Testing the limiting coefficient of friction with an adjustable 3-V kinematic coupling

Abstract

A 3-V kinematic coupling with adjustable vee angles, ranging from 25° to 80° inclination from the base plane, was designed and built to test the limiting coefficient of friction (CoF), which is a measure of the coupling's ability to reach center where all six constraints are engaged. For instance, the vee angles may be optimized by maximizing the limiting CoF to give more margin over the real CoF between contacting surfaces. Although, if the real CoF is too high, that is, higher than the limiting case, the coupling may fail to reach a stable center position. For this experiment, the coupling was configured over a wide-ranging combination of vee angles and several loading conditions for four different surface conditions. The changes in configuration and/or loading condition were made in a logical manner to map out the boundary of incipient sliding between sliding and non-sliding outcomes for a given surface condition. Three of four surface conditions were very clean, either bare 440C stainless steel, thin dense chromium plating, or the same with a proprietary polymer lubricant. The CoF for clean surfaces was too high at about 0.4–0.6 for the coupling to center except for the most favorable sliding direction for which the coupling was being tested. The fourth surface condition was lubricated with vacuum grease and wiped with a clean-room cloth. This lowered the CoF to a point <0.2 where only the least favorable sliding direction could be made non-sliding. An equilibrium equation was developed for this coupling including all the variable parameters under test. Solving it for the limiting CoF for each test configuration successfully resolved the outcomes over the wide-ranging parameter space. This work supports the method used to calculate the limiting CoF and the underlying assumptions that friction forces are directed opposite to local sliding directions and proportional to the contact normal forces.