GB2090355A - Improvements relating to brake adjusters - Google Patents

Abstract

The adjuster which adjusts only at low brake pressures comprises a first rotatable shaft member (15), a second rotatable nut member (13) and a third non-rotatable member (11), said members (11, 13, 15) having a common thrust axis and said first and second members (15, 13) having a non-reversible screw-threaded connection (17) therebetween, said second member (13) being axially displaceable and rotatable relative to said third member (11) and having an abutment surface (25) which is biassed towards engagement with a non-rotatable abutment surface (27), on member (11), a fourth annular member (19) acting as a drive ring being connected with second member (13) by a reversible screw-threaded connection (21), a friction surface on said annular member (19) extending perpendicular to the thrust axis and being biassed against a complementary friction surface (37) on said third member (11), said shaft member (15) having an enlarged diameter region (15a) which is acted upon by hydraulic pressure during service brake application, to axially move said first shaft member (15) and rotate said member (15, 13) to gether said first shaft member (15) also having a friction surface (53) which can engage with a complementary friction surface (51) to cause the said second member (13) to rotate relative to said first shaft member (15) under the effect of further hydraulic pressure. <IMAGE>

Description

SPECIFICATION Improvements relating to brake adjusters The present invention relates to an automatic slack adjuster for vehicle brakes.

In brakes where the handbrake operates the same friction linings as the service brake, it is desirable to adjust the connection between a brake actuating member, such as a hydraulic piston, and the handbrake so that excessive handbrake travel will not be encountered as the friction linings wear away. N.B.

The footbrake is however adjusted automatically by the hydraulic fluid from the master cylinder reservoir following up the hydraulic piston, piston retraction being achieved by a rubber seal surrounding the piston. An automatic brake adjuster such as disclosed in our British Patent Specification No. 1,403,357, may be provided to perform this adjustment function, such an adjuster coming into operation when the travel of the brake actuating member in the brakeapplying direction, is in excess of the desired travel.

Upon such excessive travel the adjuster adjusts the connection e.g. strut, between the piston and handbrake In British Patent Specification No. 1,403,357, the adjusting mechanism comprises a nut having a non-reversible internal thread which engages a threaded shaft with a complementary non-reversible threaded from. The nut further has a reversible external thread with which an annulus or drive ring engages, the drive ring having a complementary reversible or fast thread, on its inner periphery. The adjuster assembly is completed with the provision of a main spring between a flange at one end of the nut and a washer towards the other end, a secondary spring being located between the washer and the drive ring.All of the above components are housed within a sleeve-like container, the springs biassing the drive ring into engagement with a friction surface provided at one end of the sleeve-like container, the friction surfaces on the drive ring and container being complementary and tapered.

The expression "reversible screw-threaded connection" between two members, as used hereinbefore and hereinafter, refers to a connection wherein axial displacement of one member causes the other member to rotate and vice-versa whereas a "nonreversible screw-threaded connection" between two members is one in which axial force applied to one member cannot normally cause relative rotation between the members. Whether a screw-threaded connection is reversible or not depends upon the pitch and fiank angles of the thread and the coefficient of friction between the members.

In use, when adjustment is required, the container of the adjuster of British Patent Specification No.

1,403,357, and the push rod, move with the brake actuating member e.g. hydraulic piston, the push rod moves away from the cam actuator. When the pressure over the push rod area is sufficient to overcome the force of the return spring, it moves towards the cam and pulls the nut through the drive ring. As the drive ring is held by a cone clutch, the reversible screw thread connection with the drive ring and nut, rotates the nut on the push rod thread.

Adjustment thus occurs. On release of the brake pressure, the push rod is moved back by the return spring so that the drive ring rotates on the nut against a small bowed washer. Further adjustments can occur in a similar manner. There are however certain disadvantages with this construction of adjuster. Firstly the drive ring and container have tapered friction faces and thus cannot cater for radial movements, so that the resultant load will hinder the operation of the reversible screw thread connection with the nut. Secondly the adjuster is load sensitive and is thus susceptible to caliper deflection, i.e. the adjuster will or will continue to adjust during caliper deflection under heavy braking. This latter disadvantage can result in the brakes being held on so that heat can be generated and reduced vehicle performance is only possible.Also, excessive fuel consumption and possible brake fluid vaporisation can result Alternatively the designer is forced to increase the built in clearance and thus handbrake travel is increased.

