GB2162604A - Braking pressure generator for a hydraulic brake system - Google Patents

Abstract

A braking pressure generator (1, 21) for a hydraulic vehicle brake system, wherein a booster piston (15) slidable by hydraulic pressure is displaceable in the actuating direction, wherein the booster piston (15) is designed as a stepped piston with a smaller-diameter portion (16) close to the pedal, wherein the booster piston (15) includes a blind-end bore remote from the pedal in which a master cylinder piston (43) is sealedly guided, wherein the smaller-diameter portion (16) of the booster piston (15) confines an annular chamber (18) diminishing in volume upon brake application, and wherein there is connected to the annular chamber (18) a valve (30) controllable by the pressure in the pressure chamber (14) of the hydraulic power booster, said valve permitting closure of a connection between the annular chamber (18) and an unpressurized supply reservoir (9). A direct connection can be established between the annular chamber (18) and the working chamber (22) via the valve (30) and by circumventing a sealing collar. <IMAGE>

Description

SPECIFICATION Braking pressure generator for a hydraulic brake system The present invention relates to a braking pressure generator, for a hydraulic vehicle brake system, wherein a booster piston slidable by hydraulic auxiliary pressure is displaceable in the actuating direction, wherein the booster piston is designed as a stepped piston with a smaller- diameter portion close to an actuating pedal, wherein the booster piston includes a blind-end bore remote from the pedal in which a master cylinder piston is sealedly guided, wherein said smaller-diameter portion of the booster piston confines an annular chamber diminishing in volume upon brake application, and wherein there is connected to the annular chamber a valve controllable by the pressure in a booster chamber of the hydraulic power booster, said valve permitting closure of a connection between the annular chamber and an unpressurised supply reservoir.

A braking pressure generator having the abovementioned features is described in West German patent application P 34 08 873.3 (UK Application No. 8504647). The braking pressure generator according to that application comprises a pedal-actuatable piston rod arranged coaxially to a booster piston. Parallel to the booster piston a so-called brake valve is disposed which is connected via a lever arrangement with the pedal-actuatable push rod. The face of the booster piston close to the pedal limits a pressure (booster) chamber which, depending on the position of the brake valve, is connected with an unpressurised supply reservoir or, alternatively, with an auxiliary energy source.

The booster piston is designed substantially as a stepped piston with an annular chamber being created between the piston step with the smaller diameter and the housing of the braking pressure generator, the volume of said annular chamber diminishing upon brake application. The annular chamber normally (with the brake in released condition and the auxiliary pressure source not intact) communicates via a two-way/two-position directional control valve with an unpressurised supply reservoir. The two way/two-position directional control valve can be controlled hydraulically, while the pressure from the pressure chamber of the hydraulic power booster can be fed to the hydraulic control drive.

If an actuating force is applied to the brake pedal in the brake system described, the piston rod connected to the brake pedal is displaced relative to the booster piston so as to actuate the lever mechanism, which at first causes a hydraulic connection between the pressure chamber of the hydraulic power booster and the unpressurised supply reservoir to be interrupted. In case of the force at the brake pedal being increased, the brake valve eventually adopts a position in which the pressure chamber of the hydraulic power booster is supplied with hydraulic pressure from an auxiliary pressure source.The pressure now prevailing in the pressure chamber of the hydraulic power booster, upon having exceeded a predeterminable level, is finally capable of displacing the booster piston in actuating direction, which in turn causes the tandem master cylinder connected downstream of the hydraulic power booster to be pressurized.

Thus, actuation of the brake circuits connected to the tandem master cylinder is effected.

As already explained, during a like brake actuation the volume of the annular chamber between the booster piston and the housing of the braking pressure generator diminishes. The pressure now prevailing in the pressure chamber of the hydraulic power booster, apart from displacing the booster piston, also causes the two-way/two-position directional control valve connected to the annular chamber to be switched into a blocking position so that the annular chamber is separated from the unpressurised supply reservoir.The volume of pressure fluid enclosed in the annular chamber in this operating condition is additionally supplied, upon further displacement of the booster piston, via respective sealing collars to the working chamber of the master cylinder which is connected downstream of the hydraulic power booster, in such a way as to result in a relative displacement between the booster piston and the master cylinder piston, with the master cylinder piston travelling a longer way than the booster piston.

