Lubricant Influence on Slow Speed Wear in Gears

In gears running under conditions of high load and low circumferential speed the lubricant film thickness is often not sufficient to fully separate the mating surfaces. Wear of the gear flanks can occur and can even result in total gear failure. For a given gear geometry the main influencing parameters are load, circumferential speed and lubricant. The wear characteristics of lubricants depend on many factors. Viscosity, base oil type and additives have great influence. In slow speed gears mostly lubricants with anti-wear (AW) additives reducing wear by forming chemical and/or physical protection layers are used. For testing the wear capacity of lubricants a suitable test method was developed using a modified FZG back-to-back gear test rig. The FZG slow speed wear test C/0.05/90:120/12 is run under very low circumferential speeds of v = 0.05 m/s and 0.57 m/s and at two different test temperatures oil = 90 °C and 120 °C. The test consists of three parts and is run in several intervals. The complete test run takes 120 h. The test gears are of C-PT type, the same as for the FZG pitting test. Since the development of the test many different lubricants of different fields of application were investigated. The test results show obvious differences. Linear, degressive and progressive wear rates were found. Lubricants of the same base oil type and the same viscosity even with the same type of additive can totally differ in their wear behaviour. Also at different temperatures the wear performance of lubricants can be different. Due to activation of additives some lubricants show decreasing wear rates with increasing temperature whereas for other lubricants the temperature seems to have no influence. With the test method C/0.05/90:120/12 a suitable tool for the determination of the wear behaviour of lubricants is available. Candidate lubricants can be discriminated and ranked according to their wear characteristics under mixed and boundary lubrication conditions. The results of the slow speed wear test can be introduced into a calculation method. This calculation method allows the evaluation of the expected wear rate in a gear in practice when operated with the tested lubricant. An exemplary recalculation of the wear damage in a railway gearbox showed close correlation between calculation and practice. Substantial improvements were achieved by using a lubricant with better wear performance which was evaluated in the slow speed wear test. Utjecaj maziva na trošenje zupčanika... B.-R. Hoehn, K. Stahl, K. Michaelis 18 goriva i maziva, 51, 1 : 5-28, 2012.