Hydraulically-operated actuator assemblies for vehicle brakes

An actuator assembly includes first and second components which are movable away from each other to initiate application of a brake, upon pressurization of a pressure space between the components. An automatic adjuster assembly co-operating with the components comprises an axially extending member including a piston portion working in a bore in the first component of which the end of the bore remote from the pressure space is vented to atmosphere, a friction ring co-operating with the member and coupled to the component for relative movement through a distance corresponding to desired braking clearances, a spring for biassing the member away from the first component to maintain it in engagement with the other component when the pressure space is pressurized and until the pressure acting on the member overcomes the force in the spring whereafter the member moves in the opposite direction until that movement is arrested by the co-operation of the ring with the component upon release of the brake, the spacing between the components is determined by the co-operation between the component and the member.

This invention relates to hydraulically-operated actuator assemblies for 
vehicle brakes of the kind comprising a piston and cylinder assembly which 
includes a pair of components which are movable relatively away from each 
other to initiate application of the brake upon pressurisation of a 
pressure space between the components, and an automatic adjuster assembly 
co-operates with the two components to determine the relative retracted 
positions of the components, whereby to establish and maintain braking 
clearances within predetermined limits. 
In hydraulically-operated actuator assemblies of the kind set forth it is a 
problem to arrange that such adjuster assemblies, in use, do not respond 
to deflections of parts of the brake when the brake is applied, otherwise 
there may be a tendency for the adjuster assemblies to "over adjust" and 
excessive braking clearances may have to be provided to compensate for 
such "over adjustment". 
In GB-B-2 148 425 we have disclosed an hydraulic actuator comprising a pair 
of oppositely acting hydraulic pistons working in a bore and incorporating 
an automatic slack adjuster assembly which is housed in aligned bores in 
the pistons and acts as a strut to determine the relative retracted 
positions of the pistons. The adjuster assembly comprises a clamp member 
movable with one of the pistons and an adjuster member, the members 
including interengaging parts which co-operate with each other to form a 
releasable detent, in combination with a spring ring for biassing the 
interengaging parts into engagement. The piston and the clamp member are 
provided with interengaging inclined faces to provide a wedge action and 
enhance the engagement of the interengaging parts when the bore is 
pressurised, and a stop loaded by a spring is provided in the bore with 
which the adjuster member is adapted to co-operate when movement of that 
member in a brake-applying direction exceeds a distance sufficient to take 
up the braking clearances. Additional movement of the clamp member in the 
same direction is operative to increase the effective length of the strut 
with the detent released until the force of the engagement of the 
interengaging parts at a predetermined point exceeds the force of the 
spring. The detent is re-engaged to prevent further relative axial 
movement between the two members whereafter, further movement of the strut 
with the piston is accommodated by movement of the stop against the 
loading in the spring. The adjusted retracted position defines a position 
to which the piston can retract when the brake pressure is relieved and 
which determines the braking clearances, no relative movement between the 
clamp member and the adjuster member taking place when the brake pressure 
is so relieved. 
In GB-B-2 148 425 the said predetermined point is chosen as the point at 
which the brake is fully applied and beyond which the parts of the brake 
begin to deflect. In such a case the adjuster assembly is said to be 
load-insensitive since it will adjust only as a result of excessive piston 
movement due to wear of brake linings and not in response to excessive 
piston movement due to deflection of the parts of the brake after the 
brake had been fully applied. 
The adjuster assembly incorporated in the actuator assembly of GB-B-2 148 
425 is relatively complex in construction. 
According to our invention in an hydraulically-operated actuator assembly 
of the kind set forth the automatic adjuster assembly comprises an axially 
extending member co-operating with the two components and including an end 
portion which is received in a bore in one of the components, means for 
preventing relative axial movement between the axially extending member 
and the other component at least until the braking clearances have been 
taken up, a ring co-operating with the member and also coupled to the said 
one component for relative movement through a limited distance 
corresponding to the braking clearances and determined by movement between 
first and second stops, relative movement between the components through a 
distance greater than the said limited distance causing the ring to slide 
on the member, whereafter, upon release of the brake, the ring co-operates 
with the said one component to determine an adjusted release position for 
the components. 
