Dual circuit brake force regulating device

The brake force regulator device is for a dual circuit brake system in which the function of the regulator is suppressed if an uncontrolled brake circuit fails. A locking piston is provided which serves this purpose as well as at the same time to release a differential pressure warning signal should one of the two circuits fail. The advantages achieved with the arrangement disclosed are that only a small amount of pressure fluid is drawn from the intact brake circuit in case of the failure of the other circuit so that the locking piston performs a dual function.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
Brake force regulators for dual circuit brake systems. 
2. Prior Art 
Brake force regulators for dual circuit brake systems are known in the 
prior art, specifically from German published application DT-OS No. 
2,261,341. In that prior art brake force regulator a locking piston is 
disposed coaxially with a stepped piston in a second bore in the housing 
and extends with a push rod into a regulator chamber positioned in front 
of the larger of the stepped end surface piston. In the closing direction 
of that brake force regulator, the locking piston is exposed to the 
pressure of the fluid supplied to the wheel brake cylinders and in the 
opening direction, to the pressure in the second brake circuit. In that 
device when both brake circuits are intact the locking piston is held in 
abutment against a housing stop disposed in the closing direction of the 
brake force regulator. 
It has been found that if the brake force regulator of the published German 
application is in use a system and the second brake circuit fails, 
pressure from the first brake circuit will act on the stepped piston and 
the locking piston in the closing direction of the brake force regulator. 
The stepped piston is thereby shifted into its end position lying in the 
closing direction. The locking piston follows this movement, pushes a 
closure member open with the aid of its push rod so that pressure fluid 
continues to flow freely between a control chamber communicating with the 
first circuit and a regulator chamber in communication with the wheel 
brake cylinders. The desired effect, namely to cut the brake force 
regulator out of operation in the event of failure of the second brake 
circuit, is thus fully accomplished. However, it is a disadvantage, in the 
device of the published German application, that the regulator chamber, 
which becomes enlarged owing to the displacement of the stepped piston, 
receives the brake pressure fluid from the first intact brake circuit. 
Similarly, pressure fluid from the first intact brake circuit is required 
for shifting the locking piston. This is of particular importance since, 
in the case of a brake circuit failure, the brake pedal travel for 
actuating the master cylinder is, in all cases, larger than normal. This 
effect is further intensified if the intact brake circuit draws more 
pressure fluid than is the case when both circuits are intact. 
It is also known that as a check the proper functioning of the brake 
circuits, to supply differential pressure warning switches which are 
frequently included in dual circuit brake systems. The effect of such a 
switch is that a warning signal is given to the vehicle operator whenever 
a brake circuit has failed. It is similarly known that differential 
warning switches include a switch piston which is centered in a middle 
position and which is acted on on its opposite sides or faces by the brake 
circuit pressure of each of the two brake circuits. When either of the 
brake circuits fails the switch piston is displaced by the action of the 
pressure in the intact circuit which releases a differential pressure 
warning indication to signal the operator that one of the brake circuits 
has failed. It will be seen therefore that the prior art differential 
warning switches, in the event of failure of a brake circuit, also 
require, for their operation, additional fluid under pressure from the 
intact brake circuit. Thus when equipped with a dual circuit brake system 
having the brake force regulator known from the German published 
application DT-OS No. 2,261,341 and including a differential pressure 
warning switch of the type described above, additional pressure fluid is 
required for displacing the stepped piston, the locking piston and the 
switch piston of the differential pressure warning switch if one of the 
brake circuits has failed. Even though the individual amounts of brake 
fluid required are relatively small, when they are added together they 
cause a considerable loss in pedal travel and may even involve exhaustion 
of the master cylinder. 
Furthermore in the brake force regulator described in the published 
application the locking piston is in no position to assume the function of 
the switch piston for the differential pressure warning switch since, in 
the normal case, the locking piston is not held in a middle position but 
instead rests against a stop on the housing. Therefore, if the first brake 
circuit fails, the locking piston is unable to move, which would make it 
impossible for it to operate to give rise to a differential pressure 
warning. Furthermore, in the prior art device, it is not possible to 
center the locking piston in a middle position since it is acted on by 
different pressures above the switchover point of the brake force 
regulator and it would always release the differential pressure warning 
signal. Furthermore, in the event of the failure of the first brake 
circuit such an arrangement would still need pressure fluid for displacing 
the stepped piston and additional pressure fluid for displacing the 
locking piston. 
