Hydraulic motor vehicle servo brake

A hydraulic motor vehicle servo brake comprises a pedal-actuated master cylinder (11) with at least one master piston (24). In addition, a brake valve (17) is provided for boosting of the brake force, which brake valve upon actuation by the brake pedal (16) feeds pressure medium supplied by a hydraulic pump (18) connected to a supply reservoir (27) in a controlled way to the master piston (24). Furthermore, a pressure accumulator (45) is provided which is maintained in loaded condition by the pump (18) and is of such capacity as to ensure that it supplies sufficient pressure during the running-up phase of the pump (18) after the motor (147) is started. A control valve (46) connected to the pressure side of the pump (18) comprises a closing member 47 which normally closes a passage to a return conduit (48) leading to the supply reservoir and is pressurized by the pressure of the pump (18) against the force of a closing member spring (49). Thus the pressure of the pump can be reduced when the controlled pressure is low and the pump is in operation. Furthermore, a control valve (50) pressurized by the controlled pressure is provided. The closing member spring (49) influences the closing member (47) by way of a relief piston (53) disposed in a cylinder (52). The relief piston (53) is pressurized against the action of the closing member spring (49) by the controlled pressure. Between the relief piston (53) and the closing member (47) a week auxiliary spring (54) is disposed whose force is just sufficient to keep the closing member (47) in closing position when the relief piston (53) is in lifted position.

BACKGROUND OF THE INVENTION 
The invention relates to a hydraulic motor vehicle servo brake comprising a 
pedal-actuated master cylinder with at least one master piston, wheel 
brake cylinders connected thereto by way of at least one brake circuit, a 
brake valve inserted between the pedal and the master piston, which brake 
valve upon actuation by the brake pedal feeds pressure medium supplied by 
a motor-driven hydraulic pump connected to a supply reservoir in a 
controlled way to the master piston and directly to a brake circuit where 
it applies a controlled pressure. A pressure accumulator is maintained in 
loaded condition by the pump and being of such capacity as to ensure that 
it supplies sufficient pressure during the pump's running-up phase after 
the motor is started. A control valve is connected to the pump's pressure 
side and provided with a closing member which normally closes a passage to 
a return conduit leading to the supply reservoir and is pressurized 
against the force of a closing member spring by the pressure of the pump 
and in the direction of the force of the closing member spring by the 
controlled pressure in such a way that it reduces the pressure of the pump 
when the controlled pressure is low and the pump is in operation. A 
control valve is pressurized by the controlled pressure, which valve is 
connected to the hydraulic conduit from the pump to the master cylinder 
and in the event of there being no controlled pressure connects the pump 
to the pressure accumulator and in the event of their being controlled 
pressure separates the pump from the pressure accumulator. 
In a known motor vehicle brake of the described type (German patent 
application DE-OS No. 33 15 731) the closing member of the control valve 
is kept in idle position by a spring against the pressure of the pump 
until the pump pressure overcomes the prestress of the spring and, if 
necessary, the controlled pressure. Since the pump is switched on again 
and again between pedal actuations due to an electric switch provided at 
the pressure accumulator, in order to keep the pressure accumulator in 
loaded condition, the closing member spring must be of such strength that 
the closing member is kept in its idle position until the pressure 
accumulator is fully loaded. This means that even in case of very weak 
braking operations, such as occur for example when one has to wait at a 
traffic light, a considerable pressure, for example 50 bar, builds up 
behind the pump, although during such time only low pressure on the order 
of 5 to 7 bar is required. 
The object of the present invention is to create a motor vehicle brake of 
the type referred to in which in case of a low pressure requirement during 
a braking operation only a comparatively low pressure builds up behind the 
pump, thus not only saving energy but also avoiding unnecessary pressure 
loads on the components involved. 
SUMMARY OF THE INVENTION 
This object is achieved by the present invention in that the closing member 
spring acts upon the closing member by way of a relief piston arranged in 
a cylinder, that the relief piston is pressurized against the action of 
the closing member spring by the controlled pressure, and that between the 
relief piston and the closing member a weak auxiliary spring is disposed 
whose force is just sufficient to keep the closing member in closing 
position when the relief piston is in lifted position. Thus, the closing 
member spring is made ineffective upon occurrence of low controlled 
pressure, for example of 1 to 2 bar, and only the weak auxiliary spring is 
active, which just suffices to keep the control valve in closing position. 
Accordingly, pump pressures of low bar valve suffice to open the control 
valve and thus to avoid an unnecessarily high pressure build-up at the 
pump output. 
