Differential gearboxes with optional reduction ratios have become rare. Since the light off-road vehicles (Sport Utility Vehicles) have conquered the market, one doesn't hear much about the genuine off-road sector. It's pretty doubtful whether the multitude of all-wheel drive buyers ever use this feature at all. Thus, these special low-ratios have been saved for the vehicles which need maximum traction power on rough terrain.
Divided shaft, left free- right fixed gear
The picture series seen above shows the differential gearbox of a vehicle with the engine up front and rear-wheel drive. It could also be a utility vehicle for a building site. The mounting takes place by shortening the drive shaft behind the gearbox, which can be seen placed in the picture on the left hand side. The upper shaft is divided into two parts. Coming from the left of the gearbox, it continues to, and including, the shifting sleeve. The upper left cog is an idler-wheel, which can only be connected to the drive shaft by the shifting sleeve. The right hand shaft with the fixed gearwheel on the top right is inserted into the drive shaft but not connected to it. In the case of rear-wheel drive, the sliding collar in position on the right, joins the two shaft sections.
Only the sun-wheel is fixed, The ring-gear, together with right gear is an idler.
The all-wheel drive is engaged by shifting this sleeve to the left. Now a reduction takes place onto a somewhat larger gearwheel below, which is also not fixed to its shaft, but is connected to the planetary gears of the differential. From there a part of the torque is transferred to the front axle through the sunwheel and the shaft to which it is fixed. The remaining torque is passed by the ring-gear to the gearwheel below on the right hand side, which in this case, has no connection to its shaft. Without the ratio being reduced, it carries on further to the upper right hand cog, and thus, to the rear axle.
Different torque distribution through the planetary gearbox.
Thus, in the above shown differential, the off-road mode is always selected when the all-wheel drive is engaged, all the gear-speeds are once again reduced. It distributes the torque beteen the front- and rear axle according to a set ratio. This distribution ratio depends on the construction and can, e.g., through using larger planetary gears and a smaller sun-wheel, be altered to favour the rear axle. Should wheel-spin occur over the whole axle, this can be limited (limited-slip) or locked. In this case, the lower collar is shifted to the left connecting the lower right hand gearwheel with the shaft. From now on, the planetary set runs as a block. It is important that once good grip is again available, to disengage the locking, otherwise there is the risk of tension strain, heavy steering, higher tyre wear and damage to the axle-drive. 09/08