Hydraulic anti-skid braking systems for vehicles

In an hydraulic anti-skid braking system of the pump and de-boost type, the de-boost piston is held in an advanced inoperative position by a volume of fluid trapped in a space and supplied by a pump. A bleed device communicating with the space is held in an open position during initial bleeding of the system by a friction device which acts as a stop and which is releasable when the bleed device is subjected to a predetermined pressure. This enables initial bleeding to be carried out automatically and without an operator operation to open and close a bleed device. The device comprises a pressure-responsive detent which is located in a radial bore in the housing and has a releasable engagement at its inner end in a groove in a spool which comprises the bleed device.

This invention relates to hydraulic anti-skid braking systems for vehicles 
of the kind in which a supply of operating fluid from a supply to a 
vehicle brake is modulated by a modulator assembly in accordance with skid 
signals from skid sensing means, and an hydraulic pump incorporating at 
least one plunger working in a bore has a working chamber which is in 
communication with the modulator assembly to control brake re-application 
following skid correction. 
In the anti-skid braking systems described in GB-A-2029914 and GB-A-2069640 
the modulator assembly comprises a bore in which works a de-boost piston 
for co-operation with a control valve assembly adapted to control 
communication between the supply of operating fluid and the brake through 
an expansion chamber defined in the bore between the piston and the 
control valve assembly. Normally the piston is held in an inoperative 
advanced position by a trapped volume of fluid supplied by the pump and, 
in this position, the valve assembly is fully open and the effective 
volume of the expansion chamber is at a minimum. When a skid signal is 
produced the trapped volume of fluid is released which permits the piston 
to move into a retracted position, initially to permit the valve assembly 
to close and isolate the supply from the brake, and subsequently to 
increase further the effective volume of the expansion chamber, whereby to 
relieve the brake-applying pressure. Following correction of the skid, the 
pressure from the pump is operative to urge the piston towards its 
advanced position, initially to re-apply the brake by pressurising the 
fluid in the expansion chamber, and subsequently opening the valve 
assembly to re-establish communication between the supply and the brake. 
It is a problem in anti-skid braking systems of the kind set forth, such as 
the systems of GB-A-2029914 and GB-A-2069640, to ensure that initial 
bleeding of the system on assembly of the system on the vehicle can be 
carried out automatically and without an operator operation to open and 
close a bleed device. In addition it is also desirable to ensure that when 
the pump, and in particular the trapped volume of fluid supplied by the 
pump to support the piston in the advanced position, is bled, there is no 
danger of the bleed device remaining accidentally in an open position. 
This would cause loss of fluid from the system and consequent closure of 
the control valve assembly, ultimately rendering the pump inoperative. 
In our GB Patent publication No. 2,155,574 we have disclosed an hydraulic 
braking system of the kind set forth in which the modulator assembly 
comprises a bore in which works a de-boost piston for co-operation with a 
control valve assembly adapted to control communication between the supply 
of operating fluid in the bore between the piston and the control valve 
assembly, and the piston is held in an advanced inoperative position by a 
volume of fluid trapped in a space to which the fluid is supplied by the 
pump, a bleed device communicates with the space, friction means being 
provided for holding the bleed device in an open position during initial 
bleeding of the system, and the bleed device being adapted to close 
automatically when a fluid pressure to which it is subjected attains a 
predetermined value sufficient to overcome the friction means. 
The provision of the friction means enables us, when assembling the 
modulator, to arrange the bleed device in the open position so that 
initial bleeding of the system on assembly of the system on the vehicle 
can be carried out automatically and without an operator operation to open 
and close a bleed device. 
In the specific construction disclosed in GB No. 2155574 the bleed device 
comprises a spool working in a bore in a housing and movable between a 
first bleed position in which ports in the wall of the bore are in 
communication and a second closed position in which communication between 
the ports is cut-off, the friction means comprising a resilient locking 
member which acts between the bore and the spool, the locking member being 
located in a first position during initial assembly of the modulator 
assembly in which it acts as a releasable stop to hold the spool in the 
first bleed position, whereby initial bleeding of the system can take 
place, the force of the locking member being overcome to release the stop 
when a force comprising the pressure in the space acting on a 
pressure-responsive face on the spool attains a predetermined value, 
whereafter the locking member is carried by the spool into a second 
position as the spool is urged from the first bleed position into its 
second position, the locking member thereafter remaining in the said 
second position. 
