GB2343925A

GB2343925A - Adjusting internal shoe drum brakes for vehicles

Info

Publication number: GB2343925A
Application number: GB9924577A
Authority: GB
Inventors: Glynn Ashman; Charles Stuart Goodreid
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1998-10-21
Filing date: 1999-10-19
Publication date: 2000-05-24
Anticipated expiration: 2019-10-19
Also published as: GB9822874D0; FR2785027A1; FR2785027B1; GB2343925B; GB9924577D0

Abstract

An internal shoe drum brake incorporates an automatic adjuster comprising a plate 8 with a pivoted quadrant 13, a rotatable, toothed stud 14, a slide 30 with teeth 31, and a spring 33. During normal braking, the shoes are moved by an actuator (5, Fig.1), and the leading shoe 1, after taking up clearance 24, engages projection 23 to pull the quadrant away from the stud and rotate it anticlockwise about its pivot pin (12) which is slidable in slot 11. In this case, the quadrant pulls the slide into contact with the stud to prevent it rotating. When the brake is released, the quadrant reengages the stud, but, if over adjustment has occurred due to drum expansion (through heat or excessive pressure applied by the actuator), the quadrant and stud are able to rotate in response to drum contraction through displacement of the slide 30.

Description

2343925 IMPROVEMENTS RELATING TO INTERNAL SHOE DRUM BRAKES FOR VEHICLES

This invention relates to internal shoe drum brakes for vehicles which incorporate an automatic adjuster and are of the kind in which first and second arcuate brake shoes provided with friction linings are mounted on a stationary back-plate for application to the internal surface of a rotatable drum by actuating means acting between adjacent end of the shoes, the adjuster comprising a strut extending between the shoes adjacent to the actuated ends and comprising a lever pivotally mounted on a plate, one arm of the lever co-operating with a toothed element on the plate whilst a second arm co-operates with the first shoe, the plate co-operating with the second shoe, the arrangement being such that when the separation of the actuated ends of the shoes in the application of the brake exceeds a predetermined value the first arm of the lever is disengaged from the toothed element and the lever is moved angularly to cause its first arm to re-engage with the toothed element in a fresh position when the brake is released and thereby increase the effective length of the strut.

In the brake disclosed in GB-A-1410514 the toothed element comprises a peripherally toothed stud or spigot non-rotatably mounted in the plate. During prolonged operation of the brake of GB-A-1410514 by actuation of the actuating means, generation of excessive heat may cause the rotatable 25 drum to expand accompanied by a corresponding angular movement of the lever which, in turn, increases the effective length of the strut. When the brake is released with the drum heated the braking clearance will be correctly set. However, as the drum cools and contracts the braking clearances will be taken up causing the brake to "bind" since the adjuster 30 cannot release when subjected to drum contraction load.

2 Over adjustment can also occur if excessive braking pressure is applied which could deform the drum. After the pressure is released, the drum may return to its original configuration. However, the over adjustment 5 will then cause the brakes to bind which is undesirable.

According to our invention in an internal shoe drum brake of the kind set forth, the toothed element is rotatably mounted on the plate, and is adapted to be locked against rotation when the brake is applied for normal service braking by the actuating means, the toothed element being permitted to rotate and decrease the effected length of the strut by angular movement of the lever in the opposite direction when the strut is subjected to drum contraction load.

A locking member mounted on the strut from limited bodily movement with respect to the toothed element is normally spring urged in a direction to lock the element against rotation, the locking member being movable in the opposite direction to permit the element to rotate in response to angular movement of the lever when the strut is subjected to drum contraction load.

Preferably the locking member is urged towards the toothed element by means of a tension spring acting between the lever and the locking member.

The spring is sufficiently strong to ensure that the locking member will prevent the toothed element from rotating during normal service braking, but permits rotation of the toothed element when the lever is subjected to drum contraction load.

