GB2065810A

GB2065810A - Hydraulic Brake System

Info

Publication number: GB2065810A
Application number: GB8039767A
Authority: GB
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co OHG
Priority date: 1979-12-20
Filing date: 1980-12-11
Publication date: 1981-07-01
Also published as: GB2065810B; DE2951373C2; JPS5695749A; DE2951373A1; IT8026782A0; JPS6319381B2; IT1209291B; FR2471898A2; FR2471898B2

Abstract

A hydraulic brake system of the kind having a master cylinder, wheel cylinders and a brake slip control apparatus, with a valve device controlling fluid flow from a fluid source to the pressure chamber of the master cylinder, and with valves arranged to be actuated by the brake slip control apparatus and controlling, in the event of an imminent locked condition of a wheel, the pressure in the associated wheel cylinder independently of the pressure in the pressure chamber, the valve device being opened in dependence on a differential of pressure between the pressure in the pressure chamber and the pressure in the wheel cylinder associated with the pressure chamber, with the pressure in the wheel cylinder being required to be lower than the pressure in the pressure chamber by a predetermined amount. The valve device (106) interrupts in the open position a fluid connection (113) which is open when the valve device is closed, the fluid connection (113) connecting a chamber (35) bounded by a booster piston (2) of the brake system with an unpressurized fluid reservoir (17), and the booster piston (2) reducing the volume of the chamber (35) as it moves in the actuating direction (20). <IMAGE>

Description

SPECIFICATION Hydraulic Brake System This invention relates to a hydraulic brake system with master cylinder, wheel cylinders and a brake slip control apparatus, with a valve device controlling fluid flow from a fluid source to the pressure chamber of the master cylinder, and with valves arranged to be actuated by the brake slip control apparatus and controlling, in the event of an imminent locked condition of a wheel, the pressure in the associated wheel cylinder independently of the pressure in the pressure chamber, the valve device being opened in dependence on a difference between the pressure in the pressure chamber and the pressure in the wheel cylinder associated with the pressure chamber, the pressure in the wheel cylinder being required to be lower than the pressure in the pressure chamber by a predetermined amount.

A system has been proposed which provides for locking of the hydraulic booster piston only if the change-over pressure at which the static system is switched to the dynamic system is attained. Below this change-over point the booster piston will not be locked in position with the brake slip control apparatus in operation, so that even with additional fluid supply from the dynamic circuit into the static circuit, movement of the brake pedal through its full travel cannot be prevented.

It is an object of the present invention to simplify and improve upon such a known brake system so as to obtain a hydraulically locked condition of the booster piston also below the change-over point and to maintain the brake system checking capability prior to attainment of the change-over point.

According to the invention in its broadest aspect there is provided a hydraulic brake system with master cylinder, wheel cylinders and a brake slip control apparatus, with a valve device controlling fluid flow from a fluid source to the pressure chamber of the master cylinder, and with valves arranged to be actuated by the brake slip control apparatus and controlling, in the event of an imminent locked condition of a wheel, the pressure in the associated wheel cylinder independently of the pressure in the pressure chamber, the valve device being opened in dependence on a difference between the pressure in the pressure chamber and the pressure in the wheel cylinder associated with the pressure chamber, the pressure in the wheel cylinder being required to be lower than the pressure in the pressure chamber by a predetermined amount, characterized in that the valve device interrupts in its open position a fluid connection which is open when the valve device is closed, the fluid connection connecting a chamber bounded by the booster piston of the brake system with an unpressurized fluid reservoir, and the booster piston reducing the volume of the chamber as it moves in the actuating direction.

Because of the integration of another valve function in the valve device, the provision of a special valve for hydraulically locking the booster piston in position may be dispensed with. The diameter of the hydraulic brake booster thus remains unchanged. In a favourable design of the valve device, a stepped valve piston is movably arranged in a housing parallel to the master cylinder. The parallel arrangement results in a very low height of construction, and components can be saved by the provision of only one valve piston fulfilling several functions. Advantageously, the housing of the valve device is formed integrally with the master cylinder housing, thereby obviating the necessity to provide additional mounts.Further, this arrangement enables specific fluid connections between the valve device and the master cylinder to be designed as bores or channels in the housing so that the fluid lines otherwise needed can be saved. At the same time, potential sources of failure due to leaks are eliminated.

In order to provide several control chambers for the valve piston, the housing of the valve device is subdivided into several chambers arranged axially in series and having the valve piston extending through their partition walls in a sealed relationship thereto, with the valve piston extending into the first and last chamber of the arrangement. Thus, as a result of the effects of pressure in a chamber, the axial movement of the valve piston permits several functions to be executed in other chambers. The valve piston advantageously has a collar subdividing each chamber into an inlet chamber communicating with the pressure chamber and a control chamber communicating with the wheel cylinder.This simple design ensures that a check valve opening in the direction of the inlet chamber can be provided between inlet chamber and control chamber, the check valve being suitably formed by a bore in the collar whose opening terminating in the inlet chamber is covered by a seal sealing the collar in the housing. this design affords a simple, fluid-tight separation of a chamber into inlet and control chamber. In order to connect the inlet chambers with the fluid chamber, channels are advantageously provided in the valve piston which extend from the end mainly coaxially through the valve piston. This also enables the wheel cylinders associated with the master cylinder pressure chamber to be directly connected to the inlet chamber, this connection being interruptible by a valve.

In order to utilize the multiple valve function of the valve piston, an inlet or outlet opening adapted to be closed by the valve piston is designed as a valve seat in the first and, or, last one of the fluid-containing chambers arranged axially in series. The first chamber is connected to the pressure chamber and to the fluid flow from the fluid source, the valve piston closing the inlet opening of the pressure chamber by seating engagement with its valve seat. The most favorable spatial arrangement is obtained by connecting the last chamber to the chamber bounded by the booster piston and to the unpressurized fluid reservoir, the inlet of the chamber bounded by the booster piston being adapted to be closed by the end of the valve piston extending into the chamber.Because the last chamber is in the immediate proximity of the hydraulic booster and the first chamber is in the immediate proximity of the pressure chamber, part of the connections can be provided by bores in the housing, and the length of the remaining fluid lines required is as short as possible.

If a valve is designed such that the valve piston acts as a slide valve closing the radial inlet of the chamber bounded by the booster piston by moving axially, the relatively high pressure building up in the chamber with continued actuation will not be able to act on the valve piston in such a way as to have any effect on the valve function. Because the forces effecting the valve function act only axially and the valve piston closes the radial inlet by moving axially, the pressure developing in the inlet opening will be allowed to act on the piston only radialiy. A safe valve function is thereby ensured.

Because the valve device's first chamber which is connected to the fluid source is bounded by a piston having the valve piston extending therethrough axially movably and having acting on it a spring in opposition to the pressure of the fluid source, the valve piston's end lying in the first chamber being slightly larger in diameter than the valve piston's section extending through the piston, an emergency device is provided which, in the event of a failure of the hydraulic booster circuit, exerts an additional force on the valve piston in the closing direction. If the dynamic circuit fails, the spring force will urge the piston into abutment with the larger, stepped end section of the valve piston, thereby transmitting an additional force to the valve piston.

Advantageously, a stop is provided in the first chamber for abutting engagement with the piston acted upon by the pressure of the fluid source. In this manner, damage to the spring is avoided.

If the chamber isolated by the piston and having the valve piston extending therethrough is connected to atmosphere, it may accommodate the spring engaged between the piston and the subsequent partition wall, while it fulfils at the same time the function of a leakage chamber. In the event of leakages of the pressure chamber bounded by the piston, fluid will escape to the opening to atmosphere, and seal leakage will become visible to the eye.

If each wheel cylinder allocated to the master cylinder is assigned an inlet chamber and a control chamber which are provided between the first and the last chamber of the valve device, any number of wheel cylinders allocated to the master cylinder are pressure-controllable independently of each other.

In another embodiment of a valve, the valve piston carries a collar which is designed as a valve cone and urged into sealing engagement with a valve seat to interrupt the connection of the fluid flow to the master cylinder pressure chamber.

Such an arrangement permits a more simple, pressure-balanced valve, enabling the effective surfaces acted upon by pressure to be determined with greater accuracy. If the valve cone is pressure-balanced, it may be ieft out of account completely for the calculation of the change-over point, for instance.

By providing for the one end of the valve piston to slide in the housing in a fluid-sealed relationship thereto and to bound a chamber pressurized by the master cylinder pressure, it is possible to calculate and execute a changeover point simply and accurateiy by dimensioning its effective diameter.

Another possibility of supplying fluid to the pressure chamber of the master cylinder is provided by directing the fluid through a chamber arranged in the circumference of the master cylinder piston and connected to the pressure chamber via a check valve opening in the direction of the pressure chamber. Such an arrangement ensures a smooth adaptation of the pressure in the pressure chamber to the pressure of the dynamic brake circuit. In the inactive position of the master cylinder piston, the chamber is advantageously connected to the unpressurized fluid reservoir via a valve to be actuated by the master cylinder piston. This ensures that, even in the cut-off state of the fluid connection, the cut-off chambers are fully charged with fluid. In a simple embodiment, the valve is a tip-change valve acted upon in the closing direction by a spring.

In an embodiment incorporating a tandem master cylinder, each pressure chamber may be assigned a self-contained valve device of its own, the arrangement of the one valve device which interrupts the fluid connection acting in parallel to the arrangement of the other valve device. It is thereby achieved that only the master cylinder chamber concerned is connected to the dynamic circuit and that the booster piston is hydraulically locked in position only if both master cylinder chambers are connected to the dynamic circuit.

Advantageously, the spring which determines the pressure at which change-over to the dynamic system of the booster takes place and keeps the valve cone in seating engagement with the valve seat in the first chamber, is situated in a control chamber so that the spring is engaged between the collar of the valve piston and the subsequent partition wall towards the next chamber. Here, a helical spring could be used which when compressed requires only little space. If the spring is arranged in a control chamber in this manner, its end located in the last chamber may at the same time be designed such that the end of the valve piston and the wall opposite this end are relatively spaced so as to limit the valve travel.

Another advantageous form of accommodation of the spring determining the change-over pressure is obtained by arranging the spring coaxially in a bore in an end of the valve piston, the spring bearing against the wall opposite the bore, thereby acting upon the valve piston in the closing direction.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a hydraulic brake system with master cylinder and hydraulic booster as well as a valve device arranged parallel thereto; Figure 2 is a hydraulic brake system similar to that of Figure 1, with tandem master cylinder and two valve devices operating independently of each other; Figure 3 is a graph showing the pressure versus travel characteristics of a master cylinder constructed in accordance with the present invention; and Figure 4 is a hydraulic brake system of Figure 1 with a modified valve device.