As everybody knows, no auxiliary pressure can be built up in the pressure chamber of the hydraulic power booster in case of a failure of the auxiliary pressure source. Accordingly, the two wayitwo-position directional control valve remains in its idle position in which the annular chamber is permanently connected with the unpressurised supply reservoir. In case of a displacement of the booster piston, which is now effected exclusively by mechanical force acting on the brake pedal, the volume in the annular chamber can relieve pressure to the unpressurised supply reservoir, without any additional forces counteracting the brake application.

In a like fail condition, therefore, only the considerably smaller area of the master cylinder piston is effective, although it is ensured by appropriate dimensioning that the required minimum braking effect is achieved.

It must be regarded as less advantageous in the brake system described that when the brake is applied while the auxiliary pressure source is intact, the volume enclosed in the annular chamber is supplied via an additional sealing collar into the working chamber of the master cylinder.

Therefore, it is an object of the present invention to simplify the design of a braking pressure generator of the initially mentioned type.

According to the present invention there is provided a braking pressure generator, for a hydraulic brake system for automotive vehicles, wherein a booster piston slidable by hydraulic auxiliary pressure is displaceable in the actuating direction, wherein the booster piston is designed as a stepped piston with a smaller-diameter portion close to an actuating pedal, wherein the booster piston includes a blind-end bore remote from the pedal in which a master cylinder piston is sealedly guided, wherein said smaller-diameter portion of the booster piston confines an annular chamber diminishing in volume upon brake application, and wherein there is connected to the annular chamber a valve controllable by the pressure in a booster chamber of the hydraulic power booster, said valve permitting closure of a connection between the annular chamber and an unpressurised supply reservoir, characterised in that a direct connection can be established between the annular chamber and the working chamber via the valve and by circumventing a sealing collar.

It is further advantageous that a closing element normally sitting close to a valve seat of the valve in the connection between the working chamber and the annular chamber can be switched into an opening position of the valve gate not only by the pressure prevailing in the pressure chamber, but also by the pressure from the master cylinder. An advantageous embodiment of the valve provides that the closing element is disposed in a piston element displaceable by the pressure from the master cylinder and is preloaded by a spring against a matching valve seat provided at the piston element.On the other hand, the closing element is arranged in such a way that it can be lifted from its seat by a piston pressurisable by the pressure prevailing in the booster chamber, said piston being disposed substantially coaxially to the piston element and a::ially slidable. With regard to a simple design it is proposed that the piston element is provided with a valve seat at its end facing away from the pressurised surface. In order to separate the annular chamber from the unpressurised supply reservoir, it is proposed that, coaxially to the piston element and the piston, an annular piston is displaceably arranged which is designed as a closing element and normally kept at a distance to the piston element by a shoulder of the piston.When the brake is in released position, the piston element and the piston are disposed so as to abut to opposite housing faces.

The present invention further provides that the travel of the piston is limited b r a stop in the housing in such a way that the closing element will remain in closing position in any case as long as the piston element remains in its idle position. In addition, the design of the annular piston is such that it can be made to abut the valve seat of the piston element before the piston has performed its maximum travel. The result thereof is that in an advantageous manner the annular chamber is separated in any case from the unpressurised supply reservoir before the piston has performed its maximum travel. Thus it is ensured that, in case of normal brake actuation and corresponding travel, pressure fluid volume will escape toward the unpressurised supply reservoir.

In a further embodiment of the subject-matter of the present invention it is provided that the piston element can travel in actuating direction a maximum distance which is less than the normal distance between the piston and the closing element disposed in the piston element. It is further provided that the normal distance between the piston and the closing element disposed in the piston element is less than the sum of: a) the maximum travelling distance between the piston in idle position and the stop associated to it, and b) the maximum travelling distance of the piston element.