In advantageous embodiment of the present invention, a push rod is disposed 
at the relief piston which presses against the closing member. Although 
the control valve according to the invention in principle can also be 
equipped with a closing slide, it is advantageous if the closing member is 
a valve cone which rests on a fitting valve seat into which the pressure 
input is led. 
In order to achieve pressure compensation when there is a controlled 
pressure, it is further expediently provided that the closing member is 
connected to an intermediate piston which is guided closely in a cylinder 
portion, and that the diameter of the intermediate piston and of the 
cylinder portion is substantially smaller than that of the relief cylinder 
and the relief piston, while the cross-sectional area of the valve seat 
and of the intermediate piston shall be essentially the same. In order to 
ensure a perfect return flow to the supply reservoir, a further expedient 
provision is that the closing member is encased at a distance by a 
housing, from which an outlet leading toward the supply reservoir branches 
off. 
In order to achieve in the brake circuit directly pressurized by the 
controlled pressure substantially the same brake pressure as in the second 
brake circuit pressurized by the master cylinder, a form of embodiment 
advantageous independent of the abovementioned characteristics provides 
that into the hydraulic conduit feeding the controlled pressure to the 
first brake circuit a pressure reducing valve is insertedwhich adapts the 
controlled pressure to the pressure prevailing in the further brake 
circuit in accordance with the frictional losses of the master cylinder. 
Here the embodiment can be such that the pressure reducing valve comprises 
a piston provided with an opening and loaded against the pressure exerted 
by a pressure adjusting spring, in which piston a closing push rod with a 
closing plate is disposed opposite an outlet port and closes said outlet 
port when abutting on it and with its other end cooperates with a valve 
closing the opening in the piston in such a way that the closing push rod 
upon closing the outlet port opens the valve when there is further 
movement of the piston, with the inlet port being connected to the 
controlled pressure, the outlet port to the supply reservoir, and the 
chamber between the piston and the outlet port being connected by way of a 
connecting port to the first brake circuit.

DETAILED DESCRIPTION 
According to FIG. 1, the brake pedal 16 of a motor vehicle axially 
pressurizes a brake valve 17 arranged in the first master piston 24 of a 
tandem master cylinder 11, said brake valve 17 consisting of a valve cone 
125 pre-loaded towards a valve seat 124 against the force of a spring 121. 
The chamber around the valve push rod 126 connected to the valve cone 124 
is in communication with a hydraulic pump 18 driven by a motor 147 by way 
of radial bores 127 and an annular gap 128 between the master piston 24 
and the master cylinder 11 as well as a port 129 and a check valve 130. 
When the brake pedal 16 is pressed down, the valve cone 125 lifts off more 
or less from the valve seat 124 so that the pressure medium supplied by 
the pump 18 can pass through bores 131 in the member 124' supporting the 
valve seat 124 into the controlled pressure chamber 132 for pressurization 
of the master piston 24. The valve push rod 126 has an axial center bore 
133 which communicates with the supply reservoir 27 by way of a channel 
134 in the master piston 24, an annular chamber 135 and a hydraulic 
conduit 136. In this way the hydraulic pressure built up in the pressure 
chamber 132 can be reduced by way of the center bore 133 and cross bores 
133' connecting it with the pressure chamber 132. When the valve push rod 
126 is pushed forward, the lateral bores 133' are closed as they enter 
into a slide counterpart 124". 
The hydraulic conduit 51 coming from the pump 18 and/or the check valve 130 
comprises a branch 51' leading to a control valve 50 in which a ball valve 
138 pressurized by a piston 137 is disposed and as is described in detail 
in DE-OS No. 33 15 731. 
A helical compression spring 139 forcing the piston 137 preloads the ball 
valve 138 in its opening position. The pressure chamber 137' located on 
the side of the piston 137 facing away from the spring 139 communicates 
with the controlled pressure chamber 132 of the master piston 11 by way of 
a hydraulic conduit 140 so that the controlled pressure GD will also 
become effective in the pressure chamber 137' at the piston 137. The 
pressure in the pressure chamber 137' overcomes the preload of the helical 
compression spring 139 at relatively low controlled pressures in the order 
of no more than 1 to 2 bar. 
The ball valve 138 is inserted between the branch conduit 51' and a 
pressure accumulator 45 connected by way of a conduit 51", the capacity of 
said pressure accumulator 45 being such that it can supply the pressure 
required until starting of the pump 18, said accumulator being otherwise 
as described in detail in DE-OS No. 33 15 731. 
