-C film with thickness less than 2 nm can be obtained by the FCVA technique. As the film thickness increases the relative intensity ratio ID/IG decreases, which means that sp3 bond in the films increases. The oxide layer cleaning procedure of silicon substrate before deposition influences the growth mode and surface roughness of the films. The ultra-thin ta-C film has the lowest friction coefficient of 0.19 and excellent anti-wear properties.

Topics: Tribology , Carbon
Commentary by Dr. Valentin Fuster
2006;():23-28. doi:10.1115/IMECE2006-14726.

The present work is concerned with study the wear behavior of 1020, 1060, and 1095 steels under different wear conditions, rotating contact speeds (500,850, and 1200 rpm) and normal contact loads (50,100,150 and 200 N) using pin on wheel wear testing machine. The prepared specimens were normalized to make sure that all types of steels are in the same structure. Wear testing results were recorded by measuring weight loss at different contact times (10, 20, 30, 60 and 90 min). The aim of this paper was to study the effect of normal loads and rotating speeds and their interactions on wear behavior of steels under various continuous sliding contact times. The presented relations between the volume loss and working conditions showed that low carbon steel had wear loss higher than other carbon content. In addition, results for all types showed that lower rotating speed had higher effect than other speeds and high normal load had higher effect than other loads. The highest volume loss was observed at high normal load and low rotating speed. And the relation between wear coefficient and working conditions was also observed that wear coefficient decreases as both normal load and rotating speed increase.

Topics: Wear , Steel , Carbon
Commentary by Dr. Valentin Fuster
2006;():29-35. doi:10.1115/IMECE2006-15018.

With today's high prices for natural gas and oil, the demand for oil and country tubular goods (OCTG), with superior performance properties, is very high. Failures in OCTG can be attributed to numerous sources, for example, makeup torque, corrosion, and galling. Thread galling is the most common mode of failure. This failure often leads to leakage, corrosion of the material, and loss of mechanical integrity. The failure of OCTG eventually amounts to excessive operational costs for the gas and oil industry. The have been numerous approaches taken to improve the galling resistance of OCTG connections. The advocacy of these approaches is often achieved through experimental studies using galling testers. In this paper, it is proposed to classify the galling testers in seven distinct groups. There is a need to design and use effective galling testers to understand and improve the performance of OCTG connections. Thus, the objective of this paper was to present a concise review of literature related to the galling testers that may have applications to OCTG.

Commentary by Dr. Valentin Fuster
2006;():37-45. doi:10.1115/IMECE2006-15028.

Spur gears with asymmetric teeth have a significant potential for some applications requiring extreme performance like in the aerospace industry. In this study, the influence of tooth wear on the dynamic behavior of involute spur gears with asymmetric teeth is analyzed. The Archard's wear model was adopted in formulating and accounting for wear. Effects of gear parameters such as gear contact ratio, tooth height, mesh stiffness, and pressure angles on tooth wear are considered. These parameters are used to describe the relationship between dynamic tooth load and tooth wear. A comparison of symmetric and asymmetric teeth is also presented with respect to tooth wear. Sample simulation results, which were obtained by using an in-house developed computer program, are illustrated with numerical examples. The numerical results match well with the practical and analytical results which are available in literature. For asymmetric teeth, it was shown that the wear depth decreased with increasing pressure angle on drive side.

Commentary by Dr. Valentin Fuster
2006;():47-54. doi:10.1115/IMECE2006-13343.

This paper studies the hydrodynamic load support generated by a slip wedge of a slider bearing. The surface slip property is optimized to obtain a maximum hydrodynamic load support. A multi-linearity method was used to approach the slip control equation of two-dimensional (2-D) wall slip. It is found that the hydrodynamic pressure generated in the slip wedge is greater than that of the traditional geometrical convergent-wedge. Even though the geometrical gap is a parallel or divergent sliding gap, the slip wedge still gives rise to a very big hydrodynamic pressure. The optimized slip wedge gives rise to a maximum hydrodynamic load support as large as 2.5 times of what the Classical Reynolds theory predicts for the geometrical convergent-wedge. Wall slip usually gives a small surface friction drag.

Topics: Stress , Wedges
Commentary by Dr. Valentin Fuster
2006;():55-61. doi:10.1115/IMECE2006-13546.