We have found that the tolerances between the bore, the spool, and the 
resilient locking member are critical to ensure that the spool is able to 
move automatically into the second position when the pressure in the space 
attains the predetermined value. 
According to our present invention in an hydraulic braking system of the 
kind set forth the modulator assembly comprises a housing having a bore in 
which works a de-boost piston for co-operation with a control valve 
assembly adapted to control communication between the supply of operating 
fluid and the brake, and the piston is held in an advanced inoperative 
position by the volume of fluid trapped in a space to which the fluid is 
supplied by the pump, a bleed device communicates with the space, friction 
means being provided for holding the bleed device in an open position 
during initial bleeding of the system, and the bleed device being adapted 
to close automatically when a fluid pressure to which it is subjected 
attains a predetermined value sufficient to overcome the friction means, 
the friction means comprising a pressure-responsive detent responsive to 
fluid pressure in the space and movable between an initial advanced 
position to hold the bleed device in the open position and an inoperative 
retracted position which permits the bleed device to close automatically 
when the pressure attains the said predetermined value, the detent being 
held in its advanced position by frictional engagements with the housing 
and the bleed device, which frictional engagements are overcome by the 
force on the detent due to the pressure acting over a pressure-responsive 
face on the detent. 
Preferably the bleed device comprises a spool working in a bore in the 
housing and movable between a first bleed position in which ports in the 
wall of the bore are in communication and a second closed position in 
which communication between the ports is cut-off, the pressure-responsive 
detent comprising a piston working in a second bore in the housing normal 
to the bore in which the spool works and carrying a stem of which the free 
end is received in a complementary notch in the spool to hold the spool in 
the first bleed position and from which the stem is releasable as the 
detent moves into its retracted position. 
This facilitates construction since the tolerances between the piston, the 
housing, and the bleed device do not have to be held within critical 
limits. 
Preferably the stem has an inclined portion at its free end which 
co-operates with a complementary inclined face at the inner end of the 
notch, whereby to provide a wedge action and facilitate withdrawal of the 
stem against the frictional engagement between the inclined face and the 
notch. 
The notch preferably comprises a radial groove in the spool, and the piston 
may be retained in its bore against accidential removal by a cover plate 
of the dump valve.

The assembly illustrated in the drawing comprises a housing 1 incorporating 
a modulator assembly 2, an hydraulic pump assembly 3, and a pressure dump 
valve 4. A longitudinally extending shaft 5 projecting at opposite ends 
through the housing 1 is coupled at one end to a wheel to be braked and at 
the other end carries skid sensing means (not shown) which is enclosed 
within a cylindrical guard 6 carried from an adjacent end of the housing 
1. 
The dump valve 4 and the skid sensing means may be of any of the forms 
disclosed in GB-A-2029914, and the pump assembly 3 forms the subject of 
GB-A-2069640. These mechanisms need not be described further herein except 
to mention that the pump 3 is urged in one direction by an eccentric 5a on 
the shaft 5 and in the opposite direction by pressure from a 
pedal-operated master cylinder 15 which acts over an operating piston 3a. 
The modulator assembly 2 comprises a bore 8 extending from the dump valve 4 
and in which works a deboost piston 9. The piston 9 is normally urged into 
an inoperative position against a stop comprising a wall 10 at the closed 
end of a sleeve 11 substantially of cup-shaped outline by means of a 
spring 12, and the sleeve 11 is retained in the bore 8 by means of a 
closure 13 for the end of the bore 8 remote from the dump valve 4. 
A control valve assembly 14 housed in the sleeve 11 controls communication 
between the pedal-operated master cylinder 15 and a wheel brake 16 through 
an expansion chamber 17 defined in the bore 8 between the piston 9 and the 
control valve assembly 14 itself. 
The control valve assembly 14 comprises a first valve 18, and a second 
valve 19 which are operative sequentially. 
