GB2274316A

GB2274316A - Measurement of pad wear in vehicle brakes

Info

Publication number: GB2274316A
Application number: GB9224526A
Authority: GB
Inventors: Andrew John Ward; Ian Douglas Poole
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1992-11-24
Filing date: 1992-11-24
Publication date: 1994-07-20
Anticipated expiration: 2012-11-24
Also published as: GB9224526D0; DE4339901B4; GB2274316B; DE4339901A1; JPH06280911A

Description

2274316

DESCRIPTION MEASUREMENT OF PAD WEAR IN VEHICLE BRAKES

The present invention is concerned with the measurement of pad wear in vehicle brakes.

In vehicle brakes of the type which employ a pad which is displaced during braking against a brake disc or cylindrical drum fixed to an associated vehicle wheel, it is known to provide various means by which the state of wear of the pad(s) can be monitored.

For example, there is described in US 2002139 a system where displacement in a brake adjuster mechanism in a drum brake is transmitted via a flexible cable drive to a mechanical indicator located on the vehicle dashboard, i.e. at a distance of several metres from the brakes themselves. The displacement measured is an indication of the total brake shoe wear for both lining elements in the brake. No indication can be given of the individual lining condition, which can be important since one brake lining may wear substantially quicker than the other. The displacement of the brake adjuster mechanism is monitored by a rotary, flexible drive which provides an input to the remote indicator positioned on the vehicle dashboard. Such a system would be unreliable in practice for the following reasons. Firstly. the friction in the long length of cable needed to extend -2from the various brakes to the dashboard would be high and would cause the cable to twist, whereby the input from the adjuster mechanism would not be reproduced fully at the dashboard indicator. Secondly, since the indicator is mounted on the "sprung" or "supported" part of the vehicle, this would mean that suspension movements would apply a relative angular displacement between the two ends of the cable drive and result in an erroneous indication of input displacement. Thirdly, as mentioned above, the described sensing mechanism only allows the measurement of the total lining wear and it would be unable in principle to detect a fault in one of the brake linings, such as a seized or disabled actuator.

Japanese patent specification No. JP136021/1974 describes a pad wear warning indicator of the "maximum permitted wear condition" type wherein a warning is given only when a predetermined maximum permitted wear condition has been reached. A solid "rod" is used to transmit movement from the pad to a sensing switch when this maximum wear condition is reached. This device therefore does not measure the actual wear condition but is simply used in a conventional manner to respond to and indicate the fully worn condition. The device does use a transmission 'rod' to transfer the "wear condition" from the pad to a remote sensor -3 switch, the latter sensor switch is still mounted at a position where it would be in the high-temperature environment around the brake itself. To remove the sensor switch to a remote location out of the hightemperature environment using such a "rod" device would impose considerable constructional difficulties as the greater the distance apart the pad and the sensor are, then the more difficult it would be to align the rod with the latter devices. Vibration at the wheel would also impose severe oscillations on the connecting rod which would cause at least signal noise problems if not total loss of any readable displacement.

In accordance with a first aspect of the present invention there is provided a pad wear measurement system in which a sensing device is mounted at a location remote from the brake actuator in an area where the temperature generated by the operation of the brake does not exceed a predetermined safe working range of the sensing device.

Preferably, the mounting position of the sensing device is on an unsprung part of the vehicle, such as the axle or hub carrier, or indeed a "cool,, part of the brake mechanism itself where the aforedefined temperature condition applies.

In accordance with a second aspect of the invention, there is provided a pad wear measurement device having a sensing element which is adapted to be mounted on the unsprung part of a vehicle and to be operatively connected to the brake mechanism by a flexible cable drive.

Preferably, the sensing device is a displacement sensor, the input displacement to the sensor from the brake being provided by a flexible cable drive, such as a "Bowdenll cable, operated in a tension/compression mode.

For measuring wear on the outboard pad of a conventional reaction-type disc brake, the inner cable of the flexible cable drive is preferably connected to the (fixed) carrier of the brake assembly while the outer sheath of the cable drive is connected to the housing component which straddles the periphery of the brake disc and which carries the outboard pad.

For measuring total pad wear, i.e. the combined wear on the inboard and outboard pads, the inner cable of the flexible drive is preferably connected to the inboard pad(or to the associated spreader plate or tappet)while the outer sheath is connected to the housing carrying the outboard pad.

In a typical preferred embodiment, the length of the flexible cable drive is of the order of about 150 -5mm. This is sufficient to enable the sensor to be mounted outside the high temperature environment created by the friction between the pads and the disc during braking, yet insufficient to introduce the inaccuracies encountered with the devices formed in the prior art. In particular, by mounting the sensor on an unsprung part of the vehicle, and by using linear displacement of the cable core, the problem of "winding up" of the cable found in the prior art is avoided.