Surface topography significantly influences the behavior of lubricated contacts between highly loaded machine elements. Most oil- or grease- lubricated machine elements such as gears, rolling bearings, cams and traction drives operate in mixed lubrication conditions and the lubricant film thickness is directly related to the main practical performance parameters such as function, wear, contact fatigue and scuffing. For determination wear and especially contact fatigue, the values and distribution of the pressure in rolling contact are required. The theoretical studies usually involve the numerical solution of pressure and film thickness in the contact, using some physical mathematical model built around the Reynolds equation to describe the flow and the theory of elastic deformation of semi-infinite bodies. Such calculations can be extremely time consuming, especially when lubricant films are very thin and/or contact load very high. This study is aimed at obtaining pressure distribution within lubricated contact from measured film thickness. Lubricant film thickness distribution within the whole concentrated contact is evaluated from chromatic interferograms by thin film colorimetric interferometry. Consequently, an elastic deformation is separated from the film thickness, geometry and mutual approach of the surfaces. Calculation of the pressure distribution is based on inverse elasticity theory. EHD lubricated contact with smooth surfaces of solids was first investigated. Calculated pressure, distributions were compared with data obtained from full numerical solution to check the accuracy. The approach was also applied to surfaces with dents and their influence on distribution of pressure in lubricant film.

Commentary by Dr. Valentin Fuster
2006;():63-69. doi:10.1115/IMECE2006-16151.

Foil bearings are widely used for oil-free microturbomachinery. One of the critical technical issues related to reliability of the foil bearings is a coating wear on the top foil and rotor during start/stops. Especially for heavily loaded foil bearings, large start torque requires a large drive motor. Bearing cooling is also mandatory for certain applications because the foil bearings can generate significant amount of heat depending on operating conditions. Usually axial flow is used through the space between the top foil and bearing sleeve. In this paper, a hybrid air foil bearing with external pressurization is introduced. A flexible steel tube is attached to the backside of the top foil with orifice holes, and externally pressurized air is directly supplied to the bearing clearance to lift off the rotor before rotor spins. The hybrid operation eliminates the coating wear during start/stop cycles, reduces drag torque during starts, and eliminates axial flow cooling. The hybrid foil gas bearing was constructed using a multiple compression springs to demonstrate a feasibility of the concept. A simple analytical model to calculate top foil deflection under hydrostatic pressurization has been developed. Predictions via orbit simulations indicate the hybrid air foil bearings can have much higher critical speed and onset speed of instability than hydrodynamic counter part. Measured load capacity was slightly higher than hydrodynamic bearing even under smaller amount of air flow. In addition, the hybrid operation was very effective for bearing cooling even if the cooling flow rate was lower than hydrodynamic counterpart. The measured very small drag torque during the start/stop demonstrates the hybrid foil bearing can have near-infinite life time without wear of the bearing and rotor surface. The experimental studies show high potential of the hybrid air foil bearings for various oil-free turbomachinery, especially for heavily loaded high temperature applications.

Topics: Bearings
Commentary by Dr. Valentin Fuster
2006;():71-76. doi:10.1115/IMECE2006-16275.

In this paper, a new slip boundary condition is derived using the solution of the Boltzmann equation. The physical mechanisms of velocity slip in rarefied gas flow are discussed and emphasized. The Poiseuille flow rates predicted by the new slip model show better agreements with those calculated from the existing slip models such as 1st, 2nd, and 1.5th slip order. Based on the new slip model, a new modified Reynolds equation is also proposed to predict the pressure field in gas lubrication problem.

Topics: Lubrication
Commentary by Dr. Valentin Fuster
2006;():77-91. doi:10.1115/IMECE2006-13004.

The clearance type of bearings, where the sphere radius is smaller than that of the seat, provides a great divergence in results compared with the fitted type. The study deals with the surface roughness and the predominant centripetal inertia terms due to the shaft rotation. The solutions are presented for the un-recessed clearance type of bearings, hemispherical and partial hemispherical seats, with capillary tube and orifice restrictors. On the basis of the stochastic theory, Reynolds equation is developed. Unlike the solution of Dowson and Taylor for this type of bearings, the well known Sommerfeld substitution of journal bearings hasn't been used in this study. Expressions for the pressure distribution, load carrying capacity, volume flow rate, frictional torque, friction factor, power losses and stiffness factor are obtained. The research studies the combined effects of the centripetal inertia and the surface roughness on the bearing performance. An optimum design based on the minimum power losses, minimum flow rate and the optimal restrictor dimensions is theoretically examined. The conventional four cases of this type of bearings, dependant on the eccentricity ratio in the previous studies, have been reduced to three cases only. In addition, a unified solution for this type is developed. Finally, a general solution for the hydrostatic thrust spherical bearing is obtained where it could be applied to the, un-recessed hemispherical and partial hemispherical, clearance and fitted types of bearings.

