Introduction To Clutch :
A Clutch is an important element in power transmission. In this article, we will see brief information about what is transmission, function, and requirement of clutches, types of clutches, application, and their diagrams.
Introduction to Transmission System :
The transmission system is the system utilizing which power developed by the engine is transmitted to road wheels to propel the vehicle. In automobiles, the power is developed by the engine which is used to turn wheels. Therefore, the engine is to be connected to the transmission systems for transmitting power to wheels. Also, there should be a system utilizing which engine could be engaged and disengaged with the transmission system smoothly and without shock so that the vehicle mechanism is not damaged and passengers do not feel inconvenience. A clutch is employed in automobiles for this purpose.
- The engines employed in automobiles are of very high speed. Hence, a speed reduction is necessary to reduce the speed to moderate level as well as to get the required high torque while moving from rest. For this purpose, a gearbox is employed in automobiles.
- The figure shows the general arrangement of a power transmission system of an automobile.
- The motion of the crankshaft is transmitted to the gearbox through the clutch. The gearbox consists of a set of gears to change the speed according to the requirement. The motion is then transmitted to the propeller shaft from the gearbox through a universal joint. The purpose of the universal joint is to connect two shafts at an angle for power transmission.
- The power is transmitted to the differential unit through another universal joint. Finally, the power is transmitted from the differential to wheels through the rear end. The differential unit is used to provide the relative motion between two-run wheels while the vehicle is taking a turn.
Introduction To Clutches :
- A Clutch is a mechanism used to connect or disconnect the engine from the rest of the transmission elements. It is located between the engine and gearbox.
- During normal running and stationary position, it is always in the engaged condition. The clutch is disengaged when the driver processes the clutch pedal. The clutch is disengaged for starting, changing gears, stopping and idling.
- When the clutch is engaged, the engine will be connected to the transmission, and power flows from engine to rear wheels through a transmission system.
- When ‘the clutch is disengaged by pressing the clutch pedal, the engine will be disengaged from the transmission. Thus, the power does not flow to rear wheels while the engine is still running.
Principles of Operation of Friction Clutch
- The clutch works on the principle of friction. In Figure, the driving shaft A with flange C is rotating at ‘N’ rpm, and shaft B with the flange 0 is keyed to the driven shaft which is in stationary position when the clutch is not engaged.
- Now, an external force is applied to the flange D so that it comes in contact with flange C.
- As soon as the contact is made, they are united due to friction between them and the flange D starts rotating with flange C. The rotational speed of flange D depends on the friction between surfaces C and D which in turn proportional to the external force applied.
Functions of a Clutch:
The torque developed b the engine at the starting speed is very low. Therefore, it is not possible to start the engine under load. This requires that the transmission system should provide a means of connecting and disconnecting the engine from the rest of the transmission system. Such an operation must be smooth and without shock to the occupants of the vehicle.
Thus the two main functions of a clutch are:
1. To engage and disengage the transmission from engine to the remaining parts of the transmission. (To allow the engine to be separated from rest of the transmission system) This is required when:
(a) Starting and running the engine at a sufficiently high speed lo generate sufficient power necessary for moving the vehicle from rest.
(b) Shifting the gears so that damage to gear teeth can be avoided.
(c) Stopping the vehicle after applying brakes.
2. The second function of the clutch is to allow the engine to take up the driving load of the vehicle gradually and without shock.
Requirements of clutch:
The main requirements of a clutch are as follows:
- It should be able to transmit the maximum torque of the engine.
- It should engage gradually to avoid sudden jerks.
- It should be able to dissipate a large amount of heat generated during clutch operation.
- It should be dynamically balanced, particularly in the case of high-speed engine clutches.
- It should have a suitable mechanism to damp vibrations and to eliminate noise produced during power transmission.
- It should be as small as possible so that it will occupy minimum space.
- It should be easy to operate requiring as little exertion as possible on the part of the driver.
- It should be made as light as possible so that it will continue to rotate for any length of time after the clutch has been disengaged.
- It must be trouble-free and have longer life.
- It must be easy to inspect, adjust, and repair.
