Frictional clutches

A clutch is a device used to transmit the rotary motion of one shaft to another shaft when desired. The axes of the two shaft are coincidence.

In friction clutches, the connection of the engine shaft to the gear box shaft is affected by friction between two or more rotating concentric surface. The surface can be pressed firmly against one another when engaged and the clutch tends to rotate as a single unit.

1. Dish Clutch (Single-plate Clutch)

Single plate clutch

A disc clutch consists of a clutch plate attached to a splined hub which is free to slide axially on splines cut on the driven shaft. The clutch plate is made of steel and has a ring of friction lining on each side. The engine shaft supports a rigidly fixed flywheel.

A spring-loaded pressure plate presses the clutch plate firmly against the flywheel when the clutch is engaged. When disengaged, the springs press against a cover attached to the flywheel. Thus, both the flywheel and the pressure plate rotate with the input shaft. The movement of the clutch pedal is transferred to the pressure plate through a thrust bearing.

Figure shows the pressure plate pulled back by the release levers and the fiction linings on the clutch plate are no longer in contact with the pressure plate or the flywheel. The flywheel rotates without driving the clutch plate and thus, the driven shaft.

When the foot is taken off the clutch pedal, the pressure on the thrust bearing is released. As a result the, spring become free to move the pressure plate to bring it in contact with the clutch plate. The clutch plate slides on the splined hub and is tightly gripped between the pressure plate and the flywheel. The friction sathe linings on the clutch plate, and the flywheel on one side and the pressure plate on the other. cause the clutch plate and hence the driven shaft to rotate.

In case the resisting torque on the driven shaft exceeds the torque at the clutch, clutch slip will occur.

2. Multi-plate Clutch

Multi plate clutch

In a multi-plate clutch, the number of frictional linings and the metal plates is increased which increases the capacity of the clutch to transmit torque. Figure shows a simplified diagram of a multi-plate clutch. The friction rings are splined on their outer circumference and engage with respective splines on the flywheel. They are free to slide axially. The friction material Thus, rotates with the flywheel and the engine shaft. The number of friction rings depends upon the torque to be transmitted. The driven shaft also supports discs on the splines which rotate with the driven shaft and can slide axially. If the actuating force on the pedal is removed, a spring presses the discs into contact with the friction rings and the torque is transmitted between the engine shaft and the driven shaft. If 'n' is the total number of plates both on the driving and the driven member, the number of active surface will be 'n-1'.

3.Cone Clutch

Cone clutch

In a cone clutch (Fig. ), the contact surfaces are in the form of cones. In the engaged position, the friction surfaces of the two cones A and B are in complete contact due to spring pressure that keeps one cone pressed against the other all the time. When the clutch is engaged, the torque is transmitted from the driving shaft to the driven shaft through the flywheel and the friction cones. For disengaging the clutch, the cone is pulled back through a lever system against the force of the spring. The advantage of a cone clutch is that the normal force on the contact surfaces is increased. If F is the axial force, F, the normal force and a semi-cone angle of the clutch then for a conical collar with uniform wear theory,

F=f/sinX
=2πpr(R'-R")/sinX
=2πpr.b {sinX=R'-R"/b}

Where b is the width of the cone face. Remember as pr is constant in case of uniform wear theory which is applicable to clutches to be on the safer side, p is to be the normal pressure at the radius considered, i.e., at the inner radius it is pr and at the mean radius pR.

4. Centrifugal clutch

Centrifugal clutch

Centrifugal clothes are being increasingly used in automobiles and machines. A centrifugal clutch has a driving member consisting of four sliding blocks(fig.). These blocks are kept in position by means of flat springs provided for the purpose. As the need of the shaft increases, the centrifugal force on the shoes increases. When the centrifugal force exceeds the resisting force of the springs, the shoes move forward and press against the inside of the rim and thus the torque is transmitted to the rim. In this way, the clutch is engaged only when the motor gains sufficient speed to take up the load in an effective manner. The outer surfaces of the shoes are lined with some friction material.