GB2075140A

GB2075140A - Automatic shoe clearance adjusting device for L-T type drum brakes

Info

Publication number: GB2075140A
Application number: GB8113709A
Authority: GB
Original assignee: Akebono Brake Industry Co Ltd
Current assignee: Akebono Brake Industry Co Ltd
Priority date: 1980-05-02
Filing date: 1981-05-05
Publication date: 1981-11-11
Also published as: GB2075140B; JPS56167932A; AU526449B2; JPH0141855B2; MX151960A; AU6992681A

Abstract

A leading-and-trailing shoe type drum brake having an automatic clearance adjusting device including a strut 8 the axial length of which is adjustable on rotation of a toothed wheel 8d. A parking lever 9 and an adjuster lever 11 are both pivoted to one shoe 2 by a pin 10, and a contact point B between the levers 9 and 10 causes the adjuster lever 11 to move on operation of the parking brake. The adjuster lever 11 includes a claw 11c engageable with the wheel 8d, to effect turning thereof on release of the parking brake when the shoe clearance has become too great. An adjuster spring 12 maintains contact between the parking and adjuster levers 9 and 11, and applies a tilting force to the adjuster lever to urge the claw 11c into engagement with the toothed wheel 8d. The lever ratio between the parking and adjuster levers is maintained constant irrespective of the clearance of the shoes, and moreover the force by which the claw is urged into engagement with the toothed wheel easily is adjustable by appropriate selection of the adjuster spring 12. <IMAGE>

Description

SPECIFICATION Automatic shoe clearance adjusting device for L-T type drum brakes This invention relates to an automatic shoe clearance adjusting device for a drum brake having one leading and one trailing brake shoe, which type of drum brake will hereinafter be referred to as an 'L-T type drum brake'.

An L-T type drum brake has an anchor interposed between the opposed one ends of a pair of arcuate brake shoes and an expanding mechanism such as a hydraulic cylinder arrangement disposed between the opposed other ends of the shoes. At least one return spring serves to exert a returning force on both shoes, to contract and hold them free of the drum when the brake is released. The brake shoes are thus arranged to be expanded by the action of the expanding mechanism with the anchor serving as fulcrum when the brake is applied. L-T type drum brakes are also usually provided with an adjustment mechanism which defines the fully-contracted position of the shoes when the brake is released.

The clearance which exists between the inner surface of the drum and the linings of the shoes under a non-braking condition increases as wear of the brake shoe linings advances. In view of this, automatic shoe clearance adjusting devices have been provided, in which the return positions of the brake shoes are adjusted depending upon the wear of the linings. For instance, the adjustment mechanism may be a strut extending between the two shoes adjacent the expanding mechanism, and means may be provided automatically to adjust the length of the strut to limit the return movement of the shoes.

An example of the prior art of this type has a strut extending between a parking lever pivotally supported on one brake shoe adjacent its said other end and the other brake shoe, there also being provided an adjuster lever which is arranged to be moved by the displacement of one brake shoe and the strut, relatively separated by the parking lever during parking brake application. With this arrangement, a gear in threaded engagement with the strut is caused to rotate when the degree of movement of the adjuster lever is sufficient, in such a way as intermittently to increase the axial length of the strut.More specifically, the parking lever and the adjuster lever may be pivoted to said one brake shoe adjacent its other end (i.e. near the expanding mechanism), and between this one brake shoe and the adjuster lever, there is provided an adjuster spring which exerts a turning force on the adjuster lever. Furthermore, at the point of engagement between the strut and the parking lever, there is provided a pin which fits into a slot provided in the adjuster lever. The adjuster lever is arranged so that it is caused to turn against the force of the I adjuster spring by the movement of the strut which takes place on expanding movement of the other brake shoe during parking brake application.Then, at the time oi parking brake release, when the axial force on the strut is removed, the adjuster lever is turned in the returning direction by the force of the adjuster spring and causes the gear of the strut also to turn forward by one tooth thereof, as neces sary, so that the adjustment can automatically be accomplished.