Modifications have been conceived to overcome load sensitivity and one such modification is disclosed in our British Patent Specification No.

1,439,758. However, these modifications have required the addition of at least one extra seal and a number of extra components, and whilst the load sensitivity problem has been overcome, it has proved expensive and still does not overcome the other problems.

The aim of the present invention isto provide an adjuster which overcomes the problems outlined hereabove and does not require extra seals etc, and is thus less expensive.

According to the present invention there is provided a slack adjuster for a brake system comprising a first rotatable shaft member, a second rotatable nut member and a third non-rotatable member, said members having a common thrust axis and said first and second members having mating external and internal threads, respectively, forming a nonreversible screw-threaded connection between said first and second members, said second member being axially displaceable and rotatable relative to said third member and having an abutment surface which is biassed towards engagement with a complementary non-rotatable abutment surface, a fourth or annular member co-axial with said thrust axis, encircling said second member and being connected therewith by a reversible screw-thread connection, a friction surface on said annular member extending perpendicular to the thrust axis and being biassed against a complementary friction surface on said third member, said first rotatable shaft member having an enlarged diameter region which can be acted upon by hydraulic pressure during service brake application, to axially move said first shaft member and separate said abutment surfaces, whereby said first and second members can initially rotate together relative to said annular member, said first shaft member also having a friction surface which can engage with a complementary friction surface to cause the said second memberto rotate relative to said first shaft member under the effect of further hydraulic pressure.

In a preferred embodiment of the present invention the said third non-rotatable member is a pressed-steel cylindrical container or casing, which is press-fitted into a bore in the actuator piston of a brake. Said second or nut member is located within the container with the brake in the released or rest position, a flange on the nut member having the said abutment face which mates with a non-rotatable abutment face provided by the container and/or the actuator piston. The fourth member is the annulus or drive ring which is mounted on the nut member by the said reversible screw-thread connection and is biassed by a main spring, against a flat friction surface provided by an inwardly turned end edge of the container, said flat friction surface extending perpendicularly to the axis of rotation of the nut member and annulus.The main spring also engages the flange on the nut member and biasses the said abutment faces into engagement with one another.

The said first shaft member threadedly engages in the nut member and has an enlarged diameter region which incorporates a frusto-conical friction surface on its outer periphery and a frusto-conical recess at one axial end thereof. The frusto-conical friction surface is engaged by a non-rotatable friction ring with a complementary friction surface, the two friction surfaces being biassed into interengagement by a further spring, and a frustoconical tappet engages in the said frustoconical recess.

During normal service brake application, the actuator piston, first shaft member and nut member all move together. However, when the hydraulic pressure has increased sufficiently there is sufficient force developed over the enlarged diameter region of said first shaft member, to overcome the main spring in the container and generate a torque at the reversible connection between the drive ring and the nut member. This torque is enough to rotate the nut member and first shaft member together against the drag forces between the said friction ring and the complementary surface. At a higher hydraulic pressure the enlarged diameter region of the first shaft member is firmly held against the frusto-conical tappet and the torque generated is enough to rotate the nut member on the non-reversible screw thread on the first shaft member.Above this pressure any movement of the actuator piston merely rotates the nut member further on the first shaft member, the nut member winding itself off its seat i.e. the abutment surface, on the actuator piston.

When the brake pressure is reduced the actuator piston is forced back by caliper bridge retraction and the nut member again rotates on the first shaft member, but this time in the reverse direction. When the brake pressure falls to a low level the force of the main spring returns the nut member together with the first shaft member, back to the position in which the said abutment faces engage one another. This action causes further rotation of the nut member but not of the first shaft member which is prevented from rotating by the drag torque provided by the friction ring.

Thus, in summary, with the service brake applied, the nut member and first shaft member rotate together until the first shaft member is prevented from rotation by hydraulic pressure. Above a certain pressure the nut only rotates. As the brakes are released the nut member rotates in the opposite direction upon the first shaft member which is held initially by hydraulic pressure, and then by the drag torque provided by the friction ring.

