Clutches
A clutch is a mechanism designed to disconnect and reconnect driving and driven members. It is a device, which enables one rotary drive shaft to be coupled to another shaft, either when both the shafts are stationary or when there is relative motion between them. The need for the clutch seems mainly from the characteristics of the turning-effort developed by the engine over its lower speed range. When idling, the engine develops insufficient torque for the transmission to be positively engaged. To obtain a smooth engagement, the clutch has to be progressively engaged to take up the drive until the torque transmitted from the engine equals that required to propel the vehicle. Also the clutch disconnects the engine from the transmission to change the gear. The clutch, thus, takes up the drive smoothly and also disengages the drive whenever necessary. This chapter deals the clutches in detail.
24.1.
Clutch Fundamentals
24.1.1.
Main Functions
(a) The clutch enables smooth transmission of the rotary motion of the engine crankshaft to a stationary or slowly revolving output shaft (the gearbox shaft) without snatch.
(6) It provides a positive linkage for transmitting the maximum engine torque at high speed of the vehicle requiring direct coupling of the engine to the transmission without stalling.
(c) It enables rapid disengagement and re-engagement of the engine from the transmission while one or both are in motion, for gear-changing and emergency stops.
24.1.2.
Operation
The clutch is a friction type coupling that transmits power between the driving member and the driven member of the clutch. This is accomplished by bringing both the driving and driven members of the clutch into gradual contact. The contact is established by strong spring pressure controlled by the driver through the clutch pedal linkage. When clutch is fully engaged, the spring pressure must be sufficient to prevent slipping between driving and driven members. Therefore, transmission of power through clutch depends on the material in contact (which causes friction) and the contact pressure.
The clutch must provide smooth engagement without grabbing or chattering. This operation depends upon the coefficient of friction of the friction surface, along with the operation of cushion and action of damper spring. The clutch is usually designed to have a capacity of 125% to 150% of the maximum engine torque to handle the expected loads. The clutch starts to apply the torque as soon as the friction members touch. Torque increases until the torque provided by the engine matches the load of the drive train. Clutch torque requirements become highest during running of the engine with the driveline stationary.
Slippages occurs during engagement, which continues until the input (engine) and output (drive line) members are turning at the same speed. During slippage heat is produced, which must be absorbed by the clutch material and then dissipated into air. The amount of heat absorbed by the clutch is proportional to the time required to increase output speed to input speed. Clutch temperature is the major limiting factor in clutch capacity.
Inertia is another important factor of the clutch. The clutch output member (disc) is attached to the drive line. If the member is too heavy, its inertia keeps it spinning after releasing of the clutch, causing hard shifting and gear clashing.
The clutch rotates with the engine and drive line. Centrifugal forces on the clutch increases at a ratio of four times the engine speed. If centrifugal forces become greater than the strength of the parts, the clutch can fly apart or shatter. Clutch burst is usually designed to twice the expected maximum speed.
24.1.3.
Desired Features
(a) The force required by a clutch to separate the drive must not be excessive.
(b) The clutch friction surface should maintain a reasonable coefficient of friction under all operating conditions.
(c) The rubbing surfaces of the clutch must be correctly machined and be hard enough to resist wear but not so hard to cause scoring.
id) The rubbing surfaces must provide adequate surface area and mass to transfer and
absorb the heat generated, (e) It should have provision for adequate cooling or ventilation to dissipate generated heat. if) The clutch material should have reasonable thermal conductivity to dissipate the heat
so that distortion of the flywheel and pressure-plate is avoided. (g) It should use a friction material, which must withstand high temperatures and
clamping loads without crushing.
24.1.4.
Main Components
The clutch assembly may be divided into driving member, driven member and operating member, the three main parts.
The driving member uses a pressure-plate assembly and the flywheel both are bolted together such that the assembly rotates at the speed of the engine. The pressure plate may be coil pressure-spring type or diaphragm-spring type. The coil pressure-spring type assembly is consisted of a cover, the pressure springs, a pressure plate and release levers. All these parts are assembled inside the cover. The pressure plate requires over 7850 kPa pressure to hold the clutch disc against the flywheel. The diaphragm-spring type assembly incorporates a diaphragm spring, which works as both pressure springs and release levers.
Maximum driving forces or driving torque occurs just before the disc slips while it is clamped between the pressure plate and flywheel. This limits the maximum amount of engine torque the clutch can deliver to the drive train.
The driven member consists of a disc or plate assembly and the clutch shaft. The driven plate assembly is held between the flywheel and the pressure-plate assembly under the action of pressure springs. The power is picked up by the lined faces of the clutch disc (plate) and is transmitted through the steel hub, which is splined to the clutch shaft. The clutch-disc facings are either moulded or woven with heat resistant friction material. The lining materials are mainly asbestos fibres binder and proper reinforcements (brass or copper) for longer life. These linings are attached to the hub, which incorporates a cushioning device and a torsional vibration damping unit. The cushioning permits smooth engagement of the clutch and eliminates clutch chatter. The torsional device absorbs the torsional vibrations of the crankshaft so that they are not transmitted further.
