To meet the varying requirements of road conditions, types of load and control of the vehicle, some means must be incorporated where by the power of the engine can be transmitted to the road wheels. This is known as the transmission and the component parts of the system are illustrated in figure below.
Clutch is a mechanism for connecting and disconnecting two independently mounted revolving members. Such as, the crankshaft and the gearbox 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:
- To allow engagement of a gear when the vehicle is stationary and the engine is running.
- To transmit the engine power to the road wheels in a smooth manner when setting the vehicle in motion.
- To allow the gears to be changed, without damage to the road wheels while vehicle is in the motion.
The word clutch means, “Grip”. This mechanism functions by gripping and to have an efficient performance, a material having a high co-efficient of friction is used. Moulded asbestos linings commonly known as Ferrodo linings consisting 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 posses high coefficient and retains a fairly high value over considerable ranges of pressure, temperature and speed.
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.
(a) Cone Clutch. 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 to withdraw the internal cone, and thus disconnect the drive.
(b) 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.
The flywheel has machined face to which the clutch plate is sandwiched. The flywheel also houses the pilot bearing or bush in which the front end of the primary shaft is supported.
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.
The components of the clutch driven plate is built in three main parts as follows:
- Hub Plate. Splined to the clutch shaft, Slots for springs, slots in circumference for limits stop rivets.
- Driven Plate. Fitted around hub of hub plate, but not attached to it. Slots for springs and linings.
- Retaining Plate. Also fitted around hub of hub plate but not attached to it. It also has slots for springs and holes for limit stop rivets.
The flange of the hub plate is sandwiched between the driven plate and the retaining plate, which are attached to each other by limit stop rivets. These rivets pass through the slots in the circumference of the hub plate. When the plate is assembled the spring slots in all plates are in line, and the springs are positioned so that the ends rest against the ends of the slots in all three plates.
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 as well as providing a cushion when the drive is taken up, the springs will also damp out small variations in the driving torque the vehicle is in motion. The amount of damping is limited by the distance that the limit stops rivet travel in the slots in the hub-plate.
(a) Pressure Plate. 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.
(b) Clutch Cover. The clutch cover is bolted to the flywheel. 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.
(c) 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.
(d) 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.
(e) Primary or Clutch Shaft. Clutch shaft is a part of gearbox, which is a gearbox-driving shaft. It is splined to the hub of the clutch plate. The forward end of the clutch shaft is supported in a spigot bearing in the gearbox and carries one of the constant mesh gears. The clutch plate slides on the primary shaft splines.
Multi-Plate Clutch (Wet)
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. On the other hand, when starting from cold the oil tends to bind the plates together, thus making disengagement difficult.
Multi-Plate (Dry Clutch)
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, but metal plates are used on tractors. If the multi-plate has the cork inserts it must run in oil, the reason being that if slight slip is experienced on drive take up, the corks become scorched or charred through frictional heat and the clutch will become unserviceable because it will not maintain grip. The multi plate clutch is also used on vehicles with high powered engines, as an increase in the number of plates and therefore area of frictional contact, allows the torque to be transmitted without an increase in clutch diameter.
Booster Assisted Clutches
The booster-assisted clutch is operated by hydraulic oil pressure. It consists of a clutch cylinder and a slave cylinder. The clutch pedal is connected to the push rod of the clutch main cylinder. By operating the clutch main cylinder the hydraulic fluid pressure is applied through metal or rubber pipes to the working cylinder (clutch slave cylinder). The piston rod of slave cylinder actuates the clutch release fork. Clutch main cylinder consists of a piston, primary and secondary rubber cups, Piston return spring and check valve. Clutch slave cylinder consists of a piston, rubber cup, spring and operating rod.
Clutch Pedal Adjustment
As the clutch facings wear the free pedal travel decreases. When clutch pedal rests against the floorboard it is necessary to adjust the free travel. Lengthen or shorten the clutch control cable so that the pedal has a free travel between 3/4” to 1”, which maintains a clearance of 1/16” between the release bearing and toggle levers.
Procedure of Removal of a Clutch Assembly
(a) Remove the gearbox from the engine.
(b) Remove the bell housing.
(c) Mark both the pressure plate and flywheel so that the assembly can be installed in the same position.
(d) Loosen the screws holding clutch bracket to the flywheel in sequence diagonally to prevent distortion of bracket.
(e) Remove the screws and withdraw the assembly including driven plate from the flywheel.
Procedure of Installing of a Clutch Assembly
(a) Before assembling clutch to the flywheel first put a small amount of cup grease in the spigot bearing.
(b) Install driven plate with short end of hub towards the flywheel then place assembly in position.
(c) With clutch pilot arbour or a clutch shaft, align driven plate leaving arbour in place while tightening the clutch cover screws. Now remove the arbour (clutch alignment shaft).
(a) Clutch Slips. This occurs when the two friction surfaces do not grip each other sufficiently to convey the full power from the engine. The symptom is that the engine accelerates without a proportionate increase to the road speed of the vehicle. The probable causes are:
- Improper assembly of unit parts of clutch.
- Insufficient free pedal movement or pedal fouling to floorboard.
- Oil or grease on the linings.
- Worn friction linings, pressure plate or flywheel.
- Weak or broken thrust springs.
- Binding of clutch release mechanism.
(b) Clutch Spin. When the clutch pedal is depressed and the friction plate and gear box driving shaft continue to rotate, difficulty is experienced when engaging gears. One of the following faults may cause the trouble.
- Too much free pedal movement.
- Release levers unevenly adjusted.
- Defective clutch plate.
- Lack of lubrication on driving shaft splines.
- Bush or roller races on driving shaft spigot seized.
- Engine and gear box out of alignment.
- Badly worn graphite thrust ring on release bearing assemblies.
(c) Clutch Fierce. This is a rough engagement of the clutch when it is taking up the drive from the stationary or during gear changes. However carefully the clutch pedal is released vehicle lunches violently. The probable cause may be the following:
- Uneven wear or loose linings on the clutch plate.
- Splinces worn on sliding portions of the clutch.
- Lining badly worn.
- Bearing badly worn.
- Spigot bearing worn.
(d) Clutch Noisy when disengaged (pedal depressed).
- Clutch release bearing worn, damaged or broken.
- Clutch release bearing binding in release shaft levers.
- Crankshaft clutch pilot bearing worn damaged broken.
- Clutch, toggle levers improperly adjusted.
(e) Clutch Noisy when engaged (pedal released).
- Misalignment of transmission with engine assembly.
- Clutch driven plate hub loose on primary shaft.
- Clutch driven plate dampener spring weak or broken.
- Weak retracting springs or excessive clearance at driving lugs (Multi plate clutch).
- Cracked or distorted pressure plate.
- Loose or protruding rivets or linings badly worn or pressure plates out of adjustment.
(g) Clutch Pedal Pulsation. Clutch pedal pulsation has often termed a “nervous” pedal. When a slight pressure is applied on the pedal while the engine is running, the pedal will vibrate or bounce with every revolution of the engine. As the pressure on the pedal is increased, the pulsation will cease and the cause may be following:
- Clutch toggle levers unevenly adjusted.
- Clutch release shaft not parallel or shaft is bent.
- Clutch pedal pull back spring missing or broken.
- Misalignment between transmission and engine.
- Flywheel not properly seated on crankshaft.