With regard to the operating principle of the automatic show clearance adjusting device of this type, the present invention aims at achieving a stable operation with reliable ad justment, but with a simplified structure.

In the prior art device as discussed above, it is essential to provide the strut with a pin which engages in the slot of the adjuster lever; generally the strut has a nut and bolt arrangement with the bolt head engaged by a socket, the two brake shoes being engaged by the nut and socket respectively, in which case the pin usually is attached to the socket. This arrangement is difficult to machine and thus usually results in the need to have a relatively expensive piece. Further, the turning of the adjuster lever by the pin provided on the strut takes place about another pin provided on one of the brake shoes, these pins serving as base or cardinal points for the rotation.However, this arrangement causes the adjuster spring, which is disposed between the one brake shoe and the adjuster lever at a point spaced from these points, to have a great offset value in the axial direction of the drum. Therefore, the resilient engaging force of a claw formed on the adjuster lever and which is arranged to be in resilient engagement with the toothed pe ripheral pail of the gear of the strut, fluctuates to a large extent. Hence, the adjuster spring which exerts a return turning force on the adjuster lever must accordingly be arranged to have a relatively large spring force. This then can then give rise to the likelihood that a malfunction may occur, such as causing a reverse rotation of the gear - that is, reduc ing the length of the strut.

In addition, one of the base points for the rotation of the adjuster lever is defined by the position of the pin which is on the strut and is arranged to engage the slot of the adjuster lever but inevitably there is a certain degree of play (rolling) about the axis of the strut due to the accumulation of machining allowances. A slight degree of movement due to such play will vary the lever ratio of the adjuster lever, which is determined by the positional relation between the above-mentioned two base points and the claw. It has thus been a probiem, with the prior art device that such variations in the lever ratio tend to render the adjustment unstable because of changes in the engagement position relative to the axis of the strut at the engaging point between the claw of the adjuster lever and the gear.

According to this invention, there is provided an L-T type drum brake assembly having an automatic shoe clearance adjusting device which assembly comprises a pair of brake shoes arranged on a back plate for expanding and contracting movement, an anchor disposed between the opposed one ends of the brake shoes, an expanding mechanism disposed between the opposed other ends of the brake shoes, a return spring which exerts a spring force on the brake shoes to urge the shoes to contract, a parking lever pivoted on one brake shoe adjacent the expanding mechanism, a strut mechanism the length of which is adjustable and arranged between the parking lever and the other brake shoe to limit the contracting movement of the pair of brake shoes depending upon the length thereof, the strut mechanism including a gear the rotation of which effects adjustment of the length of the strut, and an adjuster lever which is pivoted on the one brake shoe about the same axis as the parking lever so as to lie substantially parallel thereto, the adjuster lever having a first arm extending generally parallel to the axial direction of the strut mechanism which the first arm is provided with a claw arranged resiliently to engage the gear to effect rotation thereof on movement of the adjuster lever, and the adjuster lever having a second arm which extends generally parallel to the parking lever which second arm has a stop-part formed thereon for engagement by the parking lever on operation thereof, there being an adjuster spring arranged between the stoppart of the adjuster lever and said one brake shoe which adjuster spring is arranged to exert a resilient force of the stop-part of the second arm resiliently to urge the stop-part against the parking lever and to exert a tilting force on the claw of the first arm so that the claw part is resiliently urged in contact with the gear of the strut mechanism.

It will be appreciated that in this invention, any variation of the distance between base points for rotation of the adjuster lever is prevented by arranging for the rotation of the adjuster lever to be transmitted directly from the parking lever and not through engagement with the strut. Moreover, the resilient force engaging the claw of the adjuster lever with the gear of the strut can be lessened by improving the engaging relation between the adjuster lever, the parking lever and the strut to stabilize adjustment, to prevent erroneous action and to simplify the shape of the strut for easier machining and reducing in cost thereof.

