GB1588263A - Hydraulic brake actuator having a wear adjusting device - Google Patents

Description

(54) A HYDRAULIC BRAKE ACTUATOR HAVING A WEAR ADJUSTING DEVICE (71) We, AUTOMOTIVE PRODUCTS LIMITED, a British Company of Tachbrook Road, Leamington Spa, Warwickshire, CV3 1 3ER do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to internal shoe drum brakes.

It is desirable to provide internal shoe drum brakes with automatic adjusters that maintain the surfaces of the brake linings at a substantially constant clearance from the internal surface of the brake drums, so as to compensate for wear to linings during the service life of the brakes.

In one form of drum brake which has a leading and trailing brake shoes, one pair of adjacent ends ofthe pair ofbrake shoes are separated by a double ended hydraulic actuator and the other ends of shoes remote from the actuator pivot an abutment on the brake back plate. A known form of automatic adjuster acts between the opposed pistons in the double acting hydraulic actuator, so as to effectively increase the distance between the two pistons.

One method of achieving this is to have a co-axial strut linking the two pistons and for the strut to be connected to each piston by screw-threads of opposite hand. The strut is then caused to rotate by various means so as to increase the distance between the pistons. The means of rotating the strut are various, and cone clutches operating between the strut and one of the pistons have been commonly used and have been arranged so that the adjustment takes place when the brakes are applied.

Accordingly there is provided a hydraulic brake actuator including an automatic brake adjuster and which in use applies a braking pressure to a friction brake member spring-loaded for return, said actuator comprising a piston whose at-rest position is determined by abutment against a co-axial internal strut, said strut having one end portion with a non-reversible screw threaded connection of one hand with an actuator member and the other end portion with an oppositely handed reversible screw threaded connection with a nut, the piston and nut being releasably inter-engageable by positively engageable clutch means and relatively movable apart to disengage said clutch means only when the relative piston movement exceeds a pre-determined amount on application of the brake, allowing a shoulder on the piston to contact the nut and rotate the nut along the reversible screw thread, so that when the brakes are released the return movement of the piston under said spring load causes said clutch means to reengage and hold the nut against rotation on the reversible thread, the consequent springloaded axial thrust on the nut then causing the strut to rotate on its non-reversible thread to move the strut axially relative to said actuator member into a new abutment position.

Preferably the piston comprises a piston member having a stepped co-axial through bore closed by a tappet member in abutment with the external larger diameter mouth of the bore and the nut is housed within the bore.

Conveniently the positvely engaging clutch means comprises teeth on the nut which engage with like teeth on the tappet member.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Fig. 1 is a section through a hydraulic double acting wheel cylinder according to this invention, and is at the brake at-rest position; Fig. 2 is the wheel cylinder as shown in Fig.

1 with the brakes applied, and with a condition of excess clearance between brake linings and intemal surface of the drum; and Fig. 3 is the wheel cylinder as shown in Fig.

1 with the brakes released and the strut rotated so as to take up the excess clearance.

With reference to Fig. 1 and Fig. 2 of the accompanying drawing, a double ended wheel cylinder 11 is attached to the back plate (not shown) of a drum brake. In a leadingitrailing shoe brake drum the double ended wheel cylinder acts between adjacent ends and of a pair of arcuate lining brake shoes 14 and 15 and when the brakes are applied a pair of opposed pistons 10 and 12 are urged so as to separate the ends of the two brake shoes, so as to push the lining of the brake shoes against the internal surface of a brake drum.

The first piston 12 is sealingly slidable in the cylinder 11 and co-operates with and is held relatively non-rotatable by the end of one brake shoe 14. The first piston 12 had an internally threaded co-axial blind bore 16 which has its open end facing into the wheel cylinder 11.

The second piston 10, comprises a piston member 13 which is sealingly slidable in the wheel cylinder and has a co-axial stepped through bore 17 which has its smallest diameter portion 18 adjacent the first piston and a medium diameter portion 20 and a largest diameter portion 19 at that end of the piston away from the first piston. The second piston 10 has a tappet member 21 in the external mouth of the bore 17.

The tappet member 21 has one end face 22 which co-operates with the end of the second brake shoe 15, is held relatively non-rotatable thereby, and is arranged to abut against the piston member 13. A portion 23 of the tappet is sealingly fitted within the largest diameter portion 19 of the stepped bore 17 and a second end portion 25 of the tappet extends into the medium diameter portion 20 of the stepped bore and has a second end face 26 thereon with a plurality of circumferentially spaced radially aligned teeth 27 cut into its surface. The tappet 21 is adapted to abut against an annular shoulder 24 formed on the internal surface of the bore between the largest and medium diameter portions 19 and 20 respectively and is fixed relative to the second piston by a through pin (not shown). There is a co-axial blind bore 28 in the centre of the tappet 21, the open end of which is adjacent the teeth 27.

