Compact clutch comprising a driven shaft on which the following are mounted: - a gear wheel; - a driving bush; - a driven bush; - a coil spring with two ends (380, 382); - a first collar comprising a first recess and, on its radially outer surface, cam surfaces; - a second collar comprising a second recess and, on its radially inner surface, cam surfaces; wherein: - the gear wheel, the driving bush and the driven bush are arranged axially next to each other; - the gear wheel and the driving bush are constrained together in the circumferential direction; - the driving bush and the driven bush have the same external diameter; - the coil spring has, in the rest condition, an internal diameter slightly greater than the external diameter of the driving bush and of the driven bush; - the coil spring surrounds externally the driving bush and the driven bush; - the first collar surrounds externally the coil spring and the first recess receives the first end thereof; - the second collar surrounds externally the first collar and the second recess receives the second end of the coil spring; - the second collar may be displaced in an axial direction between a rest position and a working position. - when the second collar is in the rest position the coil spring is in the rest condition; - when the second collar is displaced into the working position: - the cam surfaces (404, 424) convert the axial displacement of the second collar into a relative rotation between the first collar and the second collar; - the relative rotation between the first collar and the second collar produces a relative rotation between the first end and the second end of the coil spring; - the relative rotation between the first end and the second end produces a tightening of the coil spring around the driving bush and the driven bush; - the tightening of the coil spring constrains together by means of friction the driving bush and the driven bush; the compact clutch being characterized in that - the gear wheel, the driving bush and the driven bush are constrained to the driven shaft in the axial direction by means of two stop rings (326, 366); in that - the driven bush is constrained to the driven shaft in the circumferential direction by means of at least one tongue; and in that - the at least one tongue ensures engagement between the driven bush and the driven shaft in the circumferential direction, while in the axial direction the driven bush may slide with respect to the driven shaft at least in one sense. - a gear wheel; - a driving bush; - a driven bush; - a coil spring with two ends (380, 382); - a first collar comprising a first recess and, on its radially outer surface, cam surfaces; - a second collar comprising a second recess and, on its radially inner surface, cam surfaces; - the gear wheel, the driving bush and the driven bush are arranged axially next to each other; - the gear wheel and the driving bush are constrained together in the circumferential direction; - the driving bush and the driven bush have the same external diameter; - the coil spring has, in the rest condition, an internal diameter slightly greater than the external diameter of the driving bush and of the driven bush; - the coil spring surrounds externally the driving bush and the driven bush; - the first collar surrounds externally the coil spring and the first recess receives the first end thereof; - the second collar surrounds externally the first collar and the second recess receives the second end of the coil spring; - the second collar may be displaced in an axial direction between a rest position and a working position. - when the second collar is in the rest position the coil spring is in the rest condition; - when the second collar is displaced into the working position: - the cam surfaces (404, 424) convert the axial displacement of the second collar into a relative rotation between the first collar and the second collar; - the relative rotation between the first collar and the second collar produces a relative rotation between the first end and the second end of the coil spring; - the relative rotation between the first end and the second end produces a tightening of the coil spring around the driving bush and the driven bush; - the tightening of the coil spring constrains together by means of friction the driving bush and the driven bush; - the cam surfaces (404, 424) convert the axial displacement of the second collar into a relative rotation between the first collar and the second collar; - the relative rotation between the first collar and the second collar produces a relative rotation between the first end and the second end of the coil spring; - the relative rotation between the first end and the second end produces a tightening of the coil spring around the driving bush and the driven bush; - the tightening of the coil spring constrains together by means of friction the driving bush and the driven bush; - the gear wheel, the driving bush and the driven bush are constrained to the driven shaft in the axial direction by means of two stop rings (326, 366); in that - the driven bush is constrained to the driven shaft in the circumferential direction by means of at least one tongue; and in that - the at least one tongue ensures engagement between the driven bush and the driven shaft in the circumferential direction, while in the axial direction the driven bush may slide with respect to the driven shaft at least in one sense.