In order to allow the adjuster lever easily to tilt, as specified above, it is preferred for at least one of the stop-part of the second arm of the adjuster lever and the parking lever at the region of the engagement therebetween to have a rounded engagement face. The adjusting spring may be arranged to lie substantially parallel to the longitudinal direction of the strut mechanism, and preferably takes the form of a helical coil spring.

To assist the prevention of a malfunction, it is preferred for there to be provided a strut spring which exerts a biasing spring force on the strut mechanism directed towards said one brake shoe. Such a spring advantageously exerts a spring force on the strut mechanism in a direction oblique to the axis of the strut mechanism.

By way of example only, two specific em bodiments of this invention will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a front view of a first embodiment of an L-T type drum brake according to this invention, with the drum removed for clarity; Figure 2 is a sectional side view of a part of the first embodiment; Figure 3 is a sectional view of a further part at the bottom of the first embodiment; Figure 4 is a front view of part of a second embodiment of an L-T type drum brake of this invention, again with the drum removed for clarity; and Figure 5 is a cross-sectional view of a lower part of the second embodiment.

Figs. 1 to 3 illustrate a first embodiment of the invention, comprising a back plate 1 on which are mounted brake shoes 2 and 3 arranged for expansion by separation of their opposed ends. An anchor 4 is carried by the back plate 1, and is engaged on opposed sides by one end of each of the shoes 2 and 3, the other ends of the shoes engaging a hydraulic cylinder device 5. Return springs 6 and 7 serve to keep the brake shoes 2 and 3 in engagement with the anchor 4 and the two ends of the cylinder device 5.

Between said other ends of the shoes 2 and 3, which are arranged for expansion on brake application there is provided a strut 8 which is disposed approximately parallel to the hydraulic cylinder 5. The relative position of the two brake shoes 2 and 3 when contracted is defined by the axial length of the strut, which is arranged to be extensible. The strut 8 comprises an adjuster bolt 8a and an adjuster nut 8b which is in threaded engagement with the adjuster bolt 8a so that relative rotation changes the axial length of the strut, the nut being engaged with an inside edge of the brake shoe 3 to prevent rotation thereof.The strut also has a socket 8c which is rotatably fitted over the head portion of the adjuster bolt 8a and has a shaft the rotation of which is restrained by the side edge of a parking lever 9, and a gear 8d which is secured to the adjuster bolt 8a, the gear having teeth formed around the circumference thereof. The axial length of the strut 8 thus increases as the adjuster bolt 8a is screwed out of the adjuster nut 8b, on rotation of the gear 8d.

The parking lever 9 is at one end pivotably mounted on one brake shoe 2 by a pin 10, the other end of the lever (the lower end as viewed on the drawing) being connected to a parking brake cable (not shown) which extends into the brake assembly from outside.

When the parking brake cable pulls on the parking lever 9, that lever turns clockwise on the pin 10, as viewed on Fig. 1, pushing the strut 8 to the left as viewed on the drawing, as a result of the turning movement of the lever 9. With the strut 8 thus pushed to the left, the other brake shoe 3 is moved in the same direction. Concurrently, the reaction causes the one brake shoe 2 to be pushed to the right, as viewed on the drawing. Accordingly, the pair of brake shoes 2 and 3 are expanded against the spring force of the return spring 7 to engage the inner circumferential face of the brake drum (not shown) to apply the parking brake.

During application of the main (service) brake, the brake shoes 2 and 3 are expanded by means of the hydraulic cylinder device 5.

