Introduction to Clutch and Types of Clutches
Clutch and types of clutches
The clutches used in a motor vehicle are almost very similar in construction and operation. There are some differences in the details of the linkage as well as in the pressure plate assemblies.
In addition, some clutches for heavy duty applications has a two friction plate and an intermediate pressure plate. Some clutches are operated by hydraulic means. The dry single plate type of friction clutch is almost used in American passenger cars.
The various types of clutches used in the automobile depend upon the type and use of friction.
Most designs of the clutches use a number of coil springs but some use a diaphragm or conical type spring. The type of friction materials also varies in the clutches of different passenger cars.
Types of Clutches
Following are the different types of clutches using in automobile industries.
- Friction clutch
- Single plate clutch
- Multiplate clutch
- Cone clutch
- Centrifugal Clutch
- Semi-centrifugal clutch
- Conical spring clutch or Diaphragm clutch
- Tapered finger type
- Crown spring type
- Positive clutch
- Dog clutch
- Spline Clutch
- Hydraulic clutch
- Electromagnetic clutch
- Vacuum clutch
- Overrunning clutch or freewheel unit
Single Clutch Plate
Single plate clutches are one of the most commonly used types of clutches used in most modern light vehicles. The clutch helps to transmit torque from engine to the transmission input shaft. As the name states it has only one clutch plate.
It consists of a clutch plate, friction plate, pressure plate, flywheel, bearings, clutch spring and not-bolts arrangement.
The single plate clutch has only one plate which is attached on splines of the clutch plate. Single plate clutch is one of the main components of the clutch. The clutch plate is simply thin metallic disc which has both side friction surfaces.
The flywheel is attached on the engine crankshaft and rotates with it. A pressure plate is bolted to flywheel through clutch spring, which provides axial force to keep the clutch engaged position, and is free to slide on the clutch shaft when the clutch pedal is operated.
A friction plate which is fixed between the flywheel and pressure plate. The friction lining is provided on both sides of the clutch plate.
In a vehicle, we operate the clutch by pressing the clutch peddle for disengagement of gears. Then springs get compressed and the pressure plate moves backwards. Now clutch plate becomes free between the pressure plate and flywheel. Due to this now the clutch is getting disengaged and able to shift the gear.
This makes flywheel to rotate as long as the engine is running and the clutch shaft speed reduces slowly and then it stops rotating. As long as the clutch peddle is pressed, the clutch is said to be disengaged, otherwise, it remains engaged due to the spring forces.
After releasing the clutch pedal the pressure plate comes back to its original position and clutch is again engaged.
The multi-plate clutch is shown in the figure. This types of clutches uses multiple clutches to make frictional contact with a flywheel of the engine. This makes transmit power between the engine shaft and the transmission shaft of a vehicle. The number of clutches means more friction surface.
The increased number of friction surfaces also increases the capacity of the clutch to transmit torque. The clutch plates are fitted to the engine shaft and gearbox shaft.
They are pressed by coil springs and assembled in a drum. Each of the alternate plates slides in grooves on the flywheel and the other slides on splines on the pressure plate. Hence, each different plate has an inner and outer spline.
The working principle of multiple clutch is the same as the working of single plate clutch. The clutch is operated by pressing the clutch pedal. The multiple clutches are used in heavy commercial vehicles, racing cars and motorcycles for transmitting high torque.
The multiple clutches have two characters dry and wet. If the clutch is operated in an oil bath, it is known as a wet clutch. If the clutch is operated dry without oil, it is known as a dry clutch. The wet clutches are commonly used in connection with, or as a part of the automatic transmission.
The figure shows the diagram of a cone clutch. It consists of friction surfaces in the form of cones. This clutch uses two conical surfaces to transmit torque by friction. The engine shaft consists of a female cone and a male cone. The male cone is mounted on the splined clutch shaft to slide on it. It has a friction surface on the conical portion.
Due to the force of spring when the clutch is engaged the friction surfaces of the male cone are in contact with the female cone. When the clutch pedal is pressed, the male cone slides towards the spring force and the clutch is disengaged.