The first valve 18 comprises a first valve member 20 in the form of a 
stepped piston having a portion of intermediate diameter working in the 
bore of the sleeve 11, an outer portion of largest diameter working in a 
blind bore 21 of the closure 13, an inner portion of smaller diameter 
carrying an annular seal 22 which defines a valve head, and an innermost 
portion of smallest diameter which projects into a circular opening 23 in 
the wall 10. The valve head 22 is engageable with a seating 24 comprising 
an annulus on the wall 10 which surrounds the opening. Normally the head 
22 is spaced from the seating 24 by means of a spring 25 which acts 
between the wall 10 and a shoulder 26 at the step in diameter between the 
intermediate and the smaller diameter portions of the piston 20. 
The piston 20 has an open-ended longitudinally extending bore 27 of stepped 
outline in which the second valve 19 is housed. The second valve 19 
comprises a valve member 28 in the form of a ball which is engageable with 
a seating 29 defined by a shoulder at the step in diameter of the bore 27. 
The ball 28 is normally urged away from the seating 29 by a probe 30 with 
which the piston 9 co-operates against the force in a light compression 
spring 31. 
In the normal inoperative position shown in the drawing the dump valve 4 is 
closed so that the piston 9 is held in an inoperative advanced position in 
which the second valve 19 is held open by the probe 30, and the first 
valve is held open by the spring 25. 
When the brake is to be applied by operation of the master cylinder 15, 
hydraulic fluid is supplied to the brake through radial ports 33 in the 
wall of the sleeve 11, and through the open first valve 18 to the 
expansion chamber 17. Simultaneously fluid also enters the through-bore 27 
through a port 32 in the wall of the piston 20 and can pass to the 
expansion chamber through the open second valve 19. Thus there is a 
substantially unrestricted flow of fluid to the brake. 
Fluid from the master cylinder acts on the shoulder 26 at the step in 
diameter, over the valve head 22, and over the outer end of the piston 20 
which is of greatest area. The unrestricted communication continues until 
the pressure from the master cylinder 15 attains a predetermined value 
such that the force acting on the piston 20 due to the pressure acting 
over the end of greatest area overcomes the force in the spring 25 plus 
the force due to that pressure acting over the shoulder 26 and the head 
22. The first valve 18 then closes, and any further pressure increase can 
only take place at a reduced rate by flow through a restricted path 
comprising the clearance between the valve member 28 and the seating 29. 
When a skid signal is received the dump valve 4 opens to release the volume 
of fluid trapped in the bore 8 so that the piston 9 can retract against 
the force in the spring 12 initially to allow the second valve 19 to close 
since the pressure at which a skid signal can be emitted is higher than 
that at which the first valve 18 will have closed. This cuts-off 
communication between the master cylinder 15 and the brake 16, and the 
retraction of the piston 9 continues to increase the effective volume of 
the expansion chamber 17, whereby to relieve the pressure applied to the 
brake 16. 
Opening the dump valve 4 also unbalances the pump 3 causing it to pump 
fluid in a closed circuit into the bore 8 from a reservoir 32 to which it 
is returned, through the dump valve 4. Since communication between the 
piston 3a and the master cylinder 15 is unrestricted, the pump 3 can move 
freely. 
At the termination of the skid signal the dump valve 4 closes to isolate 
the bore 8 from the reservoir 32 and the pump 3 is then operative to 
increase the pressure in the bore 8, with the result that the piston 9 is 
urged towards its inoperative, retracted, position. Initial movement of 
the piston 9 in this direction re-applies the brake 16 by pressurising the 
volume of fluid trapped in the expansion chamber 17, and subsequent 
movement opens the second valve 19 to establish a restricted flow from the 
master cylinder to the expansion chamber 17 through the clearance between 
the ball 28 and the seating 29. Thus the first valve 18 closes at a 
predetermined pressure independent of the movement of the piston 9. The 
second valve 19 is closed and opened by movement of the piston 9 away from 
and towards its stop 10 on the sleeve 11. When the pressure from the 
master cylinder 15 is reduced below a predetermined value, the first valve 
18 reopens to provide a free and unrestricted communication between the 
master cylinder 15 and the brake 16. 
Bleeding of the space 40 in which fluid is trapped and which acts normally 
to hold the piston 9 in the advanced position is achieved by operation of 
a bleed device 41 shown in detail in FIGS. 2 and 3 of the drawings. 
