Undulation of Surfaces Caused, by Rolling Contact

Abstract

Investigations were conducted on the undulations which occurred on the surfaces of rollers made of carbon steels (S35C, S45C, S55C) and Cr-Mo steel having hardnesses ranging from 160 to 210 H_B. Results obtained are as follows : (1) When the surface roughness of the harder roller R_max is greater than the oil film thickness h_min (based on elastohydrodynamic theory), undulations sometimes occur at Hertzian stresses lower than the shakedown limit (p_max=4.0 τ_k≒0.4 H_B which is calculated using the yield stress in a simple tension test). In this case, the amplitude of the undulations does not grow rapidly with running and running can be continued until pitting occurs because the plastic deformation is restricted to only a thin layer at the surface. (2) When h_min is greater than R_max of the harder roller, undulations never occur below a Hertzian stress of 0.4 H_B (shakedown limit). However, undulations do occur at Hertzian stresses of over (0.6∼0.7) H_B in most cases. When they did occur, running could not be continued until 2×10⁵ rotations due to growth in the amplitude of undulation and resulting large vibration and loud noise. (3) The limit stress for undulation can be increased by the following four methods ; (a) work-hardening of roller surface, (b) removal of roller surface after the first stage of running, (c) applying MoS₂ on roller surfaces at commencement of running, (d) cutting grooves on the softer roller. (4) Causes of undulation are not wear but accumulated plastic deformation at surface and / or at subsurface where reversed shear stress is maximum. (5) Undulations occur at from 10³ to 2×10⁵ rotations but never beyond 2×10⁵ rotations.