GB2319574A - A braked surface temperature sensing arrangement - Google Patents

Abstract

A temperature sensing arrangement 50, for use in monitoring the temperature of a vehicle brake disc 11 whilst evaluating braking materials, comprises a heat collector body 52 mounted on a cantilevered arm 16 which is biased to effect contact between the collector body and braked surface 12. A temperature transducer 30, typically a thermocouple junction 31, is disposed in a recess in the collector body, which is formed from compressed amorphous carbon of such hardness as to abrade by friction with the disc to bed in for good thermal contact but not wear rapidly nor contaminate the braked surface 12. Carbon obviates problems of noise and vibration experienced with known metal collector bodies. The body may provide initially a line contact (figure 3a/b), or a point contact (figure 4a/b) with the disc. The body may be connected to the arm mechanically (figure 5a/b) and/or by adhesive.

Description

Temperature Sensing Arrangement This invention relates to testing arrangements for vehicle braking systems and in particular to arrangements for sensing the temperature of a brake disc or drum of a vehicle as it is driven.

The invention is particularly, but not exclusively, applicable to road vehicles.

The invention is particularly concerned with monitoring the temperature of such brake disc or drum (hereafter referred to generally as a braked surfaced) during a testing or evaluation phase of brake system design in which the components of the braking system, including actuators and friction materials, are tested by use under road going conditions. It is known to monitor temperatures arising within braking systems in normal usage by the vehicle operator, principally by means of detecting any potentially dangerous situation resulting from malfunction and/or wear of friction materials, and to this end, it is known to include within friction lining wear monitoring arrangements, temperature sensors to signal if the friction material is becoming excessively hot, for example as described in EP-B-0297429, FR-A2107232 and US-A-4649370.

This invention is not concerned with long term monitoring of friction material condition but with temporarily monitoring of the behaviour of the braked surface itself as part of a braking system being developed.

For such braked surface monitoring it is known to provide a temperature transducer, such as a simple thermocouple formed by the junction of two dissimilar wires mounted within a 'collector' of good thermal conductivity and low thermal capacity, that is carried at the end of a cantilevered, resilient arm so that the collector can be biased with predeterrnined force against the braked surface. In one well known arrangement the collector is formed by silver solder cast around the thermocouple junction and formed into a cylindrical button of predetermined thickness covering the junction and a circular end face of predetermined diameter.

It will be appreciated that in order to monitor the thermal behaviour of the braked surface the thermocouple must respond to temperature changes thereof over a wide range of temperatures and very rapidly, and to this end the thermal efficacy of the contact between collectors and braked surface is important.

However, several other criteria must also be satisfied. Whereas each brake of a axle pair is discrete in terms of its component and position, it is necessary for the brakes thereof to be matched to the extent that the thermal behaviour of the respective braked surfaces is substantially identical, and to this end, the temperature sensing arrangements must not introduce signal difference by virtue of different efficiency of thermal contact and/or caused by differential wear, or even as a result of friction caused by bias force that is too great. It is important for successful monitoring that not only are the respective temperature transducers made with matched dimensions, but also that they are mounted to make similar degree of contact with the braked surface.

Where feasible, each arrangement is mounted such that the resilient arm is biased, at some point along its length, towards the braked surface by an adjusting screw so that the collector bias against the surface and the level of bias monitored by measuring the force required to lift the collector from the braked surface.

It is also a function of such brake testing to determine the extent to which the various contacting surfaces of the braking system emit vibrational signals, including acoustic noises, which could be distracting/disturbing to the eventual user. Accordingly, it is a required that the presence of the temperature monitoring sensors should not result in the initiating such vibrational signals. It has been found that the degree of surface contact bias is important in striking a compromise between effecting contact that is sufficient for the collector to bed-in against the moving braked surface and make good thermal contact but not resulting in excessive wear of the collector by abrasion, heat generated by friction and vibration resulting from the contact and relative movement.

It has been found that as braking systems have developed, it has become necessary for the temperature sensing arrangement to accommodate braked surface temperatures in a range extending from ambient atmospheric temperature to above 800" C.

As such temperature range has increased, such sensors have employed solder alloy with progressively higher melting point for the collector; such higher melting point is reflected in a harder metal/alloy and it is found that such temperature sensors are more prone to inducing vibrational signals, often as a high pitched squeal; such squeal is found to occur in varied circumstances and is difficult to predict, but in all instances detracts from the measurement being made. Different metals, pure and alloys, have been tried for such a collector, including metals such as copper (a good conductor) and cast iron (the same material as the braked surface) but all have proven unpredictable in respect of giving rise to vibrations under indeterminate conditions that can confuse the testing procedure.

