GB2243885A - Slack adjuster mechanism - Google Patents

Abstract

An automatic slack adjuster mechanism for adjusting the relative angular positions of an input member 1 and an output member 3 has a stepless clutch arrangement having relatively tapered co-axial faces 14, 15 with a spring coil ring 16 therebetween, the stepless clutch being engaged by jamming of the ring 16 between the two clutch faces 14, 15. Upon application of the brakes the clutch arrangement permits rotation of the output member 3 relative to the clutch element 12 if the brake clearance is in excess of a pro-determined value, while upon subsequent release of the brakes the clutch arrangement prevents rotation of the output member 3 relative to the clutch element resulting in ratcheting of a drive ring clutch face 7 over an input member clutch face 8. in one embodiment the spring coil ring 16 is a ring of purled steel wire. Rotation of ring 21 disengages clutches 7/8 and 12/13 to permit resetting of the member 3 on installation of now brake pads. <IMAGE>

Description

SLACK ADJUSTER MECHANISMS This invention relates to automatic slack adjuster mechanisms for vehicle brake systems.

In UK Patent Specification No. 2088979 B there is described an automatic slack adjuster mechanism for a vehicle brake system, the mechanism including: an input member pivotable in opposite directions about an axis to apply or release a brake-applying force; an output member also pivotable in opposite directions about said axis to transmit said force to brake applying means;; a force transmitting member threadedly engaged around the output member and having a clutch face engaged with a corresponding clutch face on the input member, the threaded engagement of the force transmitting member and the output member being of such a sense that pivotal movement of the input member in the direction to apply said force tends, by the force transmitting member being rotated with the input member through the two clutch faces, further to engage the two clutch faces and thereby move the input member in a first axial direction with the force transmitting member, and pivotal movement of the input member in the opposite direction to release said force tends, by the force transmitting member being rotated with the input member through the two clutch faces, to release the two clutch faces to permit the input member to move in an opposite axial direction with the force transmitting member; and brake-clearance determining means having a first part with respect to which the output member is rotatable about the axis and a second part normally engaged with the output member through a stepless clutch of which one clutch part is located with respect to said second clearance determining means part and of which the other clutch part is located with respect to the output member; the stepless clutch being disengaged by movement of the input member in said first axial direction and being retained engaged by movement of the input member in said opposite axial direction. In use the stepless clutch is urged to disengage when brake force is applied and is permitted to re-engage when effective brake force disappears during brake release.

Subsequent release movement of the output member is limited, by engagement of the twd parts of the stepless clutch, to a preset lost motion between the second clutch part and a datum. Progressive rotational movement of the first clutch part (and hence of the output member) relative to the second clutch part, as brake wear takes place, ensures that the brake release clearance remains substantially constant for the life of the friction material by which the brake-applying force is applied for effecting braking. The stepless clutch particularly described in GB 2088979 B is a multiplate clutch having two groups of clutch plates with the plates of one group interspersed between the plates of the other group. Such a clutch is of quite complicated construction.

According to the present invention there is provided an automatic slack adjuster mechanism for a vehicle brake system, the mechanism including: an input member pivotable in opposite directions about an axis to apply or release a brake-applying force; an output member also pivotable in opposite directions about said axis to transmit said force to brake applying means;; a force transmitting member threadedly engaged around the output member and having a clutch face engaged with a corresponding clutch face on the input member, the threaded engagement of the force transmitting member and the output member being of such a sense that pivotal movement of the input member in the direction to apply said force has the result that, by the force transmitting member being rotated with the input member through the two clutch faces, the engagement force between these two clutch faces is increased and movement of the input member together with the force transmitting member tends to take place in a first axial direction, and pivotal movement of the input member in the opposite direction to release said force has the result that, by the force transmitting member being rotated with the input member through the two clutch faces, the engagement force between these two clutch faces is reduced and the input member tends move in an opposite axial direction with the force transmitting member; and brake-clearance determining means having a first part with respect to which the output member is rotatable about the axis and a second part normally engaged with the output member through a stepless clutch of which one clutch part is located with respect to said second clearance determining means part and of which the other clutch part is located with respect to the output member; the stepless clutch being disengaged by movement of the input member in said first axial direction and being retained engaged by movement of the input member in said opposite axial direction;; said one clutch part of the stepless clutch being an inner clutch element that is fast with said second part of the brake-clearance determining means and said other clutch part of the stepless clutch being an outer clutch element that is fast with the output member, these clutch elements having relatively tapered coaxial faces one on each element and between which faces lies co-axially therewith a spring coil ring, the stepless clutch being engaged by jamming of this ring between the two faces. The stepless clutch of this slack adjuster mechanism, comprising the inner and outer clutch elements with the spring coil ring between the opposed faces of these elements, is of simple construction and yet it is able to transmit heavy loading.