Preferably a ball is used as closing element which is a cheap standard part. An improvement of the subject-matter of the present invention in addition provides that a first compression spring is inserted between the piston element and the piston and a second compression spring is inserted between the stop of the piston and the annular piston, with the first compression spring being stronger than the second compression spring. The first compression spring serves the purpose of establishing the idle position of the piston which is pressurisable by the pressure in the pressure chamber of the hydraulic power booster, while the second compression spring primarily serves the purpose of compensating the friction of the sealing disposed at the annular piston.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure I is a sectional view of the entire brake system, and Figure 2 is an enlarged view of a valve.

Identical parts have been assigned identical references.

Reference numeral 1 in Figure 1 designates a hydraulic power booster, the construction of which is described in detail in West German patent application P 34 08 873.3. The hydraulic power booster 1 can be actuated by a force applied to a brake pedal 2. The hydraulic power booster 1 is connected with an auxiliary pressure source 3 consisting essentially of a pressure accumulator 4 loaded by a pump 6 driven by an electric motor 5. The suction side 7 of the pump 6 communicates via a pressure medium filter 8 with an unpressurised supply reservoir 9. Connected to the pressure side 10 of the pressure medium pump 6 is a check valve 11 which prevents a return flow toward the pressure medium pump 6. Connected to the outlet of the check valve is a pressure relief valve 12 which, when the pressure exceeds a specified level, prevents further loading of the pressure accumulator 4 and carries away the volume delivered by the pressure medium pump towards the unpressurised supply reservoir 9.

The hydraulic power booster 1 essentially operates in such a way that, when a force is applied to the brake pedal 2, a piston rod 13 connected to the brake pedal 2 is displaced in the actuating direction, which causes a hydraulic pressure to be built up in the pressure chamber 14 of the hydraulic power booster 1 via a brake valve which is not illustrated in the drawing. The pressure chamber 14 of the hydraulic power booster 1 is limited by a booster piston 15, the booster piston 15 having a portion 16 with a smaller diameter and a portion 17 with a larger diameter. The smaller-diameter portion 16 of the booster piston 15 and the housing confine an annular chamber 18, the volume of which diminishes upon brake application due to two sealing collars 19, 20.

Downstream of the hydraulic power booster 1 a tandem master cylinder 21 is connected which comprises a working chamber 22 close to the pedal and a working chamber 23 remote from the pedal. To each working chamber a central valve 24, 25 is allocated which upon displacement of the booster piston 15 or a pressure build-up in the working chamber 22 reaches a closing position in which the working chambers 22, 23, which are normally in connection with the unpressurised supply reservoir 9, are hydraulically separated from the unpressurised supply reservoir 9. Wheel brakes 26, 27, 28, 29 are connected to the working chambers 22, 23 of the tandem master cylinder 21.

A further component of the brake system according to the present invention is a valve 30, wherein a piston 31 is pressurisable by the pressure prevailing from time to time in the pressure chamber 14 of the hydraulic power booster 1 and is displaceable toward the left as viewed in the drawing against a stop 32 (Figure 2).

By means of a first compression spring 33, the piston 31 is supported on a piston element 34, which piston element 34 is essentially bell-shaped and provided with a blind-end bore facing the piston 31. When the brake is in released position as shown in the drawing, the piston element 34 as well as the piston 31 abut the housing 35 of the valve. Inside the piston element 34 a valve gate 36 is provided which substantially consists of a valve seat 37 disposed at the piston element 34 so as to match a closing element 38, with the closing element 38 being preloaded by a compression spring 39 in the closing direction of the valve gate 36. A further valve seat 40 is provided at the opening of the piston element 34 facing the piston 31.

The drawings show that, coaxially to the piston 31, an annular piston 41 is disposed which, when the brake is in released position as shown in the drawings, is kept by a shoulder 44 of the piston 31 in a position in which a set distance is established between the annular piston 41 and the valve seat 40 of the piston element 34. The annular piston 41 is so designed as to form a further valve gate together with the valve seat 40 of the piston element 34. Between the annular piston 41 and the stop 32 a second compression spring 42 is inserted.

