Planetary gears are widely used in automotive and aerospace applications. Traditionally, automatic transmissions have four forward speeds, which can be achieved by using two planes of single pinion (sun-pinion-ring) planetary gear sets. Recently, there is a trend that transmissions have more than five forward speeds, which make compound planetary gear sets with two pinions (ring- or sun-pinion1-pinion2-ring) becoming common. Due to demands for higher power density and compactness, needle bearings are often used in these gear sets. There are significant axial thrust forces between the pinions and the carrier, which could lead to excessive wear of thrust faces and deterioration of bearing system performance. In this paper, we analyzed generation mechanism of axial thrust force in compound planetary spur gear set and calculated the axial thrust force. It was shown that axial thrust force always exists because of the torsional pendulum torque acting on the long planet. Experimental test for a compound planetary spur gear was taken to study axial thrust force and validate the theory calculation.

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