In the drawings, the housings of the master cylinder and of the valve device are shown in outline only. The valve device may be separated from the master cylinder in a distinct housing and communicate with the master cylinder via fluid inlet ports. In order to provide an advantageously designed master cylinder, the valve device is integrally formed within the housing 8 of the master cylinder so that part of the fluid connections from the master cylinder to the valve device may be designed as bores in housing 8.

In Figure 1, the valve device integrated into the housing of the master cylinder has been exaggerated in size to make the description and mode of function more clearly understood.

A hydraulic booster 1 of conventional design comprises a booster piston 2 having a coaxial blind-end bore 3 in which a central piston 5 actuatable by the brake pedal 4 slides. The booster piston 2 has on its periphery a groove 6 sealed on the right- and left-hand side by seals 7 in the housing, so that there is provided a chamber 11 enclosed between groove 6 and housing 8 of the booster and connecting with a fluid accumulator 10 via a port 9. Groove 6 has radial bores 36 opening into blind-end bore 3, and the central piston includes radial bores 12, 1 4 and substantially axial channels 13. Blind-end bore 3 is in communication with a chamber 1 6 in front of the booster piston through an opening 15, chamber 1 6 being connected to a fluid reservoir 17.Central piston 5 terminates in an enlarged section of blind-end bore 3 in which it slides in a fluid-sealed relationship thereto. This results in the formation of a fluid chamber 1 9 which is bounded by booster piston 2 and the enlarged section 1 8 of central piston 5 and receives fluid via bore 14. Enlarged section 1 8 is in abutment with a stop 21 in booster piston 2 in opposition to actuating direction 20. Via an opening 22, fluid chamber 1 9 is in communication with booster chamber 23 which is bounded by booster piston 2 and housing 8. In this arrangement, the booster piston is in abutment with a stop 24 so that the volume of booster chamber 23 is at a minimum when the arrangement is in the inactive position.

Annular surface 25 pressurized in the booster chamber transmits the pressure acting on it to a master cylinder piston 26 which is mechanically coupled with booster piston 2 or designed as a separate component and slides in a master cylinder chamber isolated from the other fluid chambers, separating this chamber into a pressure chamber 27 and an unpressurized chamber 28. The two chambers communicate with each other through a bore 29 in the master cylinder piston, with the opening of bore 29 terminating in pressure chamber 27 being closed by a lip-type seal 30 sealing master cylinder piston 26 in housing 8 in a fluid-tight manner.

Thus the effect of a check valve is obtained, permitting fluid flow solely from unpressurized chamber 28 to pressure chamber 27 while inhibiting fluid flow in the opposite direction with the lip-type seal intact. Pressure chamber 27 is connected to fluid reservoir 1 7 through a breather bore 31 terminating in the pressure chamber directly in front of the lip-type seal, and unpressurized chamber 28 is connected to fluid reservoir 17 through port 32.

Booster piston 2 extends through, and is sealed fluid-tight relative to, wall 33 of housing 8 in opposition to actuating direction 20 and terminates in an enlarged-diameter housing section 34 in which the end of the booster piston slides in a fluid-tight relationship. The fluid chamber 35 thereby provided, which is bounded by housing 8 and booster piston 2, is connected to unpressurized chamber 1 6 and fluid reservoir 1 7 via valve device 106. Movement of booster piston 2 in actuating direction 20 will result in a decrease in volume of pressure chamber 35, and the fluid will be urged into fluid reservoir 1 7.

Furthermore, booster piston 2 accommodates a travel simulator comprising two relatively slidable pistons 37, 38 and a spring 39 inserted between them. The one piston 37 is in mechanical abutment with central piston 5 while the other piston 38 connects with brake pedal 4.

The prestressed spring 39 engaged between the pistons transmits forces up to a specific amount so that the arrangement is to be regarded as mechanically rigid in this operating state, i.e., the brake pedal is directly mechanically connected with the central piston. After the specific amount, spring 39 will yield, and the two pistons 37, 38 will move relatively to each other and thereby increase the pedal travel of brake pedal 4. In this manner, brake pedal 4 is allowed to travel an increased distance although booster piston 2 is locked in position as will be explained hereinafter.

Following a predetermined stroke, both pistons will be in rigid mechanical abutment, and spring 39 cannot be compressed further. Damage to spring 39 is thus precluded.

The total arrangement is maintained in its inactive position as illustrated, by a spring 40. The spring is arranged in chamber 1 6 and bears upon wall 41 located between unpressurized chamber 28 of the master cylinder and unpressurized chamber 1 6 of the booster arrangement. This particular design was chosen because it enables the fluid reservoir 17, which is divided into two compartments 42, 43 by a wall 107, to be allocated to the arrangement such that compartment 42 is assigned to the master cylinder, while compartment 43 is assigned to the hydraulic booster device. It is thereby ensured that in the event of a leak in the hydraulic booster system the fluid for the master cylinder cannot escape, and vice versa.Therefore, in the event of a line leakage, an emergency braking operation is at all times possible because a sufficient amount of fluid continues to be available.

