GB2120333A

GB2120333A - Hydraulic brake booster

Info

Publication number: GB2120333A
Application number: GB08306714A
Authority: GB
Inventors: Helmut Steffes
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1982-05-14
Filing date: 1983-03-11
Publication date: 1983-11-30
Also published as: IT8321056A0; DE3218194C2; FR2526881B1; GB8306714D0; GB2120333B; IT1163349B; FR2526881A1; DE3218194A1

Abstract

A booster piston (8) is sealingly guided in a housing and defines a pressure chamber with its one end face. A pedal-actuatable brake valve controls the auxiliary pressure in dependence upon a pedal force and supplies it to the pressure chamber (12). To avoid the disadvantage that the brake valve must be held to extremely tight manufacturing tolerances since otherwise there occurs undesired great leakage at the brake valve, a pressure-reducing valve (37) is inserted between the auxiliary pressure source (35) and the booster piston (8) in such a manner that a piston (42) of the pressure-reducing valve (37), which piston controls an annular valve passage (48, 49), is acted upon by the auxiliary pressure via a smaller effective area in the opening direction of the valve passage, while it is acted upon by reduced pressure via a greater effective area in the closing direction. A further effective area of the piston (42) is subjected to the pressure in the pressure chamber (12) in such a way that, upon actuation of the hydraulic brake booster (6), a further force becomes effective at the pressure- reducing valve (37) in the opening direction. <IMAGE>

Description

SPECIFICATION Hydraulic brake booster The present invention relates to a hydraulic brake booster, in particular for the actuation of the master brake cylinder in a hydraulic brake system for an automotive vehicle, of the kind wherein in a housing bore a booster piston is sealingly guided which defines with one end face a pressure chamber, and wherein a pedal-actuatable brake valve is provided which determines an auxiliary pressure in dependence upon a pedal force and supplies it to the pressure chamber, the pressure chamber being in communication with an unpressurised reservoir in the brake's release position.

A hydraulic brake booster of the above kind is known, for instance, from the "ATE-Brake Handbook", 6th edition 1979, Bartsch publishing house, pages 98 to 104. The brake booster described therein is the core of a brake power boosting unit and comprises a housing which contains a cylinder bore in which a booster piston is sealingly guided. This booster piston includes at its peripheral surface a circumferential groove which latter is constantly acted upon by an accumulator pressure. High-pressure seals are disposed on both sides of this circumferential groove. An end face of the booster piston defines a pressure chamber projecting into which is a pedal-actuatable piston rod.Connected to the pedal-actuatable piston rod is a control slider which, in dependence upon the force exerted on the piston rod, modulates the accumulator pressure in the circumferential groove and supplies it to the pressure chamber. The control siider is guided in an axial bore of the booster piston.

In the brake's release position, the pressure chamber of the hydraulic power booster is connected to an unpressurised reservoir, the system assuming its inactive position as a result.

When an actuating force is transmitted via the piston rod onto the control slider, the latter is displaced in relation to the booster piston and first interrupts a connection between the pressure chamber and the unpressurised reservoir. With the control slider continuing to be displaced, control channels in the control slider and in the booster piston overlap each other so that a pressure corresponding to the pedal force reaches the pressure chamber. As soon as the pressure in the pressure chamber has exceeded a predetermined amount at which the friction forces of the annular seals sealing the booster piston are overcome, the booster piston will commence movement in the direction of the master cylinder thus causing an actuation of the wheel brakes.It has to be taken into consideration that the friction forces acting on the booster piston are not inconsiderable, since the high accumulator pressure is constantly prevailing in the circumferential groove of the booster piston. Moreover, it will appear that the booster piston is driven forwardly relatively abruptly upon attainment of the minimum pressure of response in the pressure chamber, since the static friction of the annular seals sealing the booster piston changes into a lower sliding friction. Although the control slider is fitted into the booster piston to meet the closest tolerances, there nevertheless occurs leakage which, as experience shows, is dependent upon the magnitude of the pressure of the high-pressure source available.