The essential point above the valve 30, of which Figure 2 is an enlarged view, is that the maximum travelling distance S1 of the piston 31 toward the stop 32 is shorter than the distance S3 between the piston 31 and the closing element 38. Furthermore, it is essential that the piston element 34 can travel a maximum distance S2 which is shorter than the distance between the piston 31 and the closing element 38. A further condition is that the sum of the distances S and S2 is greater than the distance S3.

The mode of operation of the brake system described is explained in more detail hereinafter, starting from the brake's released condition in which all movable parts of the brake system adopt the position shown in Figure 1 In a like operating condition the working chambers 22, 23 of the tandem master cylinder 21 communicate with the unpressurised supply reservoir 9. Furthermore, via the valve gate 40, 41 of the valve 30, there is a hydraulic connection between the annular chamber 18 of the hydraulic power booster 1 and the unpressurised supply reservoir 9. When a force is applied to the brake pedal 2, hydraulic pressure is built up - as already explained - in the pressure chamber 14 of the hydraulic power booster 1, said pressure acting on the piston 31 of the valve 30 and displacing it against the force of the first compression spring 33 toward the left as viewed in Figure 1.This displacing movement is transmitted by the action of the second compression spring to the annular piston 41, the annular piston 41 being pushed along by the compression spring 42. When the piston 31 is pressurised and the pressure in the booster chamber 14 of the hydraulic power booster has reached a certain level, the piston 31 will eventually have travelled the distance Si.

A pressure build-up in the booster chamber 14 of the hydraulic power booster 1, on the other hand, will cause the booster piston 15 to be displaced in actuating direction so that the central valves 24, 25 adopt a closing position and the working chambers 22, 23 of the tandem master cylinder 21 are separated from the unpressurised supply reservoir 9. A further displacement of the booster piston 15 or an increase of the actuating force F applied to the brake pedal 2 now causes a pressure build up in the working chambers 22, 23 of the tandem master cylinder 21 which propagates via corresponding pressure conduits to the piston element 34 and displaces the piston element 34 towards the right as viewed in the drawing, which finally causes the valve gate 40, 41 between the annular chamber 18 of the hydraulic power booster and the unpressurised supply reservoir to be closed.Upon further pressure increase in the working chambers 22, 23 of the tandem master cylinder, the closing element 38 will eventually push against the piston 31, which causes the valve gate 36 between the annular charnber 18 and the working chamber 22 to be opened. Upon continued displacement of the booster piston 15, the volume of pressure fluid escaping from the annular chamber 18 will now be fed via the valve gate 36 to the working chamber 22, thus starting a relative displacement between the master cylinder piston 43 and the booster piston 15, the master cylinder piston 43 moving faster than the booster piston 15. The great advantage of such an arrangement is that relatively short pedal travel is required to generate high master cylinder pressure.

In the event of a failure of the auxiliary pressure source 3 or an equivalent fail condition, the piston 31 is kept in abutment at the right end as viewed in the drawing by the action of the compression spring 33, since no pressure builds up in the pressure chamber 14 of the hydraulic power booster 1.

Upon brake actuation the booster piston 15 is displaced to the left as viewed in the drawing solely by the force applied to the brake pedal, while, when the piston has travelled a distance predetermined by the construction, the central valves 24, 25 in turn are closed so that the working chambers 22, 23 are separated from the unpressurised supply reservoir 9. The pressure thus built up in the working chambers 22, 23 of the tandem master cylinder 21 in turn reaches the pressurisation surface of the piston element 34 of the valve 30 and causes the piston element 34 to be displaced toward the right as viewed in the drawing.However, the travelling distance of the piston element 34 is limited in such a way that the valve gate 40, 41 remains open and the valve gate 36 remains in closed position so that the working chamber 22 still remains separated from the unpressurised supply reservoir 9.

