GB2245668A - Adjustment and centring means for a drum brake. - Google Patents

Description

Y SJL290491 1 ADJUSTMENT AND CENTRING MEANS FOR A DRUM BRAKE This

invention relates to adjustment and centring means for a drum brake, comprising a slide-through or deflection lock arranged in a fixed guide frame and axially displaceable against the tensional force of a centring spring, said lock including, for each direction of deflection, an adjusting nut into which is screwed a non-rotational threaded bolt which engages the respective end of the brake shoe, toothed adjusting wheels coaxial with and associated with each said nut and having unidirectionally operative arresting teeth, and a lever mechanism including two pawls which engage the arresting teeth of the two toothed adjusting wheels and are pivotably mounted on the guide frame and pushed by the force of a spring into a centred inoperative position, in which respect the lever mechanism, during an axial displacement of the slide- through lock and after exceeding the basic clearance, abuts against one of two stops of the slide-through lock or against one of the two toothed adjusting wheels, and one of the two pawls executes a movement relative to the associated t k_oothed adjusting wheel.

Many varied forms of adjusting devices are known which automatically compensate for wear of the brake lining after operating the brake, and which at the same time centre both brake shoes relative to the brake drum, when releasing the brake by means of a pressure-spring arrangement within a slidethrough or deflection lock.

Depending on the area of application, the quality of the lining and the constructional arrangement of servo drum brakes, each brake arrangement has a certain basic clearance. This has to be taken into account in respect SJL290491 2 of the adjusting device associated with the brake and the variable mass to be retarded by the brake.

Examples of adjusting and centring devices of this type are described in German Patent Specifications DE-OS 23 11 166 and DE-OS 31 12 550, in which respect the first device operates in conjunction with a double lever, whilst the second device operates in conjunction with two adjusting levers. In these, and also in other known adjusting devices, the clearance is set and kept constant by the following measures: 1. 2. 3.

by the tooth ratio of toothed adjusting wheels; by suitable selection of the lever gearing; and by an intermediate arrangement of spacers.

By selecting the number of spacers as in 3, the limiting distance between the adjusting lever and the toothed adjusting wheel, and thus the basic clearance, can be reduced or increased.

This has to be taken into consideration during assembly of, or sparepart nsertion into such an adjusting device. However, such a plurality of different basic clearances has proved to be disadvantageous.

Another disadvantage of path-dependent adjustment results from an effect in practical use.

According to the operational principle of a servo drum brake, the braking forces of the primary shoe are transferred via the axially displaceable arrangement of the slide-through deflection lock to the secondary shoe. Thus, the braking force of the primary shoe acts as tensile force relative to the secondary shoe, which thereby, via a self-amplifying effect, exerts nearly double the brake force of the primary shoe. If the brake is excessively loaded by one or several full 4 SJL290491 3 braking operations, this can cause elastic deformation of the brake parts and the brake drum. The extent of the elastic deformation becomes a measure requiring one or even several unwanted adjusting steps in the adjusting process, as the adjusting lever is excessively pivoted over the deflection lock by the axial brake shoe deflection movement. Several teeth of the toothed adjusting wheel are thereby skipped, resulting in the end, when releasing the brake, in an excessive adjustment and thus a reduction of the required clearance. This may result in grinding of the brake linings on the brake drum and even in destruction of individual brake parts.

In order to prevent grinding of brake linings due to excessive adjustment resulting from the aforementioned effect, and increased basic clearance is in practice established by means of spacers in order to obtain a "clearance reserve" which allows for a proportion of the elastic deformation of the brake or drum. However, a relatively large operating stroke (idle stroke) of the brake is then required to overcome this clearance, which, in turn, means a reduction in the brake-force reserve.

Adjusting devices depending on path of travel may experience an excessive brake adjustment, for example during extended downhill travel with constant braking and thus with high thermal loads and brake expansion caused by this.

It is an object of this invention to provide a forcedependent adjusting device which adjusts lining wear during a braking operation after exceeding the basic clearance.

