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Automotive Lifts: Unrestrained vs. Restrained Swing Arms

[+] Author Affiliations
Ralph L. Barnett

Triodyne Inc., Northbrook, IL

John B. Glauber

University of Illinois at Urbana-Champaign, Urbana, IL

Paper No. IMECE2009-10053, pp. 373-387; 15 pages
  • ASME 2009 International Mechanical Engineering Congress and Exposition
  • Volume 13: New Developments in Simulation Methods and Software for Engineering Applications; Safety Engineering, Risk Analysis and Reliability Methods; Transportation Systems
  • Lake Buena Vista, Florida, USA, November 13–19, 2009
  • Conference Sponsors: ASME
  • ISBN: 978-0-7918-4386-4 | eISBN: 978-0-7918-3863-1
  • Copyright © 2009 by ASME


To perform automotive maintenance, there are many makeshift ways of lifting and holding a vehicle including the use of forklifts, overhead hoists and cranes, jacks of every kind, jack stands and various ramp systems. When automobiles fall from these devices, the causes are usually obvious and we disapprovingly tolerate the risk taking. On the other hand, when a vehicle falls from a dedicated automotive lift, the accident is entirely unacceptable. This paper examines several hidden dangers associated with a particular class of lifts that are “frame engaging.” Various styles of these lifts use four cantilevered arms to elevate and support vehicles on adapter pads positioned on the arms’ free ends. If the vehicle slides off of one or more pads, it usually falls catastrophically. The cantilevered arms, when raised, are supposed to be restrained against rotation in a horizontal plane. When restrained, the arms provide a robust structural system for resisting horizontal workplace forces that tend to slide vehicles off the pads. The arms maintain the horizontal locations of the adapters by developing bending and axial planar resistance. If, on the other hand, the arms are free to pivot due to sloth or poor design, their structural behavior is dramatically transformed. The planar bending resistance of the arms completely disappears and they become direct stress diagonal truss members; the vehicle itself unwittingly becomes the truss’ tension chord. The appearance of the fixed and pivoting systems is the same; however, the truss action magnifies the horizontal forces acting on the adapter pads increasing the slip probability. Indeed, depending on the orientation of the pivoting swing arms, any finite horizontal force applied to a vehicle may lead to an unbounded tangential “slide-out” force. This is, of course, a theoretical possibility, not a practical reality.

Copyright © 2009 by ASME



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