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POWER TRAINS. UNIT-5. POWER TRAINS: General Arrangement of Clutch , Principle of Friction clutches, Torque transmitted, Constructional details, Fluid flywheel, Single plate, Multi plate and Centrifugal clutches
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UNIT-5 • POWER TRAINS: General Arrangement of Clutch, Principle of Friction clutches, Torque transmitted, Constructional details, Fluid flywheel, Single plate, Multi plate and Centrifugal clutches Gear box: Necessity for gear ratios in transmission, Synchronous gear boxes, 3,4 and 5 speed gear boxes, Free Wheeling mechanism, Planetary gears systems, over drives, fluid coupling and torque converters, Epicyclic gear box, principle of automatic transmission, calculation of gear ratios, Numerical calculations for torque transmission by clutches.
Introduction • A Transmission system uses a clutch, gear box, propeller shaft and a differential gear to transmit power from engine to the road wheels • The power may be transmitted to rear or front or all four wheels depending on the drive used • The clutch and gear box varies the ratio of torque output to torque input • The propeller shaft transmits final torque to the rear axle from gear box • A differential gear equally distributes the final torque between the road wheels.
Functions of Transmission System • It disconnects engine from driving wheels when required • The engine is connected to driving wheels when required • It changes ratio of torque output to torque input, as desired • It turns the drive through a right angle
Clutch • A clutch is a mechanical device which transmits motion from one shaft to the other which are along a straight line or same axis of rotation. • A Clutch is a machine member used to connect the driving shaft to a driven shaft, so that the driven shaft may be started or stopped when ever needed, without stopping the driving shaft.
Requirements of a clutch 1. Torque transmission.---The clutch must be able to transmit maximum torque of the engine 2. Gradual engagement----The clutch should engage gradually to avoid sudden jerks 3. Heat dissipation---The clutch should be able to dissipate large quantity of heat which is generated during the clutch operation due to friction 4. Dynamic balancing---The clutch should be dynamically balanced. This is particularly required incase of high speed engine clutches. 5. Vibration damping ---The clutch should have suitable mechanism to damp vibrations and to eliminate noise produced during power transmitted. 6. Size---The clutch should be as small as possible so that it occupies minimum space
7. Inertia -- clutch rotating parts should have minimum inertia 8. Clutch free pedal play – to reduce effective clamping load on carbon thrust bearing 9. Ease of operation – as clutch transmits higher torque , its disengagement should not tiresome the driver 10. Lightness
Used Disconnecting the engine with gear box • When starting the engine • When changing the gears • When stopping the engine
Principle of Operation • The clutch works on the principle of friction. • When two surfaces are brought in contact with each other and pressed they are united due to friction between them. • The friction depends on area of contact, pressure applied upon them and coefficient of friction on the surface of the material. • The two surfaces can be separated and brought in contact when required. • One is considered driving member and other driven member. The driving member is kept rotating. When the driven member is brought in contact with the driving member it also start rotating. • When the driven member is separated from the driving member, it does not revolve .
T=µWR Where, W- axial load applied µ-Coefficient of friction T- torque transmitted R- effective mean radius of friction surface
Working of a clutch • Clutch for a drive shaft: The clutch disc (center) spins with the flywheel (left). To disengage, the lever is pulled (black arrow), causing a white pressure plate (right) to disengage the green clutch disc from turning the drive shaft, which turns within the thrust-bearing ring of the lever. Never will all 3 rings connect, with no gaps.
Types of Clutches 1. Friction Clutches: a. Single plate clutch b. Multi plate clutch----i)Wet ii)Dry c. Cone clutch---i)External ii) Internal 2. Centrifugal Clutch 3. Semi-centrifugal clutch 4. Conical spring clutch or diaphragm clutch a. Tapered finger type b. Crown spring type 5. Positive Clutch—Dog and spline clutch 6.Hydraulic clutch 7.Electro magnetic clutch 8.Vaccum clutch 9. Over running clutch or free wheel clutch
Working • It has only one clutch plate mounted on the splines of clutch shaft. • Pressure plate is bolted to the flywheel through clutch springs and is free to slide on the clutch shaft when the clutch pedal is operated. • When the clutch pedal is pressed, the pressure plate moves back against the force of the springs and clutch plate becomes free between the flywheel and the pressure plate. • Thus, the flywheel remains rotating as long as the engine is running and the clutch shaft speed reduces slowly and finally stops rotating. • As soon as the pedal is pressed, clutch is said to be disengaged otherwise it remains engaged due to spring forces. • Used in Ford V8 truck, Tata trucks, Premier President, Hindustan Ambassador, Bajaj Matador, Mahindra Jeep
Working • Multiplate clutch consist of a number of clutch plates, instead of only one clutch plate as incase of single plate clutch. • As the number of clutch plates increased number of friction surfaces which increases the capacity of the clutch to transmit torque. • The plates are alternately fixed to the engine shaft and gearbox shaft. • They are firmly pressed by strong coil springs and assembled in a drum. • Each of the alternate plate slides in the grooves on the flywheel and other slides on the splines on the pressure plate. Thus each alternate plates have inner and outer splines. • The multi-plate clutch works in the same way as single plate clutch, by operating the clutch pedal. Multiplate clutch is used in heavy vehicles, racing cars etc, where more torque is to be transmitted.
