GB1594453A

GB1594453A - Friction pad assemblies for disc brakes

Info

Publication number: GB1594453A
Application number: GB4834676A
Authority: GB
Original assignee: Girling Ltd
Current assignee: Girling Ltd
Priority date: 1976-11-19
Filing date: 1976-11-19
Publication date: 1981-07-30
Also published as: DE2751674A1; JPS5386971A; DE2751674C2

Description

(54) IMPROVEMENTS RELATING TO FRICTION PAD ASSEMBLIES FOR DISC BRAKES (71) We, GIRLING LIMITED, a British Company, of Kings Road, Tyseley, Birmingham, West Midlands, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a friction pad assembly for use in a disc brake and incorporating a wear warning sensor, and a method of manufacturing same.

Many vehicle manufacturers offer the facility of a warning lamp on the dashboard to indicate when the disc brake pads are approaching the fully worn condition. This warning lamp is coupled by suitable circuitry with a wear warning sensor incorporated in each friction pad assembly.

One such friction pad assembly is disclosed in a first prior document where a hole is drilled in the side of the pad to receive an electrically insulated wire. As the pad wears, the approaching disc abrades through the insulation until it touches the metal wire, which is then earthed; the contact made causing the lamp to light. This method is reliable as regards the production of the signal, but is clearly expensive in requiring the drilled hole.

It has been suggested to mould the wire into the pad material, or to trap it between the material and the back plate. The problems with doing this are: (a) that it is very difficult to ensure proper closure of the material-moulding die to the back plate when the protruding wire has to be allowed for, (b) the insulation around the wire is conveniently required to be flexible to allow the wire to be connected to the wire on the car.

Cheap flexible insulation materials, e.g.

P.V.C. cannot stand the temperatures involved in curing the pad material. A rigid plastic, such as most thermosetting plastics or an epoxy resin could stand the temperature, but would be too brittle and fragile.

Also of course, in the above, the wire tends to fall away when the pad material wears out, and can find its way into dangerous places, e.g. it could jam the pad against the disc and lock the wheel.

Thus a first aim of the present invention is to provide a sensor which is insulated from the electrically conductive parts of the brake in such a way that flexibility and ability to withstand high temperatures are not required to be present together in a single component, and where the structure of the sensor remains attached to the backplate even when the pad is fully worn out, and which does not require expensive drilled holes in the pad.

This aim has been achieved in a second prior document where the metal contacts are moulded into a plastic or rubber holder, which is then pushed into a shaped recess in the backplate. These arrangements have the disadvantage as follows. Backplates are stamped from a strip of sheet metal, in which it is very difficult to stamp or punch small holes (by "small" is meant, of a diameter less than about one and a half times the thickness of the metal). Therefore, if expensive drilling or other machining is not to be used, the sensor assembly must be large in these prior constructions, far larger than it needs to be for electrical reasons, or for its own robustness.

Thus a second aim of the present invention is to provide a sensor wherein the size of the components used in the sensor is not governed by any factors other than the requirements of adequate electrical conductivity and the robustness of the sensor itself.

This second aim is achieved in a third prior document in which advantage is taken of the provision in most pad designs, of spigot holes so that the sensor is mechanical ly located in the spigot of the lining material, which does not of course become worn away. However, this method is expensive in that the small holes required in the lining material through which flexible wires are inserted, must be drilled (the wire cannot be present during moulding or its insulation would melt). The assembly of such an arrangement is also very difficult. Also a securing clip arrangement has to be provided, which adds to the expense.

Thus a third aim of the present invention is to provide a sensor wherein the essential electrical requirements, i.e. those of a) an electrically conductive part, b) a part for insulating part (a) from the backplate, and (c) a means for connecting part (a) into the cars wiring system, is all that is required in the total structure i.e. that no other parts need be provided for mechanical location, or for protection against the elements, etc.

A fourth prior document describes a method of re-using backplates, whilst just changing the lining material. In this disclosure a member is fixed in a hole in the backing plate. The member can either be the wear sensor or capable of holding same.

The member is either attached to the backing plate as per a rivet or if the member is the actual wear sensor and therefore metallic it is secured in the hole by a layer of glue which also electrically insulates the sensor from the backing plate. Some other insulation may, if desired, be used between the member and the backing plate there being no requirement for the insulation to withstand curing temperatures.

