This invention relates to a pneumatic servomotor used for assisting in the braking of a motor vehicle.
More particularly, the present invention relates to a pneumatic servomotor for an assisted braking of a motor vehicle, of the type comprising a rigid casing, in which a transverse partition wall is movable, thus defining in an airtight manner a front chamber under a first engine negative pressure, and a rear chamber under a second pressure, varying between the engine negative pressure and the atmospheric pressure; of the type including a moving piston, integral with the moving partition wall and travelling with the latter, and a control rod, moving inside the piston, in a selective manner as a function of an axial input force, exerted in the forward direction against a return force applied to the rod by a return spring; of the type according to which the control rod is biased towards either an intermediate actuation position or an end actuation position, resulting from the applying of the input force at a determined high speed; of the type comprising a plunger, arranged at the front part of the control rod inside the piston, and a three-way valve, including at least an annular seat borne by a rear section of the plunger, and capable of varying the second pressure, prevailing within the rear chamber, particularly by connecting the front chamber with the rear chamber when the control rod is in the rest position, or by gradually connecting the rear chamber with the atmospheric pressure when the control rod is actuated; of the type in which, in the end actuation position of the control rod, a finger, which is slidingly fitted on the front end of the plunger, is biased by the plunger into contact with a reaction disk, integral with the moving piston, so as to transmit the reaction force of the moving piston to the plunger and to the control rod; and of the type comprising a unidirectional clutch device, which includes a coaxial sleeve, sliding on the plunger, and axial-locking means, which are capable of locking said sleeve in an end front axial position, in which a front end annular bearing surface of the sleeve locks the finger, independently of the plunger and of the control rod.
In a well-known manner, such a design is most suitable as regards safety, in the case of an emergency braking situation.
As a matter of fact, a conventional servomotor comprises neither a finger nor a unidirectional clutch device for the finger. Thus the plunger is likely to bias directly the reaction disk, integral with the rear face of the moving piston.
In a full-braking situation, in which case a maximum braking force is applied on the control rod, the actuation of the control rod causes the finger-forming plunger to be actuated, which fact results in the maximum opening of the three-way valve and, therefore, the rear chamber is subjected to the atmospheric pressure. Thus, the moving partition wall travels forward and the end of the plunger contacts the reaction disk, integral with the rear face of the moving piston.
Therefore, the force, which is applied onto the moving piston when the control rod reaches the end of its stroke, results from the assisting force, arising from the pressure difference between each side of the moving partition wall, and from the force exerted by the finger-forming plunger onto said moving piston. Besides, the driver feels the braking reaction force, which is transmitted from the moving piston to the plunger, through the reaction disk.
As a matter of fact, it has been established that quite a number of drivers, when confronted with an emergency braking situation, underestimated the risks actually incurred and, after having jammed the brakes on, would somewhat release the braking force at the very time when a considerable force should have been maintained in order to avoid an accident.
In the case of a full-braking situation, accompanied by the swift travel of the control rod, the plunger may touch the reaction disk and therefore give the driver the feeling of a maximum braking action even before the pressure difference between the front and rear chambers actually reaches its maximum value, which may lead the driver to release the braking force even though it should be maintained so as to profit by the maximum braking force.
A servomotor, like that of the above-described type, makes it possible to eliminate such a disadvantage.
As a matter of fact, since such a servomotor comprises a sleeve, which slides on the finger, when the axial-locking means fix the sleeve in an axial position, the finger is locked into contact with the reaction disk by means of the sleeve, which results in a maximum force being maintained on the rear face of the moving piston, even though the driver may have released the braking force in part.
Now, in such a servomotor, the sleeve is resiliently drawn back into contact with the plunger, through a spring, which is interposed between the moving piston and the sleeve.
Therefore, when the driver releases the braking force exerted on the brake pedal, the axial-locking means are released too, with the result that the sleeve is violently returned into abutment against a shoulder-forming face of the plunger, thus giving rise to a noise which is most unpleasant to the passengers.
In order to cope with said difficulty, the present invention provides sound-absorbing means for the return travel of the sleeve.
Therefore, it is the object of the present invention to provide a servomotor of the above-described type, characterised in that the unidirectional clutch device comprises damping means, made of an elastomeric material and interposed between a stop-forming transverse intermediate face of the plunger and an opposite transverse face of the sleeve, so as to permit, once the input force is released and when the locking means no longer lock the sleeve, the noiseless return of the sleeve to its rear rest position.
According to other features of this invention:
the sleeve has a tubular shape, with an inner diameter corresponding to the diameter of a front section of the plunger, the opposite transverse face of the sleeve being formed by the annular rear face of the sleeve;
the stop-forming intermediate face of the plunger consists of a transverse annular front face, borne by a shoulder-forming rear section of the plunger, whereas a rear face of said rear section bears the rear annular seat of the valve;
the elastomeric damping means comprise a ring-type joint, which is fastened on the annular front face of the shoulder-forming section of the plunger;
the ring-type joint comprises at least a rear annular section and a lip-forming truncated-cone-shaped front section, the taper of which is directed backwards, and which is intended to decelerate the sleeve in the course of its return to the rest position, before it actually bears against the rear annular section;
a rear face of the annular section of the ring-type joint is stuck to the annular front face of the shoulder-forming section of the plunger;
the axial-locking means for the sleeve comprise a substantially annular key, which surrounds the sleeve with a given clearance and which is capable of being driven by the moving piston, when the input force is applied at the determined speed, so as to rock about a generally transverse axis, in such a way that it may cooperate with the periphery of the sleeve and stop the latter;
the key comprises at least one branch, which is substantially radially directed and traversed by the sleeve, and having locking means intended to cooperate with the periphery of the sleeve so as to lock it;
the key is received within a cavity, traversing the piston perpendicularly to the axis of the latter, and in which it is capable of rocking to take its locking position for the sleeve, the key being resiliently returned by a compression spring, arranged between the front transverse wall of the cavity and a front face of an upper part of the branch, so as to resume its rest position, in which its branch rests, by its upper part, on the rear transverse wall of the cavity while, in the opposite direction, the lower part of the branch bears on a support integral with the casing of the servomotor.