Wear, 55 (1979) 387 - 389 0 Elsevier Sequoia S.A., Lausanne -Printed in the Netherlands

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Letter to the Editor

Comments on “A practical interpretation of unlubricated wear data for some non-ferrous metals”

The main point in Stott and Eyre’s interesting and thought-provoking article [l] is their assumption that wear data obtained in dry unlubricated testing can be applied successfully to the prediction of the wear behaviour in bearings suffering from a complete stoppage of lubricant supply.

In our opinion continuous condition monitoring of bearings during operation (e.g. by continuously performed temperature measurements) will rapidly gain in importance (in certain fields, most notably in aircraft, it is already well-established practice). This implies that serious lubricant starva- tion is bound to be detected early. Under these conditions it may well be that the ability of the combination of bearing material, journal material and lubricant (including reaction layers, friction polymers etc.) to survive rather brief periods of inadequate lubrication rather than the dry wear behaviour of the bearing materials is the determining factor.

This consideration has led to attempts to design testing procedures for bearing materials in which the presence of (traces of) lubricant is taken into account [ 21. In these procedures a distinction is made between tests with an ample supply of lubricant which are performed at such a low sliding speed that boundary lubrication conditions prevail [3] and high speed tests per- formed under conditions of marginal lubricant supply obtained by suddenly draining the lubricant bath. The former type of test simulates the behaviour of lubricated journal bearings running at speeds far below the design speci- fication (low thermal loading); the latter type of test simulates the behaviour of lubricated journal bearings suffering from sudden stoppage of the lubricant supply (high thermal loading).

Our second comment concerns Stott and Eyre’s view that whether wear takes place primarily by adhesive, abrasive or corrosive mechanisms or by a combination of these mechanisms a low wear rate will be of major interest to the designer. In fact an equally important criterion is the process surface roughness which is established as a result of metal-to-metal contacts. In par- ticular if a bearing is designed to run under conditions of hydrodynamic lubrication this process surface roughness determines the ease of surface separation upon subsequent runs with restored lubricant supply. In practice there will undoubtedly be many cases in which a combination of a low process roughness and a relatively high wear rate is to be preferred over a combination of a high process roughness and a relatively low wear rate.

Even if the above points are taken into account (i.e. tests are per- formed in the presence of traces of lubricant and the importance of the process surface roughness is acknowledged) speed uersus load diagrams,

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as proposed by Stott and Eyre, can never be used for general design purposes because in a quantitative sense they relate only to the test equipment on which the data were obtained. The use of another test rig or/application of the materials in a real bearing will cause appreciable differences in thermal conditions with the result that the transitions will shift to different combina- tions of load and speed values. Such effects have, for instance, been shown in ref. 4.

The procedure that is to be followed in such cases is firstly to estimate the temperature that may be expected in practice (e.g. by application of a thermal network calculation [ 51) and secondly to perform tests under thermal conditions that resemble those in practice as closely as possible, possibly applying an external heat source [6].

(Received December 18,1978) A. BEGELINGER and A. W. J. DE GEE Me taal Znstituu t TNO,

P.O. Box 541, 7300 AM Apeldoorn, The Netherlands

1 G. Stott and T. Eyre, A practical interpretation of unlubricated wear data for some non-ferrous metals, Wear, 50 (1978) 285 - 297.

2 Testing of the tribological behaviour of bearing materials to be used for application in oil lubricated bearings, Working Document N67, ISO/TC123/Subgroup 2, Working Group 6,1977 (copies can be obtained from H. Tepper, DIN Deutsches Institut fur Normung e.V., Kamekestrasse 8, D-5006 Koln 1, F.R.G.).

3 A. W. J. de Gee, Selection of materials for lubricated journal bearings, Wear, 36 (1976) 33 - 61.

4 A. Begelinger and A. W. J. de Gee, Synopsis of the results from an international co- operative wear programme, Lubr. Eng., 26 (Feb. 1970) 56 - 63.

5 H. Blok, The thermal network method for predicting bulk temperatures in gear trans- missions, F’roc. 7th Round Table Discussion on Marine Reduction Gears, Stal-Laval, Finspang, Sweden, 1969, pp. 3 - 25.

6 A. W. J. de Gee, Selection of materials for tribotechnical applications - the role of tribometry, Tribol. Int., 11 (1978) 233 - 239.