Asbestos based clutch facings must not operate above 600 Kto avoid the binders to be driven out of the facing. Hence the maximum temperature limits the clutch capacity. Another limit is the maximum surface velocity at the time of engagement, which is 30.5 m/s at the mean radius. For maximum clutch disc life, the pressure plate force should not exceed 78 kPa pressures against the linings. Clutch linings operating against cast iron have a coefficient of friction between 0.28 and 0.30.
The operating member is consisted of release bearing, release lever, foot pedal, linkage and the spring with adjuster. The springs are required for proper operation of clutch. The clutch pedal is connected through its linkage to the release bearing. Pressure on the pedal moves the bearing against the release lever to disengage the clutch. Proper adjustment of the pedal is necessary to provide full release of the clutch and to allow for full wear of the clutch disc facings. Improper pedal adjustment causes erratic clutch action, excessive wear, over heating and eventual clutch failure. Proper clearance between the release bearing and the release lever is necessary for quick and easy gear shifting without clash and for circulation of air over the flywheel and pressure plate surface to dissipate heat. The clutch housing and cover are provided with opening for ventilation of air.
The clutch is said “wet plate” when it operates in oil bath otherwise it is called “dry plate”. Dry-plate clutches are more common. The advantage of using wet clutch is that it reduces the fierce engagement by allowing the engine oil to flow into the clutch housing.
24.1.5.
Driven Plate Inertia
For efficient operation of the clutch, the driven-plate must have minimum possible weight so that it produces minimum of spin, i.e. very little flywheel effect when the clutch is disengaged. Spin prevention is extremely important to align the various pairs of dog teeth of the gears, for both constant mesh and synchromesh gearing arrangement. During the engagement phase, the alignment of teeth takes place in the shortest time without causing excessive pressure, wear and noise between the initial chamfer of the dog teeth. Smoothness of clutch engagement is normally obtained by building some sort of cushioning device into the driven plate. For rapid slowing down of the driven plate, the diameter, centre of gravity and weight of the driven plate is kept to the minimum for a given torque carrying capacity.
24.1.6.
Driven Plate Torque Transmission Capacity
The torque capacity of a friction clutch increases with the increase of the coefficient of friction of the rubbing materials, the diameter and/or the spring thrust sandwiching the driven plate. The friction lining materials limit the coefficient of friction to the order of 0.35. Materials with higher coefficient of friction values are available, but these tend to be unstable and to snatch during take-up. Increasing the diameter of the driven -plate raises its inertia, and its tendency to continue spinning when the driven-plate is freed during disengaged position of the clutch.
Also there is a limit to the clamping pressure to which the friction lining material may be subjected if it is to maintain its friction properties over a long period of time.
24.1.7.
Multi-pairs of Rubbing Surfaces
The transmitted torque capacity of the clutch can be raised by increasing the number of pairs of rubbing surfaces. Theoretically the torque capacity of a clutch is directly proportional to the number of pairs of surfaces for a given clamping load. Since the single driven plate clutch has two pairs of friction faces, then a twin or triple driven plate clutch for a the same spring thrust should ideally exhibit twice or three times the torque transmitting capacity respectively in comparison to that of the single driven plate unit (Fig. 24.1). However, considering the difficulty in dissipating the extra heat generated in a clutch unit, a safety factor is introduced due to which the torque capacity is generally of the order 80% per pair of surfaces relative to the single driven plate clutch.
Fig. 24.1. Relationship of torque capacity, wear rate and pairs of rubbing faces for multi-plate clutch.
The increase in the number of pairs of rubbing surfaces also improves lining life because wear is directly related to the energy dissipation per unit area of contact surface. Theoretically, if the surface area is doubled as in a twin plate clutch, the energy input per unit lining area is halved for a given slip time which results in a 50% decrease in facing wear. In reality this rarely happens (Fig. 24.1) because the wear rate depends greatly on the peak surface rubbing temperature. Also the intermediate plate of a twin plate clutch operates at a higher working temperature than either the flywheel or pressure plate as they are more effectively cooled. Thus in a twin plate clutch, the intermediate plate absorbs half the energy generated during slipping and each flywheel and pressure plate absorb only quarter. Consequently, the intermediate plate and its corresponding lining faces indicate high temperatures and increased wear compared to the linings facing of the flywheel and pressure plate. Nevertheless, multi-plate clutches life expectancy is more or less related to the number of pairs of friction faces for a given diameter of clutch.
Twin driven plates are used for heavy duty applications such as those required for large trucks. Small diameter multi-plate clutches are more common for high performance cars where very rapid gear changes are required and large amounts of power are to be developed.
