Rolling Contact Surface Research Articles

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

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 107 revolutions at a Hertzian stress of 0.64 HB although the initial peak-to-valley roughness Rmax of the 160 HB roller was about thirty times the theoretical oil film thickness hmin built between the rolling contact surfaces. In this case, the surface roughness of 160 HB roller after running became less than hmin. When two cut rollers with very rough surfaces (65 μm Rmax and 20μm Rmax) and of equal hardness were used in combination, the total surface roughness of both rollers after 107 revolutions was about 30 times hmin 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 Rmax), they became smaller than hmin 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 107 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 107 revolutions at Hertzian stresses less than 90 kg/mm2 even when an initial overload (Pmax = 130kg/mm2) for 102 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.

Surface Durability of Spur Gears at Hertzian Stresses Over Shakedown Limit

Using a new type of gear-load testing machine and a disk-type rolling fatigue testing machine designed and made by the authors, the upper limits of Hertzian contact stress allowable on rolling contact surfaces were investigated. It was shown conclusively that gears and rollers made of soft carbon steels could be rotated beyond 108 revolutions at Hertzian stresses over shakedown limit (≈ 0.4 HB). In the case of gears, pits having a pitting area ratio of 0.04 percent occurred during 1.16 × 108 rotations at a Hertzian stress of 0.50 HB. However, no pitting occurred on the roller rotated through 1.20 × 108 revolutions at a Hertzian stress of 0.71 HB, although appreciable changes in texture were observed at the subsurface. In order to rotate gears or rollers at Hertzian stresses over shakedown limit, their surface must either be very smooth initially or after a short period of running, and an oil film must be formed between contacting surfaces.

Effects of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surface : 2nd Report, Effects of Cyclic Work-Hardening

Effects of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surface : 2nd Report, Effects of Cyclic Work-Hardening

A scanning electron microscope study of fracture phenomena associated with rolling contact surface fatigue failure

In a manufacturing process, studying the removed material called chip, swarf or debris, is the key to understand the process. In micro cutting process, the size effect is a dominant factor for material removal mechanism and chip generation physics. There are different types of chips but a particular type, stands out from the others due to its specificities: the spheroidal chip. It is often found in grinding and tribological tests. This type of chip has no specific classification and there are no reports of it being found in milling or micro milling research. This paper makes a review of this not so well known type of chip, to classify it by its characteristics and to present some examples of spheroidal chips obtained from micro milling operation. Spheroidal chips were obtained when micro milling slots using a 381 µm diameter TiN coated WC tool. The workpiece material was Inconel 718, a high strength refractory superalloy. SEM images were obtained to analyze and classify the chips and observe the machined surface of the slots. In the proposed classification of spherical chip, there are eight major types: Fatigue Sphere, Layer-like Sphere, Thin Plate Sphere, Dendritic-like Sphere, Partially Formed Sphere, Tadpole, Melted sphere and Hollow sphere. The thin chip is submitted to high energy, which is one of the main aspects of size effect that occurs in micro milling. Inconel 718 has low thermal conductivity and high oxidation rates, which makes it an ideal environment for the formation of spherical chip. The spheres formed can be classified as Dendritic Spheres, defined by extremely high rates of oxidation in a thin chip which leads to high heat energy causing its spherical form and dendritic microstructure.

Elastohydrodynamic Lubrication of Roller Bearings

Abstract Roller bearing endurance tests have been run on groups of bearings with L10 and L50 lives established by Weibull analysis. Bearing roller path surface finish, shaft speed, lubricant viscosity and lubricant temperature were varied. Relevant tapered roller and cylindrical roller bearing life data was selected from prior tests for comparison. Weibull plots show the effect of the variables on life and a graph comparing the ratio of L10 test life to the calculated life and the ratio of elastohydrodynamic lubricant film thickness to composite rolling contact surface finish is given. An empirical equation to predict the effect of varying lubricant and surface finish conditions on fatigue life is given.

Effects of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surfaces : 2nd Report, Effects of Cyclic Work-Hardening

Effects of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surfaces : 2nd Report, Effects of Cyclic Work-Hardening

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

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.

Effects of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surface : 1st Report Pitting Fatigue Limit of S45C

Effects of Surface Roughness and Running-in upon Pitting Fatigue in Rolling Contact Surface : 1st Report Pitting Fatigue Limit of S45C