An adjuster lever 11 is disposed on the one brake shoe 2, parallel to the front face of the parking lever 9, and also is arranged to turn on the pin 10. The adjuster lever 11 comprises a first arm 11 a which extends from the part engaging the pin 1 0 along the centre-line of the strut 8 and a second arm 11 b which extends along the parking lever 9. At the fore end of the first arm 11 a, there is formed a claw 11 c which resiliently engages the toothed periphery of the gear 8d, on the strut 8. The second arm 11 b is provided with a stop-part 11 d which is formed by turning over a part of the arm, and is arranged to engage the side edge of the parking lever 9 on the inner side thereof.

Between the stop-part ii d of the second arm 11 b and the one brake shoe 2, there is provided an adjuster spring 1 2 which is arranged approximately parallel to the axis of the strut 8. The adjuster lever 11 is urged for counterclockwise rotation, as viewed on Fig.

1, by a spring force exerted by a tension adjuster spring 1 2, to cause the stop-part 11 a to engage the inner side edge of the parking lever 9. The engaging relation of the adjuster lever 11 and the end part of the adjuster spring 1 2 as shown in the drawing exerts a force which causes the claw 11 c on the first arm 1 la resiliently to engage the gear 8d of the strut 8.

In this particular embodiment, the fore end of the second arm 1 b of the adjuster lever 11 has a part 11 e which is arranged to pass round the back side of the parking lever 9.

The parking lever 9 also is provided with a bent part 9a which is arranged to project toward the back side thereof in such a way as to correspond to the part 11 e of the adjuster lever 11. By this arrangement, the return movement of the adjuster lever 11 at the time of parking brake release can be ensured with these parts engaging each other.

The automatic shoe clearance adjusting device of the L-T type drum brake arranged as described in the foregoing operates in the following manner.

During parking brake application, when the pair of brake shoes 2 and 3 are expanded by the pivotal movement of the parking lever 9, the adjuster 11 is turned clockwise against the spring force of the adjuster spring 12, directly by the parking lever 9 because of the contact at point A between the lever 11 and the pin 10 on the brake shoe 2 and the contact at point B between the inner edge of the parking lever 9 and the stop-part 1 id.

Therefore, the claw 11 c swings upward, as viewed on Fig. 1, over the circumference of the gear 8d. During this, since the strut 8 is subjected to a strong axial force at that moment, the gear 8d is not caused to rotate by the frictional force resulting from the resilient contact of the claw 11 c of the adjuster lever 11 with the gear.

Upon release of the parking brake, the return spring 7 causes the pair of brake shoes 2 and 3 to contract. This movement of the shoes then turns the parking lever 9 back to its initial position, by the action of the strut 8.

Therefore, because of the spring force of the adjuster spring 12, the adjuster lever 11 follows the parking lever 9, and also turns back. Accordingly, the claw 11 c swings back downward', as viewed on Fig. 1, over the circumference of the gear.

At the time of the movement of the claw 1 ic if the amount of expansion and contraction has become considerable due to wear of the linings of the brake shoes 2 and 3, the claw 11 c swings upward to an extent sufficient to over-nde one tooth of the teeth formed on the gear 8d, so as to engage the next tooth. Then, at the time of parking brake release, since the strut 8 has no axial force exerted thereon, the downward movement of the claw 11 c causes the gear 8d to turn by one tooth. As a result, the adjuster bolt 8a to which the gear 8d is secured is caused to screw out from the adjuster nut 8b. The axial length of the strut 8 is increased by this, to define a new contracted position for the brake shoes.

In Fig. 2, the contact point C indicates where the claw a c of the adjuster lever 11 engages the gear 8d of the strut 8. In Fig. 1, the contact point D indicates where the parking lever 9 is in contact with the inner face of the rim of the one brake shoe 2, to prevent excessive counterclockwise movement of the lever 9.