In a particular embodiment the spring coil ring is a ring of purled steel wire.

For a better understanding of the invention and to show how the same may be carried in to effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a side view, partly in section, of vehicle brake operating equipment incorporating a slack adjuster mechanism, and Figures 2A and 2B diagrammatically illustrate two different operating conditions of the slack adjuster mechanism of Figure 1.

Referring first to Figure 1, the vehicle brake operating mechanism has a lever 1 that is for coupling to brake-actuating mechanism of a vehicle and which constitutes an input member of the slack adjuster mechanism. This lever 1 is pivotable in opposite directions about an axis A to apply or release a brakeapplying force. The lever 1 has a central boss 2 that is mounted on a hub 3 so as to be able to move axially of the hub. The hub 3 constitutes the output member of the slack adjuster mechanism and is mounted on a straight spline arrangement 4 of an operating shaft of a brake mechanism (not shown).

A drive ring 5 constituting a force transmitting member is threadedly engaged on the hub 3 via a helical spline arrangement 6. This drive ring 5 has a veetoothed clutch face 7 engaged with a corresponding veetoothed clutch face 8 on the boss 2 of the lever 1. In brakes-released condition, engagement force is maintained by a relatively light bias spring arrangement 9 which also urges the boss 2 against a face of a ground datum arm 10.

The ground datum arm 10 is part of a brake clearance determining arrangement of the slack adjuster mechanism. This arrangement has a first part constituted by the ground datum arm 10, the central portion 10A of which is rotatably mounted on the hub 3, and a second part 11 normally engaged with the hub 3 through a stepless clutch. This stepless clutch has one clutch part which is an inner clutch element 12 that is an integral part of the brake-clearance determining arrangement second part 11 and hence is located with respect to the part 11, and another clutch part that is an outer clutch element 13 that is an integral part of the hub 3 and hence is located with respect to the hub 3. The clutch elements 12, 13 have opposing co-axial plain faces 14, 15 respectively. The outer face 15 is cylindrical whereas the inner face 14 is frusto-conical so as to taper towards the outer face 15 in the direction away from the drive ring 5. The faces 14 and 15 sandwich between them a ring 16 which in the embodiment illustrated is a ring of purled steel wire. The ring 16 is co-axial with the clutch elements 12 and 13. Purled steel wire is preferred as it is of closely coiled spring formation, the wire having a slightly flattened periphery, it is extremely rigid and will support very considerable loads in its crosssection whilst being flexible and extensible in the direction of its length, and it is manufactured to a very high tolerance level. However, a similar spring coil of round wire could serve as the ring 16.

Due to the tapering relationship between the faces 14 and 15, axial movement of the clutch elements 12 and 13 towards one another serves to roll the ring 16 into jamming relationship with the opposing faces 14, 15 to lock the stepless clutch 12/13 'and therefore to make the brake-clearance determining arrangement second part 11 rotationally fast with the hub 3. A heavy stepless clutch control spring arrangement 17 normally acts to hold the stepless clutch 12/13 engaged.

