WHAT IS FINAL DRIVE?
The final drive is the last member of a transmission system that gives a final gear reduction and distribution of engine power to wheels.
In most automobile vehicles the final drives are embodied in the rear axle. But in various popular vehicles with front-wheel drive and a few special purpose vehicles with four-wheel drive, it becomes necessary to consider final drives as units independent of their position. One form of a final drive commonly used in motorcycles is the chain drive. This is the simplest and most efficient system, but it does not find more general use, because of the difficulties of cleanliness and lubrication. Other forms of the final drive commonly used in four-wheeled vehicles, like cars, buses, trucks, is the shaft drive.
In a motor vehicle, the final drive has two purposes :
1. To turn the drive through 90° so that the torque may be transmitted from the propeller shaft to the rear axles.
2. To provide a permanent gear reduction. For motor cars, the reduction is usually 8.5 to 6.1; and for commercial vehicles, it is about 12.1.
FINAL GEAR REDUCTION
The final drive consists of a bevel pinion and crown wheel. The bevel pinion is mounted on a shaft. which is connected to the propeller shaft. It meshes with the crown wheel of the differential unit. Because the pinion has less number of teeth and the crown wheel has the number of teeth, the final drive provides a fixed gear reduction. To turn the drive at 90° and to give the required speed reduction, two basic types of gearings are used:
1. Bevel gear
2. Worm and worm wheel.
Final Drive Unit:
The pinion and the crown wheel may be either straight bevel gears or spiral bevel gears. The spiral bevel gears are silent running and stronger than the straight bevel gears because of greater contact of the teeth. The spiral bevel gears are used on some motor cars and commercial vehicles, although they are being replaced by the hypoid bevel gears. In hypoid bevel gears, the pinion shaft is placed below the axis of the crown wheel.
This permits a lower position of the propeller shaft, thus allowing a chassis height. The heavier type commercial vehicle having double reduction gearing often has a spiral bevel for the first reduction and double helical gearing for the final reduction. In double reduction, however, the size of the larger gear becomes too big.
ed. Although the worm drives
The worm and worm wheel arrangement for final gear reduction is extremely strong. It gives high reductions without using large gears. The worm may be mounted above or below the worm wheel. When mounted above, it gives a good ground clearance; and when mounted below, it enables the shaft tunnel to be expensive and heavier than bevel gears, their mechanical efficiency can be high providing great care is taken in their design and manufacture.
The transversely mounted engine does not require its final drive turned through 90°. In this case, a pair of helical gears are used to obtain the required final reduction, provided the gearbox lies in the same plane as the engine.