Learner Zone
Motor Vehicle HubTransmission
A transmission or a gearbox acts as the medium that transmits power generated by the engine to the wheels via a mechanical system of gears and gear trains. A transmission allows one to apply power to a vehicle in a controlled manner, without which the car would not efficiently move.
propeller shaft
In a front-engined, rear wheel drive car, power is transmitted from the engine through the clutch and the gearbox to the rear axle by means of a tubular propeller shaft.
The rear axle must be able to move up and down on the suspension according to the road surface. The movement causes the angle of the propeller shaft and the distance between the gearbox and the rear axle to change constantly. To allow for this constant movement, splines on the front end of the propeller shaft slide in and out of the gearbox as the distance changes. The shaft also has universal joints at each end, and sometimes in the middle. The universal joints allow the propeller shaft to be flexible, while constantly transmitting power.
The last part of the transmission is the final drive, which incorporates the differential.
The differential has three functions: to turn the direction of drive through 90 degrees to the rear wheels, to allow the rear wheels to turn faster than the other when cornering and to effect a final gear reduction.
A pinion gear inside the differential is driven by the propeller shaft and has its gears bevelled. It meshes with a bevelled crown wheel so that the two gears form a 90 degree angle.
front wheels
Front wheel-drive cars use the same transmission principles as rear wheel-drive, but the mechanical components vary in design according to the engine and gearbox layout.
Transverse engines are normally mounted directly above the gearbox, and power is transmitted through the clutch to the gearbox by a train of gears.
In-line engines are mated directly to the gearbox and drive passes through the clutch in the normal manner.
In both cases, drive passes from the gearbox to a final-drive unit. In a transverse-mounted engine, the final-drive unit is usually located in the gearbox. In an in-line engine, it is usually mounted between the engine and the gearbox.
Power is taken from the final-drive unit to the wheels by short drive shafts. To cope with suspension and steering movement in the wheels, the drive shafts use a highly developed type of universal joint called a constant-velocity joint.
A constant-velocity joint uses grooves with steel ball bearings in them instead of the spider found in a universal joint, and transmits power at a constant speed regardless of the angle and the distance between the final-drive unit and the wheels.
Some cars also have drive-shaft couplings which are spider joints, and do the same job as universal joints in rear-wheel-drive cars, allowing up-and-down movement of the suspension. They are usually made of rubber bonded to metal.
rear wheels
Some cars have rear-mounted engines and gearboxes driving the rear wheels.
Power is transmitted through the clutch to the gearbox, passing to the wheels through drive shafts. The layout is similar to some front wheel-drive cars, except that no allowance need be made for steering movement of the wheels. Sometimes the shafts are connected to the flanges at the gearbox by doughnut couplings.