It is an object of the present invention to provide a braked surface temperature testing sensor of simple form which mitigates the above experienced vibrational problems of known temperature sensors.

According to the present invention a braked surface temperature sensing arrangement comprises (i) an arm resiliently deflectable between first and second ends thereof, (mounting means arranged to support the arm at said first end thereof spaced from the braked surface and biasing the second end of the arm towards the braked surface, (iii) a thermally conductive collector carried by said second end of the arm comprising a body formed by a block of abradable, non-smearing amorphous carbon having one surface secured with respect to said ann, an opposite surface comprising a rubbing surface arranged to be biased into rubbing contact with the braked surface by the mounting means, and a blind recess spaced from the rubbing surface, and (iv) a temperature transducer contained within the recess, and in thermal contact with the collector body spaced from said rubbing surface.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1(a) is a sectional elevation through a known form of temperature sensing arrangement mounted with respect to a flat vehicle brake disc, illustrating also the method by which biased towards the braked surface of the disc, Figure l(b) is a plan view of a portion of the sensing arrangement of Figure 1(a) taken along the direction b-b thereof, Figure l(c) is a sectional elevation, similar to that of Figure l(a) but of a known form of temperature sensing arrangement mounted with respect to a cylindrically curved brake drum, Figure 2(a) is a sectional elevation of a first form of temperature sensing arrangement in accordance with the present invention mounted with respect to a flat vehicle brake disc, Figure 2(b) is a sectional elevation of a fragment of Figure 2(a) on an enlarged scale showing in detail the collector for making rubbing contact with the braked surface of the disc and temperature sensor combined therein, Figure 2(c) is an end view of the collector of Figure 2(b) along the direction c, Figures 3(a) and 3(b) are sectional elevation and front views of a second form of temperature sensing arrangement, differing form that of the first form in that the collector is curved cylindrically prior to use in order to make initially a line contact with a flat or cylindrical braked surface, Figures 4(a) and 4(b) are sectional elevation and end views similar to Figures 2(a) and 2(b) of a third form of sensing arrangement in which the rubbing surface of the collector is curved partspherically prior to use in order to make initially a point contact with the braked surface, Figures 6(a) and 6(b) are sectional elevation and rear end views respectively of part of a fifth form of temperature sensing arrangement similar to the arrangement of Figures 5(a) and 5(b).

in which the collector body has a spigot part located by mechanical interlock and adhesive with respect to a reduced diameter hole in the arm, and Figure 7 is a sectional elevation of a sixth form of temperature sensing arrangement in accordance with the invention showing the temperature transducer extending through the collector body from a different surface.

Referring to Figures l(a) and l(b), a road vehicle braking system comprises a substantially planar brake disc 11 of cast iron having major surfaces 12,13 against which brake pads (not shown) carrying friction elements are pressed to bring the disc, and vehicle, to rest. In order to evaluate new braking arrangements, including new friction materials, it is required to monitor the surface temperature, and variations therein, of the disc surface. The disc surface 12,13 comprise braked surfaces and are referred to as such hereafter.

A temperature sensing unit 15 comprises an arm 16 of spring steel or the like which is resiliently deflectable between first and second ends 17,18 thereof, and mounting means 20, arranged to be mounted with respect to the stationary part of the braking system, arranged to support the arm and the first end 17 spaced from the braked surface 12 such that the arm is cantilevered to extend overlying the braked surface, preferably in the direction of rotation indicated by arrow 21. The mounting means includes an adjusting screw 22 arranged to bear on the cantilevered arm between its ends to bias the second end 18 towards the braked surface.

The temperature sensing arrangement also includes a thermally conductive collector 25 carried by the second end 18 of the arm, being a substantially cylindrical body 26 of silver solder having one surface 27 secured to the arm (by solidification of the solder) and the opposite surface 28 comprising a rubbing surface arranged to be biased into rubbing contact with the braked surface 12 by the adjusting screw 22. The rubbing surface 28 is substantially circular in end view and flat, to present a predetermined area of contact with the braked surface.