The valve device 106 comprises essentially four chambers 44 to 47 arranged in series, with chamber pairs 44-45, 456, 46-47 being separated by walls 48 to 50 having a valve piston 51 extending therethrough in a fluid-tight relationship. The valve piston terminates in chambers 44 and 47, each of its ends 52, 53 fulfilling a valve function. For this purpose, end 53 in chamber 44 is a valve cone urged into sealing engagement with the opening of a channel 56 connecting with pressure chamber 27, with the opening of channel 56 into chamber 44 forming a valve seat 55.

A piston 57 divides chamber 44 into a hydraulic chamber 58 and a chamber 59 connected to atmosphere, with the pressure in hydraulic chamber 58 keeping piston 57 in abutment with a stop 61 in chamber 44 in opposition to the force of spring 60. Valve piston 51 extends through piston 57 in a fluid-tight manner and coaxially such that the valve piston remains displaceable relative to piston 57.

However, valve cone 54 is slightly larger than the section of the valve piston extending through the piston 57, resulting in the formation of a step 62 against which the piston is urged into engagement by spring force in the absence of hydraulic pressure in chamber 58.

End surface 63 of valve piston 51 includes an axial channel 64 directing fluid from pressure chamber 27 into chambers 45 and 46 through radial channels 65, 66. These chambers, however, are subdivided into inlet chambers 69, 70 and control chambers 71,72 by respective collars 67, 68 sealed in a fluid-tight manner relative to the housing by respective lip-type seals 73, 74 in the control chamber. Furthermore, the collars include axial through-channels 75, 76 covered by respective lip-type seals 73, 74 in the inlet chamber, so that they permit a fluid flow from control chambers 71,72 to respective inlet chambers 69, 70 in their function as check valves.

Control chamber 72 further accommodates a spring 108 urging valve cone 54 into engagement with valve seat 55.

End 52 of valve piston in chamber 47 acts as a slide valve for inlet port 109 which communicates with fluid chamber 35 through line 113. Chamber 47 is also connected to chamber 16 and compartment 43 of fluid reservoir 17 and, via port 77 and line 78, to fluid pump 79 which delivers fluid into fluid accumulator 10 when electric motor 80 is turned on. Connected upstream from pump 79 is a filter 81 whose purpose is to retain any particles of dirt in the hydraulic fluid. A check valve 82 prevents evacuation of fluid accumulator 10 in the event of a leak in line 83 or the pump device. If the pressure in fluid accumulator 10 is too low, a pressure switch 84 will issue a start command to pump 79, 80, and will stop the pump again when the pressure has reached a predetermined magnitude.

The pressure generated in the accumulator also prevails in chamber 11, and actuation of brake pedal 4 causes it to be directed into chamber 1 9 and booster chamber 23 through the bores and channels in the booster piston and the central piston, opening 1 5 being closed by central piston 5. A pressure corresponding to the position of the brake pedal will build up. in booster chamber 23 and, by transmission to master cylinder piston 26, in pressure chamber 27.

Via a line arrangement 85, 86 and a solenoid valve 91 which is open in the de-energized state, booster chamber 23 communicates with wheel cylinders 89, 90 of the rear axle. Accordingly, the pressure in the booster chamber will build up in the wheel cylinders 89, 90 without delay. Via a solenoid valve 92, which is closed in the deenergized state, and a line 93, wheel cylinders 89, 90 are also connected to line 78 feeding fluid to the pump 79 or returning the fluid to fluid reservoir 1 7. Line 85 further communicates via branch line 87 and check valve 88 with the wheel cylinders of the rear axle. However, the fluid is only allowed to flow from wheel cylinders 89, 90 into the booster chamber so that in the inactive position the wheel cylinders are connected to the unpressurized fluid reservoir 17 via the booster chamber, the channel system and chamber 16.

Via line 94, the booster pressure is also directed into hydraulic chamber 58.

The pressure in pressure chamber 27 will propagate to inlet chambers 69, 70 via channels 56, 64, 65, 66. The inlet chambers are connected to respective wheel cylinders 95, 96 via respective lines 101, 103 and respective solenoid valves 97, 99 which are open in the de-energized state. The pressure in the wheel cylinders can be decreased through solenoid valves 98, 100, which are closed in the de-energized state, and through line 105 which is connected to line 78, in response to the control signals issued by an antiskid control system (not shown). The wheel cylinders themselves are connected to the control chambers 71, 72 of the valve device via lines 102, 104, so that the pressure applied to wheel cylinders 95, 96 through solenoid valve 97 to 100 also prevails in control chambers 71,72.

The mode of operation of the arrangement is the following: Under normal brake application conditions, the valve device 106 has assumed the position illustrated. The pressure supplied by the hydraulic booster prevails directly in wheel cylinders 89, 90 of the rear axle, and via annular surface 25 exposed to pressure the pressure will act in pressure chamber 27 in accordance with the effective surface ratios of the booster piston relative to the master cylinder piston 26, this pressure propagating to the wheel cylinders 95, 96 of the front axle.If the anti-skid control apparatus signals an imminent locked condition of the rear axle, closing of valve 91 and opening of valve 92 permits fluid to be discharged from the wheel cylinders 89, 90 of the rear axle, and during another period of pressure buildup a sufficient amount of fluid volume will be available because the fluid in booster chamber 23 cannot become exhausted. Via the channels in the booster piston and in the central piston, sufficient fluid will be supplied from fluid accumulator 10 into booster chamber 23.

