Description of differential and viscous coupling

(Figs 7.16 and 7.17) The crownwheel is bolted to the differential bevel gearing and multiplate housing. Speed differentiation is achieved in the normal manner by a pair of bevel sun (side) gears, each splined to a half shaft. Bridging these two bevel sun gears are a pair of bevel planet pinions supported on a cross-pin mounted on the housing cage. A multiplate back assembly is situated around the left hand half shaft slightly outboard from the corresponding sun gear (Fig. 7.16).

The viscous coupling consists of a series of spaced interleaved multiplates which are alternatively splined to a half shaft hub and the outer differential cage. The cage plates have pierced holes but the hub plates have radial slots. Both sets of plates are separated from each other by a 0.25 mm gap. Thus the free gap between adjacent plates and the interruption of their surface areas with slots and holes ensures there is an adequate storage of fluid between plates after the sealed plate unit has been filled and that the necessary progressive viscous fluid torque characteristics will be obtained when relative movement of the plates takes place.

When one set of plates rotate relative to the other, the fluid will be sheared between each pair of adjacent plate faces and in so doing will generate an opposing torque. The magnitude of this resist ing torque will be proportional to the fluid viscosity and the relative speed difference between the sets of plates. The dilatent silicon compound fluid which has been developed for this type of application has the ability to maintain a constant level of viscosity throughout the operating temperature range and life expectancy of the coupling (Fig. 7.17).

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Limited Slip differential

Speed difference across coupling jrev/min)

40 BO 120 193

Speed difference across coupling jrev/min)

Fig. 7.17 Comparison of torque transmitted to wheel having the greater adhesion with respect to speed difference between half shafts for both limited slip and viscous coupling

Speed differential action (Fig. 7.16) In the straight ahead driving mode the crownwheel and differential cage driven by the bevel pinion act as the input to the differential gearing and in so doing the power path transfers to the cross-pin and bevel planet gears. One of the functions of these planet gears is to link (bridge) the two sun (side) gears so that the power flow is divided equally between the sun gears and correspondently both half shafts (Fig. 7.16).

When rounding a bend or turning a corner, the outer wheel will have a greater turning circle than the inner one. Therefore the outer wheel tends to increase its speed and the inner wheel decrease its speed relative to the differential cage rotational speed. This speed differential is made possible by the different torque reactions each sun gear conveys back from the road wheel to the bevel planet pinions. The planet gears 'float' between the sun gears by rotating on their cross-pin, thus the speed lost relative to the cage speed by the inner road wheel and sun gear due to the speed retarding ground reaction will be that gained by the outer road wheel and sun gear.

Viscous coupling action (Figs 7.16 and 7.17) In the situation when one wheel loses traction caused by possibly loose soil, mud, ice or snow, the tyre-road tractive effort reaction is lost. Because of this lost traction there is nothing to prevent the planet pinions revolving on their axes, rolling around the opposite sun gear, which is connected to the road wheel sustaining its traction, with the result that the wheel which has lost its grip will just spin (race) with no power being able to drive the good wheel (Fig. 7.16). Subsequently, a speed difference between the cage plates and half shaft hub plates will be established and in proportion to this relative speed, the two sets of coupling plates will shear the silicon fluid and thereby generate a viscous drag torque between adjacent plate faces (Fig. 7.17). As a result of this viscous drag torque the half shaft hub plates will proportionally resist the rate of fluid shear and so partially lock the differential gear mechanism. A degree of driving torque will be transmitted to the good traction wheel. Fig. 7.17 also compares the viscous coupling differential transmitted torque to the limited slip differential. Here it can be seen that the limited slip differential approximately provides a constant torque to the good traction wheel at all relative speeds, whereas the viscous coupling differential is dependent on speed differences between both half shafts so that the torque transmitted to the wheel supplying tractive effort rises with increased relative speed between the half shaft and differential cage.

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