High torques can be transferred. The planetary gearbox allows a change of the transmission ratio under load, without interrupting the power flow. The gearwheels are provided with internal gear teeth in the ring-gear- wheel making the gearing especially compact. No additional shaft with gearwheel for the reverse gear, as in a manual gearbox, is necessary.

Planet grouping is also found in transfer gears of four-wheel drives, in the drive hubs of trucks/busses and in the starter motor.

In the simplest case, the planetary gearbox consists of a sun wheel, planet wheels, a planet wheel carrier and a ring-inside-gear (hollow)wheel. With this planetary gearbox, the sun wheel is connected in the center, with several planet wheels, to a positive fitting internal ring-gear-wheel. Sun wheel, planet wheel carrier or ring-gear-wheel can be in each case, driving, driven or braked. A gear change is possible when this set-up is changed by multiple-disc clutches and/or braking. In today's automatic gearboxes, at least a combined planetary system is generally installed.

Without an additional gear-wheel, a reverse gear is engageable using a parking brake on the planet wheel carrier. In this case, the engine must be connected to the sun wheel and the final drive to the ring-gear-wheel. 09/08