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Metal Transfer and Wear

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Greenwood, JA 


Introduction. These thoughts are offered as a reminder that Tribology is not all about the normal contact of fractal surfaces, and indeed, not all about elastic contact of rubber and polymers, or even about dry contact. Machines do still contain metal surfaces sliding past each other, hopefully separated by an oil film; and sometimes, when tolerances have been pushed too far, or running with starved lubrication when the oil or grease supply is inadequate, with some metal to metal contact. Fortunately this is not always disastrous: surfaces do often run-in, so that after running with contact and a contribution of dry-contact friction, there is steady wear and contacts no longer occur. The traditional design criterion for gears and ball races was, and still is, the : the ratio of the predicted film thickness for smooth surfaces to the rms roughness. Certainly a of 3 or more usually leads to full-film lubrication: but to anyone with the slightest background in surface roughness this is an absurd rule. Assuming, as is usually done, that the predicted smooth-surface film thickness refers to the distance between the mean planes of the roughness, the rms roughness says nothing about the how much contact there will be. And if running-in is successful, and the high points of the surface wear away, the rms (and the ) may hardly change, but there will be successful operation. But when will running-in be successful? What determines when instead of running-in there will be scuffing, and disaster?



transfer layer, wear fragments, riