Effects of Surface Roughness and Form Factor on Rolling Contact Fatigue

Abstract

Rolling contact fatigue tests were conducted under lubricated conditions using rollers made of carbon steels and a grey cast iron having surface roughnesses ranging from 0.2 to 220μm Rmax (peak-to-valley roughness) and form actors ranging from 0.6 to 0.9 Results obtained are as follows: (1) In the case of the equal hardness combination cf annealed carbon steels (160 HB), no pitting occurred after 107 rotations at a Hertzian stress of 0.71 HB (113 kg/mm2) even when the sum of initial surface roughnesses (Rmax1+Rmax2) was greater than nine times the calculated oil film thickness (h ) based on elastohydrodynamic lubrication theory. (2) Also in the case of unequal hardness combinations, no pitting occurred on the softer rollers having an artificial V-shaped groove of 120μm in depth and a form factor of 0.9 at a Hertz Ian stress of 0.7 HB if the surface roughness of harder roller was smaller than h<ming. (3) In the case of cast iron, many cracks arrested in the subsurface were observed in the rollers which were finished by turning and had feed marks of 0.18 mm in pitch. (4) Cast iron rollers finished by grinding or superfinishing were apt to pit rather than the ones finished by turning. This may be ascribed to the enclosement of lubricating oil at the contact band because the flaky graphite at the surface can be taken as the surface can be taken as cracks corresponding to the ones produced on the steel rollers. (5) From the author's experiments, it is estimated that in the case of materials having many inclusions and brittleness, putting cracks occur first at subsurface under a Hertzian stress smaller than the one which causes work hardening at the subsurface where the reversed shear stress is maximum.