The automatic shoe clearance adjusting device of this embodiment has the following features. Firstly, the base or cardinal points A and B of rotation oz the adjuster lever 11 are kept constant, because of the parallel relation between the parking lever 9 and the lever 11, shown in Fig. 1. Therefore, the lever ratio of the adjuster lever 11 is not changed with respect to the lever 9. Secondly, the resilient contacting force of the claw 1 c of the adjuster lever 11 applied to the gear 8d of the strut 8 can be maintained stable, or be lessened. The first feature derives from the fact that the adjuster lever 11 is arranged to be turned directly by the parking lever 9, with the same radii of rotation for the adjuster lever 11 and the parking lever 9, determined by the contact points A and B which always remain unchanged.Therefore, the lever ratio for the adjuster lever also remains constant, allowing stable and reliable adjustment irrespective of the speed of operation of the parking brake. As for the second feature, it derives from the arrangement of the adjuster spring 1 2. When the spring is in a position offset from the surface of the adjuster lever 11, the spring force is received in the vicinity of the part where the adjuster spring is connected to the adjuster lever 11 (see Fig. 2) with the bent stop-part 11 d engaging the inner edge of the parking lever 9. A tilting force is exerted on the claw 11 c of the adjuster lever 11, by virtue of the bent end part 1 2b formed at the end of the adjuster spring 1 2 engaged with the adjuster lever 11.

Therefore, it is possible to set the tilting force at a desired value by means of the bent end part 1 2b formed distinctly from the main body 1 2a of the adjuster spring 12, so as to have a relatively great tilting force to cause the adjuster lever 11 to be turned within the plane of Fig. 1. Therefore, the gear 8d can appropriately be prevented by this from being caused to turn in a direction which would shorten the shaft length of the strut 8.

Furthermore, to have the tilting force exerted on the adjuster lever 11 in a stable manner, it is preferred in a practical application to have the engaging contact point B between the adjuster lever 11 and the side edge of the parking lever 9 formed with at least one of the engaging faces having a curved arcuate shape or a spherical shape.

Figs. 4 and 5 show a second embodiment of this invention, in which a spring 1 3 is added to the first embodiment. Like part for the two embodiments are given similar reference characters and these parts will not be described again. Referring now to Figs. 4 and 5, it is a feature of this embodiment that a tension spring 1 3 is arranged between the adjuster nut 8b of the strut 8 and the one brake shoe 2. In addition to that, the spring 1 3 is arranged to exert a spring force on the strut at a slanting angle relative to the axis of the strut 8.

In other words, the spring 1 3 is offset by 11 within a plane perpendicular to the axis of the drum (Fig. 4) and 12 within a plane parallel to the axis of the drum (Fig. 5). Under this condition, the strut 8 has a force F exerted thereon in the axial direction of contraction, as shown in Fig. 5, and another force H exerted thereon in a direction perpendicular to the axis thereof. The spring 1 3 is further arranged to exert some force on the strut 8 in the rotational direction about its own axis.

The arrangement described above brings about the following advantages. Firstly, with the force F exerted on the strut in the axial contraction direction, mal-operation due to a change in the mode of brake operation can be prevented. In the adjusting device of the preceding embodiment, when the claw 11 c of the adjuster lever 11 is moved during parking brake application, the gear 8 is restrained against rotation by an axial force exerted on the strut 8, giving rise to frictional resistance.

Therefore, when there is no transmission of force to the strut during the operation of the parking lever 9, as will occur if the parking brake also is applied after the main or service brake has been applied, there is a considerable probability that the gear 8d will be turned when the parking brake is applied. To solve this problem, this embodiment is arranged to have the spring 1 3 exert an axial force F on the strut, to restrain rotation of the gear 8d by the action of the claw 1 c, should there be a reduction in the frictional resistance due to the mode of operation as explained above.

Furthermore, in this embodiment a force H which is perpendicular to the axis of the strut 8 is exerted by offsetting the spring 1 3. By this, the tendency of the gear 8d to rotate on applying the parking brake is further reduced, because a wrenching force is applied to the screwed engagement between the adjuster bolt 8a and the adjuster nut 8b. This allows the spring force of the spring 1 3 to be set at a smaller value.