SUMMARY OF THE INVENTION 
The object of this invention is to improve upon the brake force regulator 
of the type referred to above so that in the event of failure of the 
second brake circuit no additional pressure fluid is required from the 
first brake circuit except that required to displace the locking piston in 
the brake force regulator and furthermore permits the use of the locking 
piston for activating the differential pressure warning switch. 
In the brake force regulator of this invention the aforegoing object is 
achieved because the locking piston is acted on in the opening direction 
by the uncontrolled pressure in the first brake circuit and in the closing 
direction by the pressure of the second brake circuit so that the axially 
displaceability of the locking piston from a middle position in the 
opening direction is limited by the stepped piston. Furthermore a tripping 
pin is supported against the locking piston in a radial direction to 
activate a differential pressure warning switch by means of two ramps 
provided adjacent a mid-section of exterior surface of the locking piston. 
In the device of this invention the full pressure from the two brake 
circuits always acts on the locking piston. Thus it is centered in a 
middle position without the risk of a faulty indication above the 
switchover point of the brake force regulator. Furthermore the arrangement 
of parts of the brake force regulator of this invention, including the 
limitaton on the axial displaceability of the locking piston on by the 
stepped piston, is such that the stepped piston moves in the opening 
direction if the second brake circuit fails, thereby stopping the stepped 
piston in its rest position. Since the stepped piston cannot be moved in 
the closing direction no additional pressure fluid can be received in the 
regulator chamber. Therefore, in the brake force regulator embodying the 
invention, only a small amount of pressure fluid is required for 
displacing the locking piston, thereby fulfilling all necessary functions, 
including a differential pressure warning indication. Thus all of the 
functions are performed by the device of this invention and the 
disadvantages occurring in the prior art are thus avoided in a simple 
manner.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As illustrated in the FIGURE, a housing 1 is provided with a stepped bore 2 
forming sections A, B, C and D. In order to understand more clearly the 
operation of the device the arrow 3 is pointing in the direction of 
closing and the arrow 4 points in the opening direction of the brake force 
regulator illustrated. A fluid port 5 is connected to and communicates 
with the first brake circuit (not shown) and the opening through fluid 
port 5 terminates at the end of bore section B lying in the closing 
direction. Fluid port 6 communicates with and is connected to the second 
brake circuit (not shown) terminates at the end of bore section B in the 
opening direction. A third fluid port 7 terminates at the end of bore 
section D lying in the opening direction. Fluid port 7 serves to connect 
the regulator of this invention to wheel brake cylinders of the controlled 
wheels. 
It will be noted that the diameters of the individual bore sections A 
through D are different, with the diameter of bore section B corresponding 
to the diameter of bore section D in the illustrated embodiment. The 
diameter of bore section C is smaller than diameters of bore sections B 
and D. The diameter of bore section A is unimportant as regards the 
functioning of the illustrated brake force regulator since it merely 
accommodates a governing spring 8 resting with one end against housing 1 
and with the other end against stepped piston 9. Stepped piston 9 has an 
end portion 10 which is in sealed and sliding engagement with bore section 
D. Stepped piston 9 further has an extension 11 extending from the end 
portion 10 in the closing direction into bore B through bore C wherein it 
is sealed in a sliding relationship and guided thereby. Sealed to and 
slidable on extension 11, there is provided a locking piston 12 which is 
designed as an annular piston and is in turn in sealing and sliding 
engagement with bore section B of housing 1. This arrangement defines, in 
bore sections A and B a control chamber 13 and an annular chamber 14. 
The axially displaceability of locking piston 12 relative to extension 11 
of the stepped piston 9 is limited by stops 15 and 16 which may be 
circlips mounted in grooves on extension 11 of the stepped piston 9. A 
centering spring 17 is accommodated in annular chamber 14 resting with one 
end against housing 1 and with the other end against locking piston 12 
thereby biasing locking piston 12 against stop 15 in the closing 
direction. 