This object is achieved, according to the invention, by SJL290491 4 providing adjustment and centring means as detailed in the first paragraph hereto which is characterised in that the excessive axial displacement of the deflection or slide-through lock is accompanied by rotation of the toothed adjusting wheel to bring about screwing-out of a threaded bolt, the engagement between the pawls and the toothed adjusting wheels being such that upon actuation of the brake each pawl engages a steep flank of the arresting teeth arrangement of the respective toothed adjusting wheel, and upon release of the brake it engages a flat flank of the arresting teeth arrangement, and in that the pawls are hingedly connected to the lever mechanism and forced into an inoperative position relative to the lever mechanism by the force of a spring, in which respect the pawls are pivotable relative to the lever mechanism if the countering force of the associated toothed adjusting wheel exceeds the pre-tensional force of the spring.

With the inventive arrangement, when brake forces are applied, i.e. when the linings abut against the brake drum, a further adjustment of the adjusting parts and of the load-dependently coupled mount between a control lever and associated adjusting lever is prevented. Under the load of abutment of the brake shoes and a further deflection movement due to the elastic brake deformation, the hinge-like connection existing between the control lever and the adjusting lever bends against the pre-tensional force of a coiled spring.

Thus that portion of the operating path which takes place due to tensioning of the brake and causes elastic deformation of the brake parts, is not taken into account, or is ineffective, when the adjustment is made. In this way, the basic clearance in each brake can be kept uniform and very small, the varying number of spacers required hitherto for setting the basic t SJL290491 clearance is dispensed with, and the idle stroke is minimised. The last mentioned advantage also makes it possible to use operating cylinders of smaller stroke than hitherto in accordance with the brake dimension.

Further details, advantages and features of the invention are contained in the following description and the accompanying drawings, to which specific reference is made with respect to disclosing all details which are not described in the text. In the drawings:

Fig. 1 is a view partially in cross-section, of a first embodiment, in accordance with the invention, of a drum brake adjusting and centring device with the brake released; Fig. 2 is a side view of the device in Fig. 1 from the right; Fig. 3 is a top view of the device in Fig.

Fig. 4 is a top view of the device in Fig. 1, at the start of the deflection movement; Fig. 5 is a top view of the device in Fig. 1, upon continued deflection movement; Fig. 6 is an illustration similar to that of Fig. 1 of a modified embodiment of an adjusting and centring device in accordance with the invention; Fig. 7 is a top view of a second embodiment of an adjusting and centring device in accordance with the invention with the brake released; Fig. 8 is a view of the device in Fig. 7; 1 SJL290491 6 Fig. 9 is a view similar to Fig. 8, at the start of the deflection movement; and Fig. 10 is a view similar to Fig. 8, upon continued deflection movement.

Between brake shoe ends (not shown), which lie opposite a spreading device of a servo drum brake, is arranged a guide frame 1 which is fixedly connected to a brake cover plate (not shown) and has an approximately Ushaped cross-section. in this guide frame 1 is mounted a slide-through or deflection lock which is displaceable in its axial direction, thereby transferring the brake operating force from the respective primary shoe to the secondary shoe. The deflection lock is constructed as an adjusting and centring device which, with wear of the brake lining of one of the brake shoes, increases its axial length relative to its centred position at the worn side, thereby keeping constant the basic clearance for each of the two brake shoes.

At each brake-shoe end there is a respective threaded bolt 2 which is nonrotationally mounted and s crewed into an associated adjusting nut 3. The two adjusting nuts 3 are rotatably mounted in a displacement sleeve 4 and each have a collar 5 against which the displacement sleeve 4 abuts.

In its middle region, the displacement sleeve 4 accommodates a centring spring 6. This centring spring 6 is supported at each end, via a flanged bush 7 or via two ring halves (split ring) against the guide frame 1, thus effecting, via the flanged bush or split ring which themselves abut against the collar 5 of the adjusting nut 3, centring of the deflection lock comprising the threaded bolts 2, the adjusting nuts 3 and the displacement sleeve 4. The brake force is transferred SJL290491 7 from the threaded bolt 2 nearest the primary shoe via the respective adjusting nut 3, its collar 5, the displacement sleeve 4 and the identical parts symmetrically arranged on the other side.

