Automatic adjusters for hydraulically operated disc brakes

In an hydraulically operated disc brake a stop is incorporated for determining the retracted position of the piston, the stop being movable towards the disc to compensate for wear of the friction pads and also acting as an anti knock-back device. The stop comprises a plate having an aperture through which a pin projects, the pin being fixed to the cylinder or to a second piston in the cylinder. An abutment member provides an abutment for the plate, and a first spring acts between the abutment member and the plate so as to urge the plate into a locked tilting position inclined with respect to the pin. A second spring acts on the abutment member to urge it in a direction away from the pad assembly.

This invention relates to hydraulically-operated disc brakes for vehicles 
of the kind in which a piston working in an hydraulic cylinder is adapted 
to apply a friction pad assembly to a rotatable disc, and a stop is 
incorporated for determining a retracted position for the piston which, in 
turn, defines a brake clearance between the pad assembly and the disc in 
the off position of the brake, the stop being movable automatically 
towards the disc to compensate for wear of the friction pad assembly. 
When a disc of a disc brake of the kind set forth is rotating in the off 
position of brake, due to disc run out, for example caused by slack in the 
bearings of the wheel with which the disc is rotatable and irregularities 
in the disc itself, the disc may contact the friction pad assembly 
intermittently in turn to transmit a force to the piston in a direction to 
force or "knock-back" the piston in the cylinder by an amount greater than 
the brake clearance. Under such circumstances upon a subsequent brake 
application the volume of hydraulic fluid required to pressurise the 
cylinder to apply the friction pad assembly to the disc is greater than 
that normally required to pressurise the cylinder when only the brake 
clearance has to be taken up. This results in the pedal travel being 
excessive or unacceptably long. 
In hydraulically-operated disc brakes of the kind set forth for vehicles 
the stop also tends to act as an anti knock-back device to prevent the 
piston from being knocked back in the cylinder. 
In one known disc brake of the kind set forth the stop comprises a collar 
which is engageable frictionally with the piston. In another known disc 
brake the stop comprises a ring forming an abutment stop for the piston 
and engaging frictionally with a pin fixed with respect to the cylinder. 
In both these known constructions utilising friction devices a spring is 
incorporated to retract the piston positively in order to establish the 
brake clearance at the termination of a brake application. 
The use of friction device also to act as an anti knock-back device imposes 
a limitation on the operation of the brake since the friction is operative 
in both directions. Thus where the friction is of sufficient magnitude to 
resist the effect of knock-back and the brake is to be applied, the 
pressure of the hydraulic fluid in the cylinder may have to be raised to 
overcome the friction. This may mean that the pressure has to be increased 
to an unacceptable value. 
In another known disc brake incorporating a mechanical adjuster, a stop in 
the adjuster comprises a rod having a one-way locking engagement with the 
wall of an opening in a plate which acts on a piston through a point 
contact. The plate tilts under the action of springs to lock onto therod 
at the termination of a brake application, thereby automatically 
compensating for wear of the braking surfaces. A mechanical adjuster has 
the disadvantage that it must be robustly constructed in order to carry 
the full clamp load at the brake and in fact it is not very rigid due to 
the point contact between the plate and the piston. Under load the point 
contact may deflect and allow extra actuator travel. Mechanical adjusters 
which are known as "load-line" adjusters are capable of supporting or 
providing the full clamp load even when the brake is off. They therefore 
present a very high resistance to knock-back and must be provided with 
built-in slack to counteract the effect of the spurious knock-back. 
According to our invention in an hydraulically-operated disc brake of the 
kind set forth for vehicles the stop comprises a plate having an aperture 
through which projects a pin fixed to the cylinder, or to a component in 
the cylinder such as another piston to which said piston is relatively 
movable, and of a section complementary to that of the aperture, an 
abutment member providing an abutment for the plate, a first spring acting 
between the plate and the abutment member to urge the plate into a locked 
tilting position inclined with respect to the pin, and a second spring 
acting on said abutment member to urge it in a direction away from the pad 
assembly, an abutment on the piston limiting movement of the abutment 
member. 
Preferably the second spring acts directly on the piston and the abutment 
member to urge the abutment member into engagement with a shoulder on the 
piston. 
The second spring acts at all times to return the piston to its initial 
position if "knock-back" occurs while the brake is inoperative. 
Since the knock-back force is taken by the second spring the only 
additional operating pressure necessary is that required to overcome 
friction in the seal of the piston and the frictional force of the 
engagement between the plate and the pin, which is minimal. Since the 
second spring acts to return the piston to a position to maintain the 
brake clearance substantially constant no additional fluid and no 
additional pedal travel is required to compensate for the effects of 
knock-back. 
The components of the stop may be enclosed in a caged retainer, the second 
spring acting between one end of the retainer and the abutment member so 
as to urge the abutment member and the retainer into engagement with a 
shoulder on the piston, a clearance being provided to allow limited axial 
movement of the retainer with respect to the piston. 
The shoulder may comprise a circlip engaged in a groove in the piston. 
The inner end of the pin is preferably enclosed by a sleeve which is 
axially movable relative to the pin to release the plate for pad removal 
or service. The sleeve is of a length slightly less than the distance 
between the plate and the piston which remains constant whilst the plate 
is in the locked tilted position. The difference between length of the 
sleeve and the said distance is chosen so that it is preferably greater 
than the distance through which the piston is likely to be retracted 
because of knock-back. Thus there is no tendency for the plate to be 
released from its locked tilted position which otherwise would allow the 
brake clearance to increase thereby requiring additional fluid and an 
increased pedal travel on the next brake application. 
When the brake is of the kind in which a fixed caliper straddles the 
peripheral edge of an axially fixed disc and friction pad assemblies are 
applied to opposite faces of the disc by pistons working in cylinders in 
opposed limbs of the caliper, the piston are all identical in construction 
and are as described above.