A hydraulic booster of the kind initially referred to is furthermore known from German printed and examined patent application 1 9 07 104. The device described therein is substantially of the design explained above and its object is in addition to achieve a continuous forward movement of the booster piston upon actuation of the control slider.

To accomplish.this aim, the device described in DE-AS 1 9 07 104 proposes to arrange for a positive, overlapping engagement after separation of the pressure chamber from the unpressurised reservoir which overlapping, upon further brake actuation, results in an increase in static pressure which is sufficient to overcome the friction forces of the annular seals sealing the booster piston.

It has to be regarded as a disadvantage in the device just described that manufacturing tolerances have to be met extremely precisely with respect to the positive overlapping engagement in order to achieve the effect desired.

This entails correspondingly high manufacturing expenses. Besides, there is the further disadvantage in this hydraulic power brake booster that the hydraulic auxiliary pressure is constantly prevailing in the circumferential groove of the booster piston and at the control slider of the brake valve, respectively. That is to say, the problem of leakage has not been solved by any means. This has to be considered in connection with the fact that in up-to-date vehicular brake systems with hydraulic power boosting, there appears a tendency to use still higher pressures in the auxiliary energy source.

In contrast thereto, it is an object of the present invention to improve upon a hydraulic power booster of the kind initially referred to such that there results a smooth response upon application of the brake and in that leakage is reduced to a minimum. Apart therefrom, the power booster is desired to be of straightforward design and reliable in operation.

According to the invention in its broadest aspect, a hydraulic brake booster of the kind referred to is characterised in that a pressurereducing valve is arranged between the auxiliary pressure source and the booster piston, in that a piston, controlling a valve passage, of the pressure-reducing valve is acted on via a small effective area by the auxiliary pressure in the opening direction of the valve passage and via a larger effective area by the reduced pressure in the closing direction, and in that a further effective area of the piston is subjected to the pressure in the pressure chamber such as to generate a further force in the opening direction.

From this arrangement it results in' a favourable manner that the annular seals sealing the booster piston will only be acted upon by a comparatively low pressure during operation so that the friction forces taking effect on the booster piston are correspondingly small. A correspondingly smooth response of the hydraulic brake booster will thereby be accomplished. Moreover, the leakage at the control slider will be reduced to an inevitable minimum amount. In addition, a like arrangement allows the control slider of the brake valve to meet less close tolerances: in certain circumstances, the control slider may even be provided with rubber seals which results in a considerable cost reduction while affording a minimum leakage.

A favourable improvement will be furthermore achieved if a stepped piston is slidably guided in the housing bore of the pressure-reducing valve and a circumferential annular chamber which communicates with the pressure chamber provided between the larger and the smaller diameter of the stepped piston. This embodiment has as a consequence that the booster piston is acted on by a comparatively low pressure during operation of the brake system. The pressure force prevailing in the pressure chamber of the hydraulic power booster which is a standard for the intensity of braking will be made use of to adapt the pressure at the booster piston to the pressure requirement in the pressure chamber. The effective areas in the circumferential annular chamber may be dimensioned as desired.

If there is arranged at the end face of the smaller-diameter stepped piston a valve closure member which, in cooperation with a valve seat, controls the communication between the auxiliary pressure source and the booster piston, there will be realized a constructively simple valve between the auxiliary pressure source and the booster piston. To avoid undesired leakage at the pressure-reducing valve, annular seals will be arranged on both sides of the circumferential annular chamber. These annular seals do not set up substantial friction forces against the actuating pressures so that the pressure-reducing valve usually is quickly responsive and reacts immediately to marked variations in pedal force.

Alternatively, the control piston can be guided in the housing bore of the pressure-reducing valve in a self-sealing manner which results in a further simplification of the device. The annular seals will be made economically, resulting, on the one hand, in a simplification in manufacture and, on the other hand, in a reduction of the number of parts to be fitted.

In another advantageous improvement of the invention, a pressure-relief valve is provided which is acted upon by the pressure in the pressure chamber, on the one hand, and by the pressure supplied to the booster piston, on the other hand, and which opens towards the pressure chamber if the differential pressure exceeds the opening pressure. The employment of a like pressure-relief valve is expedient in particular in those operating conditions in which the hydraulic brake booster is supported by the maximum auxiliary energy and in which the pedal force is subsequently reduced relatively quickly. Without the pressure-relief valve, it could be the case that the pressure of the auxiliary energy source prevails in its full amount between the annular seals of the booster piston.

