Rolling Contact Fatigue of Rollers With Surfaces Similar to those of Hobbed Gear Teeth (For 160-300 HB Carbon Steel)

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The outer surfaces of test rollers were cut on a hobbing machine using a flycutter having a semicircular or straight tip. The surfaces of rollers thus finished have three dimensional roughnesses constituted from two relative motions, namely, rotation of roller during one rotation of flycutter (roughness at transverse section) and the feed of flycutter during one rotation of roller (roughness at axial section). When the test roller (Brinell hardness = 160 HB) finished by the flycutter was combined with a superfinished roller with a hardness of 420 HB and was rotated under pure rolling conditions, no pitting occurred up to 10^7 revolutions at a Hertzian stress of 0.64 HB although the initial peak-to-valley roughness R_<max> of the 160 HB roller was about thirty times the theoretical oil film thickness h_<min> built between the rolling contact surfaces. In this case, the surface roughness of 160 HB roller after running became less than h_<min>. When two cut rollers with very rough surfaces (65 μm R_<max> and 20μm R_<max>) and of equal hardness were used in combination, the total surface roughness of both rollers after 10^7 revolutions was about 30 times h_<min> although no pitting occurred on the driver and only a single pit, small in size, occurred on the follower. However, when the circumferential roughnesses of the pair of rollers were comparatively small (15μm R_<max>), they became smaller than h_<min> due to shifting of the contact points on the roughness peaks of rollers during a long period of running and pitting did not occur until 10^7 revolutions at a Hertzian stress of 0.71 HB. When the cut rollers with a hardness of 230 or 300 HB were combined with a 420 HB superfinished roller, many pits occurred before 10^7 revolutions at Hertzian stresses less than 90 kg/mm^2 even when an initial overload (P_<max> = 130kg/mm^2) for 10^2 revolutions was applied for smoothing the surface of cut rollers. It was concluded that rolling fatigue strength of cut rollers was extremely affected by running-in abilities of roller materials.
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Rolling Contact Fatigue of Rollers With Surfaces Similar to those of Hobbed Gear Teeth (For 160-300 HB Carbon Steel)

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