Gearboxes
The Gears
First Gear
With the annulus locked by the brake band and no. 1 sunwheel rotating, the planet wheels attempt to spin but are prevented by the locked annulus. By using the internal teeth of the annulus they can spin only by rolling and thus the planet spindles cause rotation of the planet carrier, which forms part of the output shaft, this now rotates in the same direction as the input shaft.
The speed of the output shaft in relation to the input is governed by the designed ratio e.g. 4.28:1. which means that the input shaft revolves 4.28 times for each revolution of the input shaft.
The figure also shows what is taking place in the other gear train when the no. 1 brake band is applied.
Second Gear
In this case the second gear is applied after the brake band for the first gear has been released as shown below. When the second brake band is applied for the following happens:-
- the first gear annulus is released
- the second gear annulus is stopped
- the sunwheel for no.2 gear is rotating
- the three planet wheels of no.2 gear are rotating in the opposite direction to that of the input shaft
- as the annulus is stopped, but with the sunwheel revolving, the planet wheel carrier revolves
- as the planet wheel carries, which is directly connected to the annulus of the first gear, revolves, it causes the no.1 gear annulus to revolve
- as the first gear annulus and first gear sunwheel are already revolving in the same direction, this added rotation causes no. 1 gear planet wheels to rotate more quickly, this in turn causes the output shaft to rotate at a greater speed than in first gear.
Third Gear
Third gear is obtained from the added motion which the third gear train passes to second gear and on to first gear. Summarising: second gear is used to speed up first gear for second gear range. Third gear is used to speed up second gear which in turn will further speed up first gear for third gear range.
When third gear is selected a brake band stops the third gear drum to which is attached the third gear sunwheel.
The planet wheels of third gear are connected to the annulus of second gear and the second gear planet wheels are connected to the third gear annulus as well as to first gear annulus.
Rotation of sunwheel no. 2 will cause second gear planet wheels to rotate about their own axis. They in their turn will cause the second gear annulus to rotate, which in its turn will cause third gear planet wheels to roll round the stationary third gear sunwheel. This will cause the third gear annulus to rotate, causing second gear planet wheels to roll round second gear annulus.
The combined movements of the third and second gear annulus will cause the second gear planet wheels to move faster, thus the first gear annulus will revolve faster which in its turn increases the speed of the output shaft.
Fourth Gear
The fourth gear is not engaged by applying a brake band but by means of a clutch which connects the sunwheel of the third gear train to the input shaft and thus locks all the running gears together, which then revolve with the main shaft. It should be noted that there is no rotation of any of the planet wheels around their pins. This arrangement gives a 'straight through' drive in top gear.