The pressure on the booster piston will be influenced by the arrangement described hereinabove such that it is never of an amount higher than the pressure in the pressure chamber plus the switching pressure of the pressure-relief valve. That means that the restoring springs in the power brake booster may be of weaker design which results in another reduction of the actuating force, since the booster piston starts to move at a still lower pressure in the pressure chamber when the pressure-relief valve is utilised.

Preferably, the pressure-reducing valve and the pressure-reiief valve are combined to form a one construction unit and are integrated in the booster housing. A compact unit will ensue therefrom which affords ease of assembly in the automotive vehicle, while the mounting effort is considerably reduced in contrast to separately designed unit assemblies.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Fig. 1 and Fig. 3 show a diagrammatic view of two hydraulic brake systems and Fig. 2 and Fig. 4 show pressure diagrams.

Parts which correspond to each other have been assigned like reference numerals.

Fig. 1 illustrates a hydraulic dual-circuit brake system with a tandem master cylinder 1. Brake circuits 4, 5 branch off from the working chambers 2, 3 of the tandem master cylinder 1 to lead to the wheel brake cylinders of an automotive vehicle.

The tandem master cylinder 1 is actuatable by a hydraulic brake booster 6 which, in turn, consists of a booster piston 8 guided in a housing bore 7.

At a portion of its peripheral surface, the booster piston 8 contains a radial groove 9 which forms together with the housing 10 a circumferential annular chamber 11. An end face of the booster piston 8, which end face is on the right hand when viewing the drawing, defines a pressure chamber 12.

Furthermore, the booster piston 8 contains an axial bore 1 3 having guided in it a control slider 14 in a self-sealing fashion. The control slider 14 communicates via an actuating piston 1 5 with a brake pedal 1 6 and has an axial channel 17 as well as two radial channels 18, 1 9. The booster piston 8 is, in addition, provided with a radial channel 20 which latter extends radially from the internal circumferential annular chamber 11 to the axial bore of the booster piston 8. Another pressure-fluid channel 21 contained in the booster piston 8 and terminating in the axial bore 1 3 is in communication with an unpressurised reservoir 22.

On the one hand, the unpressurised reservoir 22 communicates with the supply bore 23, 24 of the tandem master cylinder while, on the other hand, it acts as a reservoir for a pressure-fluid pump 26 driven by a motor 25. For this purpose, a pressure-fluid line 27 leads from the unpressurised reservoir 22 via a filter element 28 to the pressure-fluid pump 26. In addition, the unpressurised reservoir 22 is in connection with a chamber 29 which latter is defined basically by the left-hand surface of the booster piston 8 (as seen in the drawing), and by the master cylinder piston 30 assigned to the first bral < e circuit. A compression spring 31 is arranged in this chamber 29 with a view to resetting the booster piston 8.

The outlet of the pressure-fluid pump 26 is connected to a pressure accumulator 35 via a check valve 32 and pressure-fluid lines 33, 34.

Connected to the pressure accumulator 35 via another pressure line 36 is a pressure-reducing valve 37 which, via a pressure-fluid line 38, has a connection to the circumferential annular chamber 11 of the hydraulic power booster 6 and, in addiction, is connected to the pressure chamber 1 2 of the hydraulic power booster via a pressure-fluid line 39.

The pressure-reducing valve 37 comprises a housing 40 which is provided with a stepped bore 41. A stepped piston 42 is axially slidably guided in the stepped bore 41. The smaller-diameter piston portion 43 forms with parts of the housing 40 a chamber 44, while the end face of the iarger- diameter piston portion 45 defines a chamber 46.

The chambers 44, 46 are interconnected by a pressure-fluid line 47. At the smaller-diameter piston portion 43 is a valve closure member 48 which, in cooperation with a valve seat 49, is able to cause interruption of the pressure lines 36, 38.