In one construction of this fourth document the member is the actual wear sensor, then if it were attempted to construct this friction pad assembly by moulding the friction pad onto the backing plate, a problem arises in that the hole in the backplate must be large if it is not to be drilled and therefore the sensor itself must be large. If the sensor is hollow, then there is the difficulty of attaching a wiring connector to the rear annulus in a robust way, and also the hole must be sealed in the moulding die to prevent extrusion of the lining material.

If the sensor is solid, then it is itself expensive. Thus either way space is used up that could usefully be occupied by a spigot.

When the friction material is moulded onto the backing plate, the spigot holes conveniently used are approximately 21' diameter.

It is normally desirable to provide as many spigots as possible, a restriction being that the distance between the holes in the backing plate must be sufficient for ease of stamping or punching. Often four or five spigots are used.

If this number is reduced by using the above sensor then the friction material may lift from the backing plate in that area, this being a well-acknowledged cause of squeal.

Thus a further aim of the present invention is to provide a sensor in such a way that the normal spigot requirements of the lining/backing plate, are not interfered with.

If another version shown in the fourth document is considered, i.e. where the member holds the sensor and is constructed from an electrically insulating material e.g.

a plastics material, then again, more plastic is required than is necessary for satisfactory electrical insulation, and again a useful spigot space is used up.

Thus the aims of the present invention may be summed up as follows:- to provide a pad wear warning sensor which (a) is as cheap as possible to manufacture (b) remains mechanically located in the pad at all times (c) does not require the use of drilled holes (d) does not interfere wth the ordinary lining spigot requirements (e) has only its own electrical and robustness requirements to dictate its size (f) does not require the use of materials which are at the same time insulative, temperature resistant, flexible and cheap.

According to the present invention there is provided a friction pad assembly for use in a disc brake, comprising a metal backing plate to which a pad of friction material is secured, the attachment of the friction material to the backing plate at least partially comprising a number of spigots of the friction material moulded into corresponding holes through the backing plate, an electrically conductive sensor in the form of a rigid metallic stud having a cross-section smaller than one of said holes in the backing plate, being located in the friction material in the hole in the backing plate and spaced from the side of the hole, the stud being of sufficient length that it extends beyond the surface of the backing plate on each side of the backing plate whereby, in use, the sensor can act as a wear indicator for the friction material.

The stud forming the sensor may thus be electrically insulated from the metal backing plate by the friction material which fills the remainder of the hole and surrounds the stud. The holes in the backing plate are usually circular, however, any desired shape of hole can be provided. Further, the stud forming the sensor can be located at any desired position in a hole provided it is separated from the side or sides of the hole by friction material, the friction material being electrically non-conductive. The stud itself may be of any desired configuration e.g. oval or square cross-section, but is preferably of circular transverse crosssection. It can be tapered if desired and it may be solid or hollow. If hollow then an end cap is preferably provided to prevent friction material extruding therethrough during manufacture.

To connect a friction pad assembly according to the present invention, in an electrical circuit including a dashboard warning lamp, provision for the connection of a wire to the stud must be made. One preferred connection may be made by the use of a spade-type connector attached to the stud. This connector may be attached to the stud by rivetting, this being a cheap attachment method. However, a heavy force on the stud as is produced by conventional rivetting, is likely to damage the friction material which is the only support for the stud in the present invention. Thus, the connector is preferably spin rivetted onto the protruding end of the stud. A shoulder may be provided on the protruding end of the stud for ease of attachment of the connector. Besides the preferred type connector any other suitable form of permanent or detachable connector may be used, and if desired, the wire can be directly soldered to the stud.

It will be clear from the above that no insulation surrounds the stud apart from the friction material itself. This is satisfactory where the friction material has no electrically conductive constituents. However, in the case where the friction material has metallic inclusions or is water absorptive, a layer of electrically insulating material is required.

As the stud is immovably located in the friction material at all times during its life, so will be the electrically insulating layer.

Thus the cheap, high temperature plastics material e.g. an epoxy resin may be used even though it is brittle. The insulating layer may be formed separately from the stud and located over the stud before moulding of the friction material onto the backing plate, or the insulating layer may be moulded onto the stud. Alternatively the stud may be dipped into a liquid such as an epoxy resin which later sets to form an insulating layer.

One criterion for the insulating material is that it can withstand the high temperature required for curing the friction material: such materials, if cheap are brittle: the invention allows such materials to be used.

