Reason for a clutch:
It has been seen that the internal-combustion engine, unlike the steam engine, does not produce high power at low speeds; therefore the engine must be rotating at a speed at which sufficient power is developed, before the drive to the wheels is established.
(This condition rules out the use of dog clutch since the connection of rotating engine to a stationary transmission shaft would damage the transmission and jolt the vehicle.)
The clutch used must allow the drive to be taken up smoothly so that the vehicle can be gradually moved away from the stationary position.
Once moving, it will be necessary to change gear, and so a disengagement of the engine or transmission is required.
This is also is part of the clutch’s function.
These two cuties can be performed by various mechanisms; the friction system is considered to be one of the most effective and efficient.
The friction clutch
The object of a friction clutch:
- is to connect a stationary machine part to a rotating part,
- to bring it up to speed,
- to transmit the required power with a minimum of slippage,
- serves as a safety device by slipping when the torque transmitted through it exceeds a safe value, thus preventing the breakage of parts in the transmission train.
The purpose of the automotive clutch:
* used in cars with transmission that are shifted by hand (manual transmission)
- It allows the driver to couple the engine to, or uncouple the engine from the transmission.
The clutch is located just behind the engine, between the engine and the transmission.
The simplest clutch is made up of two plates forced together by powerful springs forming, in effect, one part, linking the engine to the transmission system.
· When the driver depress the clutch pedal he forces the two plates apart, (brakes the connection between engine and transmission).
· When he lets the pedal up again, the two plates come together (connect the engine and transmission).
Types of Clutches:
* In modern automobile the common used clutch is:
A single or double dry
friction disc with a diaphragm spring, manually operated, and with hydraulic or
Selection of Clutch Type:
The factors which must be taken into consideration in deciding what type of clutch is to be used are:
Torque (normal force, type of friction surfaces, number of surfaces)
Rotation speed (light, compact, internally balanced)
Available space (diameter, height)
Frequency of operation (small travel, a simple engaging mechanism, large cooling area, low inertia)
Parts of Automobile Friction Clutch
- Pressure Plate
- Clutch Cover
- Driven Plate (friction plate)
- Thrust Spring (diaphragm)
- Clutch Housing
- Release Lever
- Ball Bearing (graphite block)
- Primary Shaft (gearbox input shaft)
- Pedal Linkage
The purpose of a flywheel is to keep the speed of a machine between given limits while the machine is doing work or receiving energy at a variable rate. Disk flywheels are widely used in automobiles because of the uniformity and high strength.
There are several advantages in using the flywheel as one member of the friction clutch:
* Cast iron, of which it is usually made, is the best material for use with asbestos-base friction facings.
- Its graphite particles have a lubricating effect which prevents scoring and abnormal wear.
- Its great heat-absorbing capacity.
At the center of the flywheel is fitted a spigot bearings, which locates the front end of the gearbox primary shaft and allows for the difference in speed between the two members. This bearing (pilot bearing) can take the form of a ball race of plain bush (aligning the axis).
It also made of cast iron. The plate, as well as every other rotating part of the clutch has a flywheel effect. The plate made stiffer to distribute the pressure more evenly. Provide with more heat capacity and radiating surface.
The pressure plate must be driven from the flywheel by a number tempered spring-steel straps, one end of each of which is riveted to the cover, while the other end is bolted to pressure plate. The straps are equally spaced and extended tangentially.
Cover (clutch housing):
The cover is made by cold forming of 2.5-4 mm thick sheet steel. It is then aligned relatively to the flywheel axis with the aid of locating pins, collars or bolts.
* the clutch housing should ensure good airing:
- for cooling the friction surfaces
- to remove the wear products from them.
Openings are made in the housing subject to the condition of housing desired rigidity.
Thrust Spring (diaphragm spring):
Diaphragm spring forms a convex or dished shape while in unloaded state. Clamping the clutch to the flywheel flattens the spring and provides the necessary thrust on the pressure plate.
The advantage of diaphragm spring clutches:
- lower pedal force required,
- suitable for extra-high engine speeds, constant spring thrust and accurate balance are maintained,
- no separate release levers are required, giving improved release efficiency
- fewer parts required,
- spring load remains approximately constant as the facing wears,
- compact design, (decrease the clutch clearance limits and the clutch mass due to overlap of the function of the pressure spring and disengaging lever.
The disadvantages of the diaphragm springs:
- it is difficult to manufacturer spring for large axial forces,
- and small overall clutch dimensions.
Components of driven Plates:
Most clutch facing have a base of asbestos. The number of friction surfaces is equal to twice the number of driven discs.
Large driven plates have a tendency to spin (i.e. to continue rotating after the clutch pedal is depressed). To limit this trouble, the plate should be made as light as possible.)
A material suitable for use as a friction surface must meet the following conditions:
- it must have a high coefficient of friction
- it must not be effected by moisture and oil
- it must resist wear
- it must be capable of resisting high temperatures caused by slippages
- it must be capable of resisting high axial pressure
- should have the given rapture strength,
* There are groves in both sides of the friction-disc facings. These grooves prevent the facings from sticking to the flywheel face and pressure plate when the clutch is released. The groves break any vacuum that might form and cause the facing to stick to the flywheel or pressure plate.
** The facing or lining on the driven plate are secured by brass rivets, the heads being recessed into the lining to prevent scoring of the flywheel and pressure plate faces. As the lining wear down, the inner ends of the release leaver move away from the flywheel, and after a predetermined a mount of wear has taken place, the lever will touch the cover.
Driven plate cushioning spring:
If the clutch is in the driving position and the pedal is depressed, the driven plate will tend to jump away from the flywheel to give ‘clear’ disengagement. While in this position, the linings will be held apart, and air will be pumped between the linings to take away the heat.
During engagement, axial compression of the driven plate spreads the engagement over a great range of pedal travel and therefore makes it easier to make a smooth engagement.
Driven plate dampers:
The damped disc center using friction discs to reduce driveline vibration.
The clutch disc is designed to absorb such things as crankshaft vibration, abrupt clutch engagement and driveline shock. Torsional coil springs allow the disc to rotate slightly in relation to the pressure plate while they absorb the torque forces, therefore reducing shock of re-engagement.
It allows the driven plate to slide on the gearbox input shaft in the process of connecting and disconnecting the transmission with the engine. It is design to transfer the engine torque to the transmission input shaft.