The present invention has the advantage that adjustment is effected art a relatively low pressure but not at the higher pressures when caliper deflection occurs. Thus overadjustment is overcome. Also, as the drive ring is seated on a flat friction surface, it can move radially and the high loads on the threads, possible with prior art designs, are not encountered.

In the particular embodiment outlined hereabove, the drive ring remains seated, held by the main spring, on a flat clutch surface, and thus the drive ring can move to its correct position.

To operate a hand brake, the frusto-conical tappet is moved axially so that the braking force is passed to the actuator piston via the first shaft member and the nut member, the handbrake load being advantageously carried by the conventional non-reversible thread connection between the nut member and the first shaft member.

The present invention will now be further described, by way of example, with reference to the accompanying drawing which illustrates, in crosssection, a brake incorporating a preferred embodiment of adjuster according to the present invention.

The brake illustrated in the accompanying drawing comprises a caliper 1 of the reaction type, supported on a torque taking member 3. The caliper 1 houses in actuator bore 4, an actuator piston 5 and a slack adjuster according to the present invention, generally designated 7, the actuator piston 5 being arranged to act upon inboard friction pad assembly 9.

The adjuster7 comprises a cylindrical casing or container 11 i.e. third member, within which a nut member 13 i.e. second member, is located, the nut member 13 being mounted on a push rod 15 i.e. first shaft member, by a non-reversible screw thread connection 17. A drive ring 19 is also located within container 11, the drive ring 19 encircling nut member 13 and being connected thereto by a reversible screw thread connection 21. A flange 23 on nut member 13 provides an abutment surface 25 which is pressed by a main spring 35, in the released or rest condition of the brake, against an inturned edge 27 at one end of the container 11. Flange 23 also provides an abutment face 29 which, with the adjuster 7 correctly fitted as a press fit in blind bore 31 in actuator pistonS, engages against a shoulder 33 of the actuator piston 5, when the brake is in the released condition. Main spring 35 acts on a thrust bearing 36 to biass flange 23 against edge 27 and shoulder 33, and also biasses drive ring 19 against an inturned edge 37 at the other end of container 11.

Inturned edge 37 presents a flat friction surface to drive ring 19, which surface is perpendicular to the rotational axis of both the drive ring 19 and the nut member 13.

Push rod 15 has an enlarged diameter portion 15a which is axially slidably located within a hollow insert 39 within actuator bore 4, seal 41 sealing the push rod 15 in insert 39. The insert 39 is keyed to the caliper prevent rotation, by dowel 43, and houses a further spring 45. Spring 45 engages between a shoulder 47 on insert 39 and a friction ring 49, friction ring 49 having a frusto-conical friction surface 51 which is biassed by spring 45, against a complementary surface 53 provided on a laterally extending flange of enlarged diameter portion 15a.

Enlarged diameter portion 15a also has a frustoconical recess 55 in the axial end thereof, a frustoconical tappet 57 engaging therein. The tappet 57 is connected by a dolly 59 to a mechanical actuator shaft 61 which is rotated by operation of the handbrake.

During normal service brake operation, hydraulic fluid is fed under pressure via connector 63 into the region 65 of actuator bore 4. As the hydraulic pressure is applied, actuator piston 5, nut member 13, drive ring 19, and push rod 15, 15a move together to apply the brake. When the hydraulic pressure has been sufficiently increased (e.g. approximately 60 p.s.i.) there is sufficient force developed on enlarged diameter portion isa to overcome the main spring 35 and generate a torque at reversible screw thread connection 21. This torque is enough to rotate the nut member 13 and push rod 15, isa together against the friction of seal 41 and the drag torque existing between friction faces 51 and 53 under the influence of spring 45. At a higher pressure (e.g.

approximately 120 p.s.i.) the push rod 15, 15a is held firmly against the keyed coned tappet 57 and sufficient torque is produced at reversible screw-thread connection 21 to overcome the torque at the nonreversible screw-threaded connection 17, so that nut member 13 rotates on push rod 15. Above this hydraulic pressure any movement of the actuator piston 5 merely rotates nut member 13 further on push rod 15 due to reversible screw thread connection 21, the nut member 13 winding itself away from abutment faces 27 and 33.

