Measurement of surface wear

Details Measurement of surface wear Measurement of surface wear

2000-05-05

2001-10-16

Williams, Hezron (Department: 2856)

Measuring and testing

By abrasion, milling, rubbing, or scuffing

Reexamination Certificate

active

06301950

ABSTRACT:

This invention relates to the determination of wear of a surface of a body and in particular relates to wear of a rubbing or frictional engagement surface of a friction couple such as a brake or clutch for a vehicle or like machine.
A friction couple is defined, for the purpose of this specification, as comprising a rubbing member having a rubbing surface and a friction material member capable of coupled motion relative to the rubbing surface when pressed into frictional engagement therewith substantially normally to the direction of said coupled motion such that a frictional engagement region extends along, and transversely to, said coupled direction.
The invention is particularly, but not exclusively, concerned with friction couples that comprise brake arrangements for railway or other such large vehicles. It is furthermore particularly concerned with disc brakes wherein the rubbing member is a disc of ferromagnetic material, iron or steel, and the rubbing surface is an annular region extending circumferentially around the axle or the disc supporting member and radially between inner and outer boundaries of the disc.
Within such exemplary railway disc brakes the swept areas are large and the co-operating friction material members are hard and contain materials which are designed to give low rates of the wear to the rubbing surface, that is, wear or abrasion of the surface is intended to take place over a long interval. If the braking system is not operating as intended there may be uneven and/or accelerated wear of the rubbing surface. However, because of the comparatively low rate of wear, even when anomalous, it is often difficult to measure before it has reached such a stage where the remaining life of the rubbing member, and possibly braking system per se is compromised.
In practice, safe operation of vehicles in use can be ensured by regular inspection and maintenance scheduled at suitably frequent intervals, although the cost of each and/or over-frequent inspections may be unsound economically.
It will be appreciated that when a new or changed design of friction couple is sought, such long intervals required before the rate of wear of rubbing surface that results from inappropriate rubbing member material or co-operating friction material make it difficult to experiment with, and compare, various combinations, and in the past it has frequently been necessary to commit to a brake design for a whole fleet of vehicles before its very long term characteristics are known, or perform prolonged braking tests (equivalent to months or years in service) to achieve wear rates that are measurable within the accuracy of known apparatus and methods.
In particular it is required to determine wear that occurs at different points across the frictional engagement region, in effect to map the wear, and to provide such indication of wear it is necessary to perform surface measurements at different times, separated by periods of wear or simulated braking operation. In that respect, it is not normally possible to have measuring apparatus permanently in place, and it becomes necessary therefore to be able to make comparable mapping measurements by way of apparatus that is separably positioned with respect to the rubbing surface on each occasion, which further challenges the accuracy achievable with existing apparatus and thus contributes to the length of time that is required between measurement operations to permit surface wear to accumulate sufficiently to be measured.
Such rubbing members and potential friction materials are, of course, tested upon dynamometer type machines which subject the friction couple to a carefully monitored programme of operating conditions which simulate those likely to be encountered in use and on an accelerated scale, but it is found that the kind of wear is usually so low as to be beyond the measuring capabilities/resolution of currently available measuring instruments, bearing in mind that a rubbing member may have a diameter of about 0.75 meters and is frictional engagement region of the rubbing surface of about 200 mm between radially inner and outer boundaries, whilst the average total wear may be of the order of less than 0.1 mm after many months in use, and variations in wear considerably less.
One measuring instrument employed hitherto to determine how wear occurs across the engagement region of a substantially plane rubbing surface of a ferromagnetic brake disc, that is, to map the wear of the rubbing surface across the engagement region, takes the form of a microscope, possibly incorporating a television camera, which is held against the rubbing surface by a permanent magnet ring that permits it to be clamped to the surface whilst the surface is in the vertical orientation normally found in use (or simulation) in relation to a vehicle wheel and then moved to different positions on the surface. In combination with the microscope a plurality of shallow blind holes, each having a prescribed degree of taper, are drilled in the rubbing surface at locations across the frictional engagement region such that by placing the microscope above each hole in turn an operative is able to assess from the current diameter of the hole the remaining depth and thus the amount by which the rubbing surface has become worn in the vicinity of the hole. This method is considered less than satisfactory in many respects. Apart from the obviously slow nature of the measurement that depends directly of the number of holes and the need to rely upon interpretation by a skilled operative, there are also drawbacks due to the inherent limitation in how closely together such holes/measurements can be made and accuracy achievable due to there being a tendency for the rubbing surface to ‘flow’ at the edge of any hole and mask the true diameter and wear it implies.
A different measuring instrument for determining a measure of surface roughness by scanning a probe across it automatically is described in DE-A-3901352. The arrangement relies upon being placed on a horizontally disposed surface. EP-A2-0509809 describes a method of determining the surface roughness of a mill roll by changes in reflection of a scanned beam of optical radiation.
However, as indicated above, it is frequently desirable for such measurements in respect of friction couples of vehicles to be made with the rubbing member in a disposition corresponding to that encountered in operation, that is vertically orientated and with proximate bodies which permit only limited access to the rubbing surface, more particularly the frictional engagement region thereof.
It will be appreciated that not all such friction couples include disc brakes, or indeed brakes at all, for example drum brakes and dry-plate clutches, and preserving the generality of the foregoing in relation to all such friction couples, it is an object of the present invention to provide, for a friction couple including a rubbing surface having a frictional engagement region, a wear determining arrangement which mitigates the accuracy problems of known devices and permits accurate and more comprehensive determination of wear in less time and with less dependency upon operative skill.
According to the present invention, for a friction couple comprising a rubbing member having a rubbing surface and a friction material member capable of coupled motion relative to the rubbing surface when pressed into frictional engagement therewith substantially normally to the direction of said coupled motion such that the frictional engagement region extends along and transversely to said coupled direction, an arrangement for determining wear of the rubbing surface comprises a (i) carrier member arranged to overlie a part of the frictional engagement region of the rubbing surface and including contact datum means adapted to bear on the rubbing surface, ii) transducer means including transducer support means, arranged to support, with respect to the carrier member, a transducer operable to provide signals related to the instantaneous distance of the frictional engagement region of

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