On its outer circumferential surface, locking piston 12 has a recess 18 
defined on either end by ramps extending outwardly in an inclined 
relationship. The recess 18 normally supports the tripping pin 19 of a 
differential pressure warning switch 20. The space which is formed by 
recess 18 is in communication with the atmosphere via an opening 21 
through housing 1. In the interior of stepped piston 9 a fluid line 22 is 
provided which includes several channels and connects control chamber 13 
with a regulator chamber 23 defined by the end portion 10 of stepped 
piston 9 in bore section D and communicating with fluid port 7. Fluid line 
22 has an enlarged chamber 24 accommodating a closure member 25 which is 
biased towards its valve seat 27 by means of spring 26. Valve closure 
member 25 has a tappet 28 projecting through regulator chamber 23 which 
serves to keep it in its open position against the force of spring 26 in 
the event that stepped piston 9 abuts against housing 1 in the opening 
direction 4. 
The operation of the illustrated brake force regulator embodiment of this 
invention is as follows: 
When the braking operation is started, all of the parts are in their 
positions illustrated in the FIGURE. If both brake circuits are intact, 
pressure will begin to develop and feed pressure fluid to fluid port 5 of 
the first brake circuit and to fluid port 6 of the second brake circuit. 
In this manner the same amount of hydraulic pressure will first build up 
in control chamber 13 and in annular chamber 14. The pressure ratio 
between control chamber 13 and annular chamber 14 will continue to prevail 
even above the switchover point so that the locking piston 12 is always 
pressure balanced with both brake circuits intact, thereby imparting no 
force to stepped piston 9. 
When the pressure starts building up, since the closure member 25 is kept 
in the open position, the pressure fluid flows through fluid line 22 to 
regulator chamber 23 and onwards to the connected wheel brake cylinders 
through fluid port 7. By these means the pressure in control chamber 13 
acts in the opening direction in the cross-sectional area of stepped 
piston 9 which is determined by the extension 11, and with the fluid line 
22 open, the same pressure which prevails in regulator chamber 23 will act 
in the closing direction on the cross sectional area of the stepped piston 
which is determined by end portion 10. Thus when the switchover point is 
reached the stepped piston will be displaced in the known manner in the 
closing direction against the control force acting in the opening 
direction which is exerted by governing spring 8 until closure member 25 
closes off fluid member 22 from which moment on the pressure in regulator 
chamber 23 will be reduced in accordance with the stepped piston's surface 
ratio of extension 11 to end portion 10, before being fed to the connected 
wheel brake cylinders. Locking piston 12 follows this relatively small 
control movement of stepped piston 9 owing to the centering spring 17. 
However, this spring is so dimensioned that it is not able to displace 
locking piston 12 beyond the mid-section of recess 18 in respect of 
tripping pin 19. When dimensioning the brake force regulator properly, the 
control movement of the stepped piston 9 will be so small that no large 
displacement stroke of the locking piston 12 will occur in this case. It 
is to be understood that in the normal case that substantially the same 
pressure prevails in control chamber 13 and annular chamber 14 so that the 
locking piston 12 is pressure balanced. 
In the event the first brake circuit fails, no pressure fluid will be fed 
to fluid port 5 and consequently to control chamber 13 so that no pressure 
will build up there. In this case pressure fluid will be fed to annular 
chamber 14 through fluid port 6 thereby acting on the locking piston 12 in 
the closing direction of the brake force regulator which causes it to be 
displaced in the closing direction (arrow 3) together with stepped piston 
9. By virtue of the ramp limiting recess 18 in the opening direction the 
tripping pin 19 is thus shifted radially outward and activates 
differential warning switch 20 to provide a warning signal indicating 
brake circuit failure signal. As a result of the displacement of stepped 
piston 9 in the closing direction which occurs in this case, closure 
member 25 will also close fluid line 22. As has been pointed out above 
this is unimportant since in the event of the failure of the first brake 
circuit, pressure cannot build up in regulator chamber 23 and the wheel 
brake cylinders connected thereto. 
