GB2046871A - Vehicle drum brake springs - Google Patents

Abstract

In a vehicle drum brake vibrations of a shoe return spring (1, 2) are damped by a member (3; 4, 5) positioned loosely within or around the spring coils (1) to be free to move longitudinally of the spring and to rotate with respect to the coils. The member may be a cylindrical rod (3) within the coils or a sleeve or collar (4, 5) with a cylindrical interior surface. The damping member is made of hard material, e.g. metal, and preferably extends over at least half the coiled length of the spring. <IMAGE>

Description

SPECIFICATION Vehicle drum brakes This invention is concerned with vehicle drum brakes and more especially with reducing vibrations of shoe pull-off springs in such brakes.

Generally speaking, it is desired that the force exerted on the shoes by the return or pull-off springs in a drum brake should remain substantially constant through the normal range of extension of the spring, i.e. from a condition of brakes off with new linings to a condition of brakes applied with worn linings. For this reason a low spring rate is necessary. It is known that the spring rate of a coil spring can be reduced by increasing the number of coils or by increasing their diameter. Of course in the case of shoe return springs, the length of the spring is determined by the design of the brake, and the space available for accommodating the springs is often limited so that large diameter springs are not practical.Springs having a large number of coils are very prone to bouncing which is unacceptable in shoe return springs in view of the severe vibrations to which vehicle road wheels are subjected.

It is sometimes possible to provide lateral support for a pull-off spring by positioning it against a rigid part of the brake structure, for example an adjuster rod or actuating cylinder. However, this method of providing lateral support for a pull-off spring has its limitations and great care must be taken to ensure that the spring does not interfere with the brake operation. If a spring having a large number of coils to obtain the required low spring rate is unsupported it tends to vibrate with an amplitude which can cause fracture-producing stresses and lead to failure.

The present invention aims at providing a solution to the above problem and according to one broad aspect resides in a method of damping vibrations of a coiled shoe return spring in a drum brake, wherein damping means is positioned loosely with or around the spring coils to be freely movable relative to the spring coils.

According to the invention there is also provided a drum brake comprising a drum, a brake shoe, actuating means for applying the shoe to the drum, a helical coil return spring for pulling the shoe off the drum when released by the actuating means, and damping means loosely positioned within or around the spring coil to be movable freely relative thereto.

The damping means preferably comprises a cylindrical member which may take the form of a rod located within the spring coils, or a collar, tube or sleeve surrounding the coils. Using such vibration damping members it has been found possible to damp the oscillations of brake shoe pull-off springs to a satisfactory level at which risks of spring fracture are substantially eliminated.

It is widely known to reduce coil spring vibrations by positioning the spring coils in close frictional contact with another element, but this technique has not proved satisfactory for brake pull-off springs. An experiment was conducted with a spring having inserted with the coils a rod of rubber having such a diameter that it was at all times gripped by the spring coils, and it was found that the amplitude of the vibrations was reduced only slightly compared with the spring having no rod.

A better understanding of the invention will be had from the following detailed description which is given by way of example with reference to the accompanying drawing, in which: Figure 1 shows a shoe return spring equipped with a damping member in the form of a cylindrical rod; and Figure 2 shows and shoe return spring fitted with two damping collars.

In Fig. 1 there is illustrated a shoe return spring for a vehicle drum brake having helical coils 1 with hook portions 2 at the ends for hooking in conventional manner into holes provided in the webs of the brake shoes. Positioned with the coils 1 of the spring with radial clearance is a cylindrical rod 3 of mild steel, the length of which is preferably at least half that of the coiled spring portion. The rod is free to move axially and to rotate with the spring coils, but is retained axially within the coils, in use, by the webs of the brake shoes.

The shoe return spring shown in Fig. 2 is the same as that of Fig. 1, but in place of the rod 3 is fitted with two collars 4, 5 which surround the spring coil with radial clearance for free movement of the collars axially and around the coils. When mounted in a vehicle drum brake the webs of the brake shoes limit axial movement of the collars to retain them on the coils of the spring.

