Abstract: Reducer comprising at least one stage and associated with a nonreturn device and with an Oldham coupling. The nonreturn device consists of two tubular seats fitted one into the other and of a helical spring braking by expansion against the casing of the reducer. The Oldham cross is seated within the nonreturn device. The reducer, the nonreturn device and the Oldham coupling are seated within the casing and are retained axially in the latter by a ring capable of receiving an adapter piece for mounting the assembly in a frame. The assembly forms a compact unit which can be handled easily.