Influences on failure modes and load carrying capacity of grease lubricated gears
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Introduction For many years, greases have been used in the lubrication of ball bearings. For the lubrication of gears, however, oils still play the dominant role. In recent years, the advantages of grease over oil in certain gear applications have led to an increasing significance of grease in gear lubrication. Unlike oil lubrication, for which methods for the calculation of the gear load-carrying capacity have long since been established in standards, such as ISO 6336 or DIN 3990, calculation methods for the loadcarrying capacity of grease do not yet exist. This is mainly due to the complex interaction of the influence parameters on the load-carrying capacity of grease-lubricated gears as well as the limited availability of accessible, experimental investigations. A large portion of the conducted experimental work on grease lubrication focuses on the application of grease in the lubrication of bearings. Therefore numerous studies have concentrated on lubricant film thickness investigations with grease. A good summary of these and further, similar work is given by Lugt (Ref. 10). Nonetheless, the number of experimental investigations on the load-carrying capacity of grease-lubricated gears is steadily increasing. For example, Fukunaga (Refs. 2, 1), and in more recent years, Krantz and Handschuh (Ref. 8), and also Krantz et al. (Ref. 9) performed experiments to investigate the effect of different operating conditions and grease parameters on the failure modes of grease-lubricated gears. In the past decade several research projects were conducted at FZG with the aim of expanding general knowledge of grease-lubricated gears. The experimental work therein focuses on the influences of different grease components and operating conditions on different gear failure modes such as scuffing, wear and pitting. In this paper, selected experimental results from DGMK research projects 591(Ref. 3) 673 (Ref. 4), 670 (Ref. 14), 671 (Ref. 15) and 725 (Ref. 11), with greases ranging from NLGI 00 to NLGI 1, using the FZG back-to-back gear test rig and that have already largely been published on their own in papers and dissertations (see Refs. 12, 16, 5 and 13), are evaluated in context.