22-11-2012, 06:14 PM
TRANSMISSION
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INTRODUCTION
Transmission is the mechanism which is used to transfer the power developed by engine to the wheels of an automobile.
The transmission system of an automobile includes clutch, gear box, propeller shaft axle and wheels, etc.
Description of various types of clutches and gear boxes has been given in the following sections of this unit. The term ‘Transmission’ is used for a device which is located between clutch and propeller shaft. It may be a gear box, an over drive or a torque converter, etc.
CLUTCH
Clutch is used to engage or disengage the engine to the transmission or gear box. When the clutch is in engaged position, the engine power or rotary motion of engine crankshaft is transmitted to gear box and then to wheels. When clutch is disengaged, the engine power does not reach to gear box (and to wheels) although engine is running.
Clutch is also used to allow shifting or changing of gears when vehicle is running. For shifting gears, clutch is first disengaged then gear is shifted and then clutch is engaged. Clutch has to be disengaged to stop the vehicle and also at the time of idling.
PRINCIPLE OF CLUTCH
It operates on the principle of friction. When two surfaces are brought in contact and are held against each other due to friction between them, they can be used to transmit power. If one is rotated, then other also rotates. One surface is connected to engine and other to the transmission system of automobile. Thus, clutch is nothing but a combination of two friction surfaces.
SINGLE PLATE CLUTCH
A single plate is commonly used in cars and light vehicles. It has only one clutch plate which is mounted on the splines of the clutch shaft. A flywheel is mounted on the crankshaft of the engine. A pressure plate is connected to the flywheel through the bolts and clutch springs. It is free to slide on the clutch shaft with the movement of clutch pedal. When clutch is in engaged position, the clutch plate remains gripped between flywheel and pressure plate. Friction linings are provided on both the sides of clutch plate. On one side clutch plate is in touch with flywheel and on other side with pressure plate. Due to friction on both sides, the clutch plate revolves with engine flywheel. Therefore, clutch transmits engine power to clutch shaft. Clutch shaft is connected to transmission (or gear box) of automobile. Thus, clutch transmits power from engine to transmission system which inturn rotates wheels of engine.
When the clutch plate is to be disengaged, the clutch pedal is pressed. Because of this pressure plate moves back and clutch plate is disengaged from flywheel. Thus, clutch shaft stops rotating even if engine flywheel is rotating. In this position, power does not reach the wheels and vehicle also stops running. Single plate clutch is shown in Figure 4.1.
MULTIPLATE CLUTCH
Multi-plate clutch consists of more than one clutch plates contrary to single plate clutch which consists of only one plate. Friction surfaces are made in case of multi-plate clutch. Due to increased number of friction surfaces, a multi-plate clutch can transmit large torque. Therefore, it is used in racing cars and heavy motor vehicles witch have high engine power. The clutch plates are alternatively fitted with engine shaft and the shaft of gear box. He plates are firmly held by the force of coil springs and they assembled in a drum. One plate slides in the grooves on the flywheel and the next plate slides on spines provided on pressure plate. Thus, each alternate plate slides in grooves on the flywheel and the other on splines of pressure plate. If we take two consecutive plates, then one has inner and other has outer splines.
When the clutch pedal is pressed, the pressure plate moves back against the force of coil spring, hen the clutch plates are disengaged and engine flywheel and gear box are decoupled. However, when clutch pedal is not pressed the clutch remain in engaged position and the power can be transmitted from engine flywheel to the gear box. This type of clutch has been shown in Figure 4.2.