The stepped piston 42 has furthermore a radial circumferential groove 50 which together with parts of the housing 40 forms an annular chamber 51 which latter overlaps the step in the stepped bore 41 and communicates via the pressure-fluid line 39 with the pressure chamber 12 of the hydraulic power booster 6. The stepped piston 42 carries annular seals 52, 53 on both sides of the radial circumferential groove 50.

The mode of operation of the brake system illustrated in Fig. 1 will now be described in more detail the starting point of all considerations being the brake's release condition in which no force is exerted on the brake pedal 1 6. It will be assumed furthermore that the pressure accumulator 35 is first unpressurised, as may be the case after a relatively long period of standstill of the automotive vehicle, for instance. Under these conditions, the booster piston 8 is placed in an end position on the right side, when viewing the drawing, due to the force of the restoring spring 31. The stepped piston 42 having no spring force acting on it is in an undefined axial position in the stepped bore 41.When putting the automotive vehicle into operation, the pump 25 starts, thus causing the pressure-fluid pump 26 that is mechanically coupled to the motor 25 to suck pressure fluid from the unpressurised reservoir 22 and to supply it via the check valve 32 to the pressure accumulator 35. In addition, the chambers 44, 46 are exposed to the pressure of the pressure accumulator. This causes displacement of the stepped piston 42 into a lefthand end position, as seen in the drawing.Upon attainment of the prescribed maximum pressure (for example 1 80 bar) in the pressure accumulator 35, the pressure in the pressure chamber 44 or in the circumferential annular chamber 11 of the hydraulic power booster 6, respectively, will amount to 35 bar, for instance, when the diameter of the stepped piston 42 is suitably dimensioned, while the valve closure member 48 bears against the valve seat 49 and separates the pressure-fluid lines 36, 38 one from the other. Due to its connection to the unpressurised reservoir 22, the pressure chamber of the hydraulic power booster or of the circumferential annular chamber 51 of the pressure-reducing valve 37, respectively, is unpressurised.

With an actuating force acting on the brake pedal 60, the control piston 1 5 with the control slider moves to the left in relation to the booster piston 8 thus first causing closure of the pressurefluid channel 21 which is in communication with the unpressurised reservoir 22. Upon continued movement of the control slider 14, the radial channel 1 8 in the control slider will be moved into overlapping engagement with the radial channel 20 in the booster piston 8; pressure fluid out of the circumferential annular chamber 11 flows via the radial channel 18, the axial channel 17 and the radial channel 19 into the pressure chamber of the hydraulic brake booster 6, while thecircumferential annular chamber 51 of the pressure-reducing valve 37 will be pressurised via the pressure-fluid line 39.By this pressurisation of the circumferential annular chamber 51, a force will be exerted on the stepped piston 42 which corresponds to the product of meternd pressure force and the surfaces' difference between smaller.

piston portion 40 and larger piston portion 45.

This force acts on the valve 48, 49 of the pressure-reducing valve in the opening direction.

As a result of the pressure introduced into the pressure chamber 12, the booster piston will be displaced to the left, as seen in the drawing, which results in a change in the position of booster piston 8 relative to control slider 14, whereby a specific breathing position will be obtained in which the pressure-fluid channel 21 leading to the unpressurised reservoir 22 is closed and the radial channel of the control slider 14 is moved to a minimal extent into overlapping engagement with the radial channel 20 of the booster piston.

Corresponding to the movement of the booster piston of the hydraulic brake booster 6, the master cylinder pistons 30, 54 of the tandem master cylinder will also be displaced to the left, as seen in the drawing, as a result whereof the working chambers 2, 3 will decrease and a corresponding braking pressure will be generated in the brake circuits 4,5 and in the wheel brakes connected to the brake circuits, respectively.

As a result of the circumferential annular chamber 51 being pressurised by the pressure prevailing in the pressure chamber of the hydraulic brake booster 6, a new condition of equilibrium will occur at the pressure-reducing valve 37 in which condition the pressure in the pressure-fluid line 38 and in the circumferential annular chamber 11, respectively, will be increased in dependence upon the pressure in the pressure chamber.