Preferably the end portion comprises a piston which works in the bore in 
the said one component, and the end of the bore remote from the pressure 
space is vented to atmosphere, a spring being provided for biassing the 
axially extending member relatively away from the said one component to 
maintain the member in engagement with the other component when the 
pressure space is pressurized and until the pressure in the pressure space 
which acts on the end of the member remote from the piston portion is 
sufficient to overcome the force in the spring whereafter the member moves 
in the opposite direction away from the second stop until that movement is 
arrested by the co-operation of the ring with the said one component, upon 
release of the brake the spacing between the components defining the 
adjusted position is determined by the co-operation between the said other 
component and the member. 
The point at which the member moves in the said opposite direction is 
chosen as the point at which fluid pressure in the pressure space will 
rise rapidly as soon as the braking clearances have been taken up. Thus 
any relative movement between the components after this point has been 
attained, for example due to deflection of parts of the brake, will have 
no effect on the adjuster assembly and the desired braking clearances. Our 
adjuster is therefore load-insensitive. 
The spring may act between the piston portion and the base of the bore in 
which it works. In a modification in which the member extends into a 
clearance bore in the said other component, the spring may be housed in 
the clearance bore and act between complementary abutments on the member 
and on the component. 
In another construction the axially extending member is fast with the other 
component, and the second stop is perforated. 
This enables the system to be bled fully since air can escape past the 
second stop through the perforations, even though the ring may be in 
co-operation with it. 
Preferably the second stop comprises a sprag washer through which the 
member extends and which has a spragging engagement with the wall of a 
counterbored recess in the one component, the sprag washer being movable 
in the counterbore only in an inward direction. 
This facilitates assembly since relative movement between the two 
components when the actuator assembly is assembled acts to urge the spring 
washer into the counterbored recess and into its operative position to 
define the second stop as the two components move into co-operation. 
In the construction described above the ring may comprise a resilient 
friction ring which is urged into a wedging engagement with the member to 
define the adjusted release position. 
When the piston and cylinder assembly comprises first and second opposed 
pistons working in a common bore in a cylinder, the member projects from 
one piston into a bore in the other piston, and the ring is retained in a 
counterbored recess in the said one piston, the braking clearances being 
defined by movement of the ring in the recess through a distance defined 
by the spacing between the first and second stops. 
Conveniently the first stop comprises a face at the base of the recess. 
Preferably the recess has a wall including a portion of frusto-conical 
outline, and the first stop comprises a part of the portion of 
frusto-conical outline which applies a wedge action to the ring, in turn 
to clamp it against the rod, the second stop comprising a circlip through 
which the member extends into the said other piston. 
Although our actuator assembly may be used to operate any suitable vehicle 
brake, either disc or drum, it is particularly convenient to utilize it 
for initiating operation of any known self-energizing disc brake of the 
spreading type, in which the assembly is installed within a housing of the 
brake and the components act between radial lugs on a pair of pressure 
plates, the relative anuglar movement between which is accompanied by a 
relative axial movement to urge rotatable friction discs into engagement 
with spaced opposed braking surfaces in the housing.

The hydraulic actuator assembly illustrated in FIG. 1 of the accompanying 
drawings is adapted to initiate application of any known brake of the 
self-energising spreading type. 
The hydraulic actuator assembly comprises a cylinder 9 which is carried 
from the housing of the brake and has a longitudinally extending 
open-ended bore 10 in which work a pair of opposed pistons 11, 12 each 
provided with a seal 13 adjacent to its inner end. The axis of the 
cylinder 9 is tangential to the pressure plates of the brake and lies in a 
plane which is angled with respect to a transverse plane parallel to the 
plane of each plate. 
An automatic slack adjuster assembly 20 is incorporated in the actuator. 
As illustrated the adjuster assembly comprises a member 21 in the form of a 
rod which co-operates with both pistons 11, 12. One end of the rod 21 
comprises a piston portion 22 which carries a seal 23 and works in a 
longitudinally extending bore 24 in the piston 11, and the other end of 
the rod 21 extends into a clearance bore 25 in the piston 12 and normally 
engages with a face 26 at the inner end of the bore. The end of the bore 
24 remote from a pressure space 27 defined in the bore 10 between the 
pistons 11, 12 is vented to atmosphere through a radial port 28. 
