In contrast to usual stepped transmissions, free-wheeling epicyclic transmissions have no free bearing forces. If a shaft is rigidly braked, it acts as a torque support. The other two shafts move relative to each other in a firm speed ratio, which can be determined from the basic equation. Based on coupling of the individual center shafts with the engine, output, and rigid transmission housing, a planetary train allows for six different ratios, two of which reverse the direction between the engine and output sides.
In addition, a direct gear can be realized if the third shaft is linked with the engine or the output. The entire planetary train then rotates as a block. The ratios i=2, i=0.5 or i=-1 cannot be realized by a simple spur-gear planetary train. In order to achieve this, the number of teeth on the hollow gear has to correspond with that of the sun gear. The situation can be set right using conical gear wheels or by assembled epicyclic transmissions.
The favorable, symmetrical design as well as a low construction volume and low weight with a simultaneously high maximum transferable torque, as a result of the distribution of forces over several meshings (several planetary wheels), led to a large number of applications of planetary transmissions in motor vehicles (automatic transmission, transfer cases in all-wheel drive systems, wheel-hub transmission).