If the anti-skid control apparatus executes a pressure decrease to avoid locking of a front wheel, the pressure decrease in wheel cylinder 95, for example, will result in the adjusted pressure prevailing also in control chamber 71.

Because in this operating state, valve 97 (which is open when de-energized) is closed, the full pressure of pressure chamber 27 will prevail in chamber 69, while the reduced pressure of wheel cylinder 95 will prevail in chamber 71.The difference of pressure will cause a force to act on collar 67 which displaces valve piston 51 to the right, thereby lifting valve cone 54 clear of valve seat 55. The dynamic pressure of the hydraulic booster 1 supplied to hydraulic pressure chamber 58 via line 94 is thus directly fed into pressure chamber 27 and, via channels 64, 65, 66 into inlet chambers 69, 70. Displacement of valve piston 51 will cause its end 52 in chamber 47 to cut off inlet port 109 of pressure chamber 35, so that booster piston 2 is hydraulically locked in position in the actuating direction.Thus any further increase in the force applied to brake pedal 4 will only result in a fully opened condition of the control mechanism of the hydraulic booster 1.

Thus it acts as a brake control valve in this position.

With the continuing application of force, a travel is simulated to brake pedal 4 by simulator 37, 38, 39 which is accomplished by compression of spring 39.

When the front-axle wheel concerned has recovered from the imminent locked condition, the pressure in wheel cylinder 95 will be built up again by opening of solenoid valve 97. Thus, the pressure in control chamber 71 will again also approximate the pressure in inlet chamber 69 and the force acting on collar 67 and keeping valve piston 51 in the open position will be reduced.

When the pressure in wheel cylinder 95 has largely approached the pressure in inlet chamber 69, the force of spring 108 will again move valve piston 51 into the inactive position illustrated, and valve cone 54 will be urged into sealing engagement with valve seat 55, so that chamber 58 is again isolated from pressure chamber 27. At the same time, inlet 109 of the line communicating with chamber 35 is again open so that booster piston 2 again becomes movable.

This mode of operation of the arrangement ensures that the fluid discharged through valves 98, 100 in the presence of an anti-skid control operation does not cause exhaustion of pressure chamber 27, because, during the period of fluid discharge, pressure chamber 27 is connected to the fluid flow of the hydraulic booster via valve device 106, so that fluid cannot become exhausted because of the pump 79 and fluid accumulator 10 arrangement. When the fluid discharged via valves 98, 100 has been recovered again in pressure chamber 27 by means of the dynamic fluid circuit of the hydraulic booster, valve device 106 will close again and the dualcircuit brake system is again split into a static circuit receiving its fluid from pressure chamber 27 and a dynamic circuit receiving its fluid from the hydraulic booster.

By the hydraulic blocking of chamber 35 it is ensured that master cylinder piston 26 is not moved in the actuating direction in the sene of reducing the volume of pressure chamber 27 when the pressure drops abruptly in pressure chamber 27 as may be the case, for example, when valves 97 and 99 open simultaneously following a pressure decrease. Thus, the static brake circuits holds a constant fluid volume, irrespective of the number of control cycles performed at wheel cylinders 95, 96 of the front axle.

Figure 3 shows that the point at which a marked pressure increase sets in, in pressure chamber 27 is at about 40 bar while the pedal travel is becoming smaller. This occurs likewise under the control of valve device 1 06. Assuming an uncontrolled braking, the pressure in pressure chamber 27 will act on end surface 63 of valve piston 51. This force acts in opposition to spring 108, which thus determines the pressure level causing displacement of the valve piston. In the embodiment shown, the point at which this condition sets in, as shown at 40 bar by way of example, is that at which valve cone 54 will then lift clear of valve seat 55 and connect the previously static brake circuit directly to the dynamic brake circuit of hydraulic booster 1.In this state, hydraulic booster piston 2 will be hydraulically locked in place by interlocking of chamber 35, so that brake pedal 4 travel can only be produced by simulator 37, 38, 39 when central piston 5 has already bottomed in blindend bore 3. This would have the advantage of the vehicle operator having available a relatively large brake pedal travel up to 40 bar, permitting him a proportional brake application. However, if he requires a pressure exceeding 40 bar which initiates a stronger or panic brake application, the system will be switched to a brake pressure control valve after the pressure has reached 40 bar, the brake pressure control valve comprising a booster piston, a central piston and their relevant channels.The distance which the brake pedal travels in this case has been kept very small deliberately so that even under panic braking conditions the vehicle operator is not required to produce an excessive pedal travel.

Moreover, in the event of failure of a hydraulic brake system, the possibility for the vehicle operator to initiate an emergency braking action is at all times ensured. For instance, if the hydraulic system of the booster fails, he can actuate the master cylinder piston 26 directly through the brake pedal and cause a pressure to develop which exceeds 40 bar by far, because in the event of failure of the hydraulic booster pressure, spring 60 will keep piston 57 in abutment with step 62 of the valve piston. Spring 60 may be so designed that the pressure to be built up at wheel cylinders 95, 96 of the front axle is sufficient to bring the vehicle to a stop within a reasonable braking distance.Because valve piston 51 is maintained in the position shown, it need not be feared that booster piston 2 becomes hydraulically locked in position, making a further pressure increase in pressure chamber 27 impossible.

