No abstract

The different mechanisms of fretting wear in oil and grease lubrication and methods to reduce fretting wear were examined by means of thrust ball bearings in this study.Tests of fretting wear under oil lubrication were conducted. It was confirmed that highviscosity oil can reduce fretting wear at high velocity (i.e., high frequency) through the oil film formation.In the case of grease lubrication, the influence of velocity on fretting wear was significantly different for low and high viscosity greases. Grease with low-viscosity base oils could reduce fretting wear at high velocity. In contrast, grease with high-viscosity base oils could reduce fretting wear at low velocity. Grease thickeners were found to be effective in forming a layer that could prevent fretting wear. These results highlight the large differences of effective fretting wear reduction mechanisms between oil and grease lubrication.

This article describes the influence of rheological properties on the bearing torque characteristics of the lithium soap greases with five types of base oils. The greases used had different yield stress depending on the base oils even with the same thickener concentration. Measurement of bearing torque was conducted for a deep-groove radial ball bearing by using a bearing test apparatus. The bearings filled with greases initially exhibited high torque but showed gradual decrease in the torque with prolonged rotation, where the greases with higher yield stress showed larger normalized torque decrease. Observation of bearing after the rotation revealed a tendency that the greases with larger normalized torque decrease had been pushed aside in the raceway by channeling. This implied that the greases with higher yield stress tended to show channeling. On the other hand, the greases with lower yield stress circulated within the bearing by churning and showed smaller normalized torque decrease. These behaviors were explained in terms of the yield stress of the greases and the shear stress to entrain the greases into the contacts. Observation of grease structure was made with atomic force microscopy showed that the greases whose thickener network structure was distributed more densely had higher yield stress.

This paper describes the influence of fiber length of lithium-soap thickener of greases on friction under boundary lubrication conditions. Ball-on-disk sliding tests were conducted with lithium soap greases with polyol ester as the base oil. The tests revealed that the greases had a lower friction coefficient than that for the base oil. The grease with a long soap fiber structure had a lower friction coefficient than that for the grease with a short soap fiber structure. The tests with only soap fibers showed that the soap fibers had better lubricity than the base oil under the boundary conditions tested, but there was little difference in the lubricity of the longer fibers and the shorter fibers. The partial coat tests revealed that the grease with longer soap fibers was easily entrained into the contact. It was concluded that this entrainment capability of the grease led to favorable lubrication to maintain low friction coefficient.

The authors have reported on results from a sliding test, which was conducted using a ball-on-disk type point-contact sliding test apparatus under gradually increased loading. Under slow sliding conditions where formation of an oil film is not expected, friction-resistant properties of grease are better than those of base oil. This result indicates that the soap fibers in grease work to protect the rubbing surface. Grease with long soap fibers shows better frictional resistance properties and offers greater protection against friction at the rubbing surface than grease with short soap fibers. In this paper, we examined factors that influence different frictional resistance properties according to soap fiber structures in the grease. Evaluation results of the friction-resistant properties of soap fibers alone, and the evaluation results of conditions of entraining grease into the rubbing surface area, show that friction-resistant properties vary according to the soap fiber structure in grease as a result of the conditions of introducing grease into the rubbing surface area.