Atsushi Yokouchi

Research Work for Clarifying the Mechanism of White Structure Flaking and Extending the Life of Bearings

Bearings used in electrical engine-driven accessories, such as alternators and air conditioner (A/C) compressor pulleys, generate flaking related to non-metallic inclusions of unusual microstructure (white structure flaking), which occurs at one-tenth to one-twentieth of the calculated bearing life. In this paper, we will describe our investigation of the mechanism of white structure flaking, and report on how we achieved extended service life of engine-driven accessory bearings by implementing an evaluation of white structure flaking based on the discovered mechanism. We verified that the tribochemical reaction of grease, and statistic electricity generated by friction between the drive belt and pulley were the main culprits of white structure flaking. White structure flaking was further augmented by hydrogen invading the bearing steel, which further impaired bearing life.

Bearing torque characteristics of lithium soap greases with some synthetic base oils

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.

Differences in Mechanisms for Fretting Wear Reduction between Oil and Grease Lubrication

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 high-viscosity oil can reduce fretting wear at high velocity (i.e., high frequency) through 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 in effective fretting wear reduction mechanisms between oil and grease lubrication.

Influence of Soap Fiber Structure on Frictional Property of Lithium Soap Grease

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.Copyright

Lubricating Characteristics of Fluorine Grease Containing Silica Nanoparticles

Electrical motor bearings for use in automotive control units are sometimes exposed to high-temperature operating conditions. Therefore, these bearings are sometimes packed with fluorine grease in order to meet the demand for long service life performance. In recent years, there is growing demand for lower initial torque in these same bearings without sacrificing service life performance. This paper describes the effects of fluorine grease containing silica nanoparticles on initial bearing torque of electrical motor bearings. Silica nanoparticles are effective in helping to reduce amounts of initial bearing torque and help stabilize bearing torque due to improved channeling characteristics.Copyright