The handing of the helical spline arrangement 6 is such that pivotal movement of the lever 1 in the direction to apply brake-applying force (out of the paper as viewed in Figure 1) has the result that the engagement forces acting on the clutch 7/8 increase in proportion to the frictional resistance of the brake mechanism and the associated brake pull-off springs, thereby tending to move the drive ring 5 and hence the lever 1 in a first axial direction (to the right in Figure 1). Such movement is initially prevented by the action of the heavy spring arrangement 17.

The ground datum arm 10 has fixed to it diametrically opposite dowels 18 (only one of which can be seen in the Figures), against each of which abuts, in a brake-released condition (Figure 2A), one face 19A of a corresponding slot 19 in the inner clutch element 12. This face 19A is urged away from the dowel 18 in a brake-application direction by the action of a torsion spring 20 having one end 20A anchored in the inner clutch element 12 and the other end 20B anchored in the central portion 10A of the ground datum arm 10. In the brakes-released condition movement of the face 19A away from the dowel 18 is prevented by engagement of the stepless clutch 12/13 exerting a torsional resistance greater than the torsional effect of the spring 20.

Operation is as follows.

Brake Application With the initial forward motion of a push rod (not shown) of the vehicle brake actuating mechanism the lever 1, drive ring 5 and hub 3 together with the brake operating shaft (not shown) of the vehicle brake mechanism rotate with the clutch 7/8 held engaged by a force proportional to the input force. At this stage the stepless clutch 12/13 is also engaged, it being held engaged by the action of the control spring arrangement 17 which is greater than the vectored input force transmitted through the helical spline arrangement 6.

During the initial movement the assembly 11/12 rotates in a brake-application direction (out of the paper in Figure 1) so that the face 19A of each slot 19 moves away from the associated dowel 18 with the torsion spring 20 unwinding. Thereafter increasing braking force is applied by further forward motion of the actuator push rod without any further effect on the clearance setting mechanism.

If brake clearance is as desired face 19B comes into abutment with the dowel 18 simultaneously with effective contact commencing between brake friction material and braking surface, and thereupon the stepless clutch 12/13 fully disengages due to the sudden increase in reactive force. Alternatively, should excess clearance exist face 19B comes into abutment with the dowel 18 before the effective contact occurs and in these circumstances the stepless clutch 12/13 slips until effective braking contact is achieved, whereupon the stepless clutch disengages.

At the moment of complete disengagement of the stepless clutch 12/13 a clearance which previously existed at C (Figure 1) between a radial face of the boss 2 and an opposing face of the hub 3 is fully taken up and thereafter the lever 1, drive ring 5 and hub 3 rotate together as one unit.

Brake Release The lever 1, drive ring 5 and hub 3 remain locked together until the high brake force and any rigging stretch is released by retraction of the actuator push rod. As release progresses the spring arrangement 17 asserts itself to re-engage the stepless clutch 12/13, thereby moving the lever 1 to the left (Figure 1) and re-establishing the clearance at C.

Continuing movement of the lever 1 in the release direction permits the brake pull-off springs to establish a brake clearance to the extent allowed by the length of the slots 19 (for example 5-8 of operating shaft movement). Thereafter further release movement of the lever 1 results in ratcheting of the drive ring clutch face 7 over the lever clutch face 8 until full actuator travel has been completed.

Take up of Excessive Clearance If, as is the case when a worn brake pad has been replaced, excessive clearance exists, this will be taken up automatically either wholly or in part during the first brake application, with any remaining excessive clearance being taken up by the next one or more brake application(s), the maximum take up per brake application being the full braking stroke less the preset clearance. Thus, upon brake application with excessive clearance present, as soon as faces 19B abut dowels 18 the actuator push rod continues to move the lever 1 and drive ring 5 to complete the brake application with the stepless clutch 12/13 slipping, allowing the outer clutch element 13 to move whilst the inner clutch element 12 is held stationary by the ground datum ring 10.

