AUTOMATIC ADJUSTER FOR INTERNAL SHOE DRUM BRAKES

Abstract: The brake has an automatic adjuster on at least one brake shoe (11) which is resiliently biased for retraction from the drum (D). The adjuster comprises an adjusting lever (121) pivotally connected to said one brake shoe (11), a spring loaded pawl (124) having a row of teech (123a), said pawl also being pivotally connected to said one brake shoe (11) and being co-operable with ratchet teeth (122a) formed on a curved face (122) of an adjusting lever (121) to permit rotation of the adjusting lever relative to the pawl in one direction only, abutment means (19) which co-operates with the adjusting lever to limit return movement of said one brake shoe during operation and means (25) which acts on the adjusting lever to rotate the adjusting lever in the direc­ tion permitted by the pawl and ratchet teeth to take up excessive clearance between said one shoe (11) and the brake drum (D) in response to wear of the shoe linings. The face (122) on the adjusting lever (121) describes a curve of changing radial distance from the axis (27a) of the pivotal connection (27) of the adjusting lever.

Description: INTERNAL SHOE DRUM BRAKES 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 toothed pawl which co-operates with ratchet teeth formed on the lever and abutment means which cooperates 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 crossstrut 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 of the foregoing kind have been made in large numbers and generally function well. However, a problem associated with such a brake is that the teeth on the pawl tend to wear in one place. This is due to the fact that the same few teeth on the pawl engage, over the range of adjustment consecutive fresh teeth on the adjusting lever and the wear on those few pawl teeth is relatively great. An object of the present invention is to provide an internal shoe drum brake intended to mitigate the foregoing problem According to the invention there is provided an internal shoe drum brake assembly having an automatic adjuster on at least one brake shoe which is resiliently biased for retraction from the drum, the adjuster comprising an adjusting lever pivotally connected to said one brake shoe, a spring loaded pawl having a row of teeth, said pawl also being pivotally connected to said one brake shoe and being co-operable with ratchet teeth formed on a curved face of the adjusting lever to permit rotation of the adjusting lever relative to the pawl in one direciton only, abutment means which co-operates with the adjusting lever to limit return movement of said one brake shoe during operation and means which acts 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, characterised in that the face on the adjusting lever describes a curve of changing radial distance from the axis of the pivotal connection of the adjusting lever. With such an arrangement, teeth on the adjusting lever, will at one end of the range of brake adjustment, interengage pawl teeth over one section of the row, and at the other end of the adjustment range will engage pawl teeth further along the row. In that manner, the likelihood of a small number of the pawl teeth being subject to excessive wear compared to the remainder is substantially reduced. Preferably the curved face on the adjusting lever describes an arc struck from a point offset from the axis of the pivotal connection of the adjusting lever. In such a case, the point about which the arc is struck may be between the axis of the pivotal connection of the adjusting lever and an actuator for the brake shoes. The arc struck about the said point preferably intersects an arc struck about the axis of the pivotal connection of the adjusting lever at one end of the curved face of the adjusting lever. Preferably, the row of teeth on the pawl is formed on a curved face which describes a curve of changing radial distance from the axis of the pivotal connection of the pawl. In that case the curved face on the pawl may describe a further arc struck about a pivot point offset from the axis of the pivotal connection of the pawl. The pawl is preferably arranged between the adjusting lever and the point about which the further arc is struck. A brake assembly in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a known brake assembly which is a variant of the type subject of British Patent No. 1,084,413, Fig. 2 illustrates part of a brake assembly in accordance with the invention, and Figs. 3 and 4 illustrate the relative positions of the pawl and adjusting lever in two different operating positionsof the Fig. 2 brake assembly. Referring to Fig. 1 a drum brake assembly comprises a pair of brake shoes 11 and 12 mounted on a back plate 13. A double ended hydraulic wheel cylinder 14 is located between one pair of adjacent ends of the shoes 11 and 12 and an abutment 15 is located between the other pair of adjacent ends. pair of pull-off springs 16 and 17 act to pull the brake shoes away from the brake drum (not shown). A hand brake lever 18 is pivoted to the one end of the shoe 12. A strut 19 extends between the two shoes 11 and 12 at the end portions thereof adjacent the wheel cylinder 14. Each end portion of the strut 19 has a slot therein, one of the slots for receiving the hand brake lever 18 and the web 20 of the brake shoe 12, and the other slot for receiving the web 23 of the other brake shoe 11 and an adjusting lever 21. The strut is biased against the shoe 12 and lever 18 by a spring 22. The adjusting lever 21 is located behind the web 23 of the brake shoe 11 and has one end pivotally connected thereto by a pin 27. The free end of adjusting lever 21 has an arcuate face which describes an arc A-A struck about the axis of pin 27. The arcuate face has ratchet teeth 31 on a spring loaded pawl 24, pivotally connected to the shoe web 23 by a pin 28. The strut 19 has a projection 25 thereon that extends into an aperture 26 in the adjusting lever 21. The projection 25 loosely engages in the aperture 26 to form a lost motion connection which provides for the normal service clearance between the brake shoe linings and the drum. In operation of the brake, the spring 22 always biases the strut towards the web 20 of shoe 12. If there is excessive clearance between the shoe linings and the drum then when the shoes move apart on application of the brakes, the lost motion connection between the strut 19 and the adjusting lever 21 is taken up and then the projection 25 on the strut will cause the adjusting lever 21 to pivot anti-clockwise relative to the shoe. The spring loaded pawl 24 will allow the adjusting lever 21 to pivot anti-clockwise to take up the excess clearance at the end of the strut, but not clockwise and therefore adjust the shoe to drum clearance. A problem associated with such a prior known brake drum assembly is that the same few teeth on the pawl tend to engage, over the range of adjustment, consecutive fresh teeth on the adjusting lever which causes the pawl teeth to wear relatively quickly. Referring now to Fig. 2 the same reference numerals will be used as in Fig. 1 for the same components. The brake shoe 11 has an adjusting lever 121 pivotally connected by a pin 27 having an axis 27a to the web 23 at the one end of the shoe. The adjusting lever 121 has a curved ratchet face 122 the teeth 122a of which engage like teeth 123a on a curved face 123 of a spring loaded pawl 124. The pawl 124 is pivotally connected to the web 23 by a pin 28. The ratchet face 122 describes an arc which is struck from a point P1 offset from the axis 27a of pivot 27 towards the actuator 14. This has the effect of producing a ratchet face having a curve thereon from axis 27a which describes a path of smoothly changing radial distance/ in an anti-clockwise direction. The curve A-A, which passes through the root of the teeth on the outer end of the ratchet, is an arc struck from the axis 27a and it can be seen that the ratchet face 122 on the arc struck from P1 passes through the root of the teeth and intersects the curve A-A at the outer end of the ratchet (the term outer end" of the ratchet is "outer" with respect to the radial distance from the centre of the brake assembly). The pawl 24 has a row of teeth 123a which lie on a curved face 123 which describes an arc of the same radius as the arc described by the face 122 on the adjusting lever 121. The arc described by face 123 is struck from a point P2 offset from an axis 28a of a pivot pin 128 for the pawl. In operation, the spring 22 always biases the strut 19 towards the shoe 12 (not shown in Fig. 2). When, as a result of excess clearance between the shoes and a drum D, the adjusting lever 121 is pivoted anticlockwise to ride over the teeth of pawl 24, the pawl rotates slightly anticlockwise due to the curve of the ratchet face on the adjusting lever 121. Figs. 3 and 4 illustrate the relative positions of the adjusting lever and pawl after the adjusting lever has moved incrementally from the Fig. 1 position (as shown in Fig. 3) through an angle < . In Fig. 3 the teeth on the adjusting lever 121 engage the pawl teeth over row section X and in Fig. 4 over row section Y. Therefore the teeth on the adjusting lever 121 do not engage the same few teeth on the pawl 124 throughout the range of adjustment thereby reducing wear and risk of damage to the adjustment mechanism.