Two embodiments of our invention are illustrated in the accompanying 
drawings in which: 
FIG. 1 is a longitudinal section through a disc brake for a vehicle; and 
FIG. 2 is a longitudinal section similar to FIG. 1 but showing some 
modifications. 
In the disc brake illustrated in FIG. 1 of the drawings a caliper 1 of 
generally U outline straddlng a portion of a peripheral edge of an axially 
fixed rotatable disc (not shown) is mounted on a fixed part adjacent to 
the disc. Friction pad assemblies 2,3 are located in opposite limbs 4,5 of 
the caliper and each assembly comprises a pad 6 of friction material for 
engagement with one face of the disc, and a rigid backing plate 7 carrying 
the friction pad 6. 
Pistons 8,9 working in opposed cylinders 10,11 in the limbs act to apply to 
friction pads 6 to the disc when the cylinders are pressurised with 
hydraulic fluid. 
Each piston 8,9 is of hollow cup-shaped outline and incorporates adjuster 
stop means 12 for determining the retracted position of the piston and for 
compensating for knock-back. Since both adjuster stop means are identical 
only that incorporated in the piston 8 will be described. 
As illustrated the limb 4 is provided with a circular pin 13 which is fast 
in the end wall of the cylinder 10 and extends towards the closed inner 
end of the piston 8. A compression spring 14 acts between the said closed 
inner end of the piston 8 and an abutment member 15 of top-hat outline 
through which the pin 13 extends and which is normally urged at its 
peripheral edge into engagement with a circlip 16 in the wall of the 
piston 8. 
A locking plate 17 through which the pin 13 extends has a circular opening 
18 of a diameter slightly greater than that of the pin 13. A lug 19 on one 
side of the plate 17 projects radially through an opening 20 in the skirt 
of the abutment member 15 and is engaged by the flange of the member 15 
normally to hold the plate 17 in an inclined position frictionally locked 
onto the pin 13 against the loading in a light compression spring 21 
acting between the abutment member 15 and the plate 17. Thus the plate 
acts as a one-way clutch normally to preclude movement of the abutment 
member 15 towards the closed end of the cylinder 10. 
A sleeve 22 encircles the inner end portion of the pin 13 and is of a 
length less than the distance between the plate 17, when in the inclined 
locked position, and the closed end of the piston 8 which is recessed and 
which acts on the braking plate 7. 
In a normal off position of brake the components are in the relative 
positions illustrated. 
When the brake is applied by pressurising the cylinders the abutment 
members 15 are advanced with the pistons 8,9 permitting the plates 17 to 
move into positions generally normal to the pins 13 and, if the pads 6 
have worn, to be moved axially with respect to the pins 13. In either 
case, when the brake is released and the piston 8 retracted, retraction of 
the abutment member 15 due to the effect of the compression spring 14 
tilts the plate 17 into the inclined position in which it locks onto the 
pin 13, in turn normally determining the retracted position of the piston 
8 by the presence of the compression spring 14. 
Should the piston 8 suffer the effects of "knock-back" as defined herein, 
the pistons 8 can retract into the cylinder 10 with the circlip 16 moving 
away from the flange of the abutment member 15. 
The compression spring 14 acts at all times to return the piston into its 
initial positon thereby ensuring that substantially no increase in 
hydraulic fluid or pedal travel is required during the next brake 
application. 
Conveniently the movement of the plate 17 in tilting into locking 
engagement with the pin 13 at the termination of a brake application can 
be made to correspond to the normal brake running clearance between the 
pad 6 and the disc. 
To release the plate 17 for removal or replacement of the pad assembly 2 by 
forcing the piston 8 back into the cylinder, the sleeve 22 is effective to 
disengage the plate 17 from the pin 13 to facilitate further movement of 
the piston 8 in the same direction. 
In the embodiment of FIG. 2 the abutment member 15 comprises a dished 
pressing 23 and the spring 14 abuts against a cup-shaped retainer 24 
having an inturned rim 25. The components are retained with the retainer 
and the rim 25 engages with the pressing 23 so that the components 
comprise a sub-assembly for ease of assembling the brake and which has a 
limited axial movement with the piston 8. This limited axial movement 
together with the engagement of the plate with the pin 13 define the 
normal running brake clearance. 
The construction and operation of the embodiment of FIG. 2 is otherwise the 
same as that of FIG. 1 and corresponding reference numerals have been 
applied to corresponding parts.