Furthermore, a control valve 46 is connected to the controlled pressure by 
way of the conduit 140, the pressure chamber 137' and a conduit 141. The 
conduit 141 leads to a pressure chamber 53' located on one side of a 
piston 53 arranged in a cylinder 52. On the other side of the piston 53 a 
closing member spring 49 is active which by way of the piston 53 and a 
push rod 55 acts upon an intermediate piston 58 arranged in a narrowed 
cylinder portion 59. Between the intermediate piston 58 and the piston 53 
having a substantially larger surface a weak auxiliary spring 54 is 
inserted whose force is just sufficient to displace a closing member 47 
secured by a rod 58' to the intermediate piston 58' to the intermediate 
piston 58 into its closing position on a valve seat 56. 
On its side facing away from the cylinder 52, where the annular seat 56 is 
disposed, the control valve 46 comprises a pressure inlet port 57 
connected with the output of the pump 18. 
The closing member 47 is encased by a housing 60 at a radial distance all 
around having a lateral outlet 61 which is connected with the supply 
reservoir 27 by way of a hydraulic conduit 48. 
At the brake pedal 16 an electric switch 143 is provided which, in the same 
way as a further switch 145 provided at the pressure accumulator 45, 
connects an electric line 144 to ground when one of the switches or both 
are closed. The switch 145 is closed when the accumulator 45 is discharged 
to such an extent that it can no longer provide sufficient pressure. The 
switch 143 is closed when the brake pedal 16 is pushed down. 
A relay 146 provided with a make contact is excited upon closure of one of 
the switches 143, 145 or upon closure of both switches 143, 145 and thus 
switches on the pump motor 147, which is indicated by a broken connecting 
line 146'. 
According to FIG. 1, in the master cylinder 11 comprising two annular steps 
83, 83' protruding outward at an axial distance the master piston 24 
expanding radially outward in corresponding steps is disposed so as to be 
axially slidable. Accordingly, the master piston 24 comprises step-type 
enlargements 24', 24"" and the master cylinder 11 step-type enlargements 
11', 11". 
Gaskets 96 ensure the required sealing between the individual pressure 
chambers 22, 25, 135, 132 to be separated from each other. 
On its side facing away from the pedal 16 the master piston 24 is provided 
with a cylindrical axial bore 78 so that it comprises an annular portion 
24'" into which a central peg 80 firmly secured on the master cylinder 
bottom 79 extends from said master cylinder bottom 79. On the inner wall 
of the annular portion 24'" a collar is disposed as check valve 23, said 
collar in a sealing function sitting closely to the outer wall of the peg 
80 and at its back being limited axially by a ring 89 which is adjacent to 
the annular chamber 22 surrounding the peg 80, with the chamber 22 in turn 
communicating by way of a hydraulic conduit 101 and a check valve 22' with 
the supply reservoir 27. Since the collar is designed at the same time as 
check valve 23, it opens a flow path when the pressure in the annular 
chamber 22 is higher than the pressure in the pressure chamber 76 of the 
master piston 24. In front of the check valve 23 formed by the collar, 
between the peg 80 and the annular portion 24'" there is an annular gap 90 
to allow unimpeded passage of flow medium between the annular chamber 22 
and the pressure chamber 76. 
At its front end the peg 80 comprises an axial recess 91 in which a 
readjusting spring 82 extending to the bottom 24" of the master piston 24 
is arranged. Furthermore, the recess 91 contains an axial stop pin 92 
which at one end is disposed at the peg 80 and with its head provided at 
the other end engages behind a stop sleeve 93 secured to the bottom 24" of 
the master piston 24. This limits the return movement of the master piston 
24 caused by the readjusting spring 82. 
Instead of designing the collar 23 as check valve, a check valve 23' 
functioning accordingly can also be provided at the rear end of the stop 
pin 92 in the peg 80, said valve 23' being connected by way of a bore 23" 
with the annular chamber 22 and also adjoining to the pressure chamber 76. 
From the pressure chamber 76 the hydraulic conduit 97 provided in the peg 
80 branches off toward a diagonal brake circuit II; the hydraulic conduit 
97 leads to the closing valve SO of the two wheel brake cylinders 12, 13 
of the one diagonal. The wheel brake cylinders 12, 13 are further 
connected by way of an opening valve SG to the hydraulic line 87 leading 
to the supply reservoir 27. 