Further displacement of the control slider 1 4 to the left, as seen in the drawing, will again disturb this condition of equilibrium, the pressure in the circumferential annular chamber 11 being increased in each case. With the support of the maximum auxiliary force of the hydraulic power booster 6, the valve 48, 49 of the pressurereducing valve 37 will be opened completely so that like pressures prevail in the chambers 44, 51, 46, 11 and 12. Upon release of the brake pedal 1 6, all movable parts of the brake system will return to their positions shown in Fig. 1.

In the event of failure of the pressure in the pressure accumulator 35, the brake circuits 4, 5 can be supplied with pressure without difficulty, though considerably increased force has to be applied on the pedal. In this case of interference, the control piston 1 5 together with the control slider 14 will be displaced in the axial bore 13 of the booster piston until it abuts on the bottom of the axial bore. Upon further increase of the booster force at the brake pedal, the booster piston 8 and the master cylinder pistons 30, 54 of the tandem master cylinder 1 will now be displaced to the left, as seen in the drawing, causing the brake circuits 4, 5 to be subjected to pressure purely mechanically.In this arrangement, the chambers 44, 51,46 of the pressure-reducing valve 37 are unpressurised so that the stepped piston 42 assumes any position whatsoever in the stepped bore 41 so that the emergency operation of the brake system is not impaired.

Fig. 2 shows the pressure distribution in the chambers 44, 51 and in the chambers 11, 12, respectively. It will be assumed in this pressure diagram that the pressure accumulator 35 is charged to its maximum value, for instance 1 80 bar. The effective areas of the pressurereducing valve 37 are so dimensioned that in the inactive position (brake's release position), i.e.

with the pressure chamber 12 unpressurised, a pressure of 35 bar will be present in the circumferential annular chamber 11. As soon as pressure fluid flows into the pressure chamber 12 of the hydraulic brake booster 6, the pressure in the circumferential annular chamber 51 of the pressure-reducing valve 37 will rise, which has as a consequence that the pressure in the circumferential annular chamber 11 of the hydraulic power booster 6 is constantly adapted to the pressure in the pressure chamber 1 2 until there will be equality of pressure in the circumferential annular chamber 11 and in the pressure chamber 12 upon attainment of the brakes' fully applied position.In the pressure diagram illustrated, the pressure in the chamber 44 of the pressure-reducing valve 37 and in the circumferential annular chamber 11 of the hydraulic brake booster 6, respectively, is designated by pt, while the pressure in the chamber 51 and in the pressure chamber 12, respectively, is designated by p2. The characteristic curve drawn in the pressure diagram illustrates the state of affairs described hereinabove.

In the hydraulic brake system shown in Fig. 1, it may occur in the event of an abrupt change from the brakes' fully applied position to the brakes' release position that the entire pressure of the pressure accumulator 35 (180 bar) prevails in the circumferential annular chamber 11, while the pressure chamber of the hydraulic brake booster 6 is already connected to the unpressurised reservoir 22. This high difference in pressure between the circumferential annular chamber 11 and the adjacent chamber 12, 29 produces likewise considerable friction forces at the booster piston 8 which have to be compensated by correspondingly strongly dimensioned restoring springs 31.To avoid this high pressure difference, according to Fig. 3 a hydraulic brake system is so designed as to interpose a pressure-limiting valve 55 between the circumferential annular chamber 11 and the pressure chamber 12 of the hydraulic brake booster 6. Since the embodiment of Fig. 3, with the exception of the pressure-limiting valve 55, does not differ from that illustrated in Fig. 1, its design will not be dealt with in detail herein.

The pressure-limiting valve 55 is adapted to open from the internal circumferential annular chamber 11 towards the pressure chamber 1 2 of the hydraulic power booster 6, if the pressure in the internal circumferential chamber 11 is higher by the opening pressure of the pressure-limiting valve. Below this differential pressure and in the event of pressure equality at the pressure-limiting valve 55, respectively, the pressure-limiting valve 55 has a pure blocking function.

