Epicyclic Gear Trains

Figure 10.5 shows an elementary epicyclic gear train together with its schematic diagram suggested by Levai. IThe train consists of a central gear 2 and an epicyclic gear 4, which produces epicyclic motion for its points by rolling around the periphery of the central gear. A crank arm 3 contains the bearings for the epicyclic gear to maintain the two gears in mesh. These trains are also called planetary or sun-and-planet gear trains. In this nomenclature, gear 2 of Fig. 10.5 is called the sun gear, gear 4 is called the planet gear, and crank 3 is called the planet carrier. Figure 10.6 shows the train of Fig. 10.5 with two redundant planet gears added. This produces better force balance; also, adding more planet gears allows lower forces by more force sharing. However, these additional planet gears do not change the kinematic characteristics at all. For this reason we shall generally show only a single planet in the illustrations and problems in this chapter, even though an actual machine would probably be constructed with planets in trios. The simple epicyclic gear train together with its schematic designation shown in Fig. 10.7 shows how the motion of the planet gear can be transmitted to another central

gear. The second central gear in this case is gear 5, an internal gear. Figure 10.8 shows a similar arrangement with the difference that both central gears are external gears. Note, in Fig. 10.8, that the double planet gears are mounted on a single planet shaft and that each planet gear is in mesh with a separate sun gear rotating at a different speed.

In any case, no matter how many planets are used, only one planet carrier or arm may be used. This principle is illustrated in Fig. 10.6, in which redundant planets are used, and in Fig. 10.9, where two planets are used to alter the kinematic performance.

According to Levai, 12 variations are possible; they are all shown in schematic form in Fig. 10.10 as Levai arranged them. Those in Fig. 1O.lOa and Fig. 10.1Oc are the simple trains in which the planet gears mesh with both sun gears. The trains shown in Fig. 1O.lOb and Fig. 10.1ad have planet gear pairs that are partly in mesh with each other and partly in mesh with the sun gears.