GB2294740A

GB2294740A - Return spring for a vehicle drum brake

Info

Publication number: GB2294740A
Application number: GB9522409A
Authority: GB
Inventors: John Patrick Bayliss
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1994-11-01
Filing date: 1995-11-01
Publication date: 1996-05-08
Also published as: GB9522409D0; GB9421911D0

Abstract

An internal shoe-drum brake is provided with a single return spring 10, of omega outline. The spring comprises a single component of resilient material comprising a bridge piece 11 extending between and across shoes 1, 2 and opposed end portions 12 contiguous with the bridge piece and each defining an anchorage with a respective shoe. Each end portion 12 is adapted to apply to each respective shoe a force having a first component adapted to hold the friction lining carried by that shoe out of engagement with the drum in an "off" position of the brake, and a second component adapted to hold the shoe against a back plate for stability. <IMAGE>

Description

IMPROVEMENTS IN VEHICLE BRAKES This invention relates to improvements in internal shoe-drum brakes for vehicles of the kind in which a pair of arcuate shoes carrying friction linings for engagement with a rotatable drum are mounted on a relatively stationary back plate.

It is known to provide shoe-drum brakes of the kind set forth with automatic adjusters to compensate for wear of the frictions linings. Adjustment is effected in opposition to the force in a brake return spring or springs which act in a direction to hold the linings out of engagement with the drum in an "off" position of the brake, and the shoes against abutment means on the back plate for stability.

The brake shoe return spring or springs may be arranged to prevent the brake shoes from moving away from the back plate. This effect, which is known as "hold down", may be increased by suitably shaping end portions of the return spring and connecting the end portions to the ends of the brake shoes in such a manner that a couple is applied to each shoe by an adjacent end portion of the spring.

A return spring of this kind is known from GB 1 199 191. However, the position of application of the horizontal load on the brake shoes allows for very little load to be applied to the abutments with the back plate. Lowering the point of application of the return spring end portions results in the provision of an excessively stiff spring. Such a spring has attendant tolerance problems, and there are also difficulties in the manufacture of such spring end portions.

According to our invention in an internal shoe-drum brake of the kind set forth provided with a return spring, the spring comprises a single component of resilient material comprising a bridge piece extending between and across the shoes, and opposed end portions contiguous with the bridge piece and each defining an anchorage with a respective shoe, each end portion applying to each respective shoe a force having a first component adapted to hold the friction lining carried by that shoe out of engagement with the drum in an "off" position of the brake, and the second component adapted to hold the shoe against the back plate for stability.

The spring is loaded in both bending and torsion.

The spring has a horizontal aspect of low rate which arises as a result of extra length of the spring due to its shape.

"Hold down" forces arise as a result of a couple provided by the spring. Restoring forces are applied by a wall of a clevis of each of a pair of hydraulic actuating pistons and an abutment plate.

Preferably the spring is of omega shape having a bridge piece curved in two planes mutually at right angles to each other and connected to the end portions by curved connecting parts, and each end portion is of L-outline having first and second arms which terminate in distal fingers, the fingers being cranked with respect to the plane of the end portions and located in openings in webs of the shoes to urge the shoes away from the drum and with the arms adjacent to the bridge piece applying an inwardly directed force to the shoes in a direction radial to the drum.

The curve of the bridge piece in one of the planes is substantial and is radial to the drum. This ensures a relative lower rate for the spring. The curve in the of the other planes is relatively minor and is chosen to provide sufficient end moments to develop the necessary hold down forces.

Since the shape and configuration of the spring is relatively simple, manufacture of the spring and the assembly of the spring into the brake is facilitated.

One embodiment of our invention is illustrated in the accompanying drawings in which: Figure 1 is a plan view of an internal shoe-drum brake of the one-leading, one-trailing, shoe type with the drum omitted for clarity; Figure 2 is a section on the line 2-2 of Figure 1; Figure 3 is a section on the line 3-3 of Figure 1; Figure 4 shows the free position of the spring being fitted to the shoes; Figure 5 is a plan of the spring in a free unstressed position; Figure 6 is a side view of the spring; and Figure 7 is a end view of the spring.

In the internal show-drum brake illustrated in the drawings a pair of arcuate shoes 1, 2, each of Tee outline having a radial web 3, carry friction linings 4 for engagement with a rotatable drum (not shown). The shoes 1, 2 are adapted to be separated at adjacent actuated ends by double-ended hydraulic actuator 5, incorporating opposed pistons 6, 7, and the non-actuated ends anchor between spaced plates defining a common abutment 8 which takes the drag on the shoes when the brake is applied.

A single return spring 10 of omega outline is incorporated to act as a return or "pull-off" spring to hold the shoes 1, 2 away from the drum in the "off" position of brake, and to hold the webs 3 against the undersides of the devises in the pistons 6, 7 and against the upper plate of the abutment 8.

The spring 10 comprises a bridge piece 11 extending between and across the webs 3 of the shoes 1, 2, and opposed end portions 12 contiguous with the bridge piece 11 each define an anchorage with a respective shoe. The bridge piece 11 is curved in two planes mutually at right angles to each other and is connected to the end portions 12 by curved connecting parts. 13. Each end portion 12 is of L-outline having first and second arms 14, 15 which terminate in distal fingers 16, the fingers being cranked in respect to the plane of the end portions 12 and being located in openings 17 in the shoe webs 3. The fingers 16 are deformed inwardly towards each other at their distal ends to define notches 18.

The spring 10 is assembled in the openings 17 by inserting each finger 16 through a respective opening 17 in a position such that the bridge piece 11 extends outwardly towards the drum. The webs 3 are then received in the piston devises and between the plate forming the abutment 8. The bridge piece 11 is therefore stressed in opposite directions in both bending and torsion to apply loading to the shoes 1, 2 in two directions. As shown in Figure 2 of the drawings the notches 18 receive edges of the openings 17 at the undersides of the webs 3, and the arms 14 act on the webs in radial directions.

The fingers are so shaped that a couple is applied to each brake shoe 1, 2 about an axis parallel with the centre line passing through the hydraulic actuator 5 and the abutments 7, and are set outwardly with respect to the ends of the shoes 1, 2. The couple is thus independent of the shoe return force applied by the spring which acts substantially radially of the brake.

Claims

1. An internal shoe-drum brake of the kind set forth provided with a return spring, in which spring comprises a single component of resilient material comprising a bridge piece extending between and across the shoes, and opposed end portions contiguous with the bridge piece and each defining an anchorage with a respective shoe, each end portion applying to each respective shoe a force having a first component adapted to hold the friction lining carried by that shoe out of engagement with the drum in an "off" position of the brake, and a second component adapted to hold the shoe against the back plate for stability.

2. A brake as claimed in claim 1, in which the spring is loaded in both bending and torsion.

3. A brake as claimed in claim 1 or claim 2, in which the spring has a horizontal aspect of low rate which arises as a result of extra length of the spring due to its shape.

4. A brake as claimed in any preceding claims, in which "hold down" forces arise as a result of a couple provided by the spring.

5. A brake as claimed in any preceding claims, in which restoring forces are applied by a wall of a clevis of each of a pair of hydraulic actuating pistons and an abutment.

6. A brake as claimed in any preceding claims, in which the spring is of omega shape having a bridge piece curved in two planes mutually at right angles to each other and connected to the end portions by curved connecting parts, and each end portion is of L-outline having first and second arms which terminate in distal fingers, the fingers being cranked with respect to the plane of the end portions and located in openings in webs of the shoes to urge the shoes away from the drum and with the arms adjacent to the bridge piece applying an inwardly directed force to the shoes in a direction radial to the drum.

7. A brake as claimed in claim 6, in which the curve of the bridge piece in one of the planes is substantial and is radial to the drum.

8. A brake as claimed in claim 6, in which the curve of the bridge piece in the other of the planes is relatively minor.

9. A brake as claimed in any of claims 6 to 8, in which the fingers are deformed inwardly towards each other at their distal ends to define notches which receive edges of the openings in the webs at the sides of the webs remote from the bridge piece.

10. A brake as claimed in any of claims 6 to 9, in which the fingers are so shaped that a couple is applied to each brake shoe about an axis parallel with a centre line passing through an hydraulic actuator constituted by the actuating pistons and the abutment.

11. A brake as claimed in any of claims 6 to 10, of the one-leading one-trailing shoe type, in which the shoes are adapted to be separated at actuated shoe ends by the pistons, and the non-actuated ends anchor between spaced plates defining a common abutment which takes the drag on the shoes when the brake is applied, the spring holds the webs against the undersides of the devises in the pistons, and against the upper plate of the abutment.

12. An internal shoe-drum brake substantially as described herein with reference to and as illustrated in the accompanying drawings.