In rolling bearings, subsurface flaking failures occur under pure rolling contact fatigue conditions. In such failures, one of the crack initiation factors is believed to be the results of repetitive orthogonal shear stresses. Accordingly, we have deemed that it is of critical importance to evaluate the shear fatigue properties, which can be obtained by torsional fatigue testing. However, up until now, it has been all but impossible to establish the shear fatigue properties of giga-cycle regimes because of conventional low loading frequency torsional fatigue testers. This is due primarily to time constraints. Ultrasonic fatigue axial loading testing has been employed for rapid evaluation of tension-compression fatigue properties. We have developed an ultrasonic torsional fatigue tester which enables for the rapid evaluation of shear fatigue properties. A loading frequency of 20,000 Hz is quite high, and as such results in a radical reduction of testing duration. It is possible to produce maximum shear stress amplitudes of about 950 MPa at the surface of specimen’s minimum diameter. This is sufficient for the evaluation of high strength rolling bearing steels over wide ranges of loading cycles.
The effects of the number of run-ins and the contact pressure on the formation behavior of real contact area for paper-based friction materials was examined through an experiment as well as particle analysis and a contact model simulation for two types of friction materials. In the particle analysis, the real contact points were treated as particles, and the number of contact points was determined together with the longest and shortest diameters, contour lengths, and complexity of the contact points. The tendency of the real contact area to increase with an increasing number of running-ins and increasing contact pressure was considered in association with the results of particle analysis. Contact phenomena of friction materials were numerically simulated by using the parameter of change in thickness of the friction materials, and the surface height distribution was approximated by using an exponential distribution function and introducing residual pre-stress force for the friction materials. The nonlinear relation between the real contact area and the normal load was reproduced, and hysteresis behavior was confirmed.
To decrease the power loss of power transmission gears, it is necessary to estimate friction coefficient at tooth meshing surface. And to prevent the failures like scuffing and pitting on tooth surface, it is necessary to decrease the flash temperature and tangential force that are proportional to friction coefficient. But there was no formula with high accuracy for estimation of friction coefficient under mixed lubrication condition considered a ratio of surface roughness and oil film thickness. In this paper, empirical validation of the new estimation formula of friction coefficient was carried out. In the contact zone under mixed lubrication, a combination of fluid friction and boundary friction exists. The new function of load ratio expressed as a simple function of the ratio of surface roughness and elastohydrodynamic lubrication minimum oil film thickness was verified. To evaluate the formula, two disc tests were conducted. The estimated values agreed well with the experimental results. The friction coefficient can be estimated by using the new function of load ratio constructed by the maximum height surface roughness. And there is no effect of average surface roughness on the friction coefficient. And there is further fact that the maximum height of grooved surface by turning can be treated as the surface with roughness height from contact surface.
This study investigated how surface roughness of steel disk coated with a Si-containing diamond-like carbon (DLC-Si) film has an effect on friction characteristics, i.e., sliding velocity dependence of friction coefficient, under oil lubricated condition via friction experiments and numerical analyses using mixed lubrication models. It was clarified to be effective for DLC-Si film to have surface roughness of RzJIS = 3 to 4 μm in order to suppress frictional vibration caused by stick-slip. In addition to the roughness of surface coated with DLC-Si film as a design guideline of sliding surface of electromagnetic clutch with excellent μ-v characteristics, it was found effective for the mating plate to have micro groove structure with depth of 10 μm and pitch of about 100 μm. It was suggested that the surface roughness and the micro groove maintain solid contact even if oil film formation enhances as sliding velocity increases, which results in boundary lubrication.