GB2272261A

GB2272261A - A drum brake adjuster

Info

Publication number: GB2272261A
Application number: GB9223087A
Authority: GB
Inventors: Kenneth Maurice Quiney
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1992-11-04
Filing date: 1992-11-04
Publication date: 1994-05-11
Anticipated expiration: 2012-11-04
Also published as: GB2272261B; GB9223087D0

Abstract

A drum brake has an adjuster comprising a lever member 24 and pawl member 32 mounted on one brake shoe 12 having mutually engaging toothed surfaces 26, 30, the pawl member 32 being urged against the lever member 24 by a spring 32. The lever member 24 is movable by a strut 22 attached to the other brake shoe 14 to operate the adjuster. A thermal locking element comprising a locking member 40 is operated via a lever 44 by a bimetallic strip 46 to prevent adjustment, when the shoe 12 heats. The bimetallic strip extends into a slot 47 in the brake shoe support 19 for friction lining 16. <IMAGE>

Description

A DRUM BRAKE The present invention relates to drum brakes and in particular to adjusters for drum brakes in which the adjuster is disabled when the drum becomes overheated, to prevent over adjustment of the brake.

The present invention provides a drum brake assembly for a motor vehicle comprising an actuator mounted between opposed brake shoe ends, each shoe having a friction material lining secured to a support, the actuator being operable to move said ends apart to apply the brakes, and an adjuster comprising a strut having one end constrained to move with one brake shoe and the other end of the strut engaging a lever member pivoted to the other brake shoe, to rotate the lever member in one direction of rotation if the brake shoe ends move apart by more than a predetermined amount, said lever member being engageable with a reaction member also pivoted to said other brake shoe and used to engage the lever member to allow rotation of the lever member in said one direction of rotation but not in the other, and a thermally responsive locking means including a thermally responsive element which operates a locking member which is engageable with one of said members, the thermally responsive element being housed in a slot in said other brake shoe support adjacent the friction material lining.

Preferably the said one member is locked by frictional engagement between the locking means and the said one member. This enables the range of positions to be a continuous range. Alternatively the locking means and said one member can have interengaging teeth such that the said one member can be locked in any of a range of discrete posi tionso Preferably the locking means includes a locking member rotatable into and out of engagement with the said one member, and the locking means may also include a lever one end of which is acted upon by the thermally responsive element. This enables a good mechanical advantage to be obtained thereby locking the said one member firmly.

The thermally responsive element which may comprise a coil or strip of bimetal or memory metal.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Fig 1 is an elevation of a known brake having an adjuster according to a firm embodiment of the present invention; Fig 2 is an enlargement of the lock means of Fig 1.

Referring to Fig 1, a drum brake comprises a back plate 8, a drum 10, two shoes 12, 13 mounted on the backplate each having friction facings 16, 17 mounted on the support platform 19 thereof. The shoes 12,13 have one end supported against a fixed abutment 20, the other ends of the shoes 16 and 17 are movable apart by a hydraulic actuator 21 located therebetween. A strut 22 acts between the other ends of the shoes to limit the distance the shoes move together under the bias of a return spring 38.

The strut 22 is constrained to move with one shoe 13 by a spring 23 in the conventional manner. The other shoe 12 has an adjuster lever 24 mounted thereon pivotable about an axis X. The lever 24 has teeth 26 on its free end 28 which engage with teeth 30 on an adjuster pawl 32 also mounted on said other shoe 12. The pawl 32 is rotatable about an axis Y and is urged against the adjuster lever 24 by a spring 34. A projection on the strut 22 engages with play in an aperture 36 in the adjuster lever 24 and acts against the adjuster lever to hold the shoe 12 away from the shoe 13. The shoes are urged together and against the strut 22 by the spring 38.

When the shoes are urged apart, for example by the hydraulic cylinder 21 acting between the ends of the shoes remote from the fixed abutment 20 the strut 22 is held against the shoe 13 by the spring 23. If the friction facings 16, 17 are worn and the shoes move far enough for the projection on the strut 22 to engage the edge of the aperture 36 to cause the lever 24 to rotate anticlockwise as shown in Fig 1. The pawl 32 rotates clockwise against the force of the spring 34 and, if the adjuster lever moves far enough the teeth 26,30 on the adjuster lever and pawl ride over one another and the adjuster lever 24 moves one tooth or more further that the pawl 32. The teeth 26, 30 prevent the adjuster lever 24 from rotating back again when the shoes 12, 14 are released so the strut 22 holds the shoes 12, 14 further apart ready for the next actuation.

Referring also to Fig 2, the locking means comprises a locking member 40 rotatably mounted in a hole 42 in the shoe 12, and a lever 44 having one end connected to the locking member, and its other end connected to a bimetallic strip 46. The bimetallic strip 46 is in the form of a coil having its inner end mounted on the shoe 12 and its outer end connected to the lever 44. The coiled bimetallic strip 46 is mounted on the shoe 12 in a slot 47 in the shoe platform 19 so that it is adjacent the friction facing 16 so that heat generated in the friction facing is passed easily to the bimetallic strip.

The locking member 40 is cylindrical but has one surface portion cut away to form a flat or concave engagement surface 48. The locking member is mounted such that when the friction facing is cool, the engagement surface 48 is just clear of a curved surface 50 on the pawl 32, the curved surface having as its centre of curvature the axis of rotation 'Y' of the pawl 32.

As the brake becomes hotter the bimetallic strip 46 coils up tighter and moves end of the lever 44 leftwards rotating the engaging element 40 clock wise as shown in Fig 2. When the brake reaches a certain temperature, the engagement surface 48 engages with the curved surface 50 on the pawl and prevents rotation of the pawl, which in turn locks the adjustment lever 24. This prevents over adjustment which would otherwise be caused by the shoes moving excessively far out due to expansion of the drum. Because the curved surface 50 on the pawl 32 and the engagement surface 48 on the engagement element 40 engage in frictional contact the pawl 32 and lever 24 can be locked in any of a continuous range of positions.

Claims

1. A drum brake assembly for a motor vehicle com prising an actuator mounted between opposed brake shoe ends, each shoe having a friction material lining secured to a support, the ac tuator being operable to move said ends apart to apply the brakes, and an adjuster compris ing a strut having one end constrained to move with one brake shoe and the other end of the strut engaging a lever member pivoted to the other brake shoe to rotate the lever member in one direction of rotation if the brake shoe ends move apart by more than a predetermined amount, said lever member being engageable with a reaction member also pivoted to said other brake shoe and used to engage the lever member to allow rotation of the lever member in said one direction of rotation but not in the other, and a thermally responsive locking means including a thermally responsive element which operates a locking member which is engageable with one of said members, the ther mally responsive element being housed in a slot in said other brake shoe support adjacent the friction material lining,

2. A brake drum assembly according to Claim 1 wherein said one member is locked by fric tional engagement between the locking means and the said one member.

3. A brake drum assembly according to Claim 1 or Claim 2 wherein the locking means includes a locking member rotatable into and out of en gagement with said one member.

4. A drum brake assembly according to any forego ing claim wherein said locking means includes a lever, one end of which is acted upon by the thermally responsive element.

5. A drum brake according to any foregoing claim wherein the locking means includes a coiled bimetallic strip.

6. A drum brake according to any foregoing claim wherein the locking means is arranged to en gage a surface on a said one member which has a centre of curvature substantially on the axis of rotation of said one member.

7. A drum brake according to any foregoing claim wherein the reaction member is a pawl.

8. A drum brake according to any one of claims 1 to 7, wherein said one member is the reaction member.