Onto each adjusting nut 3 a coaxial toothed adjusting wheel 8 is mounted, the toothing of which consists of a unidirectionally operating toothed arrangement 9 (see Fig. 2). The orientation of this toothed arrangement 9 depends on the orientation of the thread between the respective threaded bolt 2 and adjusting nut 3. The arrangement is, in any case, such that in the course of a deflection movement, i.e. when the brake is operated in a manner to be described later, a rotary movement of the toothed adjusting wheel 8 is effected which causes the associated threaded bolt 2 to be screwed out.

Two L-shaped control levers 10, 11, which are offset by 900 relative to one another, are mounted on the guide frame 1 so as to be alternately pivotable by means of respective bolts 12, 13, which are firmly attached to the guide frame 1. Each control lever 10, 11 has an angled lateral projection 14 facing towards the deflection lock, which projection 14 abuts against a part of guide frame 1. A portion 15 of each control lever 10, 11 extends from the respective bolt anchorage 12, 13 over an area of the U-shaped guide frame 1. This portion 15 of each of the two control levers 10, 11 serves both for guidance and support of the lever mechanism on the guide frame 1.

In the illustrated embodiment, the adjustment of the deflection movement takes place at the toothed adjusting wheels 8. For this purpose, both control levers 10, 11 have a slide protrusion 16. The distance (or abutment gap) between each of these two slide protrusions 16 and the associated front surface of the respective toothed t SJL290491 8 4L adjusting wheel 8 represents the basic clearance L.

An adjusting lever 17, 18 is hinged to each control lever 10, 11 by way of a rivet or screw connection 19, so that both parts can pivot relative to one another around this pivot point.

Each adjusting lever 17, 18 extends, by way of an angular lug 20 serving as a support or a stop, behind a part of its associated control lever 10, 11. Furthermore, an engagement finger 21 extends radially outwards at an angle from each adjusting lever 17, 18. A coiled spring 22 is diagonally arranged between these two fingers 21. The shape of the adjusting levers 17, 18 is such that they abut against the steep arresting flank of the respective toothed arrangements 9 by way of a pawl-like end portion 23 remote from their respective pivot points 19.

On the one hand, with the brake released, the coiled spring 22, which is pre-tensioned and arranged between the shanks 21, causes the adjusting levers 17, 18 to abut via their lugs 20 against the control levers 10, 11 in such a manner that the lateral projections 14 of the latter abut under spring pressure against the flanged bush 7 or the guide frame 1. On the other hand, the pawl-like end portions 23 of the adjusting levers 17, 18 are pushed under light pressure into tooth bases of the arresting teeth arrangements 9 and held against the steep tooth flanks.

With reference to Figs. 3 to 5 the described device operates as follows:

When the deflection lock is displaced during actuation of the brake, from the left to the right against the pre-tensional force of the centring spring 6, whilst 1 SJL290491 9 exceeding the basic clearance L, the left toothed adjusting wheel 8 presses against the left slide protrusion 16 and pushes it to the right so that the left control lever 10 is rotated clockwise around its bolt 12. The lateral projection 14 of the lever 10 thus lifts off from the guide frame 1, whilst the other control lever 11 still supports itself via its lateral projection 14 against the guide frame 1, thus maintaining the pre- tensional force of the spring between the two control levers 10, 11.

Upon pivoting of the control lever 10 and owing to pretensioning of the spring 22, the adjusting lever 18 is also pivoted and its pawl end 23, which abuts a steep flank of the arresting teeth arrangement 9, turns the right hand toothed adjusting wheel 8, thereby screwing the associated threaded bolt 2 out of its adjusting nut 3 (see Fig. 4).

With abutment of the brake shoes against the brake drum and increasing brake forces, no further turning or adjustment will occur as the thread friction between the adjusting nut 3 and the threaded bolt 2 exceeds the adjusting force. In a further deflection movement due to brake expansion, the control lever 10 is pivoted further, but due to the hinge like connection 19 between the levers 10, 18, these fold inward against the force of the coiled spring 22. The lug 20 of adjusting lever 18 then lifts off, as a support or stop, from the control lever 10 (see Fig. 5).