A co-axial strut 29 of a.constant overall diameter, has a first end portion 31 which is externally threaded with a left-handed thread, and makes a non-reversible screw-threaded engagement with the internally threaded bore 16 of the first piston 12. A second end portion 32 is externally threaded with a coarse right-handed spiral and makes a reversible screw-threaded engagement with a nut 33 which is threaded thereon and is slidable within the blind bore 28 in the tappet 21. A portion 34 of the strut intermediate the two screw-threaded end portions 31 and 32 has a non-threaded surface. The strut 29 is slideable in the smallest diameter portion 18 of the stepped bore 17 in the piston member 13 and the strut 29 and first piston 12 form a member of overall variable length, the second end of the strut can abut with the end of the blind bore 28 of the tappet 21 if the member so formed is of sufficient overall length.

The nut 35 is slideable and rotatable within the medium diameter portion 20 of the stepped bore 17. The nut is located between the shoulder 35, formed between the smallest diameter 18 and medium diameter 20 portions of the bore 17, and the second end face 26 of the tappet. An end face 36 of the nut, has circumferentially spaced radially aligned teeth 37 cut into its surface which co-operate and engage with the teeth 27 on the tappet. The other end face 38 of the nut is flat and has a sintered P.T.F.E. coating so as to reduce frictional engagement with the shoulder 35 when the two surfaces abut.

The distance between the end face 26 and the shoulder 35 is inter-related with the dimensions of the nut 33 and the depth of interengagement between the teeth 27 on the tappet and the teeth 37 on the nut. The dimensions of the nut 33 are such that a fall back space located between said other end face 38 on the nut 33 and the shoulder 35, when the nut is fully engaged with the tappet, is such that the fall back space is slightly deeper than said depth of interengagement and also includes an allowance for manufacturing tolerances. The depth of the inter-engagement of the teeth 27 and 37 is equal to the sum of the required clearances between the brake linings and the internal surface of the brake drum.

The adjuster works in the manner described below.

With the double acting wheel cylinder in the "at rest" position illustrated in Fig. 1, if the total clearance between the brake linings and the internal surface of the brake drum is less than the fall back space (previously defined) then when hydraulic pressure is introduced in the wheel cylinder and the two opposed pistons 10 and 12 move apart, the piston member 13 will move towards the brake shoe 15 and will tend to lift the tappet 21 in abutment with shoulder 24 away from the nut 33 such that the teeth 27 and 37 begin to disengage. Since the movement is less than the fall back, the teeth will not fully disengage, the shoulder 35 of the piston will not abut the other end face 38 of the but 33 and therefore the adjuster will remain in its relative position.

When the total clearances between the brake linings and the internal surface of the brake drum exceeds or equals the fall back then, as in Fig. 2, the piston member 13 moves towards the shoe 15 and lifts the tappet 21 away from the nut 33 such that the teeth 27 of the tappet disengage and come clear of the teeth 37 on the end face of the nut. Further movement of the piston member 13 towards the shoe 15 brings the shoulder 35 into engagement with said other end face 38 of the nut 33. Since the end face 38 is covered in P.T.F.E. and is substantially friction free, the hydraulic thrust member 13 causes the nut 33 to rotate on the threaded portion 32 and axially to move the nut 33 along the strut away from the piston 12.

When the brakes are released, the pull off springs (not shown) act between the two brake shoes 14 and 15 so as to urge the pistons 10 and 12, respectively together. On the immediate release of hydraulic pressure from within the cylinder 11, as shown in Fig. 3, the pull off springs push the piston member 13 and tappet 21 towards the first piston 12 and since the nut 33 is engaged with threaded portion 32 of the strut, the teeth 27 of the tappet are pulled by the springs into interlocking engagement with the teeth 37 on the nut. The nut may ro tate sufficiently to effect such interlocking.

Since the nut 33 is rotationally locked relative to the tappet 21, which is itself locked rotationally fast relative to the brake shoe 15, then the load in the pull off spring will cause the strut 29 to rotate. Since the threaded portion 31 of the strut 29 is in left-handed screwthreaded engagement with the first piston 12, and the second threaded portion 32 is in righthanded screw-threaded engagement with the now locked nut 33, the load in pull off springs will cause the strut to rotate such that it unscrews itself from the first piston 12 so as to increase the overall length of the member constituting the first piston 12 and the strut 29.

The rotation will continue to unscrew the strut 29 from the first piston 12 until the second end of the strut abuts the blind end of the bore 28 in the tappet 21. When said rotation has finished, the brake is fully adjusted having a clearance equal to the fall back distance in the inter-engaging teeth.