The sensor can be connected electrically to an electronic unit at an appropriate position, in or near the vehicle cab, for driving a suitable driver display and/or diagnostic connection point.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig.1 is a diagrammatic sketch illustrating the operating principle of a first embodiment of pad wear indicating system in accordance with the present invention; Fig.2 is a diagrammatic sketch illustrating the operating principle of a second embodiment of pad wear indicating system in accordance with the present invention; Fig.3 is a diagrammatic sketch illustrating a -6third embodiment in accordance with the present invention; and Figs 4a, 4b and 4c show in more detail three different possibilities applicable to the embodiment of Fig. 2.

Referring first to Fig.1 there is shown a typical disc brake system having inboard and outboard pads 10,12 and a housing 14 which straddles the periphery of a brake disc 16 fixed relative to the associated vehicle wheel (not shown). The outboard pad 12 is fixed to the housing 14 whereas the inboard pad 10 is slidable in a carrier 13 and is coupled to the housing 14 by way of an adjuster 18. usually in the form of a hydraulically adjustable piston but alternatively mechanically or pneumatically, adjustable. In use, when actuating fluid is applied to the cylinder 18, the.inboard pad 10 is applied directly to the disc 16. The reaction on the housing 14 via the cylinder 18 causes the housing to be displaced such as to apply the outboard pad to the other side of the disc. Thus, in applying the brakes, there is caused to take place a relative displacement of the housing 14 and the carrier 13.

In the first embodiment of the present invention illustrated in Fig.1, this displacement is measured using a flexible coaxial cable drive 26 whose inner -7cable 28 is attached at 20 to the carrier 13 and whose outer sheath 22 is attached to the housing 14 at 24. The other end of the flexible cable drive 26 is coupled to a sensor device 30 which is positioned on the brake device but at a location remote from the high-temperature environment existing around the pad/disc contact region. The sensor 30 can comprise, for example, a linear or rotary potentiometer. An electrical signal from the sensor can be passed by a cable 32 to an electronic control unit 34 which processes the electrical signal and drives an electronic display 36 in or near the vehicle cab and/or provides a diagnostic output for connection to a condition analysis system during servicing of the vehicle.

It will be understood that the connection of the flexible cable in the aforegoing manner to the carrier and housing provides a measurement of wear on the outboard pad 12 alone. Measurement of this particular pad is beneficial to measuring the overall pad wear, i.e. the inboard and output pad wear combined, since the outboard pad has the most variable wear characteristics and measuring overall wear can not distinguish between the wear on each pad. Thus, if only overall pad wear is measured, the measured wear can only be divided between the two pads equally, -8which may not give a true picture of the actual relative wear of the two pads.

Thus, the use of the flexible cable drive 26 enables the sensor 30 to be mounted remote from the high temperature areas of the brake but still enables direct connection. This also enables greater flexibility for mounting the wear indicator system to a range of brake designs with only minor modifications. Furthermore, greater flexibility is available for different vehicle installations.

Referring now to Fig.2, there is shown a similar brake to that shown in Fig.1 but which is fitted, in addition to the outboard pad wear measurement system of Fig.1, with an additional system for measurement of total pad wear. Equivalent parts in Figs. 1 and 2 are given the same reference numerals. The additional system comprises a second flexible cable drive 38 whose inner cable 40 is attached to the metal backing plate 42 of the inboard pad 10 at 44, and whose outer sheath 46 is connected to the housing 14 at 48. It will be understood that displacement of the inner cable of this second flexible drive relative to its outer sheath will increase in accordance with the total pad wear, i.e. the combined wear of the inboard and outboard pads 10,12. The other end of the second flexible drive 38 is also connected to the sensor 30 -9and drives a second potentiometer therein to provide a second electrical signal which is passed to the electronic control unit 34 for processing and display on the indicator 36 (as required).

Whereas Fig. 2 shows the connection of the inner cable 40 directly to the backing plate of the inboard pad 10, this is not necessarily the best place to connect the cable in practice as it would have to be broken and re-made at each pad change. Two alternatives overcome this problem, which are illustrated in Figures 4a, 4b and 4c. The pad assembly usually comprises three individual components, namely a pad 80, a spreader plate 82 and a tappet 84. The function of the pad 80 is of course to abut the brake disc for braking purposes. The spreader plate 82 and tappet 84 serve as transmission elements in the inboard pad drive mechanism. The spreader plate 82 transmits the input force from the tappet to the pad back plate and effectively acts to "spread" the load over the backplate to prevent bending within the pad. The tappet 84 acts to provide the axial force generated by an input actuator to the pad 80 or spreader plate 82. All three components 80, 82, 84 move in an identical manner during braking and therefore either of the three can be used as the input element to the cable 38. The benefit of using either -lo- af the two latter components 82, 84 is that, during a pad change, they remain in situ and therefore the cable connection can remain intact. Fig. 4a shows the inner cable 40 connected to the pad backplate 86, as in Fig. 3, Fig. 4b shows the inner cable 40 connected to the spreader plate 82 and Fig. 4c shows the inner cable 40 connected to the tappet 84.

