Epicyclic Gearbox – Diagram , Working , Advantages of Epicyclic gearbox
In ordinary gearing, the axes of various gears are fixed. These gears are simply rotated about their axes. In the case of epicyclic gearing, at least one of the gears not only rotates about its own axis but also rotates about some other axes. The two arrangements of the epicyclic gear trains are shown in Figure 3.27.
In Figure 3.27 (a) arrangements, a spur pinion called sun wheel is an integral part of shaft which is made to rotate freely about its own axis X- X. The pinion shaft is mounted on bearings in the frame. Also, a crankshaft or carrier arm is also made to rotate freely about the same axis x-x. This arm is connected with a spur pinion called planet wheel by crankpin. The planet wheel is freely to revolve about its own axis y-Y. When the carrier arm is rotated on its bearings, the planet wheel will also rotate bodily about the axis X- X.
The planet wheel is meshed with the spur pinion and also meshed with an internally toothed ring called annulus. The annulus is one part of the frame as a fixed unit. The annulus is circular and concentric with the axis X-X. It is an epicyclic train of gearing. It provides a definite and fixed speed or gear ratio between the shaft attached to the sun wheel and the shaft of the carrier arm.
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In Figure 3.27 (b), the sun wheel (1) is a part of integral shaft which is freely to rotate about its axis in the frame. This sun wheel is meshed with the teeth A of the compound planet wheel. Now, the compound wheel freely revolves on the pin of the carrier arm. The carrier arm called shaft is mounted in the frame. It is free to revolve about the sun wheel.
The portion B of the compound planet wheel is meshed with the sun wheel (1). When the carrier arm shaft is turned, the shaft attached to the sun wheel (1) will also be driven in the same direction as the carrier arm rotates. But its speed is low. The epicyclic or sun and planet type gearbox have no sliding dogs or gears to engage. But different gear speeds can be obtained by tightening brake bands on the gear drum. Hence, gear changing is simplified.
Figure 3.:28 shows an epicyclic gearbox. The compound gear 1, 3, 5 is located to a pin fixed on a wheel A. The compound gear is freely rotated on the pin. Gears 1, 3 and 5 are meshed with three different gears 2, 4 and 6 respectively. They are connected in turn to drums 7, 8 and 9. The drums 7 and 8 have brakes in their outer circumference and a number of clutch plates are provided on drum 9. A hub is attached to the flywheel spigot shaft N in which a number of clutch plates are fitted. For obtaining top speed, the member 10 is pressed against clutch plates. It will engage the clutch by connecting the shaft N to the output shaft P directly. Thus, the top speed gear is obtained.
For obtaining low speed, the clutch is disengaged and brake is applied on drum 8. It is done with the help of a gear change lever. This action locks the gear 4 by decreasing the speed of gear 2. Hence, the output shaft is reduced.
For obtaining reverse speed, the clutch is disengaged and brake is applied on drum 10. It locks the gear 6 by reversing the direction of rotation of 2 with respect to the input shaft. Thus, the speed of 2 is also reduced.
Advantages of epicyclic gearbox:
1. The planetary gears are in constant mesh. Hence, dog clutches or sliding gears are not used.
2. External contracting hand brackets or multi-plate clutches of relatively small dimensions are used for changing the gears.
3. It is a more compact unit because the planetary gear operates within a ring gear with its external surface of cylindrical form.
4. It is distributed over several gear wheels instead of having the load on only one pair of gears.
5. A greater area of gear tooth contact can be obtained due to distribution of loads.
6. In comparison to the three or four speed gearbox, gears and gear housings of this gearbox can be made smaller in overall dimensions.
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