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Planetary Power Axle

Drive shafts more daintily dimensioned, less stressed

From the crankshaft to the drive-wheel, the torque becomes higher and higher in the lower gears. Assuming that the utility vehicle features from 1000- to just under 3000 Nm on the clutch, this value can easily be tenfold or more on the axle drive shafts. Here the external planet-gears in the wheel hubs offer an alternative to reduce the strain on the differential gear and the axle drive shafts. Moreover, they allow a small crown wheel in the axle-drive and therefore more ground clearance, even when using smaller wheel sizes. Unfortunately, up to now, they can not always be combined with disc brakes.

Substantially smaller crown wheel, more ground clearance

The crown wheel has shrunk down almost to the size of the bevel wheel which then has perhaps a few more teeth. The gear ratio lies between 1 and 1.5. This brings only almost one fifth of the cardan shaft torque to the axle drive shafts which can also then be dimensioned accordingly smaller. In the wheel hubs they end in a sun wheel which is connected with 3 to 5 planet wheels. These roll on a stationary hollow gear-ring and thus drive the wheels. External planet-groups can be found in all drive wheels, in the case of utility vehicles with four-wheel drive, also in the front axle. They can be recognised from the outside by considerably larger diameters of the wheel hub.

Neither weight - nor efficiency advantage

The disadvantage of the external planetary gearboxes is that at least two of them are necessary, one on each drive side. This practically cancels out the weight advantage caused by the size reduction of the driven axle. Above all, these axles are considerably more expensive to produce. It may be interesting to compare vehicles with regard to their drag torque. There are utility vehicles which almost brake when the clutch is pressed, and others which carry on rolling for some distance. There are also possibilities here for fuel saving. 08/09

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