Commentary by Dr. Valentin Fuster
2006;():93-104. doi:10.1115/IMECE2006-13026.

Analytical solutions are not available for spherical bearing problems except for very specialized cases. However, this study offers a theoretical analysis, using the first order perturbations, to evaluate the frequency dependent stiffness and damping characteristics of compensated hydrostatic thrust spherical bearing including the surface roughness, the shaft rotation and the recess volume fluid compressibility effects. The dynamic stiffness and damping coefficients are presented for capillary tube and/or office compensated bearing. Results are obtained for various vibration frequencies or squeeze parameters (frequency parameters) and recess volume fluid compressibility parameters in addition to the other usual bearing design parameters. The study shows that both of the surface roughness and the centripetal inertia have slight effect on the stiffness and the damping coefficients while the recess volume fluid compressibility parameter has the major effect on the bearing dynamic characteristics.

Commentary by Dr. Valentin Fuster
2006;():105-108. doi:10.1115/IMECE2006-13691.

There is a range of mechanical components in tribological systems that operate under oscillatory sliding contact. An example of interest is the so-called pin-bushing joint interface that is typically lubricated with grease. Pin-joints are often heavily loaded and the relative sliding motion at the interface between the pin (shaft) and inner surface of the bushing generates considerable amount of heat. As a result, the surface and near-surface materials can quickly deteriorate and often trigger premature catastrophic failures. Nevertheless, the severity of the temperature rise is strongly dependent upon a series of design factors including various geometric variables, materials properties, and operating conditions. In this paper, the development of a fractional factorial experimental design for evaluation of the effect of five important design factors in pin-bushing assembly is reported. The factors of special interest include the clearance ratio (C/R), aspect ratio (L/D), oscillating angle, RPM, and applied load. The performance response is the interface temperature rise. The factorial analyses presented provide quantitative information on the factors that tend to have the greatest influence on the thermal behavior of a pin-bushing assembly.

Topics: Heat , Design
Commentary by Dr. Valentin Fuster
2006;():109-116. doi:10.1115/IMECE2006-13720.

For several decades, asperities of nominally flat rough surfaces were considered to be points higher than their immediate neighbors. Recently, it has been recognized that this model is inadequate. To address the issue, a new multiple point asperity model, called η-point asperity model, is introduced in this paper. In the new model, asperities are composed of η neighboring sampled points with η-2 middle points being above a certain level. When the separation between two surfaces decreases, new asperities with higher number of sample points, η, will come into existence. Based on the above model, the height and curvature of η-point asperities are defined and their distributions are found. The model is developed for Gaussian surfaces and for the general case of autocorrelation function (ACF). As a case study, the exponential ACF is applied to the new model, which is shown to produce remarkably good agreement with measurements from real and simulated surfaces.

Commentary by Dr. Valentin Fuster
2006;():117-124. doi:10.1115/IMECE2006-14130.

The asperities of rough surfaces have long been considered to be points higher than their immediate neighbors. Based on this concept, theories were developed for quantitatively understanding the nature of contact between rough surfaces. Recently it has been recognized that the above model for asperities is inadequate. Consequently, all the models that have been developed based on that model are inadequate as well. In this paper, based on a newly developed multiple-point asperity model, the elastic contact problem between nominally flat surfaces is reformulated. This leads to finding the deformed area, and load produced by the contact. The model is developed for the general form of isotropic rough surfaces with arbitrary height distribution and autocorrelation function (ACF). The microcontact areas generated by each asperity contact are considered to be circles. The Gaussian distribution of heights and exponential ACF are considered as a benchmark to compare the results of the new model with the existing models. Using results from numerical models developed by other groups, the new model is validated.

Commentary by Dr. Valentin Fuster
2006;():125-134. doi:10.1115/IMECE2006-14663.