3 When the brake is applied for parking or in an emergency, a brake applying lever pivotally mounted on the second shoe, acts on the first shoe through the strut co-operating directly with the locking member, which defines a hand brake strut, and the force is transmitted to the first shoe through the toothed element and the lever, both of which are locked against rotation by the force acting through the hand brake strut.

A brake incorporating an automatic adjuster in accordance with the present invention is illustrated by way of exa mple in the accompanying 1.0 drawings in which:

Figure I is an end view of actuated shoe ends of the brake; Figure 2 is a section on the line 2-2 'of Figure 1 with the brake in an "off" non actuated position; Figure 3 is a view similar to Figure 2 which shows the brake in an actuated position; Figure 4 is a similar section with the brake operated for parking or emergency braking by hand lever; and Figure 5 illustrates the effect of the application to the adjuster of drum contraction load.

The brake illustrated in the accompanying drawings is of the one-trailing, one-leading shoe type incorporating opposed arcuate shoes 1, 2 mounted on a stationary back plate (not shown) and carrying friction linings 3, 4 for engagement with the internal surface of a rotatable drum (not shown).

4 The ends of the shoes 1, 2 illustrated in the drawings are adapted to be separated to apply the brake by an hydraulic actuator 5 mounted on the back plate, the other ends of the shoes engaging a stationery abutment (not shown) on the back plate.

The brake incorporates an automatic adjuster comprising a strut 7 which extends across the brake between the shoes adjacent to the actuator 5.

The strut 7 comprises a plate 8 which is provided at opposite ends with slots 9, 10 for receiving the webs of respective shoes 1, 2. The end of the plate 8 adjacent to the shoe 1 is provided with an elongate opening 11 through which passes a pivot pin 12 for a bell-crank lever 13 in the form of a toothed quadrant. The quadrant 13 has a certain amount of free movement relative to the plate 8.

A serrated or peripherally toothed stud 14 is rotatably mounted in the plate 8. A first arm 15 of the quadrant 13 is directed towards the stud 14 at its free edge which is formed with teeth 18 co-operating with the teeth on the stud 14 and located on the arc of a circle with the pin 12 at its centre. A second arm 19 of the quadrant 13 projects through an opening 20 in the web of the shoe 1. The side of the arm 19 adjacent to the web of the shoe is provided with a cam surface 22 for engagement with the outer edge of the opening 20.

In the "off" position of the brake a projection 23 extending into the opening 20 is spaced from the inner edge of the opening 20 by a small clearance 24 corresponding to the desired clearance to be made between the shoes and the brake drum.

Finally a locking member comprising a hand brake strut 30 is mounted on the plate 8 for limited movement in a direction towards and away from the i toothed stud 14. One end of the hand brake strut 30 which is of arcuate outline is provided with teeth 31 co-operating with teeth on the stud 14 and the opposite end of the strut is adapted to co-operate with the web of the trailing shoe 2 through a pivot 41 when a hand brake lever 40 is rotated to applying the hand brake.

A tension spring 33 acting between the pivot pin 12 for the quadrant 13 and the hand brake strut 30 urges the teeth 18 and 31 into engagement with the stud 14. The end of the strut 30 may, alternatively, carry only a single tooth, or be a convex or planar outline.

The hand brake lever 40 for applying the brake manually for parking or in an emergency is pivotally connected to the web of the trailing shoe 2 on the pivot 41 adjacent to its actuated end, and an abutment face 41 at an intermediate point in the length of the lever 40 is adapted to co-operate with the adjacent end of the hand brake strut 30. A small nominal clearance 43 is provided between the lever 40 and the hand brake strut 30.

The clearance 43 is taken up as the hand brake lever is rotated. When the hand brake lever is not applied, the clearance allows the strut 30 to move slightly towards the lever to take up the clearance under certain conditions as described hereinafter.

