763 Inter axle with third differential

Description of forward rear drive axle (Fig. 7.27) A third differential is generally incorporated in the forward rear axle of a tandem bogie axle drive layout because in this position it can be conveniently arranged to extend the drive to the rear axle (Fig. 7.27).

Torsional twist (degl

Fig. 7.26 Relationship of relative speeds of double drive axles and the amount of transmission twist

Torsional twist (degl

Fig. 7.26 Relationship of relative speeds of double drive axles and the amount of transmission twist

Power from the gearbox propellor shaft drives the axle input shaft. Support for this shaft is provided by a ball race mounted in the casing at the flanged end and by a spigot bearing built into the integral sun gear and output shaft at the other end. Bevel planet pinions supported on the cross-pin spider splined to the input shaft divide the drive between both of the bevel sun gears. The left hand sun gear is integral with the input helical gear and is free to rotate relative to the input shaft which it is mounted on, whereas the right hand bevel sun gear is integral with the output shaft. This output shaft is supported at the differential end by a large taper roller bearing and by a much smaller parallel roller bearing at the opposite flanged output end.

A tandem axle transmission arrangement is shown in Fig. 7.28(a) where D1, D2 and D3 represent the first axle, second axle and inter axle differential respectively.

When power is supplied to the inter axle (forward rear axle) through the input shaft and to the bevel planet pinion via the cross-pin spider, the power flow is then divided between both sun gears. The drive from the left hand sun gear then passes to the input helical gear to the final drive bevel pinion helical gear where it is redirected at right angles by the crownwheel and pinion to the axle differential and half shafts.

At the same time the power flowing to the right hand sun gear goes directly to the output shaft flange where it is then transmitted to the rear axle via a pair of universal joints and a short propellor shaft.

Third differential action (Fig. 7.27) When both drive axles rotate at the same speed, the bevel planet pinions bridging the opposing sun gears bodily move around with the spider but do not revolve on their own axes. If one axle should reduce its speed relative to the other one, the planet pinions will start to revolve on their cross-pins so that the speed lost by one sun gear relative to the spider's input speed will be gained by the other sun gear.

Therefore the third differential connecting the two axles permits each axle mean speed to automatically adjust itself to suit the road operating conditions without causing any torsional wind-up between axle drives.

Third differential lock-out (Fig. 7.27) For providing maximum traction when road conditions are unfavourable such as driving over soft, slippery or steep ground, a differential lock-out clutch is incorporated. When engaged this device couples

Fig. 7.27 Final drive with third differential and lock and optional transfer gearing for front

the input shaft directly with the input helical gear and left hand bevel sun gear so that the differential planet pinions are prevented from equally dividing the input torque between the two axles at the expense of axle speed differentiation. Consequently, when the third differential is locked out each axle is able to deliver independently to the other axle tractive effect which is only limited by the grip between the road wheels and the quality of surface it is being driven over. It should be observed that when the third differential lock-out is engaged the vehicle should only be operated at slow road speeds, otherwise excessive transmission wind-up and tyre wear will result.

Front wheel drive transfer gear take-up (Fig. 7.28) An additional optional feature is the transfer gear take-up which is desirable for on-off highway applications where the ground can be rough and uneven. With the front wheel drive lock clutch engaged, 25% of the total input torque from the gearbox will be transmitted to the front steer drive axle, while the remainder of the input torque 75% will be converted into tractive effect by the tandem axles. Again it should be pointed out that this mode of torque delivery and distribution with the third differential locked-out must only be used at relatively low speeds.

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