A temperature transducer 30 comprises a K-type thermocouple formed by a spot welded junction 3 1 between two dissimilar wires 32,33, as is well known in the art, the transducer thermocouple junction being embedded within, and therefore in optimum thermal contact with, the collector body upon solidification of the silver solder material.

it will be appreciated that in use, as the collector body is of softer material than the braked surface it will after a short period of rubbing contact therewith abrade and 'bed-in' to conform and make good thermal contact by way of the rubbing surface 28. Clearly too little bias force would not ensure a good thermal contact (in part due to aerodynamics and vibrational effects in vehicle motion) nor efficient bedding in, whereas too much bias force may result in generation of heat by the friction of rubbing, accelerated wear of the collector body and possibly vibrational noise due to the rubbing. To this end, it is known to mount the sensing arrangement and set the bias with the help of a tension meter such as the spring balance shown ghosted at 35 which records the force necessary to lift the collector form the braking surface.

Such quantitative determination of the bias force is also important in determine that sensing arrangements on wheels at opposite ends of an axle are identical.

In operation, the instantaneous temperature of the disc under various braking conditions is manifested at the braking surface 12 and the heat is transferred to and from the collector body and communicated to the thermocouple junction. To follow changes rapidly, the collector body requires a good thermal conductivity and small thermal capacity.

The arrangement of Figures l(a) and l(b) for use with flat, exposed braked surface of a disc permits the arm 16 to be bent to such that the rubbing surface of the collector is set up, and wears, substantially parallel to the braked surface, that is, makes contact over the full area of the surface rather than any smaller area which would reduce thermal transfer.

In a alternative braking system, shown in Figure l(c) a brake drum 41 encloses a cylindrical braked surface 42 and the arm 16 is mounted to one end of a brake shoe platform 43. It will be seen that the enclosed environment of the brake drum does not readily facilitate biasing adjustments of the collector 251 nor of orientating it accurately with respect to the braking surface. Furthermore, a flat collector rubbing surface is not ideally matched to the braked surface for optimal thermal contact.

As discussed hereinbefore, it is found that as modern braking materials cause the braked surfaces to reach higher temperatures, such an alloy collector body composition requires to have a higher melting point, which in practice results in a harder material and a tendency for vibrational noise signals to be produced; one of the main problems with such vibrational signals is that the circumstances of occurrence, intensity and frequencies are unpredictable but invariably detract for any simultaneous monitoring of noise levels inherent in the braking system per se, that is, absent the additional temperature sensing arrangement.

Referring to Figures 2(a) to 2(c), a braked surface temperature sensor arrangement in accordance with the present invention is shown in a first form at 50, which corresponds in many respects to the arrangement 15 as described with reference to Figures l(a) and l(b) and for parts common to both, the same reference numbers are used. As can be seen most clearly from Figure 2(c) the principal difference is that the collector 51 comprises a body 52 of a solid block of material containing wholly or principally amorphous elemental carbon. The block is conveniently of generally the same cylindrical form and dimensions employed for the known body 26 in order to be readily substituted therefore. A typical body 51 has a diameter of 6.5 mm and a length of 5 mm between opposite and substantially flat surfaces 57,58 thereof, and may be formed by cutting from a rod of such material.

The temperature transducer 30, that is, thermocouple junction 31 and wires thereof, is contained within the body on a blind recess 60 which is drilled along the longitudinal axis of the cylinder from the surface 57 towards, but stopping short of, the rubbing surface 58 by a distance of some 2 mm.

The thermocouple junction and lead wires are held within the recess by a temperature resistant adhesive, such as a conventional ceramic fire cement 61, which is also employed to secure the carbon body to the end of the arm 16.

The poor thermal conductivity of the adhesive is beneficial in respect of its securing the collector body to the arm as it inhibits heat transfer between the body and arm. However, notwithstanding the poor thermal conductivity of such adhesive, the close proximity of the thermocouple junction to the body permits adequate thermal response, aided by ensuring physical abutment between the thermocouple junction and body at the base of the recess before the adhesive is inserted.

It is found that the carbon composition of the collector body eliminates the previously troublesome vibrational noises attributable to metal-to-metal contact between the collector rubbing surface and braked surface.

It will be appreciated that the carbon composition of the body must not be overly soft (as in graphite) to avoid both rapid wearing and smearing to leave a potentially contaminating deposit on the braked surface or be overly hard to avoid poor bedding-in and conforming with the braked surface, wear to the braked surface, and 'scratching' noises.

It has been found that a suitable source of carbon having the correct physical properties of wear and strength and thermal properties of conductivity and capacity, is the compressed amorphous carbon core of an arc welding electrode.

