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May 14, 2011

Single Disc or Plate Clutch

A single disc or plate clutch, as shown in Fig. consists of a clutch plate whose both sides are faced with a friction material (usually of 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 springs which are arranged radially inside the body. The three levers (also known as release levers or fingers) 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 from 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.
The axial pressure exerted by the spring provides a frictional force in the circumferentialdirection when the relative motion between the driving and driven members tends to take place. If the torque due to this frictional force exceeds the torque to be transmitted, then no slipping takes  place and the power is transmitted from the driving shaft to the driven shaft.
Now consider two friction surfaces, maintained in contact by an axial thrust W, shown in Fig (a).

Let               T = Torque transmitted by the clutch,
            p = Intensity of axial pressure with which the contact surfaces are held together,
       r1 and r2 = External and internal radii of friction faces, and
                  µ = Coefficient of friction.
Consider an elementary ring of radius r and thickness dr as shown in Fig (b).
We know that area of contact surface or friction surface,
                       =2 π r.dr
∴  Normal or axial force on the ring,
                δW = Pressure × Area = p × 2 π r.dr
and the frictional force on the ring acting tangentially at radius r,
                  Fr = µ.δW = µ.p × 2 π r.dr
∴  Frictional torque acting on the ring,
                   Tr= Fr × r = µ.p × 2 π r.dr × r = 2 π × µ .p.r.r dr
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