Cone clutch Contact surface is in the form of cone
Working • The centrifugal clutch uses centrifugal force instead of spring force for keeping it in the engaged position. Also it does not require clutch pedal to operate it. • The clutch is operated automatically depending upon the engine speed. The engine can be stopped in gear without stalling the engine. • Similarly vehicle can be started in any gear pressing the accelerator pedal. • It consists of weight A pivoted at B. When the engine speed increases the weights fly off due to the centrifugal force, operating the bell crank lever P which presses the plate C. • The movement of the plate C presses the spring E Which ultimately presses the clutch plate D on the flywheel against spring G. This makes the Clutch engaged. • The spring G keeps the clutch disengaged at low speeds at about 500 rpm. Stop H limits the movement of the weights due to the centrifugal force.
Clutch operation methods • Mechanically • Electromagnetic operation • Hydraulic operation • Vacuum operation • Clutch by wire
Clutch by wire • In this system there is no mechanical link between the clutch and the pedal • The clutch pedal is electronically controlled by means of an Electronic Control Unit (ECU) and an actuator • A sensor on the pedal measures its exact position which is transmitted to ECU • Information about the car’s behavior from other sensors is also transmitted to ECU, which then operates the clutch through the actuator • Advantages are improved drive ability, better pedal feel and less wear due to absence of mechanical linkage
Advantages of diaphragm w.r.t coil spring • More compact means of storing energy • Less affected by the centrifugal forces, it can withstand higher rotational speeds whereas coil springs distort transversely at higher speeds • Diaphragm acts as both clamping spring and release lever
Clutch Components • Clutch plate • Clutch facing • Pressure plate • Springs • Bearing
Clutch facing Requirement of a good clutch facing • Good wearing properties • Presence of good binders in it • Cheap and easy to manufacture • High co-efficient of friction • High resistance to heat
Types of Friction materials • Millboard type- asbestos sheet treated with certain impregnants • Moulded type- mixed asbestos fibers with a binding material • Woven type- cloth with certain binders • Solid woven variety- cloth is woven to required thickness • Laminated variety- layers of cloth one upon the other are held together by a binder. Stitches are provided in addition to binders
Common clutch facing materials are; • Leather- coefficient of friction- 0.27 • Cork -0.32 • Fabric-0.4 • Asbestos -0.2 • Reybestos and Ferodo- 0.35 • Non-asbestos clutch facing
Design details of single plate clutch Uniform Pressure Intensity: P= constant • Total axial load on clutch W= πPr(ro2 -ri2 ) • Total Torque Transmitted T= 2/3µ W(ro3-ri3)/(ro2-ri2) T= µWR • Effective Mean Radius R=2/3(ro3-ri3)/(ro2-ri2)
Uniform Rate of Wear: Pr= constant • Axial load on clutch W= 2πPr(ro-ri) • Torque Transmitted T= µ W(ro+ri)/2 T= µWR • Effective Mean Radius R= (ro+ri)/2 T=2µWR
Problem-1 The engine of a car employing a single plate friction clutch develops maximum torque of 150Nm. External diameter of the clutch plate is 1.2 times its internal diameter. Determine the dimensions of the clutch plate and the axial force provided by the springs. The maximum allowable pressure intensity for the clutch facings is 100KPa and coefficient of friction is 0.3. Assume uniform wear.
Problem-2 An automobile clutch has a clutch plate of 160mm inside and 240 mm outside diameter. Six springs in the clutch provide a total force of 4.8KN, when the clutch is new and each spring is compressed 5 mm. the maximum torque developed by the automobile engine is 250 Nm. Determine • Factor of safety for the new clutch and • The amount of wear of the clutch facing that will take place before the clutch starts slipping. Assume coefficient of friction for the facing is 0.3
Problem-3 Determine the size of the clutch plate suitable for an Ambassador car employing a single plate type of friction clutch and developing 37.5 KW at 4200rpm. The inside diameter of the clutch plate is 0.6 times its outside diameter and it is to be ensured that even after a loss of 30% of the engine torque due to wear of the clutch facing, the clutch does not slip. The intensity of pressure on the facing is not to exceed 70kPa. Assume µ=0.3
Problem-4 Single plate friction clutch is to be designed for an Ashok Leyland bus developing 80.85 kW at 2400 rpm. The maximum torque developed however is 376 Nm at 1600 rpm. A maximum wear of clutch facing of 3 mm is to be allowed, when the clutch must transmit at least 25% excess torque. A pressure intensity of 190kPa can be safely allowed and ratio Inside dia/ outside dia= 0.6 is considered reasonable. • Calculate clutch plate dimensions • If 9 springs are used and initial spring force is to be 1.2 times the spring force after allowable wear of 3 mm, find out the spring stiffness. Assume µ=0.35.
