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 2×10
5 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 MoS
2 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 10
3 to 2×10
5 rotations but never beyond 2×10
5 rotations.
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