Planetary Gear Fatigue Behavior in Automatic Transmission

An automatic transmission planetary gear fatigue test is used to screen lubricant performance of various automatic transmission fluids. The key use of this test is to assess the ability of a lubricant to extend or limit planetary gear system fatigue life. We study the fatigue behavior in this test and find the major failure modes are tooth macropitting, and macropitting-related tooth fracture of the sun and planetary gears (short and long pinion gears). Micropitting appears to be responsible for these gear failure modes. Macropitting is also seen on the shafts and needle rollers of the bearings. Gear tooth fracture appears to have originated from the surface as a secondary failure mode following macropitting. Bearing macropitting is initiated by geometric stress concentration. Bending fatigue failure on the sun and planetary gears also occurs but it is not a micropitting-initiated failure mode. Tooth macropitting and the related tooth fracture appear to occur independently from bearing macropitting. As expected, the directions of micro- and macro-crack propagations in mating gears go in opposite directions.