Thus, the present flexible cable drive of the present invention enables connection into various areas of the brake assembly to allow for measurement of total wear, inboard/outboard pad wear or any combination of the latter, using a twin cable arrangement as shown schematically in Fig.2.

Fig.3 shows how the flexible cable drive of the present invention can be applied to a drum type brake 50 of the conventional type having a pair of generally arcuate brake shoes 52,54 carrying brake linings 56,58 adapted to be brought into contact with the inner cylindrical surface of a brake drum (not shown) by means of an actuator 60 which pivots the brake shoes 52,54 on a housing 66 about their one ends 62,64. A pair of flexible cable drives 68,70 enter the housing 66 by way of an aperture 72. The inner cable of one flexible drive is connected to one of the brake shoes while its outer sheath is fixed to the housing 66. Likewise, the inner cable of the other flexible drive is connected to the other of the brake shoes while its outer sheath is again fixed to the housing 66. The displacement of each inner cable relative to its outer sheath is therefore dependent upon the individual wear on the respective lining 56,58 associated with this cable drive. The other ends of the cable drives 68,70 are again led to the remote sensor 30 and thence to the electrical control unit 34 and indicator 36. The positions at which the connection of the inner cables are made to the brake shoes are chosen to maximise the sensitivity of the system to lining wear.

Claims

1. A pad wear measurement device having a sensing device which is adapted to be mounted on the unsprung part of a vehicle and to be operatively connected to the brake mechanism by a flexible cable drive.

2. A pad wear measurement device as claimed in claim 1, wherein the sensing device is a displacement sensor, and input displacement to the sensor from the brake is provided by a flexible cable drive.

3. A pad wear measurement device as claimed in claim 2 wherein the flexible cable drive comprises a "Bowdenll type cable operated in a tension/compression mode.

4. A pad wear measurement device as claimed in claim 2 or 3, wherein, for measuring wear on the outboard pad of a reaction-type disc brake, the inner cable of the flexible cable drive is connected to the fixed carrier of the brake assembly while the outer sheath of the cable drive is connected to the housing component of the brake assembly which straddles the periphery of the brake disc and which carries the outboard pad.

5. A pad wear measurement device as claimed in -13claim 4 wherein, for measuring total pad wear, that is the combined wear on the inboard and outboard pads, the inner cable of the flexible drive is connected to the inboard pad, or to a component which moves with that pad, and the outer sheath is connected to the housing carrying the outboard pad.

6. A pad wear measurement device as claimed in claim 2 or 3 wherein, for measuring total pad wear, that is the combined wear on the inboard and outboard pads, the inner cable of the flexible drive is connected to the inboard pad. or to a component which moves with that pad, and the outer sheath is connected to the housing component of the brake assembly which straddles the periphery of the brake disc and carries the outboard pad.

7. A pad wear measurement device as claimed in claim 5 or 6 wherein, for establishing the wear on the inboard pad, the measured wear on the outboard pad is subtracted from the measured total pad wear.

8. A pad wear measurement device as claimed in claim 2 or 3, wherein, for measuring wear on at least one of the arcuate brake linings of a drumtype brake. the inner cable of at least one flexible cable drive is connected to the two brake shoe carrying a respective one of said linings and the outer sheath of - 14the flexible cable drive is fixed to the brake housing.

9. A pad wear measurement device as claimed in any of claims 1 to 8, wherein the sensing device is connected electrically to an electronic unit at a position in or near the vehicle cab for driving a driver display and/or diagnostic connection point.

10. A pad wear measurement device as claimed in any of claims 1 to 8 wherein the sensing device is mounted at a location remote from the brake actuator in an area where the temperature generated by the operation of the brake does not exceed a predetermined safe working range of the sensing device.

11. A pad wear measurement device as claimed in claim 10 wherein the mounting position of the sensing device is on an unsprung part of the vehicle, or a "cool" part of the brake mechanism itself where the aforedefined temperature condition of claim 8 applies.

12. A pad wear measurement system in which a sensing device is mounted at a location remote from the brake actuator in an area where the temperature generated by the operation of the brake does not exceed a predetermined safe working range of the sensing device.

13. A pad wear measurement system as claimed in claim 12, wherein the mounting position of the sensing -is- device is on an unsprung part of the vehicle, or a fecool" part of the brake mechanism itself where the aforedefined temperature condition of claim 9 applies.

14. A pad wear measurement device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15. A pad wear measurement system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.