As the brake pressure is reduced, to release the brake, the actuator piston 5 is forced back by caliper bridge retraction and nut member 13 again rotates on push rod 15, though this time in the reverse direction. When the pressure falls to a relatively low level e.g. 30 p.s.i., the force of the main spring 35 returns the push rod 15, 15a and nut member 13 back to the position wherein abutment faces 25 and 29 engage edge 27 and shoulder 33 respectively. This action causes further rotation of the nut member 13 but not the push rod 15, 15a which is prevented from rotation by the drag torque provided by friction ring 49 engaging enlarged diameter portion 15a.

Thus when the brake is applied the push rod 15 and the nut member 13 rotate together until the push rod 15 is prevented from rotating by the hydraulic pressure pressing the push rod firmly against tappet 57. Above a certain pressure only the nut member 13 rotates. When the brake is released the nut member 13 rotates in the opposite direction on the push rod 15, 15a. The push rod is first held by the hydraulic pressure and then by the drag torque provided by friction ring 49.

With the above described arrangement adjustment is effected only at relatively low pressures and not at the higher pressures when caliper deflection can occur and has in certain prior art constructions caused overadjustment. Also, drive ring 19 is seated on a flat friction face provided by inturned edge 37 so that radial movement can be accommodated to correctly position the drive ring with respect to the reversible screw thread connection 21.

Further, on hand brake operation, the shaft 61 rotates causing dolly 59 to move tappet 57. This movement is conveyed via push rod 15 to nut member 13 and actuator piston 5. Thus the hand brake load is advantageously carried by the conventional non-reversible screw thread connection 17.

The present invention thus provides a brake slack adjuster which is not load sensitive, which is relatively simple and cheap to construct, and which overcomes the major problems of the prior art adjusters.

Claims (8)

1. Aslackadjusterfora brake system comprising a first rotatable shaft member, a second rotatable nut member and a third non-rotatable member, said members having a common thrust axis and said first and second members having mating external and internal threads, respectively, forming a nonreversible screw-threaded connection between said first and second members, said second member being axially displaceable and rotatable relative to said third member and having an abutment surface which is biassed towards engagement with a complementary non-rotatable abutment surface, a fourth annular member co-axial with said thrust axis, encircling said second member and being connected therewith by a reversible screw-threaded connection, a friction surface on said annular member extending perpendicular to the thrust axis and being biassed against a complementary friction surface on said third member, said first rotatable shaft member having an enlarged diameter region which can be acted upon by hydraulic pressure during service brake application, to axially move said first shaft member and separate said abutment surfaces, whereby said first and second members can initially rotate together relative to said annular member, said first shaft member also having a friction surface which can engage with a complementary friction surface to causethe said second member to rotate relative to said first shaft member under the effect of further hydraulic pressure.

2. An adjuster as claimed in claim 1, in which said third non-rotatable member is a pressed steel cylindrical containerwhich is, in use, press fitted into a bore in the actuator piston of a brake.

3. An adjuster as claimed in claim 2, in which said second member is arranged to be located within the container when the brake is at rest, a flange on the second member providing said abutment face which mates with a non-rotatable abutment face provided by the container.

4. An adjuster as claimed in claim 2 or 3, in which said fourth annular member is a drive ring which is mounted on said second member by the said reversible screw-thread connection and is biassed by a main spring, against a flat friction surface provided by an inwardlyturned end edge of the container.

5. An adjuster as claimed in claim 4, in which said flat friction surface extends perpendicularly to the axis of rotation of the said second and fourth members.

6. An adjuster as claimed in claim 3 or claim 4, in which the main spring also engages the flange on said second member and biasses the said abutment faces into engagement with one another.

7. An adjuster as claimed in any of claims 1 to 6, in which said first shaft member threadedly engages in said second member and has an enlarged diameter region which incorporates a frusto-conical friction surface on its outer periphery and a frustoconical recess at one axial end thereof, the frustoconical friction surface being engaged by a nonrotatable friction ring with a complementary friction surface, the two friction surfaces being biassed into interengagement by a further spring, and a tappet engaging in said frusto-conical recess.

8. A slack adjuster for a brake system constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.