A 3D elastic-plastic rough contact (EPC) solution and code is developed using a modified semi-analytical method. The total surface deflection is induced by the contact pressure and plastic strain. A purely elastic contact field and a residual field arising from the plastic deformation are simulated iteratively to gain the final approximate solution for the elastic-plastic rough contact. Frictionless normal contact between a rigid ball and an elastic-plastic half space with polished, turned, and honed rough surfaces was numerically simulated using the developed EPC code. The distributions of surface pressures, real contact area, total stresses, residual stresses, residual displacements, and plastic strains are obtained through simulation. The effects of surface roughness, wavelength, and plastic hardening behavior upon the calculated results are analyzed.

Commentary by Dr. Valentin Fuster
2006;():135-138. doi:10.1115/IMECE2006-15208.

The mechanical properties of CrN films coated by radio frequency (rf) magnetron sputtering method were investigated. CrN films were coated on stainless steel, silicon wafer and glass substrates using sputtering of a Cr target in nitrogen ambient. The films were coated by varying the deposition temperature, nitrogen partial pressure and rf power density. The films coated were characterized by nanoindentation method, microhardness, optical, and corrosion tests. In order to use CrN as mechanical coating material, the surface roughness, hardness and adhesion properties have to be determined. The film properties were measured using atomic force microscopy and nanoindentation method and analyzed as a function of deposition conditions. It was found that these properties can be varied by changing the deposition conditions.

Commentary by Dr. Valentin Fuster
2006;():139-145. doi:10.1115/IMECE2006-15264.

Fuel cells represent a promising energy alternative to the traditional combustion of fossil fuels. In particular, solid oxide fuel cells (SOFCs) have been of interest due to their high energy densities and potential for stationary power applications. One of the key obstacles precluding the maturation and commercialization of planar SOFCs has been the lack of a robust sealant. This paper presents a computational model of leakage with the utilization of mica-based compressive seals. A finite element model is developed to ascertain the macroscopic stresses and deformations in the interface. In conjunction with the finite element model is a microscale contact mechanics model that accounts for the role of surface roughness in determining the mean interfacial gap at the interface. An averaged Reynolds equation derived from mixed lubrication theory is applied to model the leakage flow across the rough, annular interface. The composite model is applied as a predictive tool for assessing how certain physical parameters (i.e., seal material composition, compressive applied stress, surface finish, and interfacial conformity) affect seal leakage rates.

Commentary by Dr. Valentin Fuster
2006;():147-151. doi:10.1115/IMECE2006-15603.

It is widely known that surface roughness and other geometric imperfections can impose a limit on the positioning precision of actuators. The case of a hard cylinder rolling on a hard layer is investigated here, as it is applicable to friction drives. It is proposed that a soft interfacial layer between the drive roller and drive rail can mitigate the effects of geometric imperfections on positioning accuracy. Positioning accuracy is characterized by way of a 'roughness torque', which is the maximum torque that the roller sustains due to roughness of the rail. It is assumed that lower roughness torque values lead to greater positioning accuracy. Finite element analysis and an analytic approach are employed to investigate the situation.

Commentary by Dr. Valentin Fuster
2006;():153-160. doi:10.1115/IMECE2006-15604.

The rolling/sliding contact of a hard cylinder on a viscoelastic layer is re-examined. The one-dimensional Maxwell model, with the addition of a parallel spring, is used to model the normal stiffness of the viscoelastic layer A solution for the pressure distribution is presented. It is shown that the maximum tractive force that the cylinder can sustain before complete sliding is a function of the sense and magnitude of the rolling velocity. Two regimes of loading are considered - constant cylinder normal force and constant cylinder indentation.

Commentary by Dr. Valentin Fuster
2006;():161-165. doi:10.1115/IMECE2006-15636.

Silicon oxides are widespread in microelectronics and microelectromechanical systems (MEMS) applications. One form of this material that has been suggested as a dielectric in MEMS applications is a carbon-doped form of silicon oxide that can be produced in thin coatings. However, the mechanical properties and wear resistance of these coatings is unknown, and coatings of interest are difficult to characterize because they are very thin. A test methodology has been previously described using extremely sharp diamond tips on a stainless steel cantilever in an atomic force microscope, and this method allows direct calculation of an effective material flow strength at penetration depths as small as twenty nanometers. A number of forms of carbon-doped and undoped silicon dioxide have been evaluated using this methodology. Size effects on material properties are evaluated, and correlations between test methods are presented.

