373Compound epicyclic overdrive gear train

For only small degrees of overdrive (undergearing), for example 0.82:1 (22%), the simple epicyclic gearing would need a relatively large diameter annulus ring gear; about 175 mm if the dimension of the gear teeth are to provide adequate strength. A way of reducing the diameter of the annulus ring gear for a similar degree of overdrive is to utilize a compound epicyclic gear train which uses double pinion gears on each carrier pin instead of one size of pinion. By this method, the annulus diameter is reduced to about 100 mm and there are only 60 teeth compared to the 96 teeth annulus used with the simple epicyclic gear train.

To transmit power, the sun gear is held still whilst the input shaft and planet carrier are rotated. This compels the large planet gear to roll around the stationary sun gear and at the same time forces each pair of combined pinion gears to revolve about their carrier pin axes.

Consequently the small pinion gear will impart both the pinion carrier orbiting motion and the spinning pinion gear motion to the annulus ring gear so that the output shaft will be driven at a higher speed to that of the input shaft.

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