The main advantage of using cone clutch is that the normal force acting on the friction surface is greater than the axial force, as compared to the single plate clutch. That’s why the normal force acting on the friction surface is equal to the axial force.
The cone clutches are basically becoming old because of some disadvantages.
- Let’s consider the angle of the cone is made smaller than 20°, the male cone tends to bind in the female cone and it becomes difficult to disengage the clutch.
- A small amount of wear on the cone surfaces has a considerable amount of axial movement of male cones, for which it will be difficult to allow it.
The below figure shows a centrifugal clutch. To keep the clutches in the engaged position centrifugal clutch uses centrifugal force, instead of spring force. In this types of clutches, the clutch is operated automatically depending upon the engine speed. That’s why no clutch pedal is required to operate the clutch.
This made so easy to the driver to stop the vehicle in any gear without stalling the engine. Similarly, you can start the vehicle in any gear by pressing the accelerator pedal.
Working of Centrifugal clutch
- It consists of weights A pivoted at B.
- When the engine speed increases the weights fly off due to the centrifugal force, operating the bell crank levels, which press the plate C.
- The movement of plate C presses the spring E, Which ultimately presses the clutch plate D on the flywheel against the spring G.
- This makes the clutch engaged.
- The spring G keeps the clutch disengaged at low speeds at about 500rpm.
- The stop H limits the movement of the weights due to the centrifugal.
The semi-centrifugal clutch uses centrifugal force as well as spring force for keeping it in the engaged position. The figure shows a semi-centrifugal clutch. It consists of levers, clutch springs, pressure plate, Friction lining, flywheel and clutch plate.
Construction of semi-centrifugal clutch:
A semi-centrifugal clutch has levers and clutch springs which are arranged equally on the pressure plate. The springs of the clutch are designed to transmit the torque at normal engine speed. While the centrifugal force helps in torque transmission at higher engine speed.
At normal engine speeds, when the power transmission is low, the springs keep the clutch engaged, the weighted levers do not have any pressure on the pressure plate.
At high engine speed when the power transmission is high, the weights fly off and the levers also exert pressure on the plate, keeping the clutch firmly engaged.
This types of clutches consist of less stiff springs, so that the driver may not get any strain while operating the clutch. When vehicle speed decreases the weights fall and the lever does not apply any pressure on the pressure plate.
Only the spring pressure is applied to the pressure plate which is enough to keep the clutch engaged. An adjusting screw is fitted at the end of the lever, by means of which the centrifugal force on the pressure plate can be adjusted.
The diaphragm clutch consists of a diaphragm on conical spring which produces pressure on the pressure plate for engaging the clutch. The spring may be finger or crown type attached on the pressure plate.
Tapered finger type spring is shown in the figure. In this types of clutches, the engine power is transmitted from crankshaft to flywheel. The flywheel has friction lining and it is connected to the clutch as shown in the figure. The pressure plate is provided behind the clutch plate because the pressure plate applies the pressure on the clutch plate.
In diaphragm clutch, the diaphragm is a conical shape of the spring. When we press clutch pedal the outside bearing moves towards the flywheel pressing the diaphragm spring which pushes the pressure plate backwards.
By doing this the pressure on plate removes and the clutch will get disengaged. When we release pressure on clutch peddle the pressure plate and diaphragm spring will come back to its normal position and clutch will get engaged.
- This types of clutches have no release levers because the spring acts as a series of levers.
- The driver does not need to apply such heavy pedal pressure to hold the clutch disengaged as with the coil spring type in which the spring pressure increases more when the pedal is depressed to disengage the clutch.
Dog and spline clutch
A dog is a type of clutch it is used to lock two shafts together or to connect a gear and a shaft. The two parts of the clutch are one is dog clutch which has external teeth and another one is a sliding sleeve which has internal teeth.
Both shafts are designed in such a way that one will rotate another one at the same speed and will never slip. When the two shafts are connected then you can say the clutch is engaged. To disengage the clutch, the sliding sleeve moves back on the splined shaft to have no contact with the driving shaft.
The dog and splined clutch are mostly used in manual transmission vehicles to lock different gears.
