Servomotor for an emergency braking

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

• said partition wall (14) including integral piston (22) with a control rod (38) that moves in a bore inside said piston (22), said control rod movement being selective as a function of an axial input force exerted in the forward direction against a return force applied to the rod (38) by a return spring (40);
• said axial input force biasing said control rod (38) towards either an intermediate actuation position or an end actuation position as a function of a rate of speed at which said input force is applied to said control rod;
• a plunger (46) located in said bore in said piston (22) and arranged at a front end of said control rod (38) inside the piston (22), and a three-way valve (50), including at least an annular seat (48) borne by a rear section of the plunger (46), and capable of varying a second pressure (P2) prevailing within the rear chamber (18) by connecting said front chamber (16) with said rear chamber (18) when said control rod (38) is in a rest position and by gradually connecting said rear chamber (18) with said atmospheric pressure (Pa) when the control rod (38) is actuated by an input force;
• a finger (52) that is slidingly fitted on a front end of said plunger (46) and biased by said plunger (46) into contact with a reaction disk (32) carried by said moving piston (22) to transmit a reaction force from the moving piston (22) to the plunger (46) and to the control rod (38);
• and a unidirectional clutch device (56) that includes a coaxial sleeve (58) that slides on said plunger (46), and a substantially ring-shaped key (60), arranged with a given clearance around the sleeve (58) and being driven by the moving piston (22) when the input force is applied at a predetermined rate of speed, said predetermined rate of speed causing said key (60) to rotate on a generally transverse axis and co-operate with a periphery surface of said sleeve (58) to lock said sleeve in a end front axial position wherein a front end annular bearing surface (74) of said sleeve locks said finger (52), independently of the plunger (46) and of the control rod (38), said unidirectional clutch device (56) having at least one locking transverse face (76) for engagement by said sleeve, said face being axially directed rearwards to form a stop for a peg (78) located on said key (60) and extending radially towards said sleeve (58) for an axial indexing of said end locking position of the sleeve (58).