As shown in FIG. 2 the housing 1 is provided with a stepped bore 42 of 
which the inner end portion 43 of greatest diameter communicates with the 
space 40. A spool 44 of differential outline works in the bore 42. The 
spring 12 acts on the inner end of the spool 44, which is of greater 
diameter, through an abutment plate 45 to urge the spool 44 relatively 
outwardly and into an advanced position. The outer end of the spool 44 
carries a circlip 46 for engagement with the outer end of the housing 1 to 
limit movement of the spool 44 in an inwards direction. 
The spool 44 carries first and second axially spaced seals 47 and 48 of 
elastomeric material. The first seal 47 is permanently disposed within a 
portion 49 of the bore 42 which is of smallest diameter, and the second 
seal 47, carried by the portion of greater, diameter is adapted to 
co-operate with a shoulder 50 at a step at the change in diameter between 
the portion 43 of greatest diameter and a portion 51 of intermediate 
diameter to isolate a radial passage 52, leading to the dump valve 4, from 
a bleed passage 53 to atmosphere. The bleed passage 53 is disposed between 
the two seals 47 and 48 and leads from the intermediate bore portion 51. 
The shoulder 50 is inclined or otherwise radiused to facilitate the 
sealing engagement with, and the sliding movement of, the second seal 48 
into the portion 51 of intermediate diameter. 
A friction device or detent 60 is provided to hold the spool 44 in the 
retracted position shown to enable bleeding of the system to be carried 
out on initial assembly of the hydraulic system in a vehicle. 
The device 60 comprises a piston 61 carrying a fluid-tight seal 62 and 
working in a radial bore 63 in the wall of the housing 1. The piston 61 
carries a stem 64 of reduced diameter, and the stem 64 projects through a 
portion of the bore 63 which is of reduced diameter to co-operate at its 
inner end with the spool 44. 
In an advanced operative position shown in FIG. 3 of the drawings in which 
the piston 61 is assembled in the modulator assembly 2, the stem 64 is 
received in a radial groove 65 in the spool 44 with an inclined face 66 on 
the stem 62 being engaged by a complementary, similarly inclined, face 67 
at the inner end of the groove 65. The face 66 terminates in a chamfer 68, 
with a complementary chamfer 69 being provided at the outer end of the 
bore 63. This holds the spool 44 in the retracted position to enable 
bleeding to be carried out with the ports 52 and 53 in communication. 
The piston 61 is held in the advanced operative position by the friction 
between the seal 62 and the bore 63, and between the stem 64 and the spool 
44. 
When the system has been bleed initially, the pressure of hydraulic fluid 
applied to the space 40 acts on the diameter of the spool 44 which is 
defined by the seal 47, and over the effective area of the seal 62 and the 
inner end of the stem 64. When this pressure attains a predetermined 
value, the effective force acting on the piston 61 is sufficient to urge 
the piston 61 outwardly against the frictional engagement between the seal 
62 and the bore 63, and between the faces 66 and 67, and withdraw the stem 
64 from the groove. The final retracting movement of the piston 61 is 
enhanced by the ramp angle defined by the engagement of the chamfers 68 
and 69 and the pressure acting in the spool 44. The spool 44 then 
automatically moves into the closed position in which the seal 48 isolates 
the port 53 from the port 52, by the pressure acting on the spool 44 and 
the force in the spring 12, and with the circlip 46 moving away from the 
housing 1. Specifically the seal 48 initially engages with the shoulder 50 
to isolate the port 53 from the port 52, and subsequently rides up the 
shoulder 50 and into the intermediate bore portion 51 to enhance the 
sealing engagement. 
A cover plate 70 for the dump valve 4 overlaps a portion of the outer end 
of the bore 63 to prevent accidental removal of the piston 61 from the 
bore 63. 
The spool 44 can be urged inwardly against the load in the spring 12 to 
enable bleeding to be carried out manually, with movement of the spool 44 
in such a direction being limited by the engagement of the circlip 46 with 
the housing which acts as a stop. 
In such an event the friction device or detent 60 is inoperative and is 
held in its retracted position by the friction between the seal 62 and the 
bore 63.