The closing valve SO and the opening valve SG are actuated by a wheel brake 
slip control system (not illustrated), which is indicated by control 
conduits 88 shown by broken lines. In the event of a wheel slip condition 
being detected by a sensor at the diagonal brake circuit II, at first the 
closing valve SO will close in order to limit the pressure in the wheel 
brake cylinders 12, 13. If this is not sufficient to avoid the slip 
condition, the opening valve SG will open in addition. In general, the 
beginning of a wheel slip condition will entail a cyclical opening and 
closing of the valves SO and SG, which causes consumption of a certain 
amount of pressure medium which will be replenished by the pump 18 in a 
way yet to be described. 
The other diagonal brake circuit I is equally provided with a closing valve 
SO and an opening valve SG pressurized by the wheel brake slip control 
system, the opening valve SG being connected to the hydraulic conduit 87. 
The closing valve SO of the diagonal brake circuit II is connected by way 
of a hydraulic conduit 86', a pressure reducing valve 63 and a hydraulic 
conduit 86 to the hydraulic conduit 141 pressurized by the controlled 
pressure GD. 
The closing and opening valves, SO and SG respectively, of the diagonal 
brake circuit I operate in an analogous way like the corresponding valves 
of the diagonal brake circuit II in the event of a beginning wheel slip 
condition at the wheels associated to the brake circuit I. 
The pressure reducing valve 63 comprises an axially slidable piston 64 with 
a center opening 65 through which a closing push rod 66 extends which at 
its end facing away from the opening 65 bears a closing plate 66' which 
closes an outlet port 67 fluid-tight when abutting to it. 
The outlet port 67 communicates by way of the hydraulic conduits 62, 87 
with the supply reservoir 27. From the chamber 71 between the piston 64 
and the outlet port 67 the hydraulic conduit 86' branches off. 
At the end of the closing push rod 66 facing away from the outlet port 67 
there is a valve 68 which, in FIG. 1, is formed by a valve cone 68' and 
the edge of the opening 65. A spring 69 preloads the piston 64 in the 
direction of the pressure inlet port 70 connected with the hydraulic 
conduit 86, while a further spring 104 presses the valve cone 68' against 
the opening 65 in the piston 64. 
The spring 69 is so designed that the controlled pressure GD pushes the 
piston 64 in FIG. 1 downward, until the closing plate 66' closes the 
outlet port 67. Thus, the connection between the closing valve SO of the 
brake circuit I and the supply reservoir 27 is interrupted. Compressing 
the spring 104 the piston 64 then moves further downward, while the valve 
68 opens and between the edge of opening 65 and the closing push rod 66 a 
freeflow path is created in axial direction, through which a connection is 
established between the hydraulic conduits 86, 86'. The slightly reduced 
controlled pressure is now applied to the brake circuit I. The springs 69, 
104 are so designed that the pressure in the brake circuit I is reduced to 
such an extent that it corresponds to the pressure in the brake circuit 
II, which is lower than the controlled pressure because frictional losses 
occur between the master piston 24 and the master cylinder 11. 
In the event of a wheel brake slip condition being controlled in the brake 
circuit I, the amount of pressure medium consumed thereby will be 
replenished without any problem directly by way of the hydraulic conduit 
140, the pressure chamber 137', the hydraulic conduit 86, the pressure 
reducing valve 63, and the hydraulic conduit 86'. 
For immediate re-establishment of pressure in the brake circuit II, the 
relay valve 21 is provided which is connected on one side to the hydraulic 
conduit 86 and on the other side by way of a hydraulic conduit 100 to the 
annular chamber 22, which relay valve 21 in the event of a beginning slip 
condition switches over from its normal closing position as shown in FIG. 
1 into the opening position. Energizing of the relay valve 21 by the wheel 
brake slip control system (not shown) is illustrated by a control line 88' 
indicated as a broken line. When the relay valve 21 is in its open 
position, the controlled pressure GD is lead from the hydraulic conduit 86 
by way of the hydraulic conduit 100 into the annular chamber 22, from 
where the pressure medium either passes by the collar designed as check 
valve 23 or through the check valve 23' into the pressure chamber 76 and 
from there into the diagonal brake circuit II. 