On the other hand, if the static brake system fails because of leakage of lip-type seal 30, pressure is not allowed to build up in pressure chamber 27, consequently, valve piston 51 cannot be displaced in opposition to the force of spring 1 08. The hydraulic force supplied in such a case will act solely on wheel cylinders 89, 90 of the rear axle, ensuring likewise an emergency braking operation.

With the whole arrangement in the inactive position, it is further ensured that all wheel cylinders are directly connected to unpressurized fluid reservoir 17. In this arrangement, wheel cylinders 95, 96 are pressure-balanced via line 102, 104 and check valves 74 73-75, and- wheel cylinders 89, 90 via check valve 88.

For still greater safety, a brake system of this type may also include a tandem master cylinder as shown in Figure 2. This design necessitates, however, the provision of a separate valve device 106.1, 106.2 for each pressure chamber 27.1, 27.2. The valve devices correspond largely to the one of Figure 1; they are, however, equipped only with respective inlet chambers 70.1, 70.2 and respective outlet chambers 72.1, 72.2. Thus, each valve is assigned a closure spring 108.1, 108.2 of its own which determines the pressure at which the associated static system changes over to the dynamic system. In chambers 47.1, 47.2 the ends of valve pistons 51.1,51.2 act again as valve closure members in order to shut off the inlets 109.1, 109.2 of the line communicating with chamber 35.In this arrangement, it is to be noted that chamber 47.1 is connected to port 110 of unpressurized chamber 1 6 via port 112 and line 114, while chamber 47.2 is connected to port 110 of unpressurized chamber 1 6 via port 111 and the same line 114. The arrangement is interconnected such that chamber 35 is linked to chamber 1 6 via two parallel lines. The purpose of this arrangement is to achieve locking of the hydraulic booster piston 2 only if the change-over pressure has been attained in the two pressure chambers 27.1, 27.2. If the changeover pressure prevails in one pressure chamber only, booster piston 2 can be displaced further because a connection between fluid chamber 35 and fluid reservoir 1 7 is still open.

The tandem master cylinder includes a floating piston 26.2 which is actuated by the hydraulic column in pressure chamber 27.2. Provided between master cylinder piston 26.1 and floating piston 26.2 is an arrangement comprising a spring 115, a pin 11 6 including a head and secured in master cylinder piston 26.1, a cupshaped structure 11 7 having pin 11 6 extending through its bottom and being held in abutment with the rear end of the head by the force of spring 11 5. In this manner, spring 11 5 is anchored to piston 26.1, whilathe cup remains displaceable against piston 26.1 in the sense of compression of spring 11 5.Via a spring 11 8 which is disposed in pressure chamber 27.1, floating piston 26.2 is held in abutment with the arrangement comprising spring 115, pin 116 and cup 11 7. In this arrangement, spring 118 is so designed that it will not compress spring 11 5 in the inactive position shown. Floating piston 26.2 forms at the same time a boundary for an unpressurized chamber 28.1, in the same manner as piston 26.1 bounds an unpressurized chamber 28.2. The mode of function and the connections to the associated fluid chambers correspond to those of Figure 1.

Fluid reservoir 17 includes two walls 107.1, 107.2 subdividing it into three compartments 42.1,42.2 and 43. Compartment 43 connects to chamber 16, compartment 42.1 communicates on the one hand with chamber 28.2 while supplying fluid to pressure chamber 27.2 through breather bore 31.2, and compartment 42.2 is connected to fluid chamber 28.1 via a port and supplies fluid to pressure chamber 27.1 through breather bore 31.1.

The mode of function of the arrangement will not be described in greater detail because it corresponds identically to the one of Figure 1. If the pressure in the wheel cylinder associated with the pressure chamber is lowered as a result of a control signal from the anti-skid control apparatus, the relevant pressure chamber will be connected to the hydraulic fluid circuit; however, a locked condition of the hydraulic booster piston 2 will not occur because it has to remain displaceable for another pressure increase in the other fluid chamber.

Figure 4 shows a master cylinder arrangement of Figure 1 with the valve device slightly changed.

Accordingly, most of the reference numerals are identical to those of Figure 1.

The hydraulic booster is of identical design, except for a warning device 1 30 connected thereto in pressure inlet chamber 11 to indicate a failure of the hydraulic fluid circuit. Further, wall 41 has been omitted because, as will be described in more detail hereinafter, it is intended to change slightly the fluid supply compared with the embodiment of Figure 1.

The piston 51 of the valve device is of slightly different design. Thus, end 52 is designed as a valve cone adapted to be urged into engagement with a projection in chamber 47 forming a valve seat 119. A spring 127 disposed coaxially in a bore in end 52 of the valve piston and bearing against the bottom of chamber 47 maintains valve cone 52 clear of valve seat 119, keeping the valve open. Thus, the fluid flowing from chamber 35 into inlet 109 through line 113 is allowed to pass the valve and reach unpressurized chamber 16 through a line and port 110.

End 53 of valve piston 51 is likewise of different design. Chamber 44 has a reduceddiameter section 128, in which the end 53 of the valve piston extending from valve cone 54 slides and to which it is sealed. Fluid chamber 121 thus bounded by the housing and valve piston 51 communicates with pressure chamber 27 of the master cylinder which is likewise connected to wheel cylinders 95, 96 through respective lines 101 and 103 and respective solenoid valves 97, 99 which are open in the de-energized state.

