213 Fully automatic centrifugal clutch

Fully automatic centrifugal clutches separate the engine from the transmission system when the engine is stopped or idling and smoothly take up the drive with a progressive reduction in slip within a narrow rising speed range until sufficient engine power is developed to propel the vehicle directly. Above this speed full clutch engagement is provided.

To facilitate gear changes whilst the vehicle is in motion, a conventional clutch release

Fig. 2.17 Semicentrifugal clutch

Fig. 2.18 Semicentrifugal clutch characteristics

1000 2000 3000 4000 Engine speed (rev/mirt)

Fig. 2.18 Semicentrifugal clutch characteristics lever arrangement is additionally provided. This mechanism enables the driver to disengage and engage the clutch independently of the flyweight action so that the drive and driven gearbox member speeds can be rapidly and smoothly unified during the gear selection process.

The automatic centrifugal mechanism consists of a reaction plate situated in between the pressure plate and cover pressing. Mounted on this reaction plate by pivot pins are four equally spaced bob-weights (Fig. 2.19). When the engine's speed increases, the bobweight will tend to fly outward. Since the centre of gravity of their masses is to one side of these pins, they will rotate about their pins. This will be partially prevented by short struts offset to the pivot pins which relay this movement and effort to the pressure plate. Simultaneously, the reaction to this axial clamping thrust causes the reaction plate to compress both the reaction and pressure springs so that it moves backwards towards the cover pressing.

The greater the centrifugal force which tends to rotate the bobweights, the more compressed the springs will become and their reaction thrust will be larger, which will increase the pressure plate clamping load.

To obtain the best pressure plate thrust to engine speed characteristics (Fig. 2.20), adjustable reactor springs are incorporated to counteract the main compression spring reaction. The initial compression length and therefore loading of these springs is set up by the adjusting nut after the whole unit has been assembled. Thus the resultant thrust of both lots of springs determine the actual take-up engine speed of the clutch.

Gear changes are made when the clutch is disengaged, which is achieved by moving the release bearing forwards. This movement pulls the reactor plate rearwards by means of the knife-edge link and also withdraws the pressure plate through the retractor springs so as to release the pressure plate clamping load.

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