A Critical Examination of the Elastohydrodynamic Criterion for the Scoring of Gears

Abstract

A critical examination is made of the concept that gear scoring occurs when the EHD film thickness is less than several times the combined surface roughness. The well known experimental data of Borsoff, Borsoff and Godet, and Ku and Baber is analyzed using advanced EHD computational techniques. The study finds: (a) Gear scoring occurs under fully boundary-lubricated conditions and not even close to the breakdown of the EHD film. (b) Isothermal calculations of minimum EHD film thickness are in error both in magnitude and in trend with failure velocity. An analysis which takes thermal effects into account must be used. (c) Pitch point EHD film thickness does not reliably represent minimum EHD film thickness and therefore cannot be substituted for it. (d) The peak Hertz contact stress in a gear mesh may be significantly understated if calculated only at the pitch point.