Effect of Oil on the Pitting of Rollers

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Abstract
The apparatus used in this investigation was an opposed double roller machine with rollers 70 mm diameter loaded against a 30 mm 3 per cent nickel steel shaft rotated at 3000 rev/min. The viscosities of the test oils which were of two types, naphthenic and paraffinic, were between 3 and 530 centistokes. Above about 20 cS the pitting endurance limit increased with the fourth root of the viscosity alone, and was not influenced by the type of oil. The effect of viscosity is thought to be due to the damping of the pressure wave in the crack. This index (1/4) and the critical viscosity (20 cS) are probably dependent on the speed of rolling. The slope of the fatigue line, before the endurance limit, varies with the oil. This has an important bearing on the ‘time to pitting’ method of test. The pitting cracks were shown to be of two sorts, shallow and deep. This is due to there being a band of work-hardened steel at approximately the Hertzian depth. Most cracks did not penetrate this band and gave rise to shallow pits 0·002-0·004 in. deep. Those which did get through it went down at an angle of about 40° to the surface and were 0·020-0·030 in. long, giving pits of greater depth. In Appendix I the equation for the viscous pressure loss in a crack, due to the compressibility of the oil, is derived.

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