In a like fail condition, the annular chamber 18 of the hydraulic power booster remains in permanent communication with the unpressurised supply reservoir 9 via the valve gate 40, 41 of the valve 30 so that a diminution of the annular chamber 18 cannot generate a force component counteracting the actuating force.

During the brake release operation in the types of brake actuation described hereinbefore (intact auxiliary pressure supplylfaiture of auxiliary pressure supply), the movements described are reversed until each of the movable parts has again adopted the position illustrated in Figure 1.

Claims (14)

1. A braking pressure generator, for a hydraulic brake system for automotive vehicles, wherein a booster piston slidable by hydraulic auxiliary pressure is displaceable in the actuating direction, wherein the booster piston is designed as a stepped piston with a smaller-diameter portion close to an actuating pedal, wherein the booster piston includes a blind-end bore remote from the pedal in which a master cylinder piston is sealedly guided, wherein said smaller-diameter portion of the booster piston confines an annular chamber diminishing in volume upon brake application, and wherein there is connected to the annular chamber a valve controllable by the pressure in a booster chamber of the hydraulic power booster, said valve permitting closure of a connection between the annular chamber and an unpressurised supply reservoir, characterised in that a direct connection can be established between the annular chamber (18) and the working chamber (22) via the valve (30) and by circumventing a sealing collar.

2. A braking pressure generator according to claim 1, characterised in that a closing element (38) normally sitting close to a valve seat (37) of the valve (30) in the connection between the working chamber (22) and the annular chamber (18) can be switched into an opening position of the valve gate (36) not only by the pressure prevailing in the pressure chamber (14) but also by the pressure from the master cylinder.

3. A braking pressure generator according to claim 1, characterised in that the closing element (38) is disposed in a piston element (34) displaceable by the pressure from the master cylinder and is preloaded by a spring against a matching valve seat (37) provided at the piston element (34).

4. A braking pressure generator according to claim 3, characterised in that the closing element (38) can be lifted from its seat (37) by a piston (31) pressurisable by the pressure prevailing in the booster chamber (14), said piston being disposed substantially coaxially to the piston element (34) and axially slidable.

5. A braking pressure generator according to claim 3 or claim 4, characterised in that the piston element (34) is provided, at its end facing away from the pressurised surface, with a valve seat (40).

6. A braking pressure generator according to claim 4, characterised in that, coaxially to the piston element (34) and the piston (31), an annular piston (41) is displaceably arranged which is designed as a closing element and is normally kept at a distance from the piston element (34) by a shoulder (44) of the piston (31).

7. A braking pressure generator according to claim 4 or claim 6, characterised in that, when the brake is in the released position, the piston element (34) and the piston (31) abut opposite housing faces.

8. A braking pressure generator according to claim 4, claim 6 or claim 7, characterised in that the travel of the piston (31) is limited by a stop (32) in the housing in such a way that the closing element (38) will remain in closing position in any case as long as the piston element (34) remains in its idle position.

9. A braking pressure generator according to claim 6, characterised in that the annular piston (41) can be made to abut to the valve seat (40) before the piston (31) has performed its maximum travel.

10. A braking pressure generator according to claim 6, characterised in that the piston element (34) can travel in the actuating direction a maximum distance which is less than the normal distance between the piston (31) and the closing element (38) disposed in the piston element (34).

11. A braking pressure generator according to claim 6, characterised in that the normal distance between the piston (31) and the closing element (38) disposed in the piston element (34) is less than the sum of: a) the maximum travelling distance between the piston (31) in idle position and the stop (32) associated to it, and b) the maximum travelling distance of the piston element (34).

12. A braking pressure generator according to any one of claims 3 to 11, characterised in that a ball is used as closing element (38).

13. A braking pressure generator according to claim 6, characterised in that a first compression spring (33) is inserted between the piston element (34) and the piston (31) and a second compression spring is inserted between the stop (32) of the piston (31) and the annular piston (41), with the first compression spring (33) being stronger than the second compression spring (42).

14. A braking pressure generator substantially as herein described with reference to the accompanying drawings.