Thereby, end surface 63 of the valve piston is acted upon by the pressure of pressure chamber 27. At the same time, end 53 includes channels 64, 65, 66 enabling the fluid of pressure chamber 27 to be fed into inlet chambers 69, 70 directly.

Control chambers 71,72 allocated to the inlet chambers in turn communicate directly with the associated wheel cylinders 95, 96 through respective lines 1 02, 104. The step in chamber 44 towards the reduced-diameter section 128 is designed as a valve seat 120 with which the valve cone 54 is urged into sealing engagement and held in this position by spring 127. Chamber 58 which receives the pressure of the hydraulic booster through line 94 is thus in fluid-tight isolation relative to the remaining space of chamber 44, with the remaining space connecting via a port to a chamber 123 provided between master cylinder piston 26 and housing 8.

Chamber 123 is provided by a groove 122 in master cylinder piston 26, the groove being closed on the right- and left-hand side by seals, thereby sliding in housing 8 in fluid-tight engagment therewith. The left-hand shoulder of groove 122 which separates chamber 123 from pressure chamber 27 has a through bore 29 which is covered by lip-type seal 30 in pressure chamber 27, thereby providing a check valve between chamber 123 and pressure chamber 27.

Breather bore 31 terminating in front of the liptype seal and inlet 32 charging pressure chamber 1 23 with unpressurized fluid open into a chamber 124 which is connected to compartment 43 of the fluid reservoir through a port 126. Arranged in chamber 129 is a tip-change valve 125 against which a spring 1 24 bears in the closing direction.

The tip-change valve is held in the open position by the left-hand shoulder of groove 122 so that, with the brake system in the inactive state, unpressurized fluid is allowed to flow from compartment 42 into chamber 124, thereby supplying fluid to the pressure chambers connected thereto.

As already described, a suitable pressure will be built up in the wheel cylinders when the brake system is activated. When displacement of master cylinder piston 26 has taken place, tipchange valve 125 will be moved into the closed position by the force of spring 124, so that inlet port 126 into chamber 1 24 will be closed in a fluid-tight manner. The lip-type seal will have overtraveled breather bore 31 so that chamber 124 communicates with pressure chamber 123 solely via port 32.

If an imminent locked condition of the front wheels is established, the pressure in the associated wheel cylinders 95, 96 will be discharged, resulting in turn in opening of the valve device and hydraulic locking of booster piston 2. The fluid from the dynamic circuit is allowed to flow through line 94, chamber 58 and the open valve into chamber 123 where it is supplied to the associated wheel cylinders through channels 29, past lip-type seal 30 and through pressure chamber 27. Because tipchange valve 125 in chamber 129 is closed, the fluid under pressure cannot escape into fluid reservoir 1 7. When the pressure in the associated wheel cylinders 95, 96 has again adjusted itself to the master cylinder pressure, the valve piston will again be displaced by the force of spring 127, and the hydraulic fluid flow will be shut off.A pressure-balancing action will take place between chamber 123 and pressure chamber 27 until the pressure in pressure chamber 27 is adjusted to the pressure in pressure chamber 123.

In this embodiment, too, the valve device will connect the static circuit directly to the dynamic circuit from a specific pressure level onwards.

This change-over pressure is determined by the pressure on end surface 63 of valve piston 51 and by spring 127 counteracting this force.

In arrangements of this type it is worth mentioning that the static brake circuit can be checked at all times although the system is controllable by the dynamic fluid circuit of booster 1. For example, if lip-type seal 30 of master cylinder piston 26 is defective, it is not possible to build up a pressure sufficient to cause switching of the valve device to the dynamic fluid circuit. A failure of the lip-type seal will make itself felt by an increased brake pedal travel. Nevertheless, the emergency braking capability of the system will be maintained because, following overcoming of a suitable lost travel, the pressure valve in the hydraulic booster can be opened fully, with the pressure thus directed into booster chamber 23 travelling down to wheel cylinders 89, 90 of the rear axle.

If the brake system is equipped with a tandem master cylinder, the valve device may be designed such that only the pressure in the one master cylinder pressure chamber will govern the change-over to the dynamic circuit. Each master cylinder may be assigned a control chamber of its own in the valve device.

Where two valve devices are provided, it will be sufficient if only one valve device at a time locks the booster piston hydraulically. In such an arrangement, the valve devices may be interconnected such as to be capable of hydraulically locking the booster piston in the sense of a series connection.

In order not to impair the valve function, the valve travels should be kept small. Thus it is ensured that volume displacements do not cause fluctuations of pressure.

Claims

1. A hydraulic brake system with master cylinder, wheel cylinders and a brake slip control apparatus, with a valve device controlling fluid flow from a fluid source to the pressure chamber of the master cylinder, and with valves arranged to be actuated by the brake slip control apparatus and controlling, in the event of an imminent locked condition of a wheel, the pressure in the associated wheel cylinder independently of the pressure in the pressure chamber, the valve device being opened in dependence on a difference between the pressure in the pressure chamber and the pressure in the wheel cylinder associated with the pressure chamber, the pressure in the wheel cylinder being required to be lower than the pressure in the pressure chamber by a predetermined amount, characterized in that the valve device interrupts in its open position a fluid connection which is open when the valve device is closed, the fluid connection connecting a chamber bounded by the booster piston of the brake system with an unpressurized fluid reservoir, and the booster piston reducing the volume of the chamber as it moves in the actuating direction.