According to a further feature of the present invention there is provided a method of manufacturing a friction pad assembly for a disc brake, comprising the steps of forming a metal backing plate with a number of holes therein, placing the backing plate in a die of a friction pad moulding apparatus, fitting a rigid metallic stud into a location hole provided in said die in such a position that the stud protrudes through a hole in the backing plate without touching the plate, introducing friction material into the moulding apparatus and curing the friction material by heat and pressure, to mould it to the backing plate thus securing the stud in position. In a modification of this method, an electrically insulative (and brittle) material may also be introduced with the stud, if required.

The present invention will now be further described, by way of example, with reference to the drawings accompanying the Provisional Specification, in which: Figure la is a rear view of one embodiment of a friction pad assembly constructed according to the present invention; Figure ib is a cross-sectional view along line I-I Figure la; Figure 2 illustrates how the assembly of Figure la and Ib can be manufactured; Figure 3 is a cross-sectional view of part of a second embodiment of friction pad assembly constructed according to the present invention; Figure 4 is a cross-sectional view of part of a third embodiment of friction pad assembly constructed according to the present invention; Figure 5 is a cross-sectional view of part of a fourth embodiment of friction pad assembly included for illustration purposes; Figure 6 is a cross-sectional view of part of a fifth embodiment of friction pad assembly according to the present invention illustrating how a connector is attached to the stud sensor; Figure 7 illustrates the connector of Figure 6 attached to the stud sensor; Figure 8 illustrates a modified form of stud to that illustrated in Figure 7; Figure 9 is a plan view of the connector in Figure 8; Figure 10 is a cross-sectional view of part of a sixth embodiment of friction pad assembly according to the present invention, with connector attached; Figure 11 schematically illustrates how the embodiment of Figure 10 can be manufactured; and Figures 12 and 13 illustrate in crosssection, part of two further embodiments of friction pad assembly according to the present invention.

The friction pad assembly illustrated in Figures la and lb, comprises a planar metal backing plate 1 to which a pad 2 of friction material is secured. The backing plate has a number of spaced apart circular holes 3 through which the friction material extends to secure the pad 2 and backing plate 1 together. Through the centre of one of these holes 3 passes an elongate metal stud 4, the stud projecting beyond each side of the backing plate and, in use, acting as a wear indicator, the stud then being electrically connected to a dashboard warning device e.g. a lamp. The stud is cylindrical and solid, though alternatively the stud may be hollow. It is held in position by the friction material and is initially secured in this position during the manufacture of the friction pad assembly.

One basic form of manufacture of the assembly of Figures la and lb, is illustrated in Figure 2. As can be seen the backing plate 1 is first -located on a bottom die 5 of a friction pad moulding apparatus and the stud 4 is engaged in a blind bore 6 in die 5 so that it lies coaxial with a hole 3 in the backing plate 1. Friction material (shown in dots) is then introduced to fill the remainder of illustrated hole 3, so securing the stud in place. The friction material may be introduced in powder form, or may be in the form of a partially pre-shaped (but only lightly compacted) "biscuit". The friction material is subsequently cured by the application of heat and pressure.

Whilst, with reference to Figures la, 1b and 2, the stud is described as being located coaxially with respect to circular hole 3, the stud can of course be eccentrically located if so desired. The only pre-requisite is that friction material be between the stud and the side of the hole. Further, the hole 3 can be any desired configuration and the stud can also be varied in shape, by way of example three different stud configurations are illustrated in Figures 3, 4 and 5.

The stud 4a in Figure 3 is tapered like a bullet and it will be appreciated that as the stud widens towards the rear of the backing plate, friction material will be compressed between the stud and the side of hole 3 during manufacture thus firmly securing the stud in position. The stud 4b in Figure 4 also has a slight taper but in the opposite direction to stud 4a. This stud is acceptable.

However, stud 4c in Figure 5 whilst being possible, is not acceptable as the friction material would not be adequately compacted under the head 7 of the stud. Thus the stud shape should have no marked re-entrant contours, though a slight reentrant feature, as at 4b in Figure 4, may be acceptable. Another suitable stud (not illustrated) comprises a hollow tube with an end cap to prevent friction material extruding therethrough during manufacture. The stud may be slightly knurled or otherwise roughened on its surface, to improve the moulded grip.

Whilst the studs described hereabove have a generally circular cross-section, studs may be satisfactorily used which have any other desired transverse cross-section e.g.

square. Further a non-uniform cross-section may be used, though as will be evident from above a re-entrant shape is unacceptable.

To use the stud sensor of the present invention it must be electrically connected to suitable circuitry including the dashboard warning device. This electrical connection can be made by soldering a wire to the end of the stud. However preferably a connector is used, the connector being permanently or detachably attached to the stud. One suitable connector is a spade-type connector and this can be cheaply rivetted to the stud.