In the application of the brake for normal service braking the shoe ends are separated by the actuator 5. As the leading shoe 1 moves outwardly the clearance 24 between the slot 20 and the projection 23 is first taken up. If, as a result of wear of the shoe linings, the movement of the shoe I exceeds that clearance, the quadrant 13 moves outwardly with the shoe I and the toothed arm 15 of the lever is disengaged from the toothed stud 14. The quadrant 13 is then moved angularly by the shoe about the pin 12 through an angle corresponding to the movement of the shoe in 6 excess of that required for taking up the clearances. When the brake is released the toothed arm 15 of the quadrant 13 re-engages with the stud 14 in a fresh position so that the arm 15 provides a stop for the return movement of the shoe 1 spaced outwardly from the previous 5 position by the amount corresponding to wear of the shoe linings.

During the operation described above the toothed stud 14 is prevented from rotating by the hand brake strut 30 acting to lock it against rotation by the force in the tension spring 33.

During prolonged operation of the brake as described above generation of excessive heat may cause the rotatable drum to expand. A deformation of the drum may also arise if excessive pressure is applied to the brake shoes by the actuator 5. This is accompanied by the corresponding angular movement of the quadrant 13 which, in turn, increases the effective length of the strut 7. In the case of heating, when the brake is released the braking clearance will be correctly set as described below.

As the heated drum cools and contracts the braking clearance will be taken up and a drum contraction load will be applied to the shoes 1, 2. Specifically this load is applied from the leading shoe 1 onto the quadrant 13 and the leading shoe 1 rotates the quadrant 13 at least collapsing the brake slightly. The shoes 1, 2 are therefore re-adjusted substantially to drum size and the brake will then run with a zero or a minimal drag. The quadrant 13 can rotate because the hand brake strut 30 moves relatively away from the toothed stud 14, allowing the stud itself to rotate. This ensures that binding will not occur upon contraction of the brake drum upon cooling. In the case of excessive pressure, it also ensures the brake will not bind as the drum returns to its normal shape.

7 Rotation of the quadrant 13 in the reverse direction continues until the normal brake clearance 24 has been restored.

When the brake is to be applied for parking or in an emergency angular movement of the brake applying lever 40 about its pivotal connection with the trailing shoe 2 acts on the leading shoe 1 through the strut 7. The toothed stud 14 is locked against rotation between the quadrant 13 and the hand brake strut 30 thereby preventing the strut 7 from de-adjusting whilst transferring the hand brake load through the shoes, with the trailing shoe 2 applied to the drum by reaction of the lever 40 on the trailing shoe 2.

Claims

8 CLAIMS

I An internal shoe drum brake of the kind set forth, in which the toothed element is rotatably mounted on the plate, and is adapted to be locked against rotation when the brake is applied for normal service braking by the actuating means, the toothed element being permitted to rotate and decrease the effected length of the strut by angular movement of the lever in the opposite direction when the strut is subjected to drum contraction load.
2. A brake according to claim 1, in which a locking member mounted on the strut from limited bodily movement with respect to the toothed element is normally spring urged in a direction to lock the element against rotation, the locking member being movable in the opposite direction to permit the element to rotate in response to angular movement of the lever when the strut is subjected to drum contraction load.
3. A brake according to claim 2, in which the locking member is urged towards the toothed element by means of a tension spring acting between the lever and the locking member.
4. A brake according to claim 3, in which the spring is sufficiently strong to ensure that the locking member will prevent the toothed element from rotating during normal service braking, but permits rotation of the toothed element when the lever is subjected to drum contraction load.
5. A brake according to any of claims 2 to 4, in which, when the brake is applied for parking or in an emergency, a brake-applying lever pivotally mounted on the second shoe, acts on the first shoe through the strut co-operating directly with the locking member, which defines a hand 9 brake strut, and the force is transmitted to the first shoe through the toothed element and the lever, both of which are locked against rotation by the force acting through the hand brake strut.
6. An internal shoe drum brake substantially as described herein with reference to and as illustrated in the accompanying drawings.