WHAT WE CLAIM IS: 1. A hydraulic brake actuator including an automatic brake adjuster and which in use applies a braking pressure to a friction brake member spring-loaded for return, said actuator comprising a piston whose at-rest position is determined by abutment against a co-axial internal strut, said strut having one end portion with a non-reversible screw threaded connection of one hand with an actuator member and the other end portion with an oppositely handed reversible screw threaded connection with a nut, the piston and nut being releasably inter-engageable by positively engageable clutch means and relatively movable apart to disengage said clutch means only when the relative piston movement exceeds a predetermined amount on application of the brake, allowing a shoulder on the piston to contact the nut and rotate the nut along the reversible screw thread, so that when the brakes are released the return movement of the piston under said spring load causes said clutch means to reengage and hold the nut against rotation on the reversible thread, and consequent spring-loaded axial thrust on the nut then causing the strut to rotate on its nonreversible thread to move the strut axially relative to said actuator member into a new abutment position.

2. A hydraulic brake actuator as claimed in Claim 1, wherein the piston comprises a piston member having a stepped co-axial through bore closed by a tappet member in abutment with the external larger diameter mouth of the bore and the nut is housed within the bore.

3. A hydraulic brake actuator as claimed in Claim 1 or Claim 2, wherein the positively engaging clutch means comprises teeth on the nut which engage with like teeth on the tappet member.

4. A hydraulic brake actuator as claimed in Claim 2 or Claim 3, wherein the shoulder on the piston is provided by the step shoulder in the step through bore.

5. A hydraulic brake actuator as claimed in Claim 4, wherein the tappet member has a blind co-axial bore which opens into the stepped bore and in which the strut is slidable and can abut the blind end.

6. A hydraulic brake actuator as claimed in any one of Claims 1 to 5, wherein the actuator is a double ended wheel cylinder of an internal shoe drum brake and the co-axial strut has its non-reversible screw threaded connection with a second piston which constitutes the actuator member, and the strut is moved to vary the atrest distance between the two pistons.

7. A hydraulic brake actuator substantially as described herein and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. tate sufficiently to effect such interlocking. Since the nut 33 is rotationally locked relative to the tappet 21, which is itself locked rotationally fast relative to the brake shoe 15, then the load in the pull off spring will cause the strut 29 to rotate. Since the threaded portion 31 of the strut 29 is in left-handed screwthreaded engagement with the first piston 12, and the second threaded portion 32 is in righthanded screw-threaded engagement with the now locked nut 33, the load in pull off springs will cause the strut to rotate such that it unscrews itself from the first piston 12 so as to increase the overall length of the member constituting the first piston 12 and the strut 29. The rotation will continue to unscrew the strut 29 from the first piston 12 until the second end of the strut abuts the blind end of the bore 28 in the tappet 21. When said rotation has finished, the brake is fully adjusted having a clearance equal to the fall back distance in the inter-engaging teeth. WHAT WE CLAIM IS:

1. A hydraulic brake actuator including an automatic brake adjuster and which in use applies a braking pressure to a friction brake member spring-loaded for return, said actuator comprising a piston whose at-rest position is determined by abutment against a co-axial internal strut, said strut having one end portion with a non-reversible screw threaded connection of one hand with an actuator member and the other end portion with an oppositely handed reversible screw threaded connection with a nut, the piston and nut being releasably inter-engageable by positively engageable clutch means and relatively movable apart to disengage said clutch means only when the relative piston movement exceeds a predetermined amount on application of the brake, allowing a shoulder on the piston to contact the nut and rotate the nut along the reversible screw thread, so that when the brakes are released the return movement of the piston under said spring load causes said clutch means to reengage and hold the nut against rotation on the reversible thread, and consequent spring-loaded axial thrust on the nut then causing the strut to rotate on its nonreversible thread to move the strut axially relative to said actuator member into a new abutment position.

2. A hydraulic brake actuator as claimed in Claim 1, wherein the piston comprises a piston member having a stepped co-axial through bore closed by a tappet member in abutment with the external larger diameter mouth of the bore and the nut is housed within the bore.

3. A hydraulic brake actuator as claimed in Claim 1 or Claim 2, wherein the positively engaging clutch means comprises teeth on the nut which engage with like teeth on the tappet member.

4. A hydraulic brake actuator as claimed in Claim 2 or Claim 3, wherein the shoulder on the piston is provided by the step shoulder in the step through bore.

5. A hydraulic brake actuator as claimed in Claim 4, wherein the tappet member has a blind co-axial bore which opens into the stepped bore and in which the strut is slidable and can abut the blind end.

6. A hydraulic brake actuator as claimed in any one of Claims 1 to 5, wherein the actuator is a double ended wheel cylinder of an internal shoe drum brake and the co-axial strut has its non-reversible screw threaded connection with a second piston which constitutes the actuator member, and the strut is moved to vary the atrest distance between the two pistons.

7. A hydraulic brake actuator substantially as described herein and as shown in the accompanying drawings.