the design of Planetary Gear Trains Oliver K. Kelley, General Motors Corp. This paper was presented at the SAE Annual Meeting, Detroit, Jan. 14, 1959. WHY do the millions of American automatic trans- missions use planetary gears exclusively in all passenger cars and almost exclusively in the heavy- duty automatic transmissions? Obvously, there must be important reasons for this, and I would like to point out, first of all, what some of them are. Advantages of Planetary Gears 1. The planetary gear train is obviously more compact than the countershaft gear train de- signed for the same function. The planetary gear train accomplishes the entire design arrangement (lengthwise) within the length of one gear's face width only, plus a small additional dimension for the carrier's flange thickness. The countershaft arrangement of necessity must use separate input and output gearsets, which must occupy greater length. As the planetary gear train provides three or more meshing gearsets concentrically arranged around the input gear, their load sharing reduces the gear width in proportion to the number of planetary pin- ions. (Tooth contact surface stress determines the fatigue life for quiet operation; thus, the longer the working contact line, the lower the stress.) Al- though slightly lower stress must be used in the planetary design to accommodate the more-than- one-gear-meshing contact per input gear revolution which tends to reduce the hours of life, the over- whelming advantage results from lower stress of multiple pinions. This is a very significant factor. A planetary gear train also eliminates the length required to place radial load-carrying bearings forboth input and output gearsets of the countershaft design. In planetary gears there is no radial load on the input or the output member, as the multiple pinion arrangement automatically cancels it. Each pinion does carry a radial load, but here the design picture provides a natural place for locating the bearings directly under the load on the pinion shafts, thus occupying the same space as the pin- THE usefulness of planetary gear trains and the THE techniques necessary for optimum design are discussed in this paper. A simple method for calculating planetary gear ratios is described which can be used to determine quickly the potential usefulness of any planetary configurations. The author lists criteria which help to evalu- ate the potential of a planetary gear train sche- matic from the standpoints of gear noise and structural viewpoint. Detailed design of individual members include spacing of the pinions, mounting considerations, thrust direction, lubrication, and stress evaluation. VOLUME 67, 1959 495Downloaded from SAE International by University of Leeds, Monday, September 24, 2018Fig. 1 — Gear train of Buick Triple Turbine transmission ions. This is also a significant factor in reducing the overall length of the design. Although the radial dimensions in some cases may be less favorable than in countershaft design where the countershaft can be placed at will in the most advantageous position for radial space con- siderations, there can be no doubt about the general advantages of the planetary gear design in space savings. Fig. 1 pictures the gear train in the latest Ameri- can automatic transmission — the Buick Triple Turbine. Note the small space required for the gear train, which is connected to two axial flow turbine members to give two forward gear ratios and reverse within the same set. 2. The planetary gears are more efficient in gear action. A fixed countershaft dictates the existence of two sets of gears transmitting the full power and suffering the sliding action loss of the involute gear teeth twice. The two losses are: (1) the input gear train loss in proportion to the input speed times input torque, and (2) the output gear train loss in proportion to the output speed times output torque. They are substantially equal. In the planetary arrangement the gears handl