Pneumatic booster with selective and controlled stress self-assistance

A pneumatic servomotor for an assisted braking, comprising:

• a rigid casing;
• a moving airtight partition wall that defines a front chamber and a rear chamber inside said casing, said front chamber being operatively under a first pressure and said rear chamber being selectively connected to said front chamber or to a second pressure having a higher level than said first pressure;
• a pneumatic piston travelling with said moving partition wall;
• a control rod that moves inside of said piston as a function of an input force for selectively exerted in an axial actuation direction towards said front chamber and a function of a return force applied by a main spring in an axial return direction that is opposed to the axial actuation direction of the input force, said return force biasing said control rod towards a rest position while said input force biases said control rod towards an intermediate actuation position or an end actuation position, depending on whether said input force is applied at a speed that is lower or higher than a given limit speed;
• a plunger that is housed within said piston and driven by said control rod;
• a three-way valve that has an annular seat borne by a rear section of the plunger, said three-way valve connecting said rear chamber with the front chamber when the control rod is in said rest position and subjecting said rear chamber to said second pressure when said control rod is in an actuation position;
• force-transmitting means for receiving and transmitting at least one assistance force exerted by a front face on said piston when said three-way valve puts said rear chamber under said second pressure, said plunger including a front section comprising retaining means for tying said front section of said plunger to said pneumatic piston, said retaining means comprising a unidirectional clutch device for the driving of said force-transmitting means independently of said control rod and said rear section of said plunger when biased by said pneumatic piston resulting from the actuation of said control rod at a speed that is higher than a predetermined value characterised in that said unidirectional clutch device comprises a cylindrical sleeve that freely slides around a front part of said rear section of said plunger and a locking key having a central opening that surrounds said sleeve, an inner diameter that is slightly greater than an outer diameter of said cylindrical sleeve, and a first and a second indexing parts that are diametrically opposed in relation to said central opening.