764 Worm and worm wheel inter axle with third differential Fig 729

Where large final drive gear reductions are required which may range from 5:1 to 9:1, either a double reduction axle must be used or alternatively a worm and worm wheel can provide a similar step down reduction. When compared with the conventional crownwheel and pinion final drive gear reduction the worm and worm wheel mechanical efficiency is lower but with the double reduction axle the worm and worm wheel efficiency is very similar to the latter.

Worm and worm wheel axles usually have the worm underslung when used on cars so that a very low floor pan can be used. For heavy trucks the worm is arranged to be overslung, enabling a large ground to axle clearance to be achieved.

When tandem axles are used, an inter axle third differential is necessary to prevent transmission wind-up. This unit is normally built onto the axle casing as an extension of the forward axle's worm (Fig. 7.29).

The worm is manufactured with a hollow axis and is mounted between a double taper bearing to absorb end thrust in both directions at one end and a parallel roller bearing at the other end which just sustains radial loads. The left hand sun gear is attached on splines to the worm but the right hand sun gear and output shaft are mounted on a pair of roller and ball bearings.

Power flow from the gearbox and propellor shaft is provided by the input spigot shaft passing through the hollow worm and coming out in the centre of the bevel gear cluster where it supports the internally

splined cross-pin spider and their corresponding planet pinions. Power is then split between the front axle (left hand) sun gear and worm and the rear axle (right hand) sun gear and output shaft, thus transmitting drive to the second axle.

Consequently if the two axle speeds should vary, as for example when cornering, the planet pinions will revolve on their axes so that the sun gears are able to rotate at speeds slightly above and below that of the input shaft and spider, but at the same time still equally divide the torque between both axles.

Fig. 7.28(b) shows the general layout of a tandem axle worm and worm wheel drive where D1, D2 and D3 represent the first axle, second axle and inter axle differentials respectively.

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