Experimental and Numerical Study of Micropitting Initiation in Real Rough Surfaces in a Micro-elastohydrodynamic Lubrication Regime

Abstract

Micropitting is a form of surface fatigue damage that happens at the surface roughness scale in lubricated contacts in commonly used machine elements, such as gears and bearings. It occurs where the specific film thickness (ratio of smooth surface film thickness to composite surface roughness) is sufficiently low for the contacts to operate in the mixed lubrication regime, where the load is in part carried by direct asperity contacts. Micropitting is currently seen as a greater issue for gear designers than is regular pitting fatigue failure as the latter can be avoided by control of steel cleanliness. This paper describes the results of both theoretical and experimental studies of the onset of micropitting in test disks operated in the mixed lubrication regime. A series of twin disk mixed-lubrication experiments were performed in order to examine the evolution of micropitting damage during repeated cyclic loading of surface roughness asperities as they pass through the contact. Representative measurements of the surfaces used in the experimental work were then evaluated using a numerical model which combines a transient line contact micro-elastohydrodynamic lubrication (micro-EHL) simulation with a calculation of elastic sub-surface stresses. This model generated time-history of stresses within a block of material as it passes through the contact, based on the instantaneous surface contact pressure and traction at each point in the computing mesh at each timestep. This stress time-history was then used within a shear-strain-based fatigue model to calculate the cumulative damage experienced by the surface due to the loading sequence experienced during the experiments. The proposed micro-EHL model results and the experimental study were shown to agree well in terms of predicting the number of loading cycles that are required for the initial micropitting to occur.

Influence of distribution parameters of rough surface asperities on the contact fatigue life of gears

“…For surfaces of low roughness, the area with the lowest fatigue life occurs at a depth of about 0.3 Hz contact half-width from the surface, but when the roughness increases (0.5 mm < Ra < 0.9 mm), the area with the lowest fatigue life will occur near the surface, which is easy to cause micro-pitting on the tooth surface. And this trend is consistent with the results of Liu et al 12 and AL-Mayali et al 24 Influence of distribution parameters of asperities on contact fatigue…”

A Review on Micropitting Studies of Steel Gears

“…This conclusion has a considerable reference value on gear design and manufacturing processes, especially grinding and superfinishing. AL-Mayali et al [51] investigated theoretically and experimentally the micropitting initiation based on the real surface topography under the micro-EHL regime. Both the numerical and experimental results showed that the micropits formed during the running-in stage (within 10 5 cycles) since the initial surface roughness asperities were significant.…”

Lubrication-Contact Boundary Conditions for Lubrication Problems

“…However, so far, existing simulation work has either failed to address or has only partially dealt with the LCBCs as mentioned above. Furthermore, Zhou et al [25] applied EHL simulation to study lubrication between case-hardened gears, and AL-Mayali et al [26] studied micropitting initiation with experiments and numerical simulations. Hansen et al [27] proposed a new updated film parameter.…”