Declutching device having an hydraulic actuator for snap-fitting on a clutch of the pull-off type

A declutching device for snap-fitting coupling to a clutch of the pull-off type, the declutching device comprising an hydraulic actuator which itself comprises:

• a movable member;
• a fixed member having a cylindrical interface surface mounting said movable member for sliding movement along said interface surface between an extended position and a retracted position with respect to said fixed member, said fixed and movable members being in piston and cylinder relationship with each other and defining a control chamber between them;
• preloading means coupled with said movable member for biassing the latter permanently towards its said retracted position;
• and retaining means disposed operatively between said fixed and movable members for retaining said movable member in its said extended position, wherein said retaining means comprise a radially resiliently deformable locking ring, said movable member defining a first groove for containing said locking ring under precompression which biases said the locking ring to escape from said first groove, said fixed member defining a second groove recessed into said cylindrical interface surface, said second groove corresponding to said first groove and having a stepped base defining at least a first step and a second step, of different diameters and offset axially from each other, said first step being a retention step defining a first free radial depth from said cylindrical interface surface, said second step being a release step defining a second free radial depth from said cylindrical interface surface, said locking ring defining a thickness thereof, said first free depth being smaller than said thickness, and said second free depth being at least equal to said thickness.