Claims: CLAIMS 1. An internal shoe drum brake assembly having an automatic adjuster on at least one brake shoe (11) which is resiliently biasedfor for retraction from the drum (D), the adjuster comprising an adjusting lever (121) pivotally connected to said one brake shoe (11), a spring loaded pawl (124) having a row of teeth (123a), said pawl also being pivotally connected to said one brake shoe (11) and being co-operable with ratchet teeth (122a) formed on a curved face (122) of the adjusting lever (121) to permit rotation of the adjusting lever relative to the pawl in one direction only, abutment means (19) which co-operates with the adjusting lever to limit return movement of said one brake shoe during operation and means (25) which acts 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 (11) and the brake drum (D) in response to wear of the shoe lining, characterised in that the face (122) on the adjusting lever (121) describes a curve of changing radial distance from the axis of the pivotal connection (27) of the adjusting lever. 2. An assembly according to claim 1 characterised in that the curved face (122) of the adjusting lever (121) describes an arc struck from a point (P1) offset from the axis (27a) of the pivotal connection (27) of the adjusting lever. 3. An assembly according to claim 2 characterised in that the point (p1) about which said arc is struck is between the axis (27a) of the pivotal connection (27) of the adjusting lever (121) and an actuator (14 for the brake shoes. 4. An assembly according to claim 2 or 3 characterised in that the arc struck about the said point (P1) intersects an arc struck about the axis (27at of the pivotal connection (27) of the adjusting lever (121) at one end of the curved face 122 of the adjusting lever. 5. An assembly according to any preceding claim characterised in that the row of teeth (123a) on the pawl is formed on a face (123) which describes a curve of changing radial distance from the axis (28a) of the pivotal connection (28) of the pawl. 6. An assembly according to claim 5 characterised in that the curved face (123) on the pawl (124) describes a further arc struck about a point (P2) offset from the axis (28a) of the pivotal connection (28) of the pawl (124). 7. An assembly according to claim 6 characterised in that the pawl (124) is arranged between the adjusting lever (121) an the point (P2) about which the further arc is struck.

Inventors: CHARLES, JOHN MICHAEL
Applicants: AUTOMOTIVE PRODUCTS PUBLIC LIMITED COMPANY
Application year: 1983
Main classification: F16D65
IPC codes: F16D65