Micropitting in Hertzian Contacts

Abstract

This paper shows that both micropits and spalls occur near pure rolling conditions. The first are initiated at the surface and are related to asperity interaction. The second are well described in the literature and will not be discussed here. A model capable of predicting the mechanical conditions under which micropits are generated is presented. In this model the pressures on asperities are analyzed for given separations of the surfaces; elastic, elasto-plastic, or plastic deformations are deduced. Master curves can be obtained if a pressure index is used, the model requires a comprehensive description of the surface as it calls for the distributions of both profiles height and asperity tip radii of curvature. A new approach was proposed to define the peaks which carries the load in the contact. In this model a peak is defined with respect to the separation of the surfaces. Good correlation between theory and experiment was obtained for materials as different as mild and case hardened steels and with ground and “as received” treated specimen surfaces. The rate of micropit formation which depends necessarily on running-in cannot be predicted at this time. Results however clearly show that when running-in takes place, i.e. when the pressure on asperity tips decreases to a value compatible with elastic deformation, the rate of formation of micropits with time tends to zero and of course fatigue life is increased.

The Effect of a Friction Modifier Additive on Micropitting

“…The earliest mention in the tribology literature Effect of a Friction Modifier Additive on Micropitting 527 appears to be Berthe, et al (1), although it is known that the term was in use much earlier in the gear community. It is known that the phenomenon is caused primarily by the roughness of the surfaces (Spikes,et al (2); Olver (3)) and can be eliminated by sufficiently fine polishing (Cardis and Webster,(5)).…”

A Review on Micropitting Studies of Steel Gears

“…The contact fatigue performance in lubricated Hertzian contacts has been studied since decades ago by Way [14], Dowson [15], and other researchers [16][17][18]. With the view of quantifying different lubrication conditions, an important relevant parameter was defined as the specific lubricant film thickness λ, namely the lambda ratio, by Tallian [19]:…”

Roughness in elastohydrodynamic contacts

“…In the surface region, areas of high stress are produced and, for the example shown, the maximum principal stress difference is 0.93 times the Hertz pressure and occurs at a depth of 0.04b. As noted by Berthe et al [1], high contact loads that increase the Hertz pressure tend to produce spalling in which a crack forms near the point of maximum Hertz stress and a large flake eventually becomes detached. The effect of surface roughness is to increase the nearsurface stresses and this can lead to the formation of micro-pits.…”