This types of clutches are operated by electrically but the torque is transmitted mechanically. This is why this type of clutch is known as electro-mechanical clutches. Over the year, now its became electromagnetic clutch.
These clutches have no mechanical linkage to control their engagement that’s why it provides fast and smooth operation. The electromagnetic clutches are most suitable for a remote operation that means you can operate the clutch at distance.
The clutch has flywheel consists of winding. The electricity is supplied by the battery. When the electricity passes through winding it produces the electromagnetic field which causes to attract pressure plate to get engage. When the electricity supply is cutoff the clutch is disengaged.
In this clutch system the gear lever has a clutch release switch that means when the driver operates the gear lever to change gears the switch is operated cutting off the current supply to the winding which causes the clutch to disengage.
The figure shows the vacuum clutch mechanism. This type of clutches use existing vacuum in the engine manifold to operate the clutch. The vacuum clutch consists of a reservoir, non-return valve, vacuum cylinder with piston and solenoid valve.
Construction and working:
As the figure shows the reservoir is connected to the inlet manifold through a non-return valve. A vacuum cylinder is connected to a reservoir through a solenoid operated valve. The solenoid is operated from the battery and the circuit has a switch which is attached on the gear lever. The switch is operated when the driver changes the gear by holding the gear lever.
Let’s see how it works. When the throttle is opened the pressure increases in the inlet manifold due to this the
In the normal operation, the solenoid valve rod is in the bottom position of the valve as shown in the figure and the switch in the gear lever remains open. At this stage, the atmospheric pressure acts on both the side of the piston of the vacuum cylinder, because the vacuum cylinder is open to the atmosphere through the vent.
When the driver changes the gear by holding gear lever the switch gets closed. The solenoid energizes and pulls the valve up this connects one side of vacuum cylinder to the reservoir. This action opens the passage between the vacuum cylinder and the reservoir. Due to the difference in the pressure, the vacuum cylinder piston moves forward and backwards.
This piston movement is transferred by a linkage to the clutch, causing it to disengage. When the driver is not operating the gear lever, the switch is open the clutch remains engaged due to the force of springs.
The hydraulic clutch working operation is the same as the vacuum clutch. The major difference between these two is that the hydraulic clutch is operated by oil pressure whereas the vacuum clutch is operated by vacuum.
The figure shows the mechanism of a hydraulic clutch. It has fewer parts than other clutches. It consists of an accumulator, control valve, cylinder with piston, pump and a reservoir.
Working of hydraulic clutch:
The oil reservoir pumps the oil into accumulator through a pump. The pump is operated by the engine itself. The accumulator is connected to the cylinder through the control valve. The controlled valve is controlled by a switch which is attached to the gear lever. The piston is connected to the clutch by a linkage mechanism.
When the driver holds the gear lever to change the gears, the switch opens the control valve allows the oil under pressure to the cylinder. Due to the oil pressure, the piston moves forward and backwards this causes the clutch to get disengaged.
When the driver leaves the gear lever the switch is open which closes the controlvalve and the clutch will be engaged.
The freewheel unit clutches also known as spring clutch, over running clutch or one-way clutch. It is a most important part of every overdrive. The transmission of power is in one direction similar to bicycles. The freewheel unit is often mounted on behind the gearbox.
The power is transmitted from the main shaft to the output shaft from driving the output shaft when the planetary gears are in overdrive. A flywheel unit has a hub and an outer race. The hub has internal splines to connect it to the transmission main shaft.
The outer surface of the hub contains 12 cams so designed to hold 12 rollers in a cage between them and the outer race. The outer race is splined to the overdrive outer shaft.
When the hub is driven in the clockwise direction, as shown in the figure. The roller rides up the cams, and by their wedging action, they force the outer race to follow the hub. Thus the outer race moves in the same direction and at the same speed as the hub.
When the hub speed slows down, and the outer race is still moving faster than the hub, the rollers move down the cams, releasing the outer race from the hub. Thus the outer race moves independent of the hub and the unit acts as a roller bearing.
The transmission main shaft is connected to the hub and the output shaft is connected to the outer race. Thus freewheel unit can transmit power only from the main shaft to the output shaft.
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