Coupled Evolution of Load Distribution and Wear on Grease-Lubricated Lead Screw Threads

Abstract

Even under constant operating conditions, greases lubricating lead screw assemblies, as well as materials comprising their nut and screw threads, must perform over a range of contact pressures that not only varies from thread-to-thread, but also evolves with the sliding distance as the most-heavily loaded threads wear most rapidly. An example modeled here, consisting of a rigid nut and elastic screw body having flexible meshed thread pairs, describes a broad distribution of loads thread-to-thread on a new lead screw assembly that gradually evolves towards uniformity as the coupled consideration of thread loading and wear depth approaches a steady-state of equal rates of thread wear. Thread load redistributions brought on by changes in the operating conditions such as applied load, or even temperature in the case of a nut/screw pair of dissimilar materials, are similarly modeled. Such load redistributions in pre-worn lead screws may cause either the top or the bottom nut threads to become most heavily loaded, depending on whether an operating condition is increased or decreased. Sufficiently large magnitudes of such operating condition change are shown to cause pre-worn threads at either extreme of the nut to become momentarily unloaded and removed from contact.

Review led by Al Segall

KEY WORDS:

• Screw Thread Lubrication
• Wear and Failure

ACKNOWLEDGMENTS

The author wishes to acknowledge Dr. Joseph Kolly of the National Transportation Safety Board for introduction to the topic of lead screw wear, as well as doctoral candidate David Murphy at RPI for producing Fig. 1.

Notes

Review led by Al Segall