Servomotor with a finger-forming locking sleeve

A pneumatic servomotor (10) for an assisted braking of a motor vehicle comprising a rigid casing (12) in which a transverse partition wall (14) is movable in an airtight manner to define a front chamber (16) under a first engine negative pressure (P1), and a rear chamber (18) under a second pressure (P2) that varies between said engine negative pressure (P1) and the atmospheric pressure (Pa);

• a moving piston (22) integral with said moving partition wall (14), and a control rod (38) that moves inside the piston (22) in a selective manner as a function of an axial input force, said input force being exerted in the forward direction against a return force applied to said control rod (38) by a return spring (40);
• said control rod (38) being biased towards an intermediate actuation position and an end actuation position resulting from the applying of the input force at a determined high speed;
• a plunger (46) arranged at a front part of said control rod (38) inside said piston (22), and a three-way valve (50), including at least an annular seat (48) borne by a rear section of said plunger (46), and capable of varying the second pressure (P2), prevailing within said rear chamber (18), particularly by connecting said front chamber (16) with the rear chamber (18) when said control rod (38) is in a rest position and by gradually connecting said rear chamber (18) with the atmospheric pressure (Pa) when said control rod (38) is actuated;
• said control rod (38) having a finger (52) which slidingly engages the front end of said plunger (46) and is biased by the plunger (46) into contact with a reaction disk (32) that is integral with the rear face of said moving piston (22), so as to transmit a reaction force of said moving piston (22) to said plunger (46) and control rod (38);
• and a unidirectional clutch device (56), which includes a coaxial sleeve (58), sliding on the plunger (46), and a substantially ring-shaped key (60), arranged with a given clearance around the sleeve (58) and being driven by moving piston (22) when an input force is applied at said determined speed causing said key (60) to rotate on a generally transverse axis in order to cooperate with the periphery of the sleeve (58) and lock said key in an end front axial position, in which said key locks said finger (52), independently of said plunger (46) and control rod (38),
• characterised in that said finger (52) is axially integral with said sleeve (58) and in that means are provided for axial indexing of locking positions of the sleeve (58).