Investigations on the Micropitting Load Capacity of Case Carburized Gears


Investigations on the Micropitting Load Capacity of Case Carburized Gears Dr.-Ing. B.-R. Höhn, Dr.-Ing. P. Oster, Dr.-Ing. U. Schrade and Dr.-Ing. T. Tobie, Gear Research Centre (FZG) The load capacity of power transmitting gears can be limited by different failure modes. Standardized calculation methods acc. to German (DIN) or International (ISO) standard are available for rating the pitting resistance and bending strength of gear teeth. A further kind of fatigue damage is micropitting that is most frequently observed on case carburized gears. Micropitting is controlled by the conditions of the tribological system of tooth flank surface and lubricant. The oil film thickness has been found to be a dominant parameter. Lubricant of base oil and additive, operating conditions, surface roughness and gear geometry are known as important influence factors on the micropitting load capacity. In continuous work over several research projects major influences on the micropitting load capacity of gears were systematically investigated. For evaluating the influence of lubricants the FZG micropitting test was developed. Results on the influence of certain parameters such as oil temperature, surface roughness or material were determined by variation of the test conditions. Within the scope of actual research work some basic influences of gear geometry, gear size and operating conditions were investigated. For this purpose an extensive test program on spur and helical gears of different sizes and different gear geometry has been carried out. Based on the results of the previous and actual investigations an enhanced calculation method to determine the micropitting load capacity of practical gear units was developed. In accordance to the existing standardized calculation methods regarding pitting resistance and bending strength the proposed rating formulas can be used to evaluate the risk of micropitting respectively to determine a safety factor for micropitting on case carburized gears. The calculation method is based on the result of the micropitting test as a tribological parameter for the lubricant in use but enables the gear designer furthermore to take major influences as operating conditions, gear geometry and gear size of the actual application into consideration if rating the micropitting load capacity of a gear. The paper summarizes important results of the continuous experimental investigations and introduces the proposed calculation method for rating the micropitting load capacity of case carburized gears.
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