2. A hydraulic brake system as claimed in claim 1, characterized in that the valve device comprises a stepped valve piston movably arranged in a housing parallel to the master cylinder.

3. A hydraulic brake system as claimed in claim 2, characterized in that the housing of the valve device is formed integrally with the housing of the master cylinder.

4. A hydraulic brake system as claimed in claim 2, characterized in that the housing of the valve device is subdivided into several chambers arranged axially in series and having the valve piston extending through their partition walls in a sealed relationship thereto, with the valve piston extending into the first and last chamber of the arrangement.

5. A hydraulic brake system as claimed in claim 4, characterized in that the valve piston has a collar subdividing a chamber into an inlet chamber communicating with the pressure chamber and a control chamber communicating with the wheel cylinder.

6. A hydraulic brake system as claimed in claim 5, characterized in that a check valve opening in the direction of the inlet chamber is provided between the inlet chamber and the control chamber.

7. A hydraulic brake system as claimed in claim 6, characterized in that the collar includes a bore connecting the inlet chamber to the control chamber, the bore's opening terminating in the inlet chamber being covered by a seal sealing the collar in the housing.

8. A hydraulic brake system as claimed in claim 7, characterized in that the inlet chamber is connected to the pressure chamber of the master cylinder via channels.

9. A hydraulic brake system as claimed in claim 8, characterized in that the wheel cylinder associated with the master cylinder pressure chamber is connected to the inlet chamber via a valve.

10. A hydraulic brake system as claimed in claim 4, characterized in that an inlet or outlet opening adapted to be closed by the valve piston is designed as a valve seat in the first and/or last one of the fluid-containing chambers arranged axially in series.

11. A hydraulic brake system as claimed in claim 10, characterized in that the first chamber is connected to the pressure chamber and to the fluid flow from the fluid source, with the valve piston closing the inlet opening of the pressure chamber by seating engagement with its valve seat.

12. A hydraulic brake system as claimed in claim 10, characterized in that the last chamber is connected to the chamber bounded by the booster piston and to the unpressurized fluid reservoir, with the inlet of the chamber being adapted to be closed by the end of the valve piston extending into the chamber.

13. A hydraulic brake system as claimed in claim 12, characterized in that the valve piston acts as a slide valve closing the radial inlet by moving axially.

14. A hydraulic brake system as claimed in any one of the preceding claims, characterized in that the first chamber of the velve device which is connected to the fluid source is bounded by a piston having the valve piston extending therethrough axially movably and having acting on it a spring in opposition to the pressure in the chamber, with the valve piston's end being slightly larger in diameter than the valve piston's section extending through the piston.

1 5. A hydraulic brake system as claimed in claim 14, characterized in that a stop is provided in chamber for abutting engagement with the piston acted upon by the pressure of the fluid source.

16. A hydraulic brake system as claimed in claim 15, characterized in that the chamber isolated by the piston and having the valve piston extending therethrough is connected to atmosphere, and in that a spring is engaged between the piston and the subsequent partition wall.

17. A hydraulic brake system as claimed in any one of the preceding claims, characterized in that each wheel cylinder allocated to the master cylinder is assigned an inlet chamber provided between the first and the last chamber, of the valve device.

18. A hydraulic brake system as claimed in any one of the preceding claims, characterized in that the valve piston carries a collar which is designed as a valve cone, urged into sealing engagement with a valve seat and interrupts the connection of the fluid flow to the master cylinder pressure chamber.

19. A hydraulic brake system as claimed in claim 18, characterized in that the valve cone is pressure-balanced.

20. A hydraulic brake system as claimed in claim 1 8, characterized in that the one end of the valve piston slides in the housing in a fluid-sealed relationship thereto and bounds a chamber pressurized by the master cylinder pressure.

21. A hydraulic brake system as claimed in claim 18, characterized in that the fluid is directed through a chamber arranged in the circumference of the master cylinder piston and connected to the pressure chamber via a check valve opening in the direction of the pressure chamber.

22. A hydraulic brake system as claimed in claim 21, characterized in that, in the inactive position of the master cylinder piston, the chamber is connected to the unpressurized fluid reservoir via a valve actuatable by the master cylinder piston.

23. A hydraulic brake system as claimed in claim 22, characterized in that the valve is a tipchange valve acted upon in the closing direction by a spring.

24. A hydraulic brake system as claimed in any one or several of the preceding claims and incorporating a tandem master cylinder, characterized in that each pressure chamber is assigned a self-contained valve device of its own, with the arrangement of the one valve device which interrupts the fluid connection acting in parallel to the arrangement of the other valve device.

25. A hydraulic brake system as claimed in any one of the preceding claims, characterized in that the spring which determines the pressure at which changeover to the dynamic system of the booster takes place and keeps the valve cone in seating engagement with the valve seat, is situated in a control chamber and is engaged between the collar of the valve piston and the subsequent partition wall.

26. A hydraulic brake system as claimed in any one of the preceding claims, characterized in that the spring determining the changeover pressure is arranged in a coaxial bore in the end of the valve piston and bears against the opposite wall of the chamber.

27. A hydraulic brake system substantially as described with reference to the accompanying drawings.