29 Spicer twin driven plate angle spring pull type clutch Fig 214

An interesting clutch engagement and release pressure plate mechanism employs three pairs of coil springs which are inclined to the axial direction of the driven plates. These springs are mounted between the pressure plate cover housing, which takes the spring reaction, and the release lever central hub (Fig. 2.14). The axial clamping thrust is conveyed by the springs to the six to one leverage ratio release levers (six of them) spaced evenly around the release hub. These release levers span between the release hub and a large annular shaped adjustable pivot ring which is screwed inside the pressure plate cover housing. Towards the pivot pin end of the release levers a kink is formed so that it can bear against the pressure plate at one point. The pressure plate and intermediate plate are both prevented from spinning with the driven plates by cast-in drive lugs which fit into slots formed into the cover housing.

In the engaged position, the six springs expand and push the release hub and, subsequently, the release levers towards the pressure plate so that the driven plates are squeezed together to transmit the drive torque.

To release the clutch driven plates, the release bearing assembly is pulled out from the cover housing. This compels the release lever hub to compress and distort the thrust springs to a much greater inclined angle relative to the input shaft axis and so permits the pressure plate to be withdrawn by means of the retraction springs.

Because the spring thrust does not operate directly against the pressure plate, but is relayed through the release levers, the actual spring's stiffness is reduced by a factor of the leverage ratio; in this instance one-sixth of the value if the springs were direct acting.

The operating characteristics of the clutch mechanism are described as follows:

New engaged position (Fig. 2.14(a)) The spring thrust horizontal component of 2.2 kN, multiplied by the lever ratio, provides a pressure plate clamping load of 13.2kN (Fig. 2.14(a)). The axial thrust horizontal component pushing on the pressure plate does not vary in direct proportion with the spring load exerted between its ends, but is a function of the angle through which the mounted springs operate relative to the splined input shaft.

Worn engagement position (Fig. 2.14(b)) When the driven plate facings wear, the release bearing moves forward to the pressure plate so that the springs elongate. The spring load exerted between the spring ends is thus reduced. Fortunately, the inclined angle of spring axis to that of the thrust bearing axis is reduced so that as the spring load along its axis declines, the horizontal thrust component remains essentially the same. Therefore, the pressure plate clamping load remains practically constant throughout the life of the clutch (Fig. 2.14(b)).

Release position (Fig. 2.14(c)) When the clutch is released, that is when the bearing is pulled rearwards, the springs compress and increase in load, but the spring angle relative to the thrust bearing axis increases so that a greater proportion of the spring load will be acting radially instead of axially. Consequently, the horizontal component of axial release bearing load, caused by the spring thrust, gradually reduces to a value of about 1.7 kN as the bearing moves forwards, which results in the reduced pedal effort. This is shown in Fig. 2.14(c).

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