Clutch Friction Lining material and their Necessity :
The materials for clutch lining are:
5. Raybestos and Ferodo
6. Non- asbestos clutch lining material.
Necessity of clutch lining:
1. To transmit maximum power from engine flywheel transmission without jerk
2. To dissipate the heat and able to withstand higher heat generated
3. It should have a higher coefficient of friction
4. It should be cheap and easy to manufacture.
Main parts of Clutch :
- The driving members consists of a flywheel which is mounted on the engine crankshaft
- The flywheel is bolted to a cover which carries pressure plate, pressure springs, and release levers.
- As the flywheel is bolted to the cover assembly, thus, the entire assembly of the flywheel and the cover rotate all the time.
- The clutch housing and cover provided with openings so that the heat produced during the function dissipates easily
- The driven members consist of a disc or plate called a clutch plate.
- The clutch is free to slide on the splines of the clutch shaft.
- It carries friction materials on both of its surfaces.
- When the clutch plate is gripped between the flywheel and the pressure plate, it rotates the clutch shaft through splines.
The operating member consists of a pedal or lever which can be pressed to disengage the driving and driven plate.
Types Of Clutch :
Some Of Clutch Mechanism Explained Below :
Hydraulic Operated Clutch Mechanism-
A hydraulically operated clutch mechanism is shown in the figure. The mechanism consists of master and slave cylinders. The cylinders are connected by hydraulic lines. When the clutch pedal is pressed the fluid under pressure from the master cylinder reaches the slave cylinder which is mounted on the clutch itself. The fluid under pressure actuates the slave cylinder push rod which further operates the clutch release fork to disengage the clutch. In India, this type of clutch has been used in Standard 20, Swaraj Mazda, and Eicher Mitsubishi’s vehicles.
Mechanical Clutch Linkage:
The mechanical clutch linkage is shown in the fig. when the clutch pedal is pressed it pivots on the pivot point and it moves the rod further. This rod turns the cross shaft, which moves the fork lever and actuates the release bearing. This movement is further conveyed to the clutch levers to disengage the clutch. Generally mechanical leverage from 10:1 to 12:1 is employed that would require a paddle force of about 100-120 N when using travel of 75mm.
Types Of Clutches are explained :
Multi-Plate (Dry) Clutch:
It is the extension of a single plate clutch. It consists of several clutch (friction) as well as pressure plates. As the number of plates increased, the friction surfaces also increase. The increase in the number of friction surfaces increases the capacity of the clutch to transmit torque. The plates are alternately fitted to the engine shaft and gearbox shaft. They are firmly pressed by strong coil springs and assembled in a cover assembly. Each alternate plate has inner and outer splines, this each of the alternate plate slides on the splines on the pressure plate.
Working of Multi-Plate Clutch:
The pressure plates are used to apply the pressure on friction plates and the inside diameter of the pressure plate is splined while making the inside diameter splined, the rotating motion of the pressure plate is restricted. The pressure plate moves on the driven shaft axially. When we apply the pedal the pressure plates and the friction plates come in contact with each other and the speed or power is transmitted from the engine shaft to the transmission shaft.
Applications Of Multi-Plate Clutch :
This type of clutch is used in Scooters and Motor Cycles, where space availability is limited. Besides, this finds the application in some Heavy Transport Vehicles and Racing Cars where high torque is to be transmitted.
Single plate Clutch :
A single disc or plate clutch as shown in the figure consists of a clutch plate whose sides are faced with the friction material (usually ferrodo). It is mounted on the hub which is free to move axially along the splines of the driven shaft. The pressure plate is mounted inside the clutch body which is bolted to the flywheel. Both the pressure plate and the flywheel rotate with the engine crankshaft or the driving shaft. The pressure plate pushes the clutch plate towards the flywheel by a set of strong spring which is arranged radially inside the body. The three levers (also known as release levers or fingers) are carried on the pivots suspended from the case of the body. These are arranged in such a manner so that the pressure plate moves away from the flywheel by the inward movement of a thrust bearing. The bearing is mounted upon the forked shaft and moves forward when the clutch pedal is pressed.
Necessity of Single plate clutch
1) To transmit a large amount of torque single plate clutch required
2) Response time to operate is very less compared to the multi-plate clutch.
