The Importance of Variable Speeds under Extreme Pressure Loading in Molybdenum Disulfide Greases Using Four-Ball Wear Tests

Abstract

There is recent concern regarding grease behavior in extreme pressure applications. The research described here is aimed at providing good friction and wear performance while optimizing rotational speeds under extreme loading conditions. A design of experiment (DOE) was used to analyze molybdenum disulfide (MoS2) greases and their importance in reducing wear under extreme loading and various speeds conditions (schedule 1 and schedule 2 speeds). The lamellar structure of MoS2 provides very good weld protection by forming a layer that can be easily sheared under the applications of extreme pressures. An extreme load of 785 N was used in conjunction with different schedules of various rotational speeds to examine lithium-based grease with and without MoS2 for an equal number of revolutions. A four-ball wear tester was utilized to run a large number of experiments randomly selected by the DOE software. The grease was heated to 75°C and the wear scar diameters were collected at the end of each test. The results indicated that wear was largely dependent on the speed condition under extreme pressure loading, and thus a lower MoS2 concentration is needed to improve the wear resistance of lithium-based greases. The response surface diagram showed that the developed molybdenum disulfide greases exhibited both extreme pressure as well as good wear properties under various rotational speeds when compared to steady-state speed. It is believed that MoS2 greases under schedule 1 speeds perform better and provide an antiwear film that can resist extreme pressure loadings.