Automobile Clutch

Introduction to Clutch & its Functions

A clutch is a mechanism which enables the rotary motion of one shaft to be transmitted at will to second shaft, whose axis is coincident with that of first. The clutch is located between the engine and gearbox. When the clutch is engaged, the power flows from the engine to the rear wheels through the transmission system and the vehicle moves. When the clutch is disengaged (clutch pedal pressed), the power is not transmitted to the road wheels and the vehicle stops slowly while the engine is still running. [1]

A clutch is mostly useful in engaged when moving condition and disengaged when starting the engine, shifting gears, stopping the vehicle and Idling the engine.

Fig 1: Clutch location [5]

Principles of Operations of clutch:

The clutch principle is based on friction. When two friction surfaces are brought in contact with each other and pressed they are united due to friction between them. If now one is revolved, the other will also revolve. The friction between the two surfaces depends upon, (i) Area of the surface, (ii) Pressure applied upon them, and (iii) Coefficient of friction of the surface materials. One surface is considered as driving member and the other as driven member. The driving member is kept rotating. When the driven member is brought in contact with the driving member, it also starts rotating. When the driven member is separated from the driving member, it stops revolving. This is now, a clutch operates. [1]

Clutch enables the engine to keep disconnected from road wheels. The rotary motion available at the crankshaft is not transferred to road wheels. It allows the transfer of motion when desired by the driver of the automobile. It also allows the transfer of motion gradually so that the vehicle starts moving gradually. It works on the principle of friction. [2]

As the name suggests, operation of a friction clutch is based on the friction between two surfaces in close contact with each other. The two surfaces are held together by the friction force between them. Therefore, if one surface tries to turn it has to take along with it the other surface also due to the frictional force at the interface. [3]

In principle, a clutch is the same as a brake. Each of these two devices can be engaged or disengaged or, in other words, applied or released. Both clutch and brake depend on the property of friction between two surfaces provided on the rotating shafts and a stationary part in some form or the other. [3]

When the clutch is engaged, two co-axial shafts, the driving and the driven rotate together and when it is disengaged; the driven shaft slows down and may come to rest. [3]

Fig 2: Principle of Operation: Friction Clutch [4]

Consider two shafts A and B duly supported in bearing, as shown in Fig:2, and free to rotate about their common axes XY. Shaft A is driving shaft and shaft B is the driven shaft. Two discs C and D are attached at the ends of the shafts with the help of keys. Driving shaft A and disc C rotates along with it. Shaft B and disc D are at rest. If the two shafts are pressed together the surfaces of discs C and D come in contact with each other. The two surfaces are rough. Due to the movement of disc C some frictional force comes into existence When the pressure is increased frictional force between two surfaces also increases. This causes transfer of motion and the disc D also starts rotating. This means the driven shaft also starts rotating. Initially the speed of driven shaft is low. As the pressure between the surfaces of disc C and D increases the frictional force between the two also increases and so increases the rotary speed of the driven shaft. The speed of the driving and driven shaft can be equal only when the coefficient of friction between the surfaces of discs C and D is one which is not possible practically. Therefore the speed of driven shaft is always less than the speed of the driving shaft. To make the contact possible between the two surfaces of the discs C and D, force should act along the axes of the shaft. [2]

The axial force is provided by the springs. Suppose the springs, used to press the discs together, exert a total force of W Newton normal to the surface of the disc. Upon the magnitude of this force depends the magnitude of fractional force which tends to prevent the transfer of motion to the driven shaft. The magnitude of this force is μW, μ being the coefficient of friction. This force is the sum of the large number of component forces acting all over the disc surface. These forces may be considered acting on the narrow rings which divide the circular surface of the disc into a number of parts. The resultant friction m force, μW can be considered acting tangentially at a radius of R which is the mean radius of disc. This force will cause a moment about the shaft axis equal to 'μW R' Newton-meter. This moment tends to stop the driving shaft A and to drive the driven shaft B. This is the torque transmitted. [2]

The magnitude of torque depends upon the radius at which the friction force acts. The magnitude of the torque can be increased by increasing the radius of disc. The magnitude of torque also depends upon the co-efficient of friction, μ. The co-efficient of friction depends upon the nature of material of the surfaces in contact. The availability of space limits the radius of discs. The force W depends upon the springs. The torque required may be large in magnitude and it can be acquired only if certain modifications are made in the design of the clutch. The modification in the design has resulted in three principal types of clutches which are used in an automobile. These are cone clutch, single plate clutch and multi-plate clutch. [2]

Purpose of clutch: [2]

The clutch enables the rotary motion of crankshaft to be transmitted to driven shaft when desired and gradually. These can be classified as:

 Positive Clutch

 Gradual Engagement Clutch

Positive Clutch: In this type of clutch there are two positions. Either it is ‘in' when the two shafts are rigidly connected and revolve at the same speed or it is ‘out' when the shafts are entirely disconnected and there is no transfer of motion and the driven shaft is not moving. This type of clutch is not suitable for use between the engine and the gear box as the motion will be suddenly transferred Also the transfer of motion will suddenly stop. This means sudden movement of vehicle from rest and sudden stoppage of the movement. Both the situations are not desirable as sudden change of state from rest to motion or vice versa will be highly uncomfortable or even injurious to the user of the vehicle. [2]

Gradual Engagement Clutch: In this type of clutch it is possible that driving shaft is rotating and the other shaft, driven shaft, is stationary. As the engagement of clutch proceeds the speed of the two shafts gradually becomes almost equal. When the clutch is fully engaged both the shafts rotate almost at the same speed. This type of clutch is used between the engine and the gear box. In this type of clutch the transfer of motion between the two shafts depends upon the friction between the surfaces of the two shafts when these come in contact with each other. Therefore, these are also known as friction clutch. [2]

In manual transmission it is very practical, not to say ideal, to incorporate a device in the power train between the engine and the transmission. These device has the purpose of disengaging the transmission and power train from the engine before a particular gear position is selected on the gear shift lever of the automobile. This ensures smooth engagement of the driving shaft or the engine crankshaft with the gearbox input shaft. This device is called the clutch. [3]

The clutch is normally in the engaged position. That means the drivers foot is off the clutch pedal in the automobile with manual transmission. The torque capacity of the clutch at a particular speed of the vehicle is equal to or more than the developed engine torque. There is scope for slip and the engine power flows through the power train up to the wheels. [3]

When the vehicle has to be slowed down the driver shifts to a lower gear in the transmission. This can be done by first disengaging the clutch by depressing the foot pedal. When the clutch pedal is fully depressed upto the floor level, the clutch is totally decoupled from the engine. It is safer and less noisy now to change gear and reengage the clutch. The same procedure is followed for moving into a higher gear. While the clutch is being engaged, clutch plates slip before they let pass full engine torque at that speed. This spares the gearbox and drive train components from impact loads and rude shocks. [3]

While cranking the engine for start-up, it is much easier to decouple the drive components and the gearbox from the engine for obvious reasons. This is again done by disengaging the clutch. [3]

Requirement of Clutch: [4]

A clutch should perform the following functions:

1. When the transfer of power needs to be cut, the clutch must instantaneously and completely disconnect the engine from the transmission.
2. When the transfer of power is to be started, the clutch must exhibit a certain degree of sliding so that the engine and the transmission can be engaged in a gradual fashion to avoid sudden jerks.
3. When  the  engine  and  transmission  have  been  securely  connected,  the clutch  must  then transfer power without any loss or slippage.
4. It should be able to dissipate large amount of heat which is generated during the clutch operation due to friction.
5. It should be dynamically balanced specially in case of high speed engines.
6. It should be as small as possible in size so that it occupies minimum space.
7. The driven (rotating) member of the clutch should be made as light as possible so that it will not continue to rotate for any length of time after the clutch has been disengaged.
8. It  should  have  suitable  mechanism  to damp  vibrations  and  to eliminate  noise  produced during the power transmission.
9. It should have free pedal play to reduce effective clamping load on the carbon thrust bearing and wear on it.
10. It should be easy to operate requiring minimum effort on part of the driver.

Types of Clutches:

1. Friction clutch
2. Single plate clutch
3. Multi-plate clutch: Wet & Dry
4. Cone clutch: External & Internal
5. Centrifugal Clutch
6. Semi-centrifugal clutch
7. Conical spring or Diaphragm clutch: Tapered finger type & Crown spring type
8. Positive clutch: Dog clutch & Spline Clutch
9. Hydraulic clutch
10. Electromagnetic clutch
11. Vacuum clutch
12. Overrunning clutch or freewheel unit

"Above all clutches will discuss on later or upcoming article (Types of Clutches)."

Next Article: "Main Parts of Clutch"

References:

[1] A Text Book of Automobile Engineering By R. K. Rajput, Laxmi Publications. New Delhi. EAE-0697-350, C-13047/06/08, 2007, “Transmission System” p.462-527.

[2] Automobile Engineering By Sudhir Kumar Saxena, University Science Press: Laxmi Publications, New Delhi, UAE-9356-195, C.17388/09/04,2009, “Transmission System” p. 77.

[3] Automobile Engineering By Kamaraju Ramakrishna. PHI Learning Pvt Ltd, new Delhi, ISBN 978-81-203-4610-9, 2012, “Clutch” p.173-189.

[4] Automobile Engineering By Er SK Gupta, S. Chand & Co Pvt Ltd, New Delhi, ISBN 978-93-837-4691-0, Code 1010-630, 2014, 2016, “Clutches” p.627-666.

[5] Automobile Engineering By Er AK Babu & Er Ajit Pal Singh, S. Chand & Co Pvt Ltd, New Delhi, ISBN 81-219-9770-4, Code:10A 560, 2013, 2014, “Clutch” p.177-185.