Upon releasing the brake, the parts of the deflection lock are urged back into their initial positions by the action of the centring spring 6. At the same time, the coiled spring 22 ensures that the angled lug 20 of the adjusting lever 18 returns to its abutment against the control lever 10. The control lever 10 is thus turned SJL290491 back anti-clockwise around its bolt 12 until its lateral projection 14 again abuts against the guide frame 1. If the rotary path of the control lever 10 to the abutment increases due to increased lining wear by more than the tooth ratio of the arresting teeth 9 of the toothed adjusting wheel 8, then the pawl end 23 of the adjusting lever 18 slips over the slanted flank of a tooth into the next tooth base of the arresting teeth arrangement 9. In this way, the lining wear is adjusted in the next following brake operation as described above.

The adjustment of the other brake shoe is effected in the opposite braking direction by the pair of levers 11 and 17.

The force ratio between the coiled spring 22 and a (not illustrated) brake-shoe return spring is chosen so that unintentional turning back of the toothed adjusting wheel 8 is impossible when the pawl end 23 jumps over the next tooth of arresting teeth arrangement 9.

In the modified embodiment of an adjusting and centring device as shown in Fig. 6, the deflection movement and thus the start of the pivot movement for the control levers 10, 11 is initiated via the axial displacement of the flanged bushes 7. For this purpose, the lateral projection 14 of each control lever 10 and 11 has a special contour. The contour of the lateral projections 14 as abutments or supports for the control levers 10, 11 is then arranged so that they can on the one hand fixedly support themselves against the guide frame 1, while on the other hand there is a gap between the radially outward extending flange of the bush 7 and the lateral projection 14. This gap, as a distance measure or abutment gap for the axial displacement path of the deflection lock, defines the basic clearance.

tL i SJL290491 A second embodiment of an adjusting device is shown in Figs. 7 to 10. The basic construction is the same as in Figs. 2 to 5, only the control and adjusting parts differing as described below.

Similar to the adjusting device of the previously mentioned DE-OS 23 11 166, the guide frame 1 includes a control plate 24 which is mounted so as to be rotatable from alternate sides by means of bolts 12, 13. The bolts 12, 13 extend through bore holes 25, 26 in the control plate 24, which holes extends transversely and widen a little in the outward direction from the location of the respective bolt in the inoperative position of the control plate. The control plate 24 has slide protrusions 16 at both sides as stops for the toothed adjusting wheels 8. Respective coiled springs 27 engage into and extend between the recesses of the bore holes 25 and 26 and the guide frame 1, thereby holding the control plate 24 against the bolts 12, 13.

A double adjusting lever 28 is pivotably mounted on the control plate 24 via a hinged connection 19 formed by a rivet or a bolt. This double adjusting lever 28 is.also displaceably mounted on the bolts 12, 13. The double adjusting lever 28 has pawl-like end portions 23 at both ends, which are crimped and which engage the arresting teeth arrangement 9 of the two toothed adjusting wheels 8.

The hinged connection 19 serves for the mounting of a flexible spring 29 which presses the double adjusting lever 28 against the control plate 24. The flexible spring 29 has a first limb 30 which extends across the double adjusting lever 28, and has an angled end portion 31 projecting into an opening of control plate 24. A second limb 32 of the flexible spring 29 has a double angled end portion 33 which extends behind and under the 1 SJL290491 12 double adjusting lever 28. Thus, the spring 29, which is pre-tensioned, acts by its end portion 33, which engages a recess of the double adjusting lever 28, pressing the latter towards the angled end portion 31 of the other limb 30 which is fixed in an opening of the control plate 24. Consequently, when in the inoperative position, all the adjusting members are tensioned relative to each other.

The described device operates, according to Figs. 8 to 10, as follows:

When, during operation of the brake, the deflection lock is displaced towards the right and the basic clearance thus exceeded, the left toothed adjusting wheel 8 pushes against the left slide protrusion 16 and pushes same to the right. This causes the control plate 24 to be rotated clockwise around the bolt 12. The borehole 26 then slides along the bolt 13. This displacement movement is also followed by the double adjusting lever 28 via the spring force of the flexible spring 29, as this is, on the one hand, connected via its first limb end portion 31 to the control plate 24, and, on the other hand, exercises a pre-tensional force via its second limb end portion 33 on the double adjusting lever 28 so that the latter supports itself against the end portion 31 (see Fig. 9). Thus the right pawl end 23 of the double adjusting lever 28 brings about rotation of the right toothed adjusting wheel 8, causing the relevant threaded bolt 2 to be screwed out.