Since due to the pressure build-up in the pressure 76 after the switch-over 
of the relay valve 21 and the opening of the check valve 23 or 23' a 
return force is exerted on the master piston 24 which is in excess of the 
transverse force exerted from the pressure chamber 132, the pedal holding 
annular chamber 25 is provided at the annular step 83 of the master 
cylinder 11, which chamber 25 is in communication with a pedal holding 
valve 26 by way of a hydraulic conduit 32. The pedal holding valve 26 
contains a valve piston 28 sliding in a cylinder 29, the pressure side 105 
of the valve piston 28 being connected by way of a hydraulic conduit 106 
to the hydraulic conduit 100 which, with opened relay valve 21, is 
pressurized by the controlled pressure GD. At its end facing away from the 
pressure chamber 105 the valve piston 28 comprises a closing plate 75 
which is disposed opposite a connecting port 73 leading toward the supply 
reservoir 27. A readjusting spring 110 preloads the valve piston 28 
against the pressurization so that without pressurization the valve piston 
28 always adopts the position shown in FIG. 1. Upon application of the 
controlled pressure GD in the event of a beginning wheel slip to the 
pressure side 105, the valve piston 28 is displaced downwards and its 
closing plate 75 closes the connecting port 73, whereby the pedal holding 
annular chamber 25 connected by way of a hydraulic conduit 32 with the 
lateral port 74 of the pedal holding valve 26 is separated from the supply 
reservoir, which hinders the return movement of the piston 24 upon opening 
of the relay valve 21, i.e. it remains in its position adopted at that 
moment. 
The mode of operation of the vehicle brake illustrated in FIG. 1 is as 
follows: 
When, with the accumulator 45 being in loaded condition due to displacement 
of its piston 45' into its left-hand end position, the brake valve 17 is 
opened by pushing down the brake pedal 16, the pressure accumulator 45, by 
releasing the spring 45", first delivers by way of a control valve 50 the 
pressure required for generating the controlled pressure in the pressure 
chamber 132. At the same time, however, due to the closing of the switch 
143, the motor 147 starts and builds up pressure at the pump 18. Due to 
the action of the spring 139, the value 138 is still in open position. 
As soon as there is a controlled pressure of approximately 1 to 2 bar in 
the pressure chamber 132, the piston 137 of the control valve 50 is 
displaced towards the right and the ball valve 138 closes, so that the 
pressure accumulator 45 is separated from the branch conduit 51'. At the 
same time the piston 53 of the control valve 46 is displaced to the right, 
thus making the spring 49 ineffective. 
Now the pressure of the pump 18 only requires a comparatively low increase 
relative to the controlled pressure GD prevailing in the pressure chamber 
132 to open the closing member 47, whereby an unnecessary pressure 
build-up at the output side of the pump 18 is prevented and the pressure 
medium runs off to the supply reservoir 27. 
When the controlled pressure is increased, however, the controlled pressure 
at the intermediate piston 58 and the pump pressure at the pressure inlet 
57 are essentially balanced, so that the closing member 47 remains in 
abutment with the valve seat 56 and thus the pressure in the pressure 
chamber 132 can build up further. 
After a braking operation the switch 143 opens and the controlled pressure 
GD in the pressure chamber 132 disappears, whereupon the closing member 47 
is pressed against its seat 56 by considerable force exerted by the spring 
49 and the pressure accumulator 45 can be loaded again with the required 
pressure by way of the ball valve 138 now opened again, whereupon the 
switch 145, having been closed in the unloaded condition, opens and the 
motor 147 is switched off by way of the relay 146. 
According to FIG. 2 the valve 68 in the pressure reducing valve 63 consists 
of a ball 68" pressed by a spring 104' against the edge of the opening 65. 
The closing push rod 66 is axially slidable within an extension 64' of the 
piston 64 and with its closing plate 66' presses against the outlet port 
67 when the piston 64 is pushed downward. As soon as the closing plate 66' 
is seated on the outlet port 67, the closing push rod 66, in case of 
further downward movement of the piston 64, will move upward relative to 
the piston 64 and thus open the valve 68. 
FIG. 3 shows the pedal holding valve 26 in enlarged scale. In the area 
where the hydraulic conduit 106 is led in, the valve piston 28 comprises 
an annular collar 112 which surrounds the inlet port and seals it and 
ensures that the valve piston 28 starts moving downward with a jerk only 
when there is an increased controlled pressure. 
Whereas the connecting port 74 is disposed in the side of the cylinder 29, 
the connecting port 73 leading toward the supply reservoir 27 is disposed 
in the center of the bottom of cylinder 29. The blocking of the connection 
between the ports 73, 74 is effected by a closing plate 75 disposed, 
axially offset, at the lower end of the valve piston 28, which closing 
plate 75 is seated on the connecting port 73 when the valve piston 28 is 
displaced and thus tightly closes said port.