To meet the varying requirements of road conditions, types of load and control of the vehicle, some means must be incorporated in the vehicle where by the power of the engine can be transmitted to the road wheels. There by the series of components are placed in a proper sequence so that the engine power is transmitted to the rear road wheels. This arrangement is known as the transmission of engine power from the engine up to the rear/front road wheels.
MAIN COMPONENT OF TRANSMISSION SYSTEM IS AS FOLLOWS
(a) Clutch Assembly. The purpose of the clutch assembly is to connect and disconnect drive between the engine and gear box as and when required.
(b) Gear Box. The gear box is a mechanism which is used to transfer drive received from the clutch to the propeller shaft through a fixed reduction or increased ratio mechanism under varying engine and vehicle speed condition.
(c) Propeller Shaft. Propeller shaft is a device which is used to transmit the drive from the gear box to the differential assembly.
(d) Universal Joints. Universal joint is used to connect two shaft inclined to one another at a small angle and at the same time to transmit the drive in rotary motion. It is fitted between gear box and propeller shaft and between propeller shaft and differential assembly.
(e) Differential Assembly. The purpose of differential assembly is as follows.
(f) To transmit the drive received from propeller shaft to road wheels at reduced gear ratio.
(g) To transmit the drive received from propeller shaft to road wheels at right angle.
(h) To transmit the drive received from propeller shaft to road wheels in such a way that inner wheel can run at lower speed than that of outer wheel when cornering the vehicle.
PURPOSE AND LAY OUT OF TRANSMISSION SYSTEM
(a) Purpose. The purpose of transmission system is to transmit the drive received from the engine to the rear/front wheels through the series of components as per the various road and load conditions.
(b) Layout of transmission.
PURPOSE OF CLUTCH
A clutch is a mechanism for connecting and disconnecting two independently mounted revolving members Such as, the crankshaft and the gear box primary shaft. It is fitted to the flywheel at the rear end of the crankshaft, i.e., between the flywheel and the gearbox. The different purposes of the clutch are:
(a) To allow engagement of a gear when the vehicle is stationary and the engine is running.
(b) To transmit the engine power to the road wheels, when setting the vehicle in motion, in a smooth manner without shock to the transmission.
(c) To allow the gears to be changed, without damage to the road wheels while vehicle is in the motion.
PARTS OF CLUTCH PLATE
The clutch driven plate is built in three main parts as follows:
(a) Hub Plate. Splined to the clutch shaft. Slots for springs, slots in circumference for limits stop rivets.
(b) Driven Plate. Fitted around hub of hub plate, but not attached to it. Slots for springs and linings.
(c) Retaining Plate. Also fitted around hub of hub plate but not attached to it. Slots for springs. Holes for limit stop rivets.
INTRODUCTION AND TYPES OF CLUTCHES
Most of the clutches used in motor transport vehicles are of a frictional type and are classified under three main types such as, Cone clutch, single plate clutch and Multi plate clutch. Moulded asbestos linings commonly known as Ferro do linings are used as a friction material. The Ferro do used in the automobile industry consists of impregnated compressed asbestos and may have numerous fine brass wires running through it. The object of the wires is to hold the material together and conduct away the heat generated by the friction. These materials possess high co-efficient of friction and retains a fairly high value over considerable ranges of pressure, temperature and speed.
(a) Cone Clutch
(b) Multi-Plate Clutch (Wet)
(c) Multi-Plate (Dry Clutch)
(d) Single Plate Clutch
This type of clutch is not in general use today but was used for larger and earlier type vehicles. It consists of an internal and external cone, the internal cone being fitted with frictional lining of a wire and asbestos composition. A large and strong coil spring was used to hold the cones together in position. A foot operated clutch pedal was employed to overcome the spring pressure and withdraw the internal cone and thus disconnect the drive.
MULTI-PLATE CLUTCH (WET& DRY TYPE)
(a) Multi-Plate Clutch (Wet Type). The multi-plate, all metal clutch consists of a number of metal discs alternately fitted on the engine shaft and the gear box shaft. The plates are usually made of dissimilar metals such as steel plates on the engine shaft and phosphor bronze on the gear box shaft. The plates run in a bath of oil and are firmly pressed together by strong coil springs. When the clutch is disengaged the oil gets in between the plates, so that when re-engagement takes place the oil has to be squeezed out, and it provides a very smooth and gradual engagement.
(b) Multi-Plate (Dry Clutch). A later multi-plate clutch has a plate lined with friction material similar to that used on the more general single plate clutch or alternate plates, have cork inserts. The operation is the same in all types but when the plates are lined with friction material they do not run in a bath of oil. Multi-plate clutches, usually with cork inserts, are used on motor cycles.
SINGLE PLATE CLUTCH
The single plate is the most common type in use today, and it consists principally of a clutch disc, pressure plate and a cover plate bolted to the engine flywheel. By removal of the cover plate bolts, the complete clutch assembly can be removed from the flywheel.