Commentary by Dr. Valentin Fuster
2006;():167-173. doi:10.1115/IMECE2006-14142.

The aerostatic guidance is used as a super precision positioning device in the fields of semiconductor production and measurement machine, and its performance is expected to be improved. Although the study on the small guidance clearance region by which the high positioning precision may be expected is very important, there is almost no research papers published in this area to date. The purpose of this study is to enhance the calculation accuracy in the region of which the guidance clearance is comparatively small in the aerostatic guidance design. This is accomplished by considering the influence of the cutting surface roughness upon the air flow. More specifically, the amount of air flux passing through the orifice which is principal constituent of an aerostatic guidance is calculated in two separated parts. One part is the calculation done on the boundary layer flow whereas the other part on the potential flow. In this case, the calculation of the boundary layer flow is attempted using a theory in which the surface roughness is taken into account. That is, the calculation is done based on a virtual cylinder which has the same area as that of the rough inner surface of the orifice calculated based on the measured result of the surface roughness of an orifice cut surface. The truth inner surface area is calculated from the inspection result of the inner surface roughness measured by using a 3D surface roughness measuring machine. Through this method, the velocity distribution in the boundary layer is made clear where the diameter of the orifice is comparatively small, and a technique for calculating intrarubular flow is newly established. In addition, experimental apparatus to evaluate this theoretical calculation were designed and produced to confirm the flow characteristics and the load characteristic of the squeeze structure used in this study. As a result, the boundary layer velocity in the small diameter orifice which plays an important role in obtaining a high static rigidity and the pressure of the downstream side of the orifice can be calculated with precision. Thus, an attempt is made to improve the calculation precision of static rigidity in the small guidance clearance region where a high positioning precision can be expected. All of these calculation results are in a fairly good agreement with the experimental results.

Commentary by Dr. Valentin Fuster
2006;():175-180. doi:10.1115/IMECE2006-15262.

The aerostatic lead screw is a non-contact type lead screw in which the contact surface of a male screw and a female screw is supported by a pressurized air film. It is characterized by the features that: (1) there is no degradation in accuracy due to friction wear; (2) no environmental pollution is caused by a leak and scattering of lubrication fluid; and (3) maintenance is easy and simple. However, drawbacks are that vibration is liable to occur and therefore the static rigidity is low. The aerostatic lead screw is useful for semiconductor fabrication and also effective in positioning a table on which a sample is placed in a biomicroscope. This study relates to a drunkenness analysis of the aerostatic lead screw. The drunkenness of a contact-type lead screw represented by the ball screw is generally transferred directly from the male screw to the female screw. In the aerostatic lead screw, the drunkenness based on machining accuracy error of each screw flank is not directly transferred onto the female screw and is instead averaged out by balancing of load capacities among each screw flank, so that the enhanced accuracy in positioning lead screws is expected. In this paper, a balance of the load capacity of each screw flank was computed using the aerostatic guidance theory, so as to carry out a drunkenness analysis. It is found that the balancing effect plays the vital role in improvement on drunkenness of an aerostatic lead screw used in this study.

Topics: Screws
Commentary by Dr. Valentin Fuster
2006;():181-190. doi:10.1115/IMECE2006-15465.

This paper aims to study a fully coupled thermal effect in the contact parts - lip surface, rotary shaft and lubricant film - on the elastohydrodynamic behaviour of radial lip seals. The results obtained show that most operating characteristics such as reverse pumping, minimal thickness and power loss are strongly influenced by the temperature distribution in the contact zone. Therefore, all operating characteristics computed by using the local approach are lower than those obtained by using the global approach.

Commentary by Dr. Valentin Fuster
2006;():191-196. doi:10.1115/IMECE2006-15495.

A noisy mechanical seal is a grave problem, especially in water pumps designed for the automotive industry. The noisiness is often caused by dynamic instability (stick-slip behavior), which occurs when the seal lubrication changes from hydrodynamic to mixed. Starting from this hypothesis, the paper shows a theoretical model that describes the interaction between the seal disks. Therefore this model correlates the acoustic emission to the working conditions of the water pump.

Topics: Lubrication , Pumps , Water
Commentary by Dr. Valentin Fuster

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