2142Clutch cable linkage with automatic adjuster Fig 222

The release bearing is of the ball race type and is kept in constant contact with the fingers of the diaphragm spring by the action of the pedal self-adjustment mechanism. In consequence, there is no pedal free movement adjustment required (Fig. 2.22).

Fig. 2.19 Fully automatic centrifugal clutch
Fig. 2.20 Fully automatic centrifugal clutch characteristics
Fig. 2.21 Hydraulic clutch operating system with over-centre spring

When the pedal is released, the adjustment pawl is no longer engaged with the teeth on the pedal quadrant. The cable, however, is tensioned by the spring which is located between the pedal and quadrant. As the pedal is depressed, the pawl engages in the nearest vee between the teeth. The particular tooth engagement position will gradually change as the components move to compensate for wear in the clutch driven plate and stretch in the cable.

2.14.3 Clutch air/hydraulic servo (Fig. 2.23) In certain applications, to reduce the driver's foot effort in operating the clutch pedal, a clutch air/ hydraulic servo may be incorporated into the actuating linkage. This unit provides power assistance whenever the driver depresses the clutch pedal or maintains the pedal in a partially depressed position, as may be necessary under pull-away conditions. Movement of the clutch pedal is immediately relayed by way of the servo to the clutch in proportion to the input pedal travel.

As the clutch's driven plate wears, clutch actuating linkage movement is automatically taken up by the air piston moving further into the cylinder. Thus the actual servo movement when the clutch is being engaged and disengaged remains approximately constant. In the event of any interruption of the air supply to the servo the clutch will still operate, but without any servo assistance.

Immediately the clutch pedal is pushed down, the fluid from the master cylinder is displaced into


Fig. 2.22 Clutch cable linkage with automatic adjuster the servo hydraulic cylinder. The pressure created will act on both the hydraulic piston and the reaction plunger. Subsequently, both the hydraulic piston and the reaction plunger move to the right and allow the exhaust valve to close and the inlet valve to open. Compressed air will now pass through the inlet valve port and the passage connecting the reaction plunger chamber to the compressed air piston cylinder. It thereby applies pressure against the air piston. The combination of both hydraulic and air pressure on the hydraulic and air piston assembly causes it to move over, this movement being transferred to the clutch release bearing which moves the clutch operating mechanism to the disengaged position (Fig. 2.23(d)).

When the clutch pedal is held partially depressed, the air acting on the right hand side of the reaction plunger moves it slightly to the left which now closes the inlet valve. In this situation, further air is prevented from entering the air cylinder. Therefore, since no air can move in or out of the servo air cylinder and both valves are in the lapped position (both seated), the push rod will not move unless the clutch pedal is again moved (Fig. 2.23(c)).

When the clutch pedal is released fluid returns from the servo to the master cylinder. This permits the reaction plunger to move completely to the left and so opens the exhaust valve. Compressed air in the air cylinder will now transfer to the reaction plunger chamber. It then passes through the exhaust valve and port where it escapes to the atmosphere. The released compressed air from the cylinder allows the clutch linkage return spring to move the air and hydraulic piston assembly back to its original position in its cylinder and at the same time this movement will be relayed to the clutch release bearing, whereby the clutch operating mechanism moves to the engaged drive position (Fig. 2.23(a)).

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