A resilient friction snap ring 29 is retained in a counterbored recess 30 
at the inner end of the bore 24 by means of a circlip 31. The rod 21 is 
slidable through the ring 29. The wall of the recess 30 includes a 
frusto-conical portion 32 of which the end of smaller diameter terminates 
at the base 33 of the recess. The external diameter of the ring 29 is 
greater than the end of the frusto-conical portion 32 which is of smaller 
diameter, but is smaller than the end of greater diameter. The ring has a 
limited axial movement in the recess 30 between a point on the portion 32, 
which defines a first stop, and the circlip 31, which defines a second 
stop. 
In the retracted position shown in the drawing the spacing between the 
pistons 11 and 12 is defined by the engagement between the ring 29 and the 
frusto-conical portion 32 at the point defining the first stop and by 
means of which the ring 29 is clamped against the rod 21 by the wedging 
action of the portion 32. This prevents movement of the rod 21 through the 
ring 29 in a direction away from the pressure space 27. 
A compression spring 34 is housed in the outer end of the bore 24 and acts 
on the piston portion 22, normally to maintain the rod 21 in contact with 
the face 26 at the base of the bore 25. 
When the brake is to be applied hydraulically, hydraulic fluid admitted to 
the pressure space 27 urges the pistons 11, 12 relatively away from each 
other, in turn to move the pressure plates angularly in opposite 
directions to initiate application of the brake in a known manner until 
the braking clearances are taken up. During this operation, the spring 34 
acts to urge the piston portion 22 axially in order to maintain the rod 21 
in contact with the face 26, and the ring 29 is carried with the rod 21, 
relatively away from the frusto-conical portion 32. 
When no adjustment is required, the relative movement between the pistons 
11 and 12 with the rod 21 accompanying the piston 12 will only be 
sufficient to cause the ring 29 to engage with the circlip 31 at the point 
that the braking clearances are taken up. 
If, however, adjustment is required, then movement of the ring 29 with the 
rod 21 in the brake-applying direction will be arrested by the circlip 31, 
and the rod 21 will be withdrawn through the ring 29 until the braking 
clearances have been taken up. 
As soon as the braking clearances have been taken up, the fluid pressure in 
the pressure space 27 will rise rapidly, acting on the end of the rod 21. 
Since the opposite end of the rod 21 is exposed to atmosphere, the rod 21 
will move back against the force in the spring 34 and carry the ring 29 
back with it until movement of the ring 29, and the rod with it, are both 
arrested by the engagement of the ring 29 with the frusto-conical portion 
32. The wedge angle between the portion 32 and the rod 21 is so chosen 
that the rod 21 will be firmly held in that position by the action of 
friction. 
When the pressure in the pressure space 27 is released, the pistons 11 and 
12 are moved towards each other and are held in a spaced apart position 
determined by the effective length of the rod 21 namely the distance by 
which it projects from the ring 29. This distance remains constant in a 
brake-applying sequence in which no adjustment has occurred, but increases 
to compensate for wear of the friction lining when the rod 21 has slid 
through the ring 29 as described above. 
To facilitate installation of the actuator assembly in the brake, it is 
desirable to maintain the effective length of the assembly at a minimum by 
holding the rod 21 in a retracted position, and with the spring 34 
prevented from acting on it to urge the pistons 11 and 12 away from each 
other. 
This can be achieved, as shown in FIG. 2, by providing the rod 21 with an 
axial extension 40 which the spring 34 surrounds, and inserting a wire 42 
through the radial port 28 to co-operate at its inner end in a groove 41 
in the extension 40. The wire acts to hold the rod 22 in a retracted 
position and as an abutment for the outer end of the spring 34. 
After installation in the brake, the wire 42 is withdrawn from the port 28 
to release the spring 34 and the rod 40. 
In the modified construction illustrated in FIG. 3, the port 28 traverse 
the bore 24 and the wire 42 is passed through a cross-drilling 43 in the 
rod 40. 
In the actuator assembly illustrated in FIG. 4, the spring 34 comprises a 
compression spring which acts between a head 45 at the end of the rod 21 
remote from the piston portion 22 and a circlip 46 in the wall of the bore 
25. The head 45 incorporates a damper or flow control valve 47 to ensure 
that adjustment does not occur when the brake is released, thereby 
prevented any tendency for over adjustment to occur due to a sudden 
pressure drop as a result of the pedal being released quickly. 
The rod 21 is also provided with a grooved working surface 49 to enhance 
the grip between the ring 29 and the rod 21. 
