## Differential action when moving straight ahead

(Fig. 7.15) When the vehicle is moving straight ahead power is transferred from the propellor shaft to the bevel pinion and crownwheel. The crownwheel and differential cage therefore revolve as one unit (Fig. 7.15). Power is divided between the left and right hand worm wheel by way of the spur gear pins which are attached to the differential cage. It then flows to the pair of meshing worm gears, where it finally passes to each splined half shaft. Under these conditions, the drive in terms of speed and torque is proportioned equally to both half shafts and road wheels. Note that there is no relative rotary motion between the half shafts and the differential cage so that they all revolve as a single unit.

Differential action when cornering (Fig. 7.15) When cornering, the outside wheel of the driven axle will

Fig. 7.14 Comparison of internal friction expressed in terms of mechanical efficiency of both bevel pinion type and worm and spur type differentials

Drive lorque INm)

Fig. 7.14 Comparison of internal friction expressed in terms of mechanical efficiency of both bevel pinion type and worm and spur type differentials tend to rotate faster than the inside wheel due to its turning circle being larger than that of the inside wheel. It follows that the outside wheel will have to rotate relatively faster than the differential cage, say by +20 rev/min, and conversely the inside wheel has to reduce its speed in the same proportion, of say —20 rev/min.

Fig. 7.15 Sectioned views of Torsen worm and spur gear differential

When there is a difference in speed between the two half shafts, the faster turning half shaft via the splined worm gears drives its worm wheels about their axes (pins) in one direction of rotation. The corresponding slower turning half shaft on the other side drives its worm wheels about their axes (pins) in the opposite direction but at the same speed (Fig. 7.15).

Since the worm wheels on opposite sides will be revolving at the same speed but in the opposite sense while the vehicle is cornering they can be simply interlinked by pairs of meshing spur gears without interfering with the independent road speed requirements for both inner and outer driving road wheels.

Differential torque distribution (Fig. 7.15) When one wheel loses traction and attempts to spin, it transmits drive from its set of worm gears to the worm wheels. The drive is then transferred from the worm wheels on the spinning side to the opposite (good traction wheel) side worm wheels by way of the bridging spur gears (Fig. 7.15). At this point the engaging teeth of the worm wheel with the corresponding worm gear teeth jam. Thus the wheel which has lost its traction locks up the gear mechanism on the other side every time there is a tendency for it to spin. As a result of the low traction wheel being prevented from spinning, the transmission of torque from the engine will be concentrated on the wheel which has traction.

Another feature of this mechanism is that speed differentiation between both road wheels is maintained even when the wheel traction differs considerably between wheels.

Crownwheel mounting flange

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