Some tests were carried out on springs fitted with damping members as seen in Figs. 1 and 2 and an identical spring having no damping means. The vibration forces were applied in the directions of the arrows F, and the results of the tests are given in the Tables I and II.

TABLE I Inside Outside Mass Length Diameter Diameter (gm) Material (mm) (mm) (mm) Spring Steel 246 7.89 13.17 88 (Stretched) (Coils) (Coils) Rod Mild Steel 154 - 6.35 40 Collar Mild Steel 15 16.5 20.0 10 TABLE II Stiffness Frequency Input Amplitude (Kn/mm) (Hz) ('g') (mm) Cycles Spring .01036 95 2 33 failed at .16 x 106 Spring did not + Rod.0235 85 2 8 fail 1.0X 106 Spring did not + 2 - 85-92 .2 7 fail Collars 1.0 X 106 From the above it will be seen that the damping rod and collars are both very effective in reducing the amplitude of spring vibrations and can extend the spring lifetime.The reasons why these damping members are so successful are not completely understood but a possible explanation is that when the spring starts bouncing the damping member is thrown against the spring coils and moves with frictional contact against the coils. The vibrations of the spring are inevitable of a complex nature including lateral as well as longitudinal components of different modes. Nevertheless, whenever the damping member comes into contact with the coils there is a tendency for the member to be driven in rotation and energy is dissipated by frictional heating due to relative movement between the coils and damper member while in contact.

There is a natural tendency for the damper members to move axially to and fro along the spring, for which reason it is preferred that the damper members have a length (or a combined length if there are more than one) equal to at least half the length of the coiled spring portion so that there is always a part of a damper member at the middle of the spring where the amplitude of vibrations is greatest.

It is thought that the stiffness of the material of the damper members may be significant and satisfactory results have been obtained using metal, in particular mild steel, and glass-reinforced plastics (hard), whereas less satisfactory results were achieved with -softer materials.

While the use of two damping collars on the outside of the spring coils has been described it will be appreciated that more collars or a single sleeve or tube might be used instead. Similarly .more than one rod might be located within the coils and damper members inside and outside the coils could be used on the same spring.

The use of an internal damper does have the slight advantage of not demanding any additional space within the brake structure since it is contained wholly within the spring coils.

Claims (14)

1. A method of damping vibrations of a coiled shoe return spring in a drum brake, wherein at least one damping member is positioned loosely within or around the spring coils for said member(s) to be carried by the spring and be free to rotate and move longitudinally of the spring relative to the spring coils.

2. A method according to claim 1, wherein said at least one damping member comprises a cylindrical rod located within the spring coils with a narrow radial clearance therebetween.

3. A method according to claim 1 or 2, wherein said at least one member comprises a collar or sleeve with a cylindrical interior surface surrounding the spring coils with a narrow radial clearnace therebetween.

4. A method according to claim 1, 2 or 3, wherein the member(s) located either within or around the coils extend over at least half the coiled length of the spring.

5. A method according to any one of claims 1 to 4, wherein the or each damping member is made of hard material.

6. A method according to claim 5, wherein the hard material is metal.

7. A method according to claim 1 and substantially as herein described.

8. A drum brake comprising a drum, a brake shoe, actuating means for applying the shoe to the drum, a helical coil return spring for pulling the shoe off the drum when released by the actuating means, and at least one vibration damping member carried by the spring and positioned loosely within or around the spring coils to be freely rotatable and movable longitudinally of the spring relative to the spring coils.

9. A drum brake according to claim 8, wherein said at least one damping member comprises a cylindrical rod located within the spring coils with a narrow radial clearance therebetween.

10. A drum brake according to claim 8 or 8, wherein said at least one member comprises a collar or sleeve with a cylindrical interior surface surrounding the spring coils with a narrow radial clearance therebetween.

11. A drum brake according to claim 8, 9 or 10, wherein the member(s) located either within or around the coils extend over at least half the coiled length of the spring.

12. A drum brake according to any one of claims 8 to 11, wherein the or each damping member is made of hard material.

1 3. A drum brake according to claim 12, wherein the hard material is metal.

14. A drum brake according to claim 8, wherein the damping member is substantially as herein described with reference to the accompanying drawings.