The inventor obtained satisfactory results by removing the outer copper sheath from conventional, generic 6 mm and 8 mm carbon cored are welding rods obtained from Delworth and Morris (Engineers Merchants) Limited of St Georges Road, New Mills, Stockport, Cheshire, SK12 4JU. The carbon core of such rods has a Rockwell L scale hardness in the range 55 - 65, more particularly in the range 60 - 64, and is shown by x-ray analysis to comprise at least 99% carbon with traces (' 0.75% wt) iron and even smaller traces ( < 0.2% wt) other elements possibly contamination from removing the sheath.

Notwithstanding its precise chemical composition, such material is easily worked to the correct external dimensions and drilled to provide the recess, and is found to bed-in after a relatively short period of rubbing against the braked surface, corresponding to the vehicle travelling about 20 miles. Furthermore, it abrades only slowly without adversely contaminating the braked surface and the rubbing surface does not expose the recess and thermocouple within the normal brake evaluating period/distance of about 1,000 miles. It is found that the thermal conductivity and capacity, coupled with the ability to mount the thermocouple junction about 2 mm from the braked surface enables a direct substitution of collector.

As discussed above in relation to Figure l(c), it is not always possible to control very accurately the bias force and orientation of a flat collector rubbing surface, particularly within an enclosed brake drum.

Referring to Figures 3(a) and (b), the form of temperature sensing arrangement 70 has an arm 16 (or 161 )and a collector 71 of carbon composition similar to 51 but shaped such that it is of semi-cylindrical form having a longitudinal axis 72 extending in a direction substantially perpendicular to the direction of rubbing motion (21). The body of collector 71 therefore initially has a rubbing surface 78 that makes a line contact with the braking surface and the bedding-in results in abrasion of the surface of the body such that after the bedding in period there is an area of contact for thermal transfer. It will be appreciated that as abrasion continues with use the area of contact will increase to some extent over the period of use, but with the benefit of experience this can be allowed for. However, such form of contact may be particularly advantageous where the alignment of the rubbing surface with the braked surface cannot be established accurately, as within an enclosed brake drum with cylindrically curved broken surfaces, and thus notwithstanding this change in effective area of contact, any pair of collectors at opposite ends of an axle wall bed in, and wear equally.

Whereas such collector configuration is beneficial where there is only possibility of misalignment in one direction, the third form of sensing arrangement 80 illustrated in Figures 4(a) and (b) has a collector 81 of carbon compound similar to collector 51 but in which the rubbing surface 88 is arranged to make point contact (at 881) initially before abrading and bedding-in to provide an area of contact. Conveniently the rubbing surface is manufactured having a profile that is part of a sphere of large radius.

As described above the collector body is secured to the arm by means of adhesive, but alternatively a mechanical interlock may be affected. Referring to Figures 5(a) and (b) a fourth form of temperature sensing arrangement 90 comprises carbon collector body 92 similar to body 52, except for the provision of peripheral groove 93, and arms 16' similar to arm 16 except for a slot 94 having a width corresponding to the body 92 at the base of the groove 93.

The body may thus be slid into the slot and held against movement in an orthogonal direction by abutment between the body and arm. Such mechanical interlock may be supplemented by adhesive if required.

Referring now to Figures 6(a) and 6(b) a fifth form of temperature sensor 100 is shown in sectional elevation and rear end views similar to Figures 5(a) and 5(b). The sensor 100 has a collector body 101 is formed from a cylindrical body which over about 15% - 20% of its length is turned to define a spigot 102 which extends from shoulder 103 of the body and forms a sliding fit in through-aperture 104 in arm 162 The body is retained by adhesive 105 which surrounds the spigot 102 on one or both faces of the arm, the initial spigot engagement easing the positioning of the collector with respect to the arm and its retention whilst the adhesive sets, as well as providing additional shear strength.

As an alternative to the above described carbon material, the body 101, or indeed the body of any of the above described embodiments, may be formed of a material designated as graphite grade GX obtained from Graphoidal Developments Ltd, Wreakes Lane, Dronfield, Sheffield, England.

This material exhibits tensilelflexurallcompression strengths of 7-12/14-20/27-34 N/mm2, a porosity of 23% and a thermal conductivity of 173-220 W/mOC.

A cylindrical body 101 of such material having a diameter of about 7.5 mm was readily turned to give a spigot diameter of about 6.25 mm and performed satisfactory.

In all of the above, the temperature transducer has been contained in a recess drilled from the surface opposite to the rubbing surface, that is, from the arm through which it is convenient for the wires to extend and be supported. It will be appreciated, as shown in Figure 7 in a sixth form of temperature sensing arrangement 110, for the collector ill, secured to arm 163 with adhesive 112, to have a recess 113 formed from any other face provided that the temperature transducer 30 is disposed in proximity to, and good thermal contact with, the rubbing surface.

It will be appreciated that the temperature transducer may be other than a simple thermocouple effected by a metallic junction, such as a semiconductor device which is electrically insulated from the collector body.

Likewise it will be appreciated that the adhesive may be of a different form provided it can withstand the conditions found within a braking system, and if appropriate the temperature transducer may be mounted in the collector body without any adhesive, relying upon abutment or proximity with the carbon of the collector body to reflect body temperature as a transducer signal.

Also, other forms of carbon composition may be used provided they exhibit the desired physical and thermal characteristics.

Whereas the above description has concentrated upon the use of the temperature sensing arrangement in relation to a road vehicle because of the need for quiet operation, it will be appreciated that it may be used in the above described, or suitably modified, configurations with other vehicles, such as rail vehicles which may not require the lower noise properties but can still benefit from the low wear and thermal characteristics.

Claims (15)

Claims:

1. A braked surface temperature sensing arrangement comprising (i) an arm resiliently deflectable between first and second ends thereof, (ii) mounting means arranged to support the arm at said first end thereof spaced from the braked surface and biasing the second end of the arm towards the braked surface, (iii) a thermally conductive collector carried by said second end of the arm comprising a body formed by a block of abradable, non-smearing amorphous carbon having one surface secured with respect to said arm, an opposite surface comprising a rubbing surface arranged to be biased into rubbing contact with the braked surface by the mounting means, and a blind recess spaced from the rubbing surface, and (iv) a temperature transducer contained within the recess, and in thermal contact with the collector body spaced from said rubbing surface.

2 A braked surface temperature sensing arrangement as claimed in claim 1 in which the temperature transducer is secured with respect to the collector body by adhesive means.

3. A braked surface temperature sensing arrangement as claimed in claim 1 or claim 2 in which the adhesive means is arranged also to secure the collector to the arm.

4. A braked surface temperature sensing arrangement as claimed in claim 3 in which the adhesive is a heat tolerant ceramic cement.

5. A braked surface temperature sensing arrangement as claimed in any one of claims 1 to 4 in which the collector body is of cylindrical form and said rubbing surface is substantially circular in end view and substantially flat.

6. A braked surface temperature sensing arrangement as claimed in any one of claims 1 to 4 in which the collector body has a rubbing surface which, before use, is curved in at least one plane and arranged te make point or line contact in the plane of curvature and wear, as a result of rubbing against said braking surface in operation, into conformity therewith over an area of engagement.

7. A braked surface temperature sensing arrangement as claimed in claim 6 in which the collector body comprises part of a cylinder whose longitudinal axis extends in a direction substantially perpendicular to the direction of rubbing motion between the braked surface and collector.

8. A braked surface temperature sensing arrangement as claimed in any one of the preceding claims in which the collector body comprises a block of porous graphite.

9. A braked surface temperature sensing arrangement as claimed in claim 8 in which the graphite has a thermal conductivity in excess of 173 W/mOC.

10. A braked surface temperature sensing arrangement as claimed in any one of the preceding claims in which the collector body comprises greater than 99% wt compressed a amorphous carbon.

11. A braked surface temperature sensing arrangement as claimed in any one of the preceding claims in which the carbon block forming collector body has the physical and thermal characteristics of the core of a carbon cored arc welding electrode.

12. A braked surface temperature sensing arrangement as claimed in any one of the preceding claims in which the collector body has a hardness in the range 55 to 65 on the Rockwell L scale.

13. A braked surface temperature sensing arrangement as claimed in claim 12 in which the collector body has a hardness in the range 60 to 64 on the Rockwell L scale.

14. A braked surface temperature sensing arrangement as claimed in any one of the preceding claims in which the temperature sensor is disposed such as to be displaced from the rubbing surface of the collector after bedding-in with respect to the braking surface by approximately 2 mm.

15. A braked surface temperature sensing arrangement substantially as herein described with reference to, and shown in Figures 2(a) to 2(c), Figures 3(a) and 3(b), Figures 4(a) and 4(b), Figures 5(a) and 5(b), Figures 6(a) and 6(b) or Figure 7 of the accompanying drawings.