Studies of Friction in Grease-Lubricated Rolling Bearings Using Ball-on-Disc and Full Bearing Tests

• Published in: Tribology transactions 2020-01, Vol.63 (1), p.77-89
• Main Authors: Kanazawa, Yuta, De Laurentis, Nicola, Kadiric, Amir
• Format: Article
• Language: English
• Subjects: Ball bearings; Coefficient of friction; Contact; efficiency; elasto hydrodynamic lubrication; Film thickness; Friction; Friction reduction; Grease; Greases; Lithium; Low speed; Lubricants; Mathematical analysis; Performance evaluation; Power loss; Roller bearings; rolling bearing; Thickness measurement; Thrust bearings; Torque; Tribometers; Ultrasonic testing
• ISSN: 1040-2004; 1547-397X
• DOI: 10.1080/10402004.2019.1662147

This article evaluates the frictional performance of different bearing grease formulations in full rolling bearings and a ball-on-disc rig and subsequently assesses whether the ball-on-disc test results can be used to predict the grease performance in actual bearings. A selection of custom-made greases with systematically varied formulations as well as their base oils were tested. Bearing torque was measured in two different cylindrical roller thrust bearings and a thrust ball bearing. The same lubricants were tested with ball-on-disc tribometers, a mini traction machine (MTM) to measure friction and an optical elastohydrodynamic (EHD) rig to measure film thickness. Both lithium complex and diurea greases were observed to produce lower friction than their base oils within the low speed, low nominal lambda ratio region, whereas the greases and oils had the same friction at high nominal lambda ratio values. These relative trends were the same in full bearing and single-contact MTM tests. The reduction in friction was seen to be related to the level of film thickness enhancement provided by greases at lower speeds, which leads to an increase in the effective lambda ratio and hence reduced friction. By extracting the sliding torque component from the overall measured bearing torque, a plot of the friction coefficient against the effective lambda ratio was produced encompassing all bearing and single-contact tests and all lubricants and test conditions. This plot was seen to follow a general shape of a master Stribeck curve, indicating that the numerical values of the friction coefficient from ball-on-disc and full bearing tests overlap and can be related to each other using this approach over the range of conditions employed here. Thus, single-contact ball-on-disc tests can provide a fast and economical way of establishing the frictional performance of bearing greases in full bearings in terms of both relative performance rankings and quantitative values of bearing frictional power losses.