731 Salisbury Powr Lok limited slip differential

This type of limited slip differential is produced under licence from the American Thornton Axle Co.

The Powr-Lok limited slip differential essentially consists of an ordinary bevel gear differential arranged so that the torque from the engine engages friction clutches locking the half shafts to the differential cage. The larger the torque, the greater the locking effect (Fig. 7.11).

Multiclush Plate For Tractors Pictures
Fig. 7.11 Multiclutch limited slip differential

There are three stages of friction clutch loading:

1 Belleville spring action,

2 Bevel gear separating force action,

3 Vee slot wedging action.

Belleville spring action (Fig. 7.11) This is achieved by having one of the clutch plates dished to form a Belleville spring so that there is always some spring axial loading in the clutch plates. This then produces a small amount of friction which tends to lock the half shaft to the differential cage when the torque transmitted is very low. The spring thus ensures that when adhesion is so low that hardly any torque can be transmitted, some drive will still be applied to the wheel which is not spinning.

Bevel gear separating force action (Fig. 7.11) This arises from the tendency of the bevel planet pinions in the differential cage to force the bevel sun gears outwards. Each bevel sun gear forms part of a hub which is internally splined to the half shaft so that it is free to move outwards. The sun gear hub is also splined externally to align with one set of clutch plates, the other set being attached by splines to the differential cage. Thus the extra outward force exerted by the bevel pinions when one wheel tends to spin is transmitted via cup thrust plates to the clutches, causing both sets of plates to be camped together and thereby preventing relative movement between the half shaft and cage.

Vee slot wedging action (Fig. 7.11(a and b)) When the torque is increased still further, a third stage of friction clutch loading comes into being. The bevel pinions are not mounted directly in the differential cage but rotate on two separate arms which cross at right angles and are cranked to avoid each other. The ends of these arms are machined to the shape of a vee wedge and are located in vee-shaped slots in the differential cage. With engine torque applied, the drag reaction of the bevel planet pinion cross-pin arms relative to the cage will force them to slide inwards along the ramps framed by the vee-shaped slots in the direction of the wedge (Fig. 7.11(a and b)). The abutment shoulder of the bevel planet pinions press against the cup thrust plates and each set of clutch plates are therefore squeezed further together, increasing the multiclutch locking effect.

Speed differential and traction control (Fig. 7.12) Normal differential speed adjustment takes place

Coefficient of adhesion of tyre to rood

Fig. 7.12 Comparison of tractive effort and tyre to road adhesion for both conventional and limited slip differential

Coefficient of adhesion of tyre to rood

Fig. 7.12 Comparison of tractive effort and tyre to road adhesion for both conventional and limited slip differential continuously, provided the friction of the multiplate clutches can be overcome. When one wheel spins the traction of the other wheel is increased by an amount equal to the friction torque generated by the clutch plates until wheel traction is restored. A comparison of a conventional differential and a limited slip differential tractive effort response against varying tyre to road adhesion is shown in Fig. 7.12.

7.3.2 Torsen worm and wheel differential

Differential construction (Figs 7.13 and 7.14) The Torsen differential has a pair of worm gears, the left hand half shaft is splined to one of these worm gears while the right hand half shaft is splined to the other hand (Fig. 7.13). Meshing with each worm gear on each side is a pair of worm wheels (for large units triple worm wheels on each side). At both ends of each worm wheel are spur gears which mesh with adjacent spur gears so that both worm gear and half shafts are indirectly coupled together.

Normally with a worm gear and worm wheel combination the worm wheel is larger than the worm gear, but with the Torsen system the worm gear is made larger than the worm wheel. The important feature of any worm gear and worm wheel is that the teeth are cut at a helix angle such that the worm gear can turn the worm wheel but the worm wheel cannot rotate the worm gear. This is achieved with the Torsen differential by giving the

Fig. 7.13 Pictorial view of Torsen worm and spur gear differential

worm gear teeth a fine pitch while the worm wheel has a coarse pitch.

Note that with the conventional meshing spur gear, be it straight or helical teeth, the input and output drivers can be applied to either gear. The reversibility and irreversibility of the conventional bevel gear differential and the worm and worm wheel differential is illustrated in Fig. 7.14 by the high and low mechanical efficiencies of the two types of differential.

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