371 Epicyclic overdrive gearing

Epicyclic gear train overdrives are so arranged that the input shaft drives the pinion carrier while the output shaft is driven by the annular gear ring

(Figs 3.27 and 3.28). The gear train may be either of simple (single stage) or compound (double stage) design and the derived formula for each arrangement is as follows:

Simple gear train (Fig. 3.27)

Overdrive gear ratio

where A = number of annulus ring gear teeth

S = number of sun gear teeth

P = number of planet gear teeth

Compound gear train (Fig. 3.28)

where A = number of annulus ring gear teeth

S = number of sun gear teeth

Ps = number of small planet gear teeth

Pl = number of large planet gear teeth

The amount of overdrive (undergearing) used for cars, vans, coaches and commercial vehicles varies from as little as 15% to as much as 45%. This corresponds to undergearing ratios of between 0.87:1 and 0.69:1 respectively. Typical overdrive ratios which have been frequently used are 0.82:1 (22%), 0.78:1 (28%) and 0.75:1 (37%).

Example 1 An overdrive simple epicyclic gear train has sun and annulus gears with 21 and 75 teeth respectively. If the input speed from the engine drives the planet carrier at 3000 rev/min, determine a) the overdrive gear ratio, b) the number of planet gear teeth, c) the annulus ring and output shaft speed, d) the percentage of overdrive.

a) Overdrive gear ratio

Therefore P

75 21

Fig. 3.27 Simple epicycle overdrive gear train c) Output speed

3000

0.78125

d) Percentage of overdrive = (

3840 rev/min 3840 3000

3000

840 x 100 3000

4000 0.82

4878 rev/min

Example 2 A compound epicyclic gear train overdrive has sun, small planet and large planet gears with 21, 15 and 24 teeth respectively. Determine the following if the engine drives the input planet carrier at 4000 rev/min.

a) The overdrive gear ratio, b) the number of annulus ring gear teeth, c) the annulus ring and output shaft speed, d) the percentage of overdrive.

a) Overdrive _ Pl (Pl + Ps + S) gear ratio = Pl (Pl + Ps + S) + PsS

_ 24 (24 + 15 + 21) = 24 (24 + 15 + 21) + (15 x 21)

d) Percentage of overdrive =-x 100

7 6 4000

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