743 Inboard and outboard double reduction axles

Where very heavy loads are to be carried by on-off highway vehicles, the load imposed on the crownwheel and pinion and differential unit can be reduced by locating a further gear reduction on either side of the differential exit. If the second gear reduction is arranged on both sides close to the differential cage, it is referred to as an inboard reduction. They can be situated at the wheel ends of the half shafts, where they are known as outboard second stage gear reduction. By having the reduction directly after the differential, the increased torque multiplication will only be transmitted to the half shafts leaving the crownwheel, pinion and differential with a torque load capacity proportional to their gear ratio. The torque at this point may be smaller than with the normal final drive gear ratio since less gear reduction will be needed at the crownwheel and pinion if a second reduction is to be provided. Alternatively, if the second reduction is in the axle hub, less torque will be transmitted by the half shafts and final drive differential and the dimensions of these components can be kept to a minimum. Having either an inboard or outboard second stage gear reduction enables lighter crownwheel and pinion combinations and differential assembly to be employed, but it does mean there are two gear reductions for each half shaft, as opposed to a single double reduction drive if the reduction takes place before the differential.

Inboard epicyclic double reduction final drive axle (Scammell) (Fig. 7.20) With this type of double reduction axle, the first stage conforms to the conventional crownwheel and pinion whereas the second stage reduction occurs after passing through the differential. The divided drive has a step down gear reduction via twin epicyclic gear trains on either side of the differential cage (Fig. 7.20). Short shafts connect the differential bevel sun gears to the pinion sun gear of the epicyclic gear train. When drive is being transmitted, the rotation of the sun gears rotates the planet pinions so that they are forced to roll 'walk' around the inside of the reaction annulus gear attached firmly to the axle casing. Support to the planet pinions and their pins is given by the planet carrier which is itself mounted on a ball race. Thus when the planet pinions are made to rotate on their own axes they also bodily rotate about the same axis of rotation as the sun gear, but at a reduced speed, and in turn convey power to the half shafts splined to the central hub portion of the planet carriers.

Inboard epicyclic differential and double reduction axle (Kirkstall) (Fig. 7.21) This unique double reduction axle has a worm and worm wheel first stage gear reduction. The drive is transferred to an epicyclic gear train which has the dual function of providing the second stage gear reduction while at


Fig. 7.20 Inboard epicyclic double reduction final drive axle


Fig. 7.20 Inboard epicyclic double reduction final drive axle

Fig. 7.21 Inboard eplcycllc double reduction axle

the same time performing as the final drive differential (Fig. 7.21).

Principle of operation Power is transmitted from the propellor shaft to the worm and worm wheel which produces a gear reduction and redirects the drive at right angles and below the worm axis of rotation (Fig. 7.21). The worm wheel is mounted on the annulus carrier so that they both rotate as one. Therefore the three evenly spaced planet pinions meshing with both the annulus and the sun gear are forced to revolve and move bodily on their pins in a forward direction. Since the sun gear is free to rotate (not held stationary) it will revolve in a backward direction so that the planet carrier and the attached left hand half shaft will turn at a reduced speed relative to the annulus gear.

Simultaneously, as the sun gear and shaft transfers motion to the right hand concentric gear train central pinion, it passes to the three idler pinions, compelling them to rotate on their fixed axes, and in so doing drives round the annulus ring gear and with it the right hand half shaft which is splined to it.

The right hand gear train with an outer internal ring gear (annulus) does not form an epicyclic gear train since the planet pins are fixed to the casing and do not bodily revolve with their pins (attached to a carrier) about some common centre of rotation. It is the purpose of the right hand gear train to produce an additional gear reduction to equalize the gear reduction caused by the planet carrier output on the left hand epicyclic gearing with the sun gear output on the right hand side.

Differential action of the epicyclic gears The operation of the differential is quite straight forward if one imagines either the left or right hand half shaft to slow down as in the case when they are attached to the inner wheel of a cornering vehicle.

If when cornering the left hand half shaft slows down, the planet carrier will correspondingly reduce speed and force the planet pinions revolving on their pins to spin at an increased speed. This raises the speed of the sun gear which indirectly drives, in this case, the outer right hand half shaft at a slightly higher speed. Conversely, when cornering if the right hand half shaft should slow down, it indirectly reduces the speed of the central pinion and sun gear. Hence the planet pinions will not revolve on their pins, but will increase their speed at which they also roll round the outside of the sun gear. Subsequently the planet pins will drive the planet carrier and the left hand half shaft at an increased speed.

7.4.4 Outboard double reduction axles

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