ALL machines, particularly modern machines, are constructed and operated with the general understanding that they will at some time wear out. Wear is combated in many ways but never more successfully from both scientific and economic points of view than when all the major parts of a machine wear out together. This state of affairs is obviously preferable to having a machine which will never wear out at all; for even if such an article could be made, the cost of its construction and operation would be uneconomical. As is well known, many machines do not wear out before they become obsolete, but again this is rarely either a sound economic or scientific proposition. Not only is loss of capital involved but an inaccurate estimation of operational life is a failure in the application of engineering knowledge. If this outlook can be sustained for machines generally it is certainly vital and of the highest importance for prime movers such as aero engines with which this paper is chiefly concerned. An aero engine today must give definite life for definite cost and in no other branch of engineering are quality and reliability so vital and cost, by reason of its very magnitude, so deeply considered. Hence no one has attempted to construct an aero engine which could be economically scrapped after its first overhaul life and not even the most optimistic manufacturers have devised a non‐wearing, everlasting product. As might be expected, a reasonable compromise is the general rule but it is however a noticeable fact that modern engines are becoming more and more resistant to wear and if anything there is a general tendency to the second alternative referred to above. The term “wear” in the general sense covers a multitude of physical changes and chemical reactions, some of which it is hoped to consider later, but if for a moment this expression is held to embrace the deterioration of highly stressed steel parts from prolonged fatigue, marked recent improvements can be reported. Some years ago it was common to replace certain aero engine parts for no other reason than that they were “time expired”. It was considered in such cases that the parts after a certain life had passed the safe limit of their fatigue resistance, although the parts themselves and the rest of the engine would be in what appeared a perfect condition. Developments over the past few years have made it possible to eliminate such precautions and in most modern engines all parts can be relied upon to maintain a safe margin of fatigue resistance for the useful life of the complete engine. This is an isolated example of how the wearing qualities, or more accurately, the endurance, of engines has been improved and it is of interest to note that these advances in the selection and better use of materials have nearly been equalled by improvements in actual surface wear resistance. It can be safely asserted that bearing combinations are available today which will resist surface wear indefinitely under the heaviest possible conditions of loading, providing the loading is not associated with other factors which cannot as yet be overcome. It is therefore an object of this paper, in addition to considering wear generally, to examine these factors as their interest lies in the obstacles they present to the production of a machine which will approach the ideal of being completely worn out after a given life.
MCB UP Ltd
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