Effect of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surface : 1st Report, Pitting Fatigue Limit of 0.45% Carbon Steel

Abstract

Using a rolling fatigue testing machine which was designed and made by the authors, the effects of surface roughness and running-in upon pitting fatigue in lubricated rolling contact surfaces were studied and found to be of the following two categories. In the first category, no pitting occurred on a roller of 5μm Hmax initial roughness after 2×107 rotations under a Hertzian stress of 113kg/mm2 (=0.71 HB, HB=Brinell hardness of test roller before running), while in the second one, pitting did occur on the 5μm Hmax roller even under a very low Hertzian stress such as 40kg/mm2 (=0.25 HB). In order to prevent occurrence of pitting under a Hertzian stress two to three times greater than that reported in many past publications, the sum of the peak-to-valley roughnesses (Hmax) of a pair of test rollers before or after a short period of load running must be smaller than the oil film thickness calculated from Dowson's equation derived from the elastohydrodynamic lubrication theory for a pair of rollers having a very smooth surface. Moreover, relationships of the shakedown limit proposed by K.L. Johnson to metal flow and cange in hardness are also discussed. A new test roller having artificial cracks was also used to observe the starting points of cracks which caused pitting.