Clutch, Automotive Chassis and Transmission

This chapter covers a total of 10 % from your syllabus. Here, you'll get to know about all the most important topics and concepts that are useful and will be coming surely into the exams. Here the language of the concepts is kept easy to make it easier for you to understand everything.

1. Necessity of Clutch:

A clutch is important for the transmission of power of shafts and machines that should be started and stopped frequently. Its application is also found in cases during which power is to be delivered to machines partially or absolutely loaded. The force of friction is employed to start out the driven shaft from rest and gradually brings it up to the right speed without excessive slipping of the friction surfaces.

In cars, clutch is employed to attach the engine to the driven shaft. it should be noted that -

1. The contact surfaces ought to develop a resistance force that will obtain and hold the load with fairly low pressure between the contact surfaces.
2. the warmth of friction ought to be quickly dissipated and tendency to grab should be at a minimum.
3. The surfaces ought to be backed by a fabric stiff enough to ensure a fairly uniform distribution of pressure.


2. Functions of a Clutch:

The primary function of clutch is to transmit the torque from the engine to the drivetrain.

Smoothly deliver the power from the engine to enable smooth vehicle movement while perform everything quietly.

To reduce drive-related vibration.



3.  Types of Clutches:

* Single Clutch Plate


Single plate clutches are majorly used in most modern light vehicles. The clutch helps to transmit torque from engine to the transmission input shaft. As its 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. The clutch plate is simply thin metallic disc which consists of friction surfaces on both sides.
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 the sides of the clutch plate for best of its working.


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Pic 1


Working:

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.



Multiplate Clutch

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. More 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.


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Pic 2


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.
While 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.

* Concept of Dry and Wet clutches:


Usually there are multiple clutches that have one of the two characters dry or 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.


* Centrifugal Clutch

The below figure shows a centrifugal clutch. To keep the clutches in engaged position centrifugal clutch uses centrifugal force, inspite of spring force. In this type, the clutch is operated automatically depending upon the engine speed. That’s why there's 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.


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Pic 3


* Working of Centrifugal clutch

It consists of weights A pivoted at B.
When the engine speed increases, it lets 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.

* Semi-Centrifugal Clutch


 In case of a  semi-centrifugal clutch it 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.
These types of clutches consist of less no. of stiff springs, so that the driver should 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.



* Diaphragm Clutch
The diaphragm clutch has a diaphragm on conical spring which produces pressure on the pressure plate for engaging the clutch. The spring may be of finger or crown type attached onto 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.



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Pic 4

Advantages:

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.

*Hydraulic clutch

The working operation of a hydraulic clutch 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 how the mechanism of a hydraulic clutch works. It has fewer parts than other clutches. It consists of an accumulator, control valve, cylinder with piston, pump and a reservoir.


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Pic 5


*Working of hydraulic clutch:

The oil reservoir pumps the oil into accumulator through a pump. Here 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.


* Fluid Coupling:

Fluid coupling is also known as hydraulic coupling. It is a hydrodynamic device which is basically used to transfer rotational power from one shaft to another by the use of transmission fluid. It is used in automotive transmission systems, marine propulsion systems and in industries for power transmission. It is used as an alternative for the purpose of mechanical clutch.

- Main Parts

It has three main components

i) Housing: It is also known as the shell. It consists of an oil-tight seal around the drive shaft. This also protects the impeller and turbine from outside damages.
ii) Impeller or pump: It is a turbine which is connected to the input shaft and called as impeller. This is also known as pump because it acts as a centrifugal pump.
iii) Turbine: It is connected to the output shaft to which the rotational power is to be transmitted.

The impeller is directly connected to the prime mover (internal combustion engine) which is here the power source. And the turbine is connected to the output shaft where rotation power is needed to be transmitted. The impeller and turbine are enclosed in an oil-tight sealed housing. This housing consists of transmission fluid.


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Pic 6


* Working Principle

The working principle of fluid can be explained by the taking two fans in which one is connected to the power supply and other is not. When the power switched is ON, the air from the first fan is starts to blow towards the second fan (which is not connected to the power source). Initially when the first fan is blowing at lower speed, it doesn't able to drive the second fan. But as the speed of the powered fan increases gradually, the speed of air striking the blades of second fan also increases and it starts to rotate. And after some time it acquires the same velocity of that of the first fan.


On the same principle the fluid coupling works. In that case, the impeller act as first fan and the turbine act as second fan. Here both the impeller and the turbine are enclosed in an oil tight housing. The impeller is connected the input shaft of the prime mover and the turbine with the output shaft. When the impeller is moved by the prime mover, the fluid in housing experiences centrifugal force and due to curved vanes of the impeller the fluid directed towards the turbine blades. As the fluid strikes the turbine blades it starts rotating. With the increase in the speed of impeller, the velocity of the turbine increases and becomes approximately equal to the impeller speed. The fluid after passing through the turbine blades again return to the impeller.



* Working of Fluid Coupling
As the prime mover moves gradually, it rotates the impeller of the coupling. The impeller acts as a centrifugal pump and throws the fluid outward and directs it towards the turbine blade.
As the high moving fluid strikes the turbine blades, it also starts rotating, after striking on the blades, the direction of the fluid is changed and it is directed towards the impeller again. The blades of turbine are designed in such a way that it can easily change the direction of the fluid. It is the changing of direction of the fluid that makes the turbine to rotate.
As the impeller's speed increases, the speed of the turbine also increases. After sometime the speed of both impeller and turbine becomes equal. In this way power is transmitted from one shaft to another by the use of fluid coupling.
In the same way torque converter works but the difference is that it has a stator placed in between the impeller and the turbine for its torque multiplication.


* Application
It is used in automotive industries for the purpose of transmission of power from the engine to the wheel as alternative of clutch.
It is used in marine propulsion systems.
It is used in various industries for the power transmission.

*Friction Disc

Friction discs are made up of high quality friction materials and bonded, usually with rivets, to a metal plate. These make up the disc brake pads of the braking component. The other parts of a typical disc brakes usually include calipers attached to pistons and rotors which are responsible for motion. These friction discs are used to slow or stop the motion of drive shafts which will in turn stop the wheels from rotating. As pressure is applied to the braking pedal, the calipers cause the discs to close around to the rotors. This pressure can be a result of hydraulic, mechanical or pneumatic power depending on the designing of the engine, but either way, when enough friction has been created, the rotors will stop moving.


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Pic 7


Dust is created as the plates rub against each other and slowly results in breaking down of their surfaces. This is the major reason why asbestos is no longer widely used for disc plate manufacturing or in clutch materials as asbestos dust is injurious when inhaled. Other materials used for friction discs usually include woven fibers such as Kevlar or aramid and ceramic materials which are durable enough to withstand the pressures of friction but not so hard as to cause component damage.

Note: For any queries or questions you can directly mail me. Or just dm me on instagram @chakitg. 

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Link for Previous Chapters are given below:
Chassis, its fabrication and Automotive body. Automotive Chassis and Transmission


Regards: 

Er. Chakit Gupta


Clutch, Automotive Chassis and Transmission Clutch, Automotive Chassis and Transmission Reviewed by Oyetechy.com on November 02, 2019 Rating: 5

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