AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 7. CLUTCH
The power produced by the engine (which we shall study in detail in the second semester) is delivered to the wheels in order to move the car forward by the power train. It consists of many sub-systems. These are connected together in a definite sequence right from the engine to the wheels. The first component in the power train is the clutch. It works like a switch to connect or disconnect the power developed in the engine to the next component in the power train. It is used mainly during gear changing.
7.0
Objectives After going through this lesson, you will be able to:
i. List the components of the power train and state the function of each. ii. Describe the construction and working of the clutch. iii. Classify clutches into main types used in automobiles.
7.1
Introduction
A layout of the components of the power transmission system on the chassis frame of a rear wheel driven vehicle is shown in Fig. 7.1. The rotary motion of the engine crankshaft is transmitted through the clutch to the gearbox which consists of a set of gears to change the vehicle speed as per requirement. From the gearbox, power is transmitted through a universal joint to the propeller shaft and then to the differential through another universal joint. Finally, power is transmitted to the rear wheels through the rear axles, which are also called half-shafts. In this lesson, we will look at the first component of the power transmission system namely, the clutch.
Fig. 7.1: Layout of power train.
7.2
Principle of Working of Clutch
The clutch is used in the transmission system between the engine and the gearbox to engage or disengage the engine from the gearbox, as required. It works on the principle of friction. When two discs are in contact with each other under pressure and one disc is rotated, the other one also rotates due to friction between the discs. In case of a single plate clutch, which is generally used in four-wheelers, one disc is the flywheel, which is mounted on the engine crankshaft and rotates with it, and the other one is the clutch plate.
7.3
Parts of the Clutch
Clutch consists of the following main parts• Flywheel – it is mounted on the engine crankshaft and is driven by the engine • Clutch cover - it is bolted to the flywheel and rotates with it; it carries the pressure plate and pressure spring(s) etc • Clutch plate – it is pressed between the flywheel and the pressure plate and carries the clutch shaft in its hub • Pressure plate – it exerts pressure over the clutch plate and presses it against the flywheel • Release bearing – it exerts pressure over the spring(s) to pull back the pressure plate and disengage the clutch • Release fork – it pushes the release bearing forward
Fig. 7.2: Parts of clutch.
Fig. 7.3: Clutch assembly.
Self-check Questions 1. Which is the first component of the power train? When is it required? 2. What is the placement of the clutch?
3. On what principle does the clutch work? 4. Which clutch is used in 4-wheelers?
7.4
Working of the Clutch
Pressure is applied onto the clutch plate by the pressure plate, with the help of springs. The amount of pressure to be applied depends upon the power or torque that is to be transmitted. The clutch normally remains in the engaged position. In order to disengage the clutch, the clutch pedal is pressed. This pulls back the pressure plate and the pressure on the clutch plate is removed. Once the pressure is removed, the clutch plate and flywheel are separated and the flywheel keeps on rotating without rotating the clutch plate. As a result, the input shaft to the gearbox, which is held in the clutch plate via splines, also stops rotating. In this position, the engine keeps running but the power is not transmitted to the gearbox. Clutch is used primarily for easy gear changing.
7.5
Clutch Springs
Pressure is applied onto the clutch plate by the pressure plate either due to coil springs or a diaphragm spring. The Maruti 800 car uses a diaphragm spring type clutch.
Fig. 7.4: Diaphragm spring. Fig. 7.5: Coil springs.
7.6 Types of Clutches Used In Automobiles Clutches used in automobiles may be single plate or multi-plate type. The single plate clutch has only one clutch plate held between the flywheel and the pressure plate. The clutch plate has a friction lining on both its sides and carries the clutch shaft within its hub. It is generally of the dry type.
A multi-plate clutch may be dry or wet type. Wet type clutch is generally used in two-wheelers. Its components are immersed in oil, which helps in heat dissipation and cushions the shock and vibration during engagement. The multi-plate clutch is compact and occupies less space. It consists of a clutch drum and a clutch hub. There are 2 types of discs – the friction disc and the steel disc. One type is held in the drum and the other type in the hub. Pressure is applied by coil springs. Torque is transmitted from the clutch drum to the hub. When the clutch is to be disengaged, spring pressure is released and the clutch hub becomes free thus power transmission is stopped.
Fig. 7.6: Types of clutch (a) Single plate clutch. (b) Multi plate clutch.
The two types are shown in figure above. The material of friction lining for each type is different. Asbestos lining is used for dry type clutch whereas cork lining is used for wet type clutch.
Self-check Questions 5. 6. 7. 8. 9.
The clutch normally remains in the ________________ position Name the types of springs used in single plate clutch. Clutch used in 2-wheeler may be immersed in oil. State whether True/False. Lining of dry clutch is made of ___________. Lining of wet clutch is made of _______________
7.7
Assignments
7.7.1 Class Assignment 1. Discuss why clutch is not used in a bicycle. 2. Discuss what would happen if the clutch started slipping
7.7.2 Home Assignment 1. List names of five vehicles and the type of clutch used by each..
7.8
Summing Up
Clutch serves to permit or restrict engine power from being transmitted to the transmission. It is the first component of the power train and comes just after the engine. It is used mainly for gear changing. Clutch used in automobiles can be of different types namely, single or multi-plate and further dry or wet. Single plate dry clutch is generally used in cars whereas multiplate dry or wet type is used in 2-wheelers. Material of friction lining in case of dry clutch is asbestos and in case of wet clutch it is cork.
7.9 1. 2. 3. 4. 5. 6. 7. 8. 9.
7.10 1. 2. 3. 4.
7.11
Possible Answers to Self-Check Questions clutch; for easy gar changing it is placed between the engine and gearbox friction single plate dry clutch engaged coil, diaphragm True asbestos cork
Terminal Questions List the components of the transmission system and give the function of each. What is the function of the clutch in a car? How does it work? Describe the construction of the single plate dry clutch and explain its working. Classify clutches into its main types and state where each is used.
References and Suggested Further Reading
1. AA BOOK OF THE CAR. Drive Publications Ltd, London. 2. Maruti 800 Service Manual 3. http://www.howstuffworks.com
7.12
Glossary
Asbestos
A friction material.
Clutch
Device used to engage or disengage the transmission from the engine.
Cork
A friction material.
Diaphragm spring
Steel plate suitably cut at the centre which acts like a spring.
Dry clutch
Clutch which is not immersed in oil.
Friction
Resistance to sliding of two bodies in contact with each other.
Multi-plate clutch
Clutch assembly having many friction plates and steel discs alternately placed, one type connected to the clutch hub and the other to the clutch drum.
Single plate clutch
Clutch assembly having a single friction plate.
Splines
Teeth which permit two mating members slide axially, but not in rotation, relative to each other.
Torque
Rotary force.
Wet clutch
Clutch which is immersed in oil.