Secondly, because a rotating force is applied to the strut 8 by the offset of the spring 13, rattling can be prevented. With the rotating force applied, the strut which has some degree of play about its axis relative to the parking lever 9 and the other brake shoe 3 can be biased in one direction about its axis, and thus can be prevented from rattling. The unvarying lever ratio of the adjuster lever 11 and the restriction on turning by the biasing arrangement around the axis of the strut 8 stabilizes the M value (Fig. 1), which is determined by the positional relation between the claw clinic 11 c and the gear 8d, so that the adjustment can always be effected to a constant degree.

As described in the foregoing, the embodiments automatic shoe clearance adjusting de vice for L-T type drum brakes, according to the present invention, not only are capable of having the rotating force of the adjuster lever and the resilient contact force of the claw on the gear set at adequate values independently of each other, but also are capable of allowing a constant adjustment to be obtained, with a stable and reliable operation. In addition to these advantages, the simplified shape of the strut facilitates the manufacture of the device and permits a reduction in cost.

Claims

1. An L-T type drum brake assembly having an automatic shoe clearance adjusting device which assembly comprises a pair of brake shoes arranged on a back plate for expanding and contracting movement, an anchor disposed between the opposed one ends of the brake shoes, an expanding mechanism disposed between the opposed other ends of the brake shoes, a return spring which exerts a spring force on the brake shoes to urge the shoes to contract, a parking lever pivoted on one brake shoe adjacent the expanding mechanism, a strut mechanism the length of which is adjustable and arranged between the parking lever and the other brake shoe to limit the contracting movement of the pair of brake shoes depending upon the length thereof, the strut mechanism including a gear the rotation of which effects adjustment of the length of the strut, and an adjuster lever which is pivoted on the one brake shoe about the same axis as the parking lever so as to lie substantially parallel thereto, the adjuster lever having a first arm extending generally parallel to the axial direction of the strut mechanism which first arm is provided with a claw arranged resiliently to engage the gear effect rotation thereof on movement of the adjuster lever, and the adjuster lever having a second arm which extends generally parallel to the parking lever which second arm has a stop-part formed thereon for engagement by the parking lever on operation thereof, there being an adjuster spring arranged between the stoppart of the adjuster lever and said one brake shoe which adjuster spring is arranged to exert a resilient force on the stop-part of the second arm resiliently to urge the stop-part against the parking lever and to exert a tilting force on the claw of the first arm so that the claw part is resiliently urged into contact with the gear of the strut mechanism.

2. An L-T type drum brake assembly according to claim 1, wherein at least one of the stop-part of the second arm of the adjuster lever and the parking lever at the region of the engagement therebetween has a rounded engagement face.

3. An L-T type drum brake assembly according to claim 1 or claim 2, wherein the stop-part of the adjuster lever engages an inner edge of the parking lever, having regard to the axis of the drum.

4. An L-T type drum brake assembly according to any of the preceding claims, wherein said adjusting spring is arranged to lie substantially parallel to the longitudinal direction of the strut mechanism.

5. An L-T type drum brake assembly according to any of the preceding claims, wherein there is provided a strut which exerts a biasing spring force on the strut mechanism directed towards said one brake shoe.

6. An L-T type drum brake assembly according to claim 5, wherein the strut spring exerts a spring force on the strut mechanism in a direction oblique to the axis of the strut mechanism.

7. An L-T type drum brake assembly according to claim 6, wherein the strut spring exerts on the strut mechanism a spring force which has a first component in the axial direction thereof and a second component perpendicular to the axis thereof.

8. An L-T type drum brake assembly according to claim 1 or claim 7, wherein the strut spring exerts a spring force on the strut mechanism which force tends to rotate the strut mechanism about the axis thereof.

9. An L-T type drum brake assembly substantially as hereinbefore described, with reference to and as illustrated in Figs. 1 to 3 or in Figs. 4 and 5 of the accompanying drawings.