A rolling bearing is an important machine element upon which industries and our daily life are based. The contact surfaces of bearing rings and rolling elements are expected to have high reliability against repeated load. As the material for steel balls used for rolling bearing such as deep groove ball bearing, high carbon-chromium material for bearing (JIS SUJ2) is widely used. The authors have been focusing on the steel ball, have developed materials to gain higher reliability, and have clarified the effect of characteristics associated with the rolling fatigue. In this report the influence of heat treatment and residual stress formed by peening process on rolling fatigue life is examined with various steel balls prepared with various heat treatment conditions (quenching & tempering temperatures), peening conditions, and manufacturing process. Consequently, we clarified the effect of hardness and metal structure on the rolling fatigue life of the steel ball, and derived the most appropriate heat treatment condition. We also clarified the residual stress applied during peening process shortened the fatigue life of steel ball, and proposed an optimum residual stress distribution.
To measure friction characteristics of lubricant films in nanometer thickness, we constituted novel ball suspension assemblies (BSAs) in which ultra-smooth glass balls 2 mm in diameter were employed and a portion of the ball was protruded through a hole fabricated at the end of the suspension. Using BSAs with different protrusion heights of 1.48, 0.81 and 0.41 mm, the sliding tests were performed under the same sliding condition, and the out-of-plane vibration of the suspension and the lateral vibration of the friction-displacement transducer comprised of parallel springs were compared in a wide speed range. From these comparisons, we derived a guideline for BSA design, i.e., reducing protrusion height to around 0.8 mm allows for significant vibration suppression and stable sliding at a high speed up to 209 mm/s. In addition, using this novel type of BSA, we measured friction forces of PFPE Zdol4000 lubricant films in nanometer thickness at loading forces of 0.2-1.6 mN, and speeds of 2.09-209 mm/s. The applicability of the Johnson-Kedall-Roberts (JKR) theory and the Derjaguin-Muller-Toporov (DMT) theory to the experimental results was discussed.
In situ observation of lubricant films formed with greases was carried out with a micro-FTIR under EHL conditions between a steel ball and a single crystal silicon disk. Lithium-stearate and aromatic-type urea compound were used as a thickener and 12 mass percent of the thickener was added in PAO. From the IR spectra, CH peak, C=O peak and NH peak were observed from Li-stearate grease and urea grease, respectively. The film thickness and the concentration of thickener were estimated from the absorbance of CH and the ratio of absorbance of C=O and CH. Two-dimensional distribution of the thickness and the concentration around EHL contact was obtained at the resolution of 50 μm. It was found that the concentrate on of thickener at EHL contact region was dependent on thickener itself. Although the concentration of Li grease decreased at EHL contact region, that of urea increased even at low entrainment speed. The thickener of urea grease was concentrated on the ball and the disk surfaces. The film thickness at EHL contact of each grease can be explained by the concentration of thickener at the contact. Lubricating characteristics of urea grease was discussed based on the concentration of thickener at EHL contact.