The rod is of smaller diameter than that of the piston portion 22 and the 
forward face 48 of the piston portion 22 defines a wear stop which is 
engageable with the ring 29 at a predetermined point in order to limit the 
spacing between the pistons 11 and 12, thereby preventing further 
adjustment beyond this point. 
The construction and operation of the assembly of FIG. 4 are otherwise the 
same as that of FIG. 1, and corresponding reference numerals have been 
applied to corresponding parts. 
The hydraulic actuator illustrated in FIGS. 5 and 6 of the drawings 
comprises a cylinder body which is carried from a housing of a brake and 
has a longitudinally extending open-ended bore 50 in which work a pair of 
opposed pistons 51, 52 each provided with a seal 53 adjacent to its inner 
end. The piston 51 has a couterbored recess 54 in its inner end into which 
a spigot 55 on the adjacent end of the piston 52 projects with substantial 
clearance when the two pistons are in engagement. 
An automatic slack adjuster 60 is incorporated in the actuator. As 
illustrated the adjuster 60 comprises a member 61 in the form of a rod 
which projects axially from the inner end of the piston 52 into a 
complementary blind bore 62 extending from the inner end of the recess 54. 
The rod 61 is retained in the piston 52 against relative axial movement. 
The rod 61 is slidable through a resilient friction snap ring or circlip 
63 which is adapted to co-operate with a frusto-conical face 64 between 
the bore 62 and a counterbored 65. 
A spragging washer 66 surrounding the rod 61 and through which the rod 61 
is freely movable, has an internal annular portion 67 with which the 
spigot 55 is engageable, and a series of angularly spaced teeth 68 
projecting radially from the outer peripheral edge of the portion 67 are 
inclined relatively towards the piston 52. This enables the washer 66 to 
slide into the counterbored recess 54, but the teeth 68 engage with the 
wall of the recess 54 to prevent movement of the washer 66 in the opposite 
direction with respect to the piston 51. 
The friction ring 63 has a limited movement with the rod 61 between the 
face 64, which defines a first stop, and the washer 66, which defines a 
second stop, through a distance equivalent to brake clearances. 
The external diameter of the ring 63 is greater than the end of the 
frusto-conical portion 64 which is of smaller diameter but is smaller than 
the end of greater diameter. In a retracted position the spacing between 
the pistons 51, and 52 is defined by an engagement between the ring 63 and 
the fruto-conical portion 64, at a point which defines the first stop and 
by means of which the ring 63 is clamped against the rod 61 by the wedging 
action of the portion 64. This prevents movement of the rod 61 through the 
ring 63 in a direction towards the piston 51. 
When fluid is admitted to the cylinder bore 50 and pistons 51 and 52 move 
away from each other, the rod 61 which is retained firmly in the piston 
52, moves with the piston 52 carrying the ring 63 with it and relatively 
away from the piston 51. 
Normally, when no adjustment is required, the relative movement between the 
pistons 51 and 52 will only be sufficient to cause the ring 63 to engage 
with the spragging washer 66 when the brake is applied fully. However, 
should adjustment in fact be required, additional relative movement 
between the pistons 51,52 will cause the rod 61 to be withdrawn through 
the ring 63, after movement of the ring 63 with the rod 61 in that 
direction has been arrested by the washer 66. 
When the brake is released, the pistons 51 and 52 retract but are held in 
an adjusted, spaced apart, position by the further engagement of the ring 
63 with the frusto-conical portion 64 to clamp the ring 63 against the rod 
61 with the wedging action as described above. 
To assemble the actuator assembly the rod 61 is first made fast in the 
piston 52. The spragging washer 66 and the ring 63 are both slid onto the 
rod 61 in close proximity to the spigot 55. The piston 51, and the piston 
52 are then slid into the bore 50 until they abut at adjacent ends. This 
urges the washer 66 into the recess 54 as described above to determine the 
second stop. The ring 63 is spaced from the face 64. 
When the fluid is first admitted to the cylinder bore 50 and the pistons 51 
and 52 move apart, movement of the ring 63 with the rod 61 is arrested by 
the washer 66 as described above to determine the brake clearances upon 
release of pressure from the bore 50. 
When the system is bled, any air trapped between the rod 61 and the bore 62 
can escape through gaps between the teeth 68, even though the ring 63 may 
be in engagement with the washer 66, since these gaps are disposed 
radially outwards of the ring 63.