GB2041470A - Brake pad provided with wear warning - Google Patents

Abstract

A brake pad of a disc or drum brake comprises a supporting plate tion (1), a layer of friction material (2) and an electrical contact assembly (6) which extends through the supporting plate to project into the layer of friction material and which comprises two terminal portions (18, 19) electrically insulated from each other by material (22) and a connecting portion (20) which electrically connects the terminal portions and is located wholly within the layer of friction material (2). The connecting portion is arranged to be exposed and abraded after prolonged abrasion of the friction material by contact with a braking surface whereby electrical contact between the terminal portions is broken. The contact assembly is connected into a warning circuit which produces a first warning signal when the layer of friction material has become abraded sufficiently to allow electrical contact between the connecting portion and the braking surface, and a second warning signal when the connecting portion has become sufficiently abraded to break the electrical connection between the terminal portions of the contact assembly - see Figs. 4 and 5 for details. <IMAGE>

Description

SPECIFICATION Brake pad This invention relates to brake pads.

Conventional brake pads for motor vehicles comprise a supporting plate, which is adapted to be connected to an actuating mechanism, and a layer of friction material carried on a front face of the supporting plate and shaped to conform to a braking surface such as a brake disc or drum.

In order to provide a warning signal when the layer of friction material is about to wear out, it is known to incorporate an electrical contact in the brake pad which contacts the braking surface when the friction material has worn to a dangerously low level thereby allowing activation of an electrical warning device such as a warning lamp on an instrument panel.

Such warning devices generate only one single warning signal, so that the operator of the vehicle has no indication as to the urgency with which the brake pad requires replacement. Additionally, the device will fail to operate in the event that the electrical leads to the contact become broken.

According to the present invention there is provided a brake pad comprising a supporting plate, a layer of friction material carried on a front face of the supporting plate, an electrical contact assembly extending through the supporting plate and projecting into the layer of friction material, characterized in that the electrical contact assembly comprises two terminal portions electrically insulated from each other and extending through the supporting plate, and a connecting portion electrically connecting the terminal portions, the connecting portion being located wholly within the layer of friction material, and arranged to be exposed and abraded after prolonged abrasion of the friction material by contact with a braking surface whereby electrical contact between the terminal portions is broken.

The brake pads of the invention may be adapted for use with any conventional brake assembly, for example a disc brake or drum brake assembly.

The brake pads are incorporated in a braking system which may comprise any number of brakes. The or each brake may incorporate one or more brake pads according to the invention and the contact assembly of the or each pad is connected in an electrical circuit which produces a first warning indication when the layer of friction material in the or one of the brake pads has become abraded sufficiently to allow electrical contact between the connecting portion of the contact assembly and the braking surface of the brake, and a second warning indication when the connecting portion has become sufficiently abraded to break the electrical connection between the terminal portions of the contact assembly.

For example, the or each contact assembly may be electrically connected in series with a circuit test switch and an indicator, such as a bulb in the vehicle instrument panel. so that the indicator is activated either when the test switch is closed or when the connections portion makes electrical contact with the braking surface, thereby producing the first indication, and the indicator fails to be activated by closure of the test switch when the electrical connection between the terminal portions of the contact assembly through the connecting portions is broken, thereby producing the second indication.

Conveniently, the test switch is closed when a hand brake operating lever is actuated.

Preferably the terminal portions of the contact assembly are formed integrally with the connecting portion. In the preferred embodiment of the invention, the contact assembly comprises a block of electrically conductive material such as graphite having a bifurcation extending along part of its length, the block being mounted in an aperture in the supporting plate, and a sheet of electrically insulating material, such as ceramic or plastics material, received in the bifurcation and projecting laterally from the block into engagement with recesses in the front face of the supporting plate to retain the block in the aperture.

Desirably, electrical connection is made to the contact assembly by means of a plug removably mounted on the rear face of the supporting plate.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing, in which: Figure 1 is an "exploded" perspective view of part of disc brake pad in accordance with the invention.