As a starting point for the description of the mode of operation of the brake system will be taken the brakes' fully applied position described in connection with Fig. 1 in which position the entire pressure of the pressure accumulator 35 prevails in the circumferential annular chamber 11 as well as in the pressure chamber 12 of the hydraulic power booster. When the brake pedal 1 6 is moved abruptly from this operating position into the brakes' release position, the pressure in the pressure chamber 1 2 will fall relatively swiftly compared with the pressure in the circumferential annular chamber 11. However, on account of the pressure-limiting valve 55, the differential pressure Ap between the chambers 11, 1 2, can at the most adopt a value which corresponds to the opening pressure of the pressure-limiting valve 55. If, for example, the opening pressure of the pressure-limiting valve is set to p = 50 bar, the differential pressure between the chambers 11, 1 2 is able to adopt this value at the most; the result thereof being that lower friction forces oppose the backward movement of the control piston 8 so that the compression spring 31 in the chamber 29 is enabled to be dimensioned correspondingly weaker.

Finally, Fig. 4 illustrates the connection between the pressure in the circumferential annular chamber 11 and the pressure chamber of the hydraulic power booster 6. Also in this case it will be assumed that the highest pressure able to be produced in the pressure accumulator 35 amounts to 1 80 bar. The opening pressure of the pressure-limiting valve 55 be set to p = 50 bar. It can be seen clearly from the characteristic curve that the differential pressure referred to between the chambers 11, 12 may adopt this pressure of 50 bar at the most.

Claims

1. A hydraulic brake booster, in particular for the actuation of a master brake cylinder in a hydraulic brake system for an automotive vehicle, of the kind wherein in a housing bore a booster piston is sealingly guided which defines with one end face a pressure chamber, and wherein a pedal-actuatable brake valve is provided which determines an auxiliary pressure in dependence upon a pedal force and supplies it to the pressure chamber, the pressure chamber being in communication with an unpressurised reservoir in the brake's release position, characterised in that a pressure-reducing valve (37) is arranged between the auxiliary pressure source (35) and the booster piston (8), in that a piston of the pressure-reducing valve, said piston controlling a valve passage, is acted on via a smaller effective area by the auxiliary pressure in the opening direction of the valve passage and via a larger effective area by the reduced pressure in the closing direction, and in that a further effective area of the piston is subjected to the pressure in the pressure chamber such as to generate a further force in the opening direction.

2. A hydraulic brake booster as claimed in claim 1, characterised in that a stepped piston (42) is guided slidably in a housing bore -(41) of the pressure-reducing valve (37), and in that arranged between the larger piston portion (45) and the smaller piston portion (43) of the stepped piston (42) is a circumferential annular chamber (51) which communicates with the pressure chamber (12) of the hydraulic brake booster (6).

3. A hydraulic brake booster as claimed in claim 2, characterised in that arranged at the end face of the smaller-diameter stepped piston (42) is a valve closure member (48) which, in cooperation with a valve seat (49), governs the connection (36, 38) between the auxiliary pressure source (35) and the booster piston (8).

4. A hydraulic brake booster as claimed in any one of the preceding claims, characterised in that annular seals (52, 53) are arranged on both sides of the circumferential annular chamber (51).

5. A hydraulic brake booster as claimed in claim 4, characterised in that the stepped piston (42) is guided in a self-sealing manner in the housing bore (41) of the pressure-reducing valve (37).

6. A hydraulic brake booster as claimed in claim 1, characterised in that there is provided a pressure-relief valve (55) which, on the one hand, is acted upon by the pressure in the pressure chamber (12) and, on the other hand, by the pressure supplied to the booster piston (8), and which opens towards the pressure chamber (12) in the event of the differential pressure (Ap) exceeding the opening pressure.

7. A hydraulic brake booster as claimed in claim 6, characterised in that the opening pressure of the pressure-relief valve (55) is higher than the differential pressure (Ap).

8. A hydraulic brake booster as claimed.in claim 6 or 7 characterised in that the pressure-reducing valve (37) and the pressure-relief valve (55) are combined to form a construction unit and are integrated in the booster housing (10).

9. A hydraulic brake booster substantially as described with reference to the accompanying drawings.