FRICTION CLUTCHES

FRICTION CLUTCHES

A friction clutch has its principle application in the transmission of power of shafts and machines which must be started and stopped frequently. its application is also found in cases in which power is to be delivered to machines partially or fully loaded.the force of friction is used to start the driven shaft from rest and gradually brings it up to the proper speed without excessive slipping of the friction surfaces.

Clutch operation

Clutches are devices used to engage or disengage moving components from each other. In a vehicle, the clutch is used to connect the drive from the engine to the transmission and the driving wheels.

The proper alignment of the bearing must be maintained and it should be located as close to the clutch as possible. it may be noted that..

1. The contact surfaces should develop a frictional force that may pick up and hold the load with reasonably low pressure between the contact surfaces.

2. The heat of friction should be rapidly dissipated and tendency to grab should be at a minimum.

3. The surfaces should be backed by a material stiff enough to ensure a reasonably uniform distribution of pressure.

The different types of friction clutches are:

• Disk or plate clutches(single or multiple disc clutch).
• Cone clutches, and
• Centrifugal clutches.

Now let us see in detail about the types of friction clutches.

• Single disc or plate clutch.

A single disc or plate clutch consists of a clutch plate whose both sides are faced with a friction material. 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 springs which are arranged radially inside the body.  the three levers are carried on pivots suspended from the case of the body. these are arranged in such a manner so that the pressure plate moves away form the flywheel by the inward movement of a thrust bearing. the bearing is mounted upon a forked shaft and moves forward when the clutch pedal is pressed.

      when the clutch pedal is pressed down, its linkage forces the thrust release bearing to move in towards the flywheel and pressing the longer ends of the levers inward. the levers are forced to turn on their suspended pivot and the pressure plate moves away from the flywheel by the knife edges, thereby compressing the clutch springs. this action removes the pressure from the clutch plate and thus moves back from the flywheel and the driven shaft becomes stationary. on the other hand, when the foot is taken off from the clutch pedal, the thrust bearing moves back by the levers. this allows the springs to extend and thus the pressure plate pushes the clutch plate back towards the flywheel.

• Multiple disc clutch.

A multiple disc clutch may used when a large torque is to be transmitted. The inside discs are fastened to the driven shaft to permit axial motion(except for the last disc). The outer discs(usually of bronze) are held by bolts and are fastened to the housing which is keyed to the driving shaft. The multiple disc clutches are extensively used in motor cars, machine tools etc.

• Cone disc.

A Cone disc is extensively used in automobiles but now-a-days it has been replaced completely by the disc clutch. It consists of one pair of friction surface only. in a cone clutch, the driver keyed to the driving shaft by a sunk key and has an inside conical surface or face which exactly fits into the outside conical surface of a driven. The driven member resting on the feather key in the driven shaft, may be shifted along the shaft by a forked lever provided, in order to engage the clutch by bringing the two conical surfaces in contact. due to the frictional resistance set up at this contact surface, the torque is transmitted from one shaft to another. in some cased, a spring is placed around the driven shaft in contact with the hub of the driven. this spring holds the clutch faces in contact and maintains contact surfaces of the clutch may be metal to metal contact, but more often the driven member is lined with some material like wood, leather, cork or asbestos etc. the material of the clutch faces depends upon the allowable normal pressure and the coefficient of friction.

• Centrifugal clutch.

The centrifugal clutches are usually incorporated into the motor pulleys. it consists of a number of shoes on the inside of a rim of the pulley. the outer surface of the shoes are covered with a friction material. these shoes, which can move radially in guides, are held against the boss(or spider) on the driving shaft by means of springs. the springs exert a radially inward force which is assumed constant. the mass of the shoe, when revolving, causes it to exert a radially outward force(i.e centrifugal force). the magnitude of this centrifugal force depends upon the speed at which the shoe is revolving. a little consideration will show that when the centrifugal force is less than spring force, the shoe remains in the  the same position as when the driving shaft was stationary, but when the centrifugal force is equals to the spring force, the shoe is just floating. when the centrifugal force exceeds the spring force, the shoe moves outward and comes into contact with the driven member and presses against it. the force with which the shoe presses against the driven member is difference of the centrifugal force and the spring force.the increase of speed causes the shoe to press harder and enables more torque to be transmitted.

Posted in: F,Mechanical Engineering