GB1598921A

GB1598921A - Internal shoe drum brakes

Info

Publication number: GB1598921A
Application number: GB2973477A
Authority: GB
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1977-07-15
Filing date: 1977-07-15
Publication date: 1981-09-23
Also published as: FR2397566B1; FR2397566A1

Description

(54) INTERNAL SHOE DRUM BRAKES (71) We, AUTOMOTIVE PRO DUCTS L1MITED, a British Company of Tachbrook Road, Leamington Spa, Warwickshire, CV31 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: The invention relates to internal shoe drum brakes of the kind having an automatic adjuster on at least one brake shoe, the adjuster comprising an adjusting lever having a pivotal connection to the shoe, a spring-loaded pawl which cosperates with ratchet teeth formed on a quadrant of the lever and abutment means which co-operates with the adjusting lever to limit return movement of the brake shoe during brake operation.

Such a drum brake is described in British Patent Specification No. 1,084,413. This specification shows a leading and trailing shoe hydraulic brake with mechanical actuation provided by a lever and a cross-strut, the cross-strut co-operating with the adjusting lever which is pivotally mounted on one brake shoe and acting as the abutment means which limits return movement of both brake shoes.

Brakes as generally described in British Patent Specification No. 1,084,413 have been made in large numbers and usually function well, but the adjuster mechanism provides a shoe to drum clearance which can vary considerably according to the manufacturing tolerances employed. Since the adjusting lever and the cross-strut are usually simply blanked from sheet or strip metal, the relative positions of their inter-engaging surfaces can vary so that some brakes adjust to a shoe to drum clearance which is unacceptably large. The clearance is also affected by the fit of the pivotal connection of the adjusting lever with the brake shoe.

It is an object of the invention to provide an internal shoe drum brake of the kind described in which manufacturing tolerances have a minimised effect on shoe to drum clearance.

The invention provides an internal shoe drum brake having an automatic adjuster on at least one brake shoe which is resiliently biassed for retraction from the drum, the adjuster comprising an adjusting lever having a pivotal connection to said one brake shoe, a spring-loaded pawl having another pivotal connection to said one brake shoe and which co-operates with ratchet teeth formed on the arc of a quadrant of the adjusting lever to permit rotation of the adjusting lever relative to the pawl in one direction only, abutment means which co-operates with the adjusting lever to limit return movement of said one brake shoe during brake operation and a biassing spring acting on the adjusting lever to rotate the adjusting lever in the direction permitted by the pawl and ratchet teeth to take up excessive clearance between said one shoe and the brake drum in response to wear of the shoe lining, one of the pivotal connections incorporating a controlled amount of free play which determines the minimum shoe to drum clearance.

The invention will now be described with reference to the accompanying drawings, of which: Fig. 1 is an elevation of one example of a drum brake assembly according to the invention; Fig. 2 is a cross-section on the line II--II of Fig. 1; Fig. 3 is a scrap cross-section on the line III--III of Fig. 1; and Fig. 4 is a partial elevation of the drum brake assembly shown in Figs. 1-3 incorporating certain modifications.

Referring to Figs. 1-3, the drum brake assembly shown is of the leading and trailing shoe type, hydraulically operated and without a mechanism for mechanical actuation although mechanical actuation can be provided without extensive modification. Two brake shoes 11 and 12 are mounted on a back plate 13 and provided with a double ended hydraulic wheel cylinder 14 between one pair of adjacent ends and a fixed abutment 15 between the other pair of adjacent ends.

An adjuster lever 16 has a pivotal con nection 17 to one brake shoe 11 and is formed with ratchet teeth 18 on the arc of a quadrant. Similar teeth are provided on a portion of a pawl 19 which has a pivotal connection 21 to a brake shoe 11 and which is springloaded into engagement with the ratchet teeth 18 by a helical torsion spring 22. The pivotal connection 21 comprises a pin 23 which is a close fit in a hole in the web of the shoe 11 and is a large clearance fit in a hole in the pawl 19. The pin 23 passes through the helical portion of the torsion spring 22 and is secured by a circlip 24.

The pivotal connection 17 is similar to the other pivotal connection 21 except that the clearances are only sufficient to ensure free pivotal movement of the adjuster lever 16.

The arrangement of the pawl 19 and ratchet teeth 18 allows the adjuster lever 16 to pivot in an anti-clockwise direction only relative to the pawl 19 as viewed in Fig. 1.

A helical tension spring 25 acts as a biassing spring on the adjusting lever 16 and on brake shoe 12 to bias the adjusting lever 16.

This spring 25 also serves as a pull-off spring, being connected to brake shoe 12, and supplements another pull-off spring 26 acting between both brake shoes 11 and 12 to resiliently bias the shoes for retraction from the brake drum (not shown). In the absence of brake-applying pressure in the wheel cylinder 14, spring 25 pulls the adjusting lever into engagement with abutment means provided by a cross-strut 27 which extends between the adjusting lever 16 and brake shoe 12.

