The Wälzkörpergetriebe is a force- transmission design. The rotationally symmetrical rolling elements roll off against each other. A Wälzkörpergetriebe in the simplest case of two shafts or rollers which roll on each other ( friction gear ). The rolling radii are to achieve the desired gear ratio varied accordingly. For gear for higher powers, an oil film on the bodies prevents a direct metallic contact between them. In this film of oil to take the shear forces and the gradient of the shear forces between the driving and driven rolling elements with increasing torque.
The ratio of the angular velocities ( " speeds " ) behaves inversely as the radii. As for other transmissions and is the gear ratio for the torque transmitted to the reciprocal of the ratio of the rotational speed.
The advantage of this mechanism is that the safety clutch is just a key press, so to speak, since it comes in the overload case of slippage of the friction pair.
The main drawback lies in the fact that in normal operation, the slip is much greater than in other types of transmission, and that thus the efficiency is lower. Under certain circumstances - depending on the design - a higher wear recorded. Another disadvantage is that the high pressure forces necessary also have high bearing loads result, also increase with decreasing rolling radii necessary for the transmission of torque to pressure forces, bringing a compact construction is limited.
The company Nissan has such a transmission under the name " Extroid - CVT" implemented and used in vehicles.
Friction gears are a special kind of Wälzkörpergetriebe. In this case, a wheel rolling on another wheel, or any other body to be driven off.
A popular application was for a long time, the drive of the turntable with turntables. Could be cost-effectively at constant drive motor speed different turntable speeds via a friction gear.