31. WEAR OF A SHAFT IN ITS BEARINGS

by Sir Henry R. Ricardo

It appears that under normal running conditions of full fluid lubrication, wear of a shaft in its bearings is due primarily to the presence of grit, etc., which is imported into the bearing by the lubricating oil. These minute particles of grit, too small to be caught by any filter, then become embedded in the surface of a bearing material but may still project sufficiently to span across the oil film when it is at its thinnest and so to lap the shaft. To a secondary degree it is due to attrition, when owing to the thinness of the oil film, the high spots on both members of the bearing come into metallic contact, but with highly finished surfaces, and an ample supply of lubricant, it is doubtful whether wear by attrition is an important factor provided that the materials are compatible and are not such as will readily weld together. Clearly the higher the mean loading, the thinner will be the oil film on the loaded side of the bearing, and therefore the greater the intensity of the lapping process. Again, the harder the surface of the shaft, or the softer that of the bearing material, the more readily will particles be driven home into the latter, and so out of harm's way. Other things being equal, the rate of wear of the shaft will depend upon the difference in surface hardness between itself and the bearing material; the ideal is clearly the hardest possible shaft in the softest possible bearing.

If a soft, as opposed to a surface-hardened, steel shaft is used, then we are almost bound to employ white metal, at all events for parts of the bearing, for any other material is too hard and will lead not only to undue wear, but also to ringing and scoring of the shaft. If a flame- or induction-hardened shaft is used, then we can employ a copper-lead bearing material, provided that the loading is not too high, but we shall be on the border-line of scoring. Copper-lead, as opposed to lead-bronze, consists, in effect, of a fine copper sponge saturated with lead. In principle the copper provides the necessary physical strength, while the lead serves as the bearing material, but, of course, it is not possible in practice to separate the two functions so completely, and a considerable area of the bearing surface is represented by copper which is really too hard for anything but a very hard steel shaft. A useful compromise can be achieved by coating a copper-lead bearing with a thin deposit of pure lead and this in turn can be protected from corrosion by a still thinner deposit of indium, which appears to soak well into the lead. Also the low melting-point of indium allows of local fusion without destroying the general oil film and so, as in the case of white metal, allows of closer clearances being employed. Thus treated, a copper-lead bearing gives satisfactory results against a flame-hardened shaft and can even be used against an unhardened shaft provided the peak loading is not too high.

(from Science in Writing, compiled by T. R. Henn (Harrap))