Upon release of the brake application and after recovery of any stretch within the rigging, the reactive brake force is completely dissipated as the brake friction material leaves the braking surface. At this point the stepless clutch 12/13 is re-engaged.

Continuing release travel results in the establishment of the correct brake clearance as faces 19A reestablish abutment with dowels 18.

Resetting To enable relative rotation of the lever 1 and the hub 3 to accommodate a new brake pad replacing a worn pad, there is a reset arm 21 extending from a ring 21A rotatable mounted around the drive ring 5. Operation of this arm 21 to rotate the ring 21A causes cam faces 22 on the ring 21A to drive balls 23 against the central portion 2 of the lever 1, thereby separating the ring 21A from the lever 1. Acting on a flange 5A of the drive ring 5, the ring 21A moves the ring 5 against the action of the bias spring arrangement 9 to disengage the clutch 7/8. Further rotation of ring 21A moves the lever 1 and the ground datum arm 10 together against the action of the control spring arrangement 17 to disengage the stepless clutch 12/13. Thus the hub 3 together with the brake operating shaft of the vehicle brake mechanism are freed for the brake operating shaft to be re-set to suit the new brake pad.

Claims (6)

1. An automatic slack adjuster mechanism for a vehicle brake system, the mechanism including: an input member pivotable in opposite directions about an axis to apply or release a brake-applying force; an output member also pivotable in opposite directions about said axis to transmit said force to brake applying means;; a force transmitting member threadedly engaged around the output member and having a clutch face engaged with a corresponding clutch face on the input member, the threaded engagement of the force transmitting member and the output member being of such a sense that pivotal movement of the input member in the direction to apply said force has the result that, by the force transmitting member being rotated with the input member through the two clutch faces, the engagement force between these two clutch faces is increased and movement of the input member together with the force transmitting member tends to take place in a first axial direction, and pivotal movement of the input member in the opposite direction to release said force has the result that, by the force transmitting member being rotated with the input member through the two clutch faces, the engagement force between these two clutch faces is reduced and the input member tends to move in an opposite axial direction with the force transmitting member; and brake-clearance determining means having a first IL part with respect to which the output member is rotatable about the axis and a second part normally engaged with the output member through a stepless clutch of which one clutch part is located with respect to said second clearance determining means part and of which the other clutch part is located with respect to the output member; the stepless clutch being disengaged by movement of the input member in said first axial direction and being retained engaged by movement of the input member in said opposite axial direction;; said one clutch part of the stepless clutch being an inner clutch element that is fast with said second part of the brake-clearance determining means and said other clutch part of the stepless clutch being an outer clutch element that is fast with the output member, these clutch elements having relatively tapered coaxial faces one on each element and between which faces lies co-axially therewith a spring coil ring, the stepless clutch being engaged by jamming of this ring between the two clutch faces.

2. A slack adjuster mechanism as claimed in claim 1, wherein the spring coil ring is a ring of purled steel wire.

3. A slack adjuster mechanism as claimed in claim 1 or 2, wherein the brake-clearance determining means first part is a ground datum arm arrangement that is rotatably mounted on the output member and that is coupled to the inner clutch element of the stepless clutch via a dowel and slot arrangement, the clearance determined by the brake-clearance determining means being set by the permitted range of dowel-in-slot movement as determined by slot length.

4. A slack adjuster mechanism as claimed in claim 3, wherein the dowel and slot arrangement is urged to one limit of its movement by a torsion spring connected between the inner clutch element and the ground datum arm arrangement.

5. A slack adjuster mechanism as claimed in any one of claims 1 to 4 and including a resetting arrangement comprising a reset member rotatable with respect to the force transmitting member to bring rolling means against the input member in the sense to separate the force transmitting member from the input member and disengage the clutch faces thereof, and thereafter to move the input member to disengage the stepless clutch.

6. An automatic slack adjuster mechanism substantially as hereinbefore described with reference to the accompany drawings.