Epicyclic Transmission (Planetary Gearbox)
Epicyclic transmissions are characterized by at least three coaxial center shafts that permanently engaged with each other over gear wheels. Fig. 3-59 shows the different gear wheel positions in an epicyclic transmission with three center shafts.
Two of the shafts involved are connected with the central wheels, the sun wheel and the hollow wheel. The third one is coupled to a bar which is connected to the so-called planetary wheels which mesh with both central wheels. This design is chosen when possible as a result of its low constructional length, simplicity of the gear wheels and the point of action of loads which are in plane. This design of a circulation transmission is called spur-gear planetary transmission.
The characteristic parameter of an epicyclic transmission is the nominal ratio i0 which is defined by the ratio of hollow wheel radius to sun wheel radius.