However, a heavy force on the stud, as is probable in conventional rivetting, is likely to damage the friction material which is the only support for the stud. Thus it is preferred to spin rivet the connector to the stud.

The 'before' and 'after' condition of the spin rivetting is shown in Figures 6 and 7. Before rivetting the cylindrical stud 8 has.a blind bore 9 in its protruding end. A shaped tool 11, whilst spinning at a high speed, is brought down to engage the stud. Due to the high spinning speed the friction between the tool and the stud appears circumferentially with only a tiny axial component so that only the force to actually distort the metal has to be reacted by the friction material. As can be seen from Figure 7 the sides of the blind bore 9 are spun over to secure the connector 10.

Whilst in Figures 6 and 7 the connector 10 is secured between the stud and the friction material 2 by the spin rivetting, the stud 12 in Figure 8 has a shoulder 13 so that the connector is secured clear of the friction material by spin rivetting. The connector 10 used in Figures 6 to 8, is illustrated in plan in Figure 9. The connector has a hole 14 for engaging over the protruding end of a stud and deformed or cut out portions 15 for frictionally gripping its complimentary part (not illustrated). The connector may be of the type known as a Lucar (registered Trade Mark) connector.

In the above described embodiment of the present invention the stud is embedded directly in the friction material, no electrical insulation being provided. This is perfectly satisfactory in the case where the friction material is electrically non-conductive.

However, most friction materials tend to have metallic inclusions and/or are water absorptive. In such cases the metal stud has to be electrically insulated. This insulation can be effected in a number of ways. The stud can be dipped in a liquid such as an epoxy resin which later sets to form an insulating coating which can stand up to the subsequent curing of the friction material in the manufacture of the friction pad assembly. Alternatively as the stud will be immovably located in the friction material at all times during service, and so will be the insulation, the cheap high temperature plastics material e.g. epoxy resin may be used even though it is brittle. Figure 10 shows such an insulator which is in the form of an inverted Top Hat 16. The insulator 16 is an interference fit with the stud though alternatively the insulator can be moulded around the stud. This Top Hat shape provides an annular flange 17 which, as can be seen from Figure 11, facilitates the manufacture of a friction pad assembly according to the present invention. The flange 17 seals off the blind bore 18 in the bottom die 19 of the friction pad moulding apparatus, so that the stud can be an imperfect fit in bore 18 and even so the friction material cannot extrude into the hole.

Figures 12 and 13 illustrate two further forms of stud 20,21 wherein the studs have integral electrical connectors. The stud in each illustrated case is in the form of the connector 10 illustrated in Figure 9, but without hole 14. During the manufacture of the friction pad assemblies the studs are straight and it is only after curing of the friction material, that the protruding parts of the studs are bent over, the stud 21 of Figure 13 being bent further than the stud 20 of Figure 12. In both Figure 12 and Figure 13 the stud is provided with an insulating layer 22 within the friction material. Whilst the friction material should be strong to take the reaction force resulting from bending the studs, unlike the stud 20 of Figure 12, a bolster (not shown) is suitably positioned to take the reaction force in producing the stud 21 of Figure 13.

These latter two embodiments are particularly cheap and robust, it being necessary for the electrical connectors to be firmly fixed to the studs in all embodiments, to minimise damage caused when the friction pad assemblies are transported, and during fitting.

When a friction pad assembly according to the present invention is fitted in a brake, the annular contact area of the or each piston in the brake on the backing plate, should be arranged to not cover the back of the studs so that the wire connecting the stud to the associated electrical circuitry can be led from the brake without difficulty.

However, if this is not possible and the stud must protrude within a piston then a notch will have to be provided for the passage of the wire. This will mean orientating the piston to the correct position. However this is easy to provide with a simple key in the piston/pad assembly contact interface.

WHAT WE CLAIM IS: 1. A friction pad assembly for use in a disc brake, comprising a metal backing plate to which a pad of friction material is secured, the attachment of the friction material to the backing plate at least partially comprising a number of spigots of the friction material moulded into correspond ing holes through the backing plate, an electrically conductive sensor in the form of a rigid metallic stud having a cross-section smaller than one of said holes in the backing plate, being located in the friction material in the hole in the backing plate and spaced from the side of the hole, the stud being of sufficient length that it extends beyond the surface of the backing plate on each side of the backing plate whereby, in use, the sensor can act as a wear indicator for the friction material.