The ends of the cross-strut 27 and adjacent parts of the lever and the brake shoes 11 and 12 are notched to provide a transverse location for the cross-strut.

With no brake-applying pressure in the wheel cylinder 14, the adjusting lever 16 is biassed by springs 25 and 26 in a direction whereby the adjusting lever 16 is locked by the pawl 19 against clockwise rotation, the consequent reaction biassing the pawl 19 radially outwards relative to the drum rotational axis.

When pressure is generated in the wheel cylinder 14, spring 25 ensures that the adjusting lever 16 maintains contact with the cross-strut 27 so that as the ends of the shoes 11 and 12 adjacent the wheel cylinder 14 move outwardly, the adjusting lever 16 rotates anticlockwise relative to Fig. 1. Since pivotal connection 21 has a controlled amount of free play determined by the clearance fit of the pin 23 in the pawl 19 the effect is for the pawl 19 to rotate with the adjusting lever 16 and thus move in a generally radially inwards direction relative to the drum rotational axis.

Once the free play of pivotal connection ratchet teeth under the biassing action of spring 25. The pawl 19 rides over the ratchet teeth 18 by an amount determined by wear of the shoe lining so that when the pressure in the wheel cylinder 14 subsides the shoes return under the effects of springs 25 and 26 to the positions determined by the crossstrut 27. If lever 16 rotates by an amount equivalent to one pitch of the ratchet teeth or greater, the lever 16 remains in a new position so that excessive shoe to drum clearance is taken up.

The maximum shoe to drum clearance of both shoes 11 and 12 is determined by a combination of the free play in pivotal connection 21 and the pitch of the ratchet teeth 18. The minimum shoe to drum clearance is determined by the free play alone.

The torsion spring 22 is relatively weak compared with spring 25 as regards the effect on the movement of the pawl 19 in pivotal connection 21. Since spring 25 is connected to the adjusting lever 16 at a position between pivotal connection 17 and the abutment with strut 27 it maintains the load on pivotal connection 17 in substantially the same direction during all normal operating conditions so that any play in that connection does not affect the shoe to drum clearance. The diameter of the pin 23 can be controlled to within relatively fine limits, as can the associated holes in the web of shoe 11 and the pawl 19 so that the minimum shoe to drum clearance is affected by relatively few manufacturing tolerances which can be closely controlled.

In the modified assembly shown in Fig. 4, spring 25A acts between both brake shoes 11A and 12A to function as a normal pulloff spring and an additional spring 28A acts between shoe 12A and a position on the adjusting lever 16A between the arc of the quadrant and the abutment with the crossstrut 27A. The controlled free play is provided in pivotal connection 17A instead of pivotal connection 21A. The additional spring 28A acts as a biassing spring on the adjusting lever 16A to bias it in an anticlockwise direction relative to Fig 4 at all times. However, the adjusting lever 16A rotates about the abutment with the cross link 27A during initial outwards movement of the brake shoes 11A and 12A until the free play in pivotal connection 17A is taken up, when the pivot point transfers to the axis of pivotal connection 17A. The load of the additional spring 28A is such that while the adjusting lever 16A is pivoting on the cross-strut 27A there is insufficient tangential loading on the ratchet teeth 18A to allow the teeth to slip, but with the change in moment arms resulting from the transfer of pivot points there is sufficient tangential loading for the teeth to slip when

Claims

WHAT WE CLAIM IS:

1. An internal shoe drum brake having an automatic adjuster on at least one brake shoe which is resiliently biassed for retraction from the drum, the adjuster comprising an adjusting lever having a pivotal connection to said one brake shoe, a spring-loaded pawl having another pivotal connection to said one brake shoe and which co-operates with ratchet teeth formed on the arc of a quadrant of the adjusting lever to permit rotation of the adjusting lever relative to the pawl in one direction only, abutment means which cooperates with the adjusting lever to limit return movement of said one brake shoe during brake operation and a biassing spring acting on the adjusting lever to rotate the adjusting lever in the direction permitted by the pawl and ratchet teeth to take up excessive clearance between said one shoe and the brake drum in response to wear of the shoe lining, one of the pivotal connections incorporating a controlled amount of free play which determines the minimum shoe to drum clearance.

2. An internal shoe drum brake according to Claim 1, wherein the controlled amount of free play is provided in said other pivotal connection.

3. An internal shoe drum brake according to Claim 2, wherein said biassing spring acts on the adjusting lever to maintain a load on said one pivotal connection in substantially the same direction during all normal operating conditions.

4. An internal shoe drum brake according to Claim 3, wherein said biassing spring acts on the adjusting lever at a position between the abutment means and said one pivotal connection.

5. An internal shoe drum brake according to Claim 1, wherein the controlled amount of free play is provided in said one pivotal connection.

6. An internal shoe drum brake according to Claim 5, wherein said biassing spring acts on the adjusting lever at a position between the abutment means and the arc of the quadrant.

7. An internal shoe drum brake substantially as described herein with reference to Figs. 1-3 or with reference to Fig. 4 of the accompanying drawings.