Propeller Shaft | Functions of the Propeller Shaft
The propeller shaft connects the transmission shaft to the pinion shaft at the wheel axle. The propeller shaft is also called driveline shaft or drive shaft.
The propeller shaft carries the power from the engine, clutch and transmission unit to the driving wheels of the vehicle, through the final drive and differential unit.
Functions of the propeller shaft
In most of the automotive vehicles, the engine is located at the front and the rear wheels of the vehicle are being driven. This arrangement stipulates a longer propeller shaft to be used. In some arrangements two or three propeller shafts are used to make up the length.
In some vehicles, the engine is kept at the front and the front wheels of the vehicle are being driven. In some other vehicles, the engine is at the rear and the rear wheels are being driven. For such arrangements a short propeller shaft is used to drive each wheel.
The engine and the transmission unit are attached to the vehicle frame with some flexible mounting. The rear axle housing with differential and wheels are attached to the vehicle frame by suspension springs.
Due to the above arrangement, the transmission output shaft and the input shaft to the rear axle housing are in different planes. This compels the propeller shaft that connects these two shafts to be kept inclined.
Further, whenever the rear wheels encounter irregularities in the road, the rear axle housing moves up and down, compressing and expanding the suspension springs. As this happens, the angle between the transmission output shaft and the propeller shaft changes. Further, the length to be occupied by the propeller shaft also changes.
The variation in the length of the propeller shaft happens because the propeller shaft and the rear axle housing rotate on arcs with different points as their centres of rotation.
The rear axle housing moves in the shorter arc than that of the propeller shaft. This is because the centre of the rear axle housing arc is the point of attachment of the rear spring or control arm to the vehicle frame. This aspect causes a reduction in the length occupied by the propeller shaft as the angle between the transmission and the propeller shaft increases.
To sum up, the propeller shaft does the following functions:
1. It transmits rotary motion of the gearbox output shaft to the differential and then to the wheels through the axle shafts.
2. It transmits motion at an angle which is varying frequently.
3. It accommodates changes in length between gear box and rear axle.
In the case of cars, where the overall length of the vehicle is not too much, the propeller shaft is of single length. On the other hand, the distance between the transmission shaft and the pinion shaft of the differential is more in the case of trucks, buses and long chassis cars. In such cases, on or more intermediate propeller shaft is connected to the gearbox main shaft and the other end to be main propeller shaft. The intermediate propeller shaft is supported in a bearing unit. The bearing unit consists of a bracket, a rubber pad and a ball bearing. The bearing bracket is attached to the cross member of the vehicle frame. The intermediate shaft arrangement reduces the length of the main propeller shaft.