Figure 2 is a cross-section on an enlarged scale, taken along line ll-ll of Fig. 1.

Figure 3 is a similar cross-section to Fig. 2 showing the complete disc brake pad.

Figure 4 is a circuit diagram of a braking system in accordance with the invention, and Figure 5 is a circuit diagram of an alternative braking system in accordance with the invention.

Referring to the drawings, Figs. 1 to 3 illustrate a brake pad for a disc brake assembly. The brake pad comprises a steel supporting plate 1, which carries a layer of friction material 2 (Fig. 3) of conventional composition on its front face 3. The plate 1 defines an aperture 5 which receives an electrical contact assembly 6. The rear face 7 of the plate 1 defines a square counterbore 8 and two recesses 9, 10 each of which is undercut at 11, 1 2 for the mounting of a plug 15, as seen in Fig. 3.

The contact assembly 6 comprises a gener ally cylindricai block 1 6 of moulded graphite.

The block 1 6 is bifurcated by a diametrical slot 1 7 into two axially extending terminal portions 18, 19, which are bridged by a shamfered disc connecting portion 20. A sheet of ceramic insulating material 28 is inserted into the slot 1 7 to separate the two terminal portions 18, 1 9. The end portions of the sheet project radially from the block 1 6 and are received in respective recesses 23, 24 in the aperture 5 in the plate 1. As indicated by shading in Fig. 2, the walls of the aperture 5, recesses 23, 24 and the parts of the front and rear surfaces of the plate 1 adjacent the aperture are covered with an insulating material so that when the contact assembly 6 is positioned in the aperture 6, it is electrically insulated therefrom.This insulating material may be in the form of a sheet or coating of non-conducting plastics material In an alternative embodiment, the block 16, rather than the walls of the aperture 5 are coated with the insulating material prior to insertion of the block 1 6 into the aperture 5.

The contact assembly 6 is so positioned in the plate 1 that the connecting portion 20 of the block 16 projects from the front face of the plate 1 and, as seen in Fig. 3, lies wholly within the layer of frictional material. The pad may be conveniently manufactured by first forming the plate 1 with the aperture 5 and recesses 9, 10, coating the surfaces of the aperture 5 (and/or the surfaces of the block 16) with the insulating material, inserting the block 1 6 and the sheet 20 with the aperture 5 and moulding the layer of friction material on to the front face of the plate 1. If desired insulating material may be applied to the rear face of the place 1 to cover the exposed end of the block 1 6.

Electrical connections are made to the contact assembly by means of the plug 15, which carris two spring-loaded contacts 28, 29 each of which engages with a respective one of the terminal portions 18, 1 9 of the block 1 6. The plug 1 5 is mounted on the rear face of the plate 1 by means of spring dips 30, 31 which engage in the undercut recesses 9, 10.

In use, the brake pad is installed in a conventional brake assembly (not shown) in the normal manner. Details of the brake assembly will be well known to those skilled in the art, and have not therefore been described. Fig. 4 illustrates schematically a suitable motor vehicle braking system which incorporates brake pads in accordance with the invention. The vehicle includes two front brake assemblies 35, 36, which include braking surfaces 35, 36 of the drum type, and two other brake assemblies 37, 38 which include braking surfaces 37, 38 of the disc brake type. Each brake assembly incorporates two brake pads 40 to 47 in accordance with the invention.The rear brake pads 44 to 47 are similar to those illustrated in Fig. 1; the front brake pads 40 to 43 include a contact assembly similar to that used in the rear brake pads, but have supporting plates and layers of frictional material which conform to a drumshaped braking surface, rather than to a brake ing disc.

The electrical leads to all the contact assemblies are connected in series with a battery 50, a warning lamp 51 mounted on the vehicle instrument panel and a switch 52 which is closed when the handbrake operating level 53 of the motor vehicle is applied. The drum and disc braking surfaces 35', 36', 37', 38' are all electrically earthed to the vehicle body.

In normal operation of the vehicle, application of the handbrake lever 53 closes the switch 52, thus causing the warning lamp 51 to be illuminated, reminding the driver that the handbrake has been applied. When the lever 53 is released, the lamp 51 will not normally be illuminated.

However, as the layer of friction material on the brake pads becomes worn away, the connecting portions 20 of the contact assemblies will become exposed. If the brakes are applied with a brake in this condition, the contact assembly 6 will make electrical connection with the braking surface 35', 36', 37' or 38'.

This completes an electrical circuit through the lamp 51 and generates a first warning signal, indicating that one of the brake pads has worn to a minimum level consistent with safety, and will require replacement at the first opportunity.

As the layer of friction material continues to wear, the connecting portion will be abraded completely, and the electrical connection between the two terminal portions will be severed. In this condition, closure of the switch 52 by operation of the handbrake lever 53 will no longer cause the lamp 51 to illumi- nate, thus giving a second warning signal indicating that one of the brake pads has worn to a dangerously low level.

It will be appreciated that, in the event that one of the leads to the contact assemblies 6 or the switch 51 becomes broken, the lamp 51 will not be illuminated when the lever 53 is applied. This condition therefore indicates either a failure in the system, or a worn brake pad.

Fig. 5 illustrates an alternative circuit for a series of motor vehicle brakes 60, 61 (two only illustrated) fitted with brake pads 60', 61 ' in accordance with the invention. In this arrangement the contact assemblies of the brake pads are connected in series with a logic circuit 62 which detects fluctuations in potential between a power source 63 and earth caused on the one hand by contact of the connecting portions of one of the brake pads with its associated braking surface when the connecting portion is first exposed and on the other hand, braking of the electrical con tact between the terminal portions of the contact assembly when the connecting portion is abraded away. The two voltage level charges are used to trigger an alarm to give appropriate warning to the vehicle driver. For example, the warning may be in the form of a flashing light when the contact portion is first exposed, and in the form of a steady warning light when the contact portion has been abraded away.

Claims (8)

1. A brake pad comprising a supporting plate, a layer of friction material carried on a front face of the supporting plate, an electrical contact assembly extending through the supporting plate and projecting into the layer of friction material, characterised in that the electrical contact assembly comprises two terminal portions electrically insulated from each other and extending through the supporting plate, and a connecting portion electrically connecting the terminal portions, the connecting portion being located wholly within the layer of friction material and arranged to be exposed and abraded after polonged abrasion of the friction material by contact with a braking surface whereby electrical contact between the terminal portions is broken.

2. A brake pad according to Claim 1 wherein the terminal portions are formed integrally with the connecting portion.

3. A brake pad according to Claim 2 wherein the contact assembly comprises a longitudinally extending bifurcated block of electrically conductive material which is mounted in an aperture in the supporting plate, and a sheet of projecting laterally from the block into engagement with recesses in the supporting plate.

4. A brake pad for a disc or drum brake substantially as hereinbefore described and as illustrated in Fig. 1 to 3 of the drawings.

5. A motor vehicle braking system comprising one or more brakes, the or each brake incorporating a brake pad according to any one of the preceding Claims, the contact assemblies of the said pads being connected into an electrical circuit which produces a first warning signal when the layer of friction material in at least one of the brake pads has become abraded sufficiently to allow electrical contact between the connecting portion of the contact assembly of that pad and its associated braking surface, and a second warning signal when the connecting portion has become sufficiently abraded to break the electrical connection between the terminal portions of the contact assembly.

6. A system according to Claim 5 wherein the electrical circuit comprises a series of electrical connections through the terminal portions af the or each contact assembly, a test switch and an indicator whereby the indicator is activated either when the test switch is closed or when connecting portion of a contact assembly makes contact with a braking surface, thereby producing the first indication, and the indicator fails to be activated by the closure of the test switch when the electrical connection between the terminal portions of a contact assembly through the connecting portion is broken, thereby producing the second indication.

7. A system according to Claim 5 wherein the test switch is closed when a handbrake 'operating lever is actuated.

8. A motor vehicle braking system substantially as hereinbefore described and as illustrated with reference to Fig. 4 and 5 of the drawings.