3) It generates low heat so no need of cooling media required.
4) It should be dynamically balanced and easy to operate.
Disengaging the clutch:
When the clutch pedal is pressed down, it’s linkage forces the thrust bearing to move towards the flywheels and pressing the pressure plate away from the flywheel thereby the compression springs are compressed. This action removes the pressure from the clutch plate and the driving shaft comes to a stationary position.
Engaging the clutch:
On the other hand when the foot is taken off from the clutch pedal, the thrust bearing moves back by levers this action allows the springs to extend, and thus pressure plate pushes the clutch plate back towards the flywheel. The clutch is engaged and power is transmitted from engine to gearbox.
Application of Single plate clutch:
1. Most commonly used in cars.
2. Used in Light commercial vehicles and heavy transport vehicles.
Clutch remains usually in the engaged condition. It is required to depress the clutch pedal to disengage the clutch. When a driver or an operator drives a vehicle he is required to engage clutch by depressing clutch pedal. As the driver depresses the clutch pedal, the effort applied gets transmitted either through level or cable to clutch release fork. The fork pushes clutch release bearing towards the engine side due to which clutch release levels shown in figure get displaced getting pressure plate in the backward direction. This action creates clearance between drive and driven members resulting in disengagement of the clutch. As the driver leaves the clutch pedal it returns to its original position due to which pressure plate put thrust on the clutch plate from one side and flywheel on the other. This is how the clutch gets engaged.
- When the engine is started, the speed of the driving shaft is less, so the centrifugal force is also less. Therefore, shoes (flyweights) do not move outwards and torque is not transmitted to the rear wheel. As the speed of the engine increases, the centrifugal force also increases.
- At certain engine speed, the shoes fly off outwards due to increased centrifugal force and they come in contact with the driven member. Now both the driving and driven members rotate together and the clutch is said to be engaged.
- Thus the engine torque is transmitted to the rear wheel.
- When the engine speed decreases, the centrifugal force also decreases. Now the shoes return to their original position due to spring force which results in a disengagement of the clutch and torque is not transmitted to the rear wheel.
Application of Centrifugal Clutch :
1. Used in Automatic transmission vehicles like mopeds and two-wheelers without gear.
2. Used in semi-automatic transmission vehicles like some modern cars.
Cone clutch consists of friction surfaces in the form of a cone. The engine shaft consists of the female cone. The male cone is mounted on the splined clutch shaft. It has friction surfaces on the conical portion. The male cone can slide on the clutch shaft. Hen the clutch is engaged the friction surfaces of the male cone are in contact with that of the female cone due to force of the spring. When the clutch pedal is pressed, the male cone slides against the spring force, and the clutch is disengaged.
The only advantage of the cone clutch is that the normal force acting on the friction surfaces is greater than the axial force, as compared to the single-plate clutch in which the normal force acting on the friction surfaces is equal to the axial force. The disadvantage in the cone clutch is that if the angle of the cone is made smaller than 200 the male cone tends to bind in the female cone and it becomes difficult to disengage the clutch. Cone clutches are generally now only used in low peripheral speed applications although they were once common in automobiles and other combustion engine transmissions. They are usually now confined to very specialist transmissions in racing, rallying, or in extreme off-road vehicles, although they are common in powerboats. Small cone clutches are used in synchronizer mechanisms in manual transmissions.
Dog & Spline Clutch:
- This type of clutch is used to lock two shafts together or to lock a gear to a shaft. It consists of a sleeve having two sets of internal splines. It slides on a splined shaft with the smallest diameter splines. The bigger diameter splines match with the external dog clutch teeth on the driving shaft. When the sleeve is made to slide on the splined shaft, its teeth match with the dog clutch teeth of the driving shaft. Thus the sleeve turns the splined shaft with the driving shaft.
- The clutch is said to be engaged. To disengage the clutch, the sleeve is moved back on the splined shaft to have no contact with the driving shaft. This type of clutch does not tend to slip. The driven shaft revolves exactly at the same speed of the driving shaft, as soon as the clutch is engaged. This is also known as a positive clutch.
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