On the Necessary but Sufficient Amount of Grease to Obtain Fully Lubricated Fretting Regime in Moist Air for SUJ2 Bearing Steel against S15C Carbon Steel

Abstract

Influence of the moist air humidity (10-70%) on the fretting characteristics of steels under grease lubrication is experimentally investigated. Ball (bearing steel, JIS SUJ2) on disk (carbon steel, JIS S15C) type fretting tests for 0-20 mg of grease supplied before each test at the punctual contact between ball and disk were conducted at 50 μm slip amplitude and 6.7 Hz frequency. From the shape of the friction hysteresis one calculates the slip index, and based on it, determines the type of fretting regime against the number of fretting cycles. Influence of the grease amount and relative humidity on the friction coefficient, slip index and wear rate is interpreted based on the occurrence of the grease film breakdown. Fretting wear rate appears as unaffected by variation of the environmental humidity for an amount of grease exceeding 3 mg, this representing the minimum amount necessary to avoid the grease film breakdown. Results obtained are useful to find the amount of grease necessary but sufficient for proper lubrication, and hence to protect the environment by grease saving. In order to generalize the results obtained for ball- bearings, screws, joints, guide-ways, etc., a tribological analysis based on the Stribeck curve was performed and then, the amount of grease necessary but sufficient to obtain fully lubricated fretting regime in moist air for ball bearings was discussed in detail.

Keywords

• tribology
• fretting
• wear
• grease lubrication
• moist air
• material testing
• steel