In the event that the second brake circuit fails, annular chamber 14 will 
remain unpressurized. As a result of the pressure developed in control 
chamber 13 by the first brake circuit, locking piston 12 is displaced in 
the opening direction of the brake force regulator supporting itself, 
after having released the pressure differential warning signal, against 
stop 15 on stepped piston 9 in the opening direction. In this manner the 
force acting on stepped piston 9 in the opening direction originates not 
only from the governing spring 8 but also from the locking piston 12. In 
this particular instance the cross-sectional area of locking piston 12 
which faces control chamber 13 and that of extension 11 can be regarded as 
one surface. In the embodiment illustrated in the FIGURE, that surface is 
equal to the surface of end portion 10 which is acted on by pressure in 
regulator chamber 23 in the closing direction. Thus, as long as the same 
pressure prevails in control chamber 13 and regulator chamber 23, the 
hydraulic forces are balanced. As a consequence of the control force 
exerted by governing spring 8 on stepped piston 9 in the opening 
direction, stepped piston 9 remains in its illustrated position so that 
fluid line 22 cannot be closed by closure member 25. This ensures free 
fluid flow between control chamber 13 and regulator chamber 23 in the 
event of failure of the second brake circuit. Thus it results that always 
the same pressures build up in regulator chamber 23. In this specific 
instance, the brake force regulator is thus de-activated by locking piston 
12 which has simultaneously released the differential pressure warning 
signal. 
It should be noted that in the brake force regulator of this invention the 
control force acting in the stepped piston 9 and determining the 
switchover point may of course be generated by a force which depends on 
the axle load. Furthermore it is not necessary for the outer diameter of 
locking piston 12 to correspond to the diameter of end portion 10 of 
stepped piston 9. It is only essential that the sum of the forces acting 
in the opening direction obtained from the control force and hydraulic 
force acting on extension 11 and locking piston 12 is larger than the 
hydraulic force acting in regulator chamber 23 on end portion 10 of 
stepped piston in the closing direction. 
It will be seen that there is a particular advantage in the brake force 
regulator of the FIGURE in that the locking piston 12 is defined as an 
annular piston disposed in a sealed sliding relationship between the 
stepped piston and the bore section B of housing 1 and defining, together 
with stepped piston 9, a control chamber 13 communicating with the first 
brake circuit and an annular chamber 14 communicating with the second 
brake circuit, with the stop 16 on the stepped piston 9 in the annular 
chamber 14 limiting the axial displacement of the locking piston 12 in the 
opening direction. Thus, the stop 16 effectively positions the locking 
piston 12 limiting the displacement of the locking piston 12 in the 
opening direction. This provides for a particularly compact arrangement of 
the brake force regulator of this invention. In order to accomodate the 
locking piston 12, no additional cylinder space needs to be provided which 
would then have to be connected to the brake circuits through suitable 
lines. 
It is furthermore a particular advantage that the stop 15 provided on the 
step piston 9 within control chamber 13 limits the displacement of the 
locking piston 12 in the closing direction. Thus the centering spring 17 
disposed between the locking piston 12 and the housing 1 in bore section B 
keeps the locking piston 12 in its normal operating position against stop 
15. Furthermore, by providing the spool shaped cross-section to locking 
piston 19 there are two ramps for operating the tripping pin 19 of 
differential pressure warning switch 20. Because of this design, the 
mid-section between the two ramps of locking piston 12 which supports 
tripping pin 19, requires only one centering spring 17 which holds the 
locking piston 12 in its predetermined position relative to stepped piston 
9, which position is defined by the stop 15. The normal movement of 
control of the stepped piston 9 which is also followed by movement of the 
locking piston 12 will not cause a differential pressure warning 
indication because the tripping pin 19 is always supported on the 
mid-section of locking piston 12. If the first brake circuit fails, as 
described above, the stepped piston 9 will be displaced in the closing 
direction together with the locking piston 12. This is of no real concern 
since in this instance the first brake circuit and its related components 
are depressurized. Furthermore, in the design illustrated, the stops 15 
and 16 are provided by machining circumferential grooves on stepped piston 
9 and positioning circlips therein. This is particularly advantageous in 
regards to manufacture of the device illustrated since it eliminates the 
need for further steps on stepped piston 9 and the reduced diameter of 
stepped piston 9 and the grooves for stops 15 and 16 can be machined in a 
continuous operation. Furthermore, the arrangement of parts as illustrated 
simplifies assembly in an advantageous manner. 
It is another advantage of the device illustrated that the space defined 
between the seals of the locking piston 12 and the bore section B is 
connected to atmosphere through channel 21. This arrangement ensures that 
any damage to a seal on the locking piston will be immediately noticed by 
the vehicle operator through the differential pressure warning indication 
which will be released, and this avoids the situation occurring that such 
a failure, in combination with another fault of the brake system, would 
cause a total failure of the brake system. 
While this invention has been described in connection with a particular 
embodiment it will be apparent that the inventive subject matter can be 
modified without departing from the scope of the appended claims.