When the brake shoes abut against the brake drum, the thread friction exceeds the adjusting force as defined by the pre-tensional force of the flexible spring 29. The control plate 24 then pivots further and takes along the spring end portion 31, but a further adjustment is prevented (see Fig. 10).

SJL290491 13 Upon release of the brake, the parts of the deflection lock are again centred, and the coiled springs 27 return the control plate 24 to its initial position by pivoting anti-clockwise. The flexible spring 29 also follows this pivot movement and attempts to return into its initial position since its first limb 31 is fixed in the control plate 24. The second limb end 33 of the spring 29 consequently presses the double adjusting lever 28 so that its pawl-like end 23 moves into the next tooth base of the arresting teeth arrangement 9.

In subsequent operation of the brake, an adjustment is made if the basic clearance is exceeded.

SJL290491 14

Claims (4)

1. Adjustment and centring means for a drum brake comprising a slidethrough lock arranged in a fixed guide frame and axially displaceable against the tensional force of a centring spring, said lock including, for each direction of deflection, an adjusting nut into which is screwed a non-rotational threaded bolt which engages the respective end of the brake shoes, toothed adjusting wheels coaxial with and associated with each said nut and having unidirectionally operative arresting teeth, and a lever mechanism including two pawls which engage the arresting teeth of the two toothed adjusting wheels and are pivotally mounted on the guide frame and pushed by the force of a spring into a centring inoperative position, in which respect the lever mechanism, during an axial displacement of the slide-through lock and after exceeding the basic clearance, abuts against one of two stops of the slide-through lock or against one of the two toothed adjusting wheels, and one of the two pawls executes a movement relative to the associated toothed adjusting wheel, characterised in that excessive axial displacement of the slide-through lock is accompanied by rotation of the toothed adjusting wheel to bring about screwing-out of a threaded bolt, the engagement between the pawls and the toothed adjusting wheels being such that upon actuation of the brake each pawl engages a steep flank of the arresting teeth arrangement of the respective toothed adjusting wheel, and upon release of the brake it engages a flat flank of the arresting teeth arrangement, and in that the pawls are hingedly connected to the lever mechanism and forced into an inoperative position relative to the lever mechanism by the force of a spring, in which respect the pawls are pivotable relative to the lever mechanism if the countering force of the associated toothed adjusting 1 4 SJL290491 15 wheel exceeds the pre-tensional force of the spring.

2. Adjustment and centring means according to claim 1, characterised in that the lever mechanism consists of two identical assembly units, each comprising a control lever and an adjusting lever carrying the pawl, which units are 900 offset relative to one another, the control levers being mounted on the guide frame by respective bolts and the adjusting levers being pivotably connected to their associated control levers and held by a coiled spring in an inoperative position in which they are supported via angular lugs against the associated control lever, and each control lever having a portion laterally offset from the bolt, which portion is engaged during a deflection movement by a component connected to the slide-through lock in order to pivot the control lever.

3. Adjustment and centring means according to claim 1. characterised in that the lever mechanism consists of a control plate and a double adjusting lever with two pawls in that two bolts are mountedlon the guide frame and pass through two enlarged bore holes of the control plate, in that the double adjusting lever is pivotably connected to the control plate and held by a spring in an inoperative position relative thereto, in that the control plate is held in an inoperative position by two springs and has two areas which are laterally offset with respect to the bolts and which are engaged during a deflection movement by a component connected to the slide-through lock in order to pivot the control plate against the force of the two springs.

4. Adjustment and centring means for a drum brake substantially as hereinbefore described with reference to and as illustrated by Figs. 1 to 5, or Fig. 6 or Figs. 7 to 10 of the accompanying drawings.

Published 1991 at The Patent Office, Concept House. Cardiff Road. Newport, e t I RH. u er c pi ay b ob f rn Sales Branch, Unit 6. Nine Mile Point, CwnifelWach, Cross Keys, Newport, NPl 7HGZwPrrlinNtepd9 F rth 0 es rn e tained ro by Multiplex techniques lid. St Mary Cray. Kent.