(a) Construction. The clutch plate is lined on both sides with friction material and has central hub machined with internal splined gearbox driving shaft. Modern vehicles are equipped with cushion drive clutch plate. The purpose of the cushion drive plate is to provide a damping action against torsional vibrations or variations of driving torque between the engine and the transmissions. This is achieved by coupling the splined centre hub to the driven plate by a flexible mounting.
(b). Operation. When the clutch is engaged, the drive passes from the lining to the plate, and retaining plate (via the rivets). The springs transmit the drive from the plate to the hub-plate and in doing so they are slightly compressed, thereby cushioning the drive. It will be noted that apart from providing a cushion when the drive is taken up, the springs will also damp out small variations in the driving torque when the vehicle is in motion. The amount of damping is limited by the distance that the limit stop rivet travel in the slots in the hub-plate.
PRESSURE PLATE CONSTRUCTION AND OPERATION
The pressure plate has one face machined which presses the clutch plate on to the flywheel. Pressure springs are located between the pressure plate and the clutch cover assembly. The toggle or release levers are pivoted in such a way that whenever the levers are depressed the pressure plate is withdrawn from the fly wheel. The pressure plate usually has three or four lugs machined which fit in corresponding slots provided in the clutch cover.
(a) Clutch Cover. The clutch cover is bolted to the fly wheel. The clutch cover carries the pressure plate, toggle or release lever mechanism. The pressure plate, therefore, always revolves with the flywheel and pressure plate can revolve independently from the driven plate and driving shaft when the clutch is disengaged.
(b) Release Levers. Release levers pivot on pins in the clutch cover, the outer ends of which are positioned on pressure plate lugs whilst inner ends project towards the clutch shaft.
(c) Withdrawal Fork Bearing. Two types of withdrawal bearings (Thrust Bearing) are employed viz the Carbon graphite thrust ring and the ball bearing. The ball bearing is used on the heavier types of vehicles and usually mounted on an extension sleeve bolted to the cover centre flange. The Carbon graphite ring is used on the lighter vehicles (Cars etc.).The withdrawal bearing is carried on the withdrawal fork and faces the inner ends of toggle levers. The fork is pivoted on a ball bearing mounted in the clutch outer casings. The other end of the withdrawal fork connected to the clutch pedal by rods and levers.
DIAPHRAGM TYPE PRESSURE PLATE
Diaphragm spring type clutch has been introduced in place of the convention coil-spring type Clutch. It contains a conical diaphragm spring, with slots radiating from the centre to form release fingers. In as fitted condition, the diaphragm spring is almost flattened so that in trying to regain its conical shape, it exerts an even pressure around its outer edge, where pressure plate makes contact with it. Disengaging the clutch flexes the diaphragm spring the other way to form a reverse cone and frees the pressure plate from load. With conical diaphragm spring pedal effort is reduced. More even and uniform pressure on the clutch disc is achieved. With wear of the lining, the pressure exerted by diaphragm spring on pressure plate reduces negligibly. This feature is unique for this type of clutch which improves clutch performance and clutch life considerable. Number of components and hence, number of settings, adjustments in this type of clutch are very few as compared to coil spring type clutch, making it more reliable.
OPERATION OF DIAPHRAGM TYPE CLUTCH
(a) The operation of Diaphragm type clutch is like the action that takes place when the bottom of oil can is depressed. it dishes inward when the throughout bearing moves in against the end of the fingers, the entire diaphragm is forced against a pivot ring, causing the diaphragm to dish inward. This moves the pressure plate away from the friction disk.
(b) In engaged position the diaphragm spring is slightly dished, with the tapering fingers pointing slightly away from the flywheel .This position places spring force against the pressure plate around the entire circumference of the diaphragm spring. The diaphragm spring is shaped to exert this initial force.
(c) When clutch pedal is depressed the spring is forced to pivot about the inner pivot ring, dishing in the opposite direction. The outer circumference of the spring now lifts the pressure plate away through a series of retracting spring placed about the outer circumference of the pressure plate.
SERVICING OF CLUTCH
The major clutch services include clutch linkage adjustment, clutch removal and clutch replacement, and clutch disassembly, inspection, adjustment and reassembly If a clutch defect is develops, You must do more than just replace a worn part .You must determine what cause the part to wear and fix the trouble, so that the new part will not wear rapidly.
CHECKING OF CLUTCH FREE PLAY
One of the most common causes of rapid disc-facing wear and clutch failure is improper pedal free travel, if pedal travel is not sufficient, the clutch will not engage completely. It will slip and wear rapidly. In addition the throughout bearing will be operating continuously and will soon wear out. And gear engagement and disengagement will be noisy.
CLUTCH FREE PLAY ADJUSTMENT
As the clutch facings wear the free pedal travel decreases. When clutch pedal rests against the floor board it is necessary to adjust the free travel. Lengthen or shorten the clutch control cable or linkage so that the pedal has a free travel between ¾” to 1” which maintains a clearance of 1/6” between the release bearing and toggle levers.