2. A friction pad assembly as claimed in claim 1, in which the hole in the backing plate is circular and the stud has a circular transverse cross-section, the stud being located centrally in the hole.

3. A friction pad assembly as claimed in claim 1, in which the stud is offset from the centre of the hole.

4. A friction pad assembly as claimed in any one of claims 1 to 3, in which the stud is tapered.

5. A friction pad assembly as claimed in any one of claims 1 to 3, in which the stud is hollow.

6. A friction pad assembly as claimed in any one of claims 1 to 5, in which the stud is coated with a layer of electrically insulating material.

7. A friction pad assembly as claimed in claim 6, in which electrically insulating material is a high temperature plastics material.

8. A friction pad assembly as claimed in any one of the preceding claims, in which an electrical connector is attached to the stud.

9. A method of manufacturing a friction pad assembly for a disc brake, comprising the steps of forming a metal back plate with a number of holes therein, placing the backing plate in a die of a friction pad moulding apparatus, fitting a rigid metallic stud into a location hole provide in said die in such a position that the stud protrudes through a hdle in the backing plate without touching the plate, introducing friction material into the moulding apparatus and curing the friction material by heat and pressure, to mould it to the backing plate thus securing the stud in position.

10. A method as claimed in claim 9, in which an electrically insulating layer is provided on the stud to electrically isolate the stud from the friction material.

11. A method as claimed in claim 10, in which the insulating layer is formed separately from the stud and located over the stud before moulding of the friction material onto the backing plate.

12. A method as claimed in claim 10, in which the insulating layer is moulded onto the stud.

13. A method as claimed in claim 10, in which the stud is dipped into a liquid which sets to form the insulating layer before the stud is located in the friction pad moulding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. inverted Top Hat 16. The insulator 16 is an interference fit with the stud though alternatively the insulator can be moulded around the stud. This Top Hat shape provides an annular flange 17 which, as can be seen from Figure 11, facilitates the manufacture of a friction pad assembly according to the present invention. The flange 17 seals off the blind bore 18 in the bottom die 19 of the friction pad moulding apparatus, so that the stud can be an imperfect fit in bore 18 and even so the friction material cannot extrude into the hole. Figures 12 and 13 illustrate two further forms of stud 20,21 wherein the studs have integral electrical connectors. The stud in each illustrated case is in the form of the connector 10 illustrated in Figure 9, but without hole 14. During the manufacture of the friction pad assemblies the studs are straight and it is only after curing of the friction material, that the protruding parts of the studs are bent over, the stud 21 of Figure 13 being bent further than the stud 20 of Figure 12. In both Figure 12 and Figure 13 the stud is provided with an insulating layer 22 within the friction material. Whilst the friction material should be strong to take the reaction force resulting from bending the studs, unlike the stud 20 of Figure 12, a bolster (not shown) is suitably positioned to take the reaction force in producing the stud 21 of Figure 13. These latter two embodiments are particularly cheap and robust, it being necessary for the electrical connectors to be firmly fixed to the studs in all embodiments, to minimise damage caused when the friction pad assemblies are transported, and during fitting. When a friction pad assembly according to the present invention is fitted in a brake, the annular contact area of the or each piston in the brake on the backing plate, should be arranged to not cover the back of the studs so that the wire connecting the stud to the associated electrical circuitry can be led from the brake without difficulty. However, if this is not possible and the stud must protrude within a piston then a notch will have to be provided for the passage of the wire. This will mean orientating the piston to the correct position. However this is easy to provide with a simple key in the piston/pad assembly contact interface. WHAT WE CLAIM IS:

1. A friction pad assembly for use in a disc brake, comprising a metal backing plate to which a pad of friction material is secured, the attachment of the friction material to the backing plate at least partially comprising a number of spigots of the friction material moulded into correspond ing holes through the backing plate, an electrically conductive sensor in the form of a rigid metallic stud having a cross-section smaller than one of said holes in the backing plate, being located in the friction material in the hole in the backing plate and spaced from the side of the hole, the stud being of sufficient length that it extends beyond the surface of the backing plate on each side of the backing plate whereby, in use, the sensor can act as a wear indicator for the friction material.

apparatus.

14. A friction pad assembly for use in a disc brake, constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the drawings accompanying the Provisional Specification.

15. A method of manufacturing a friction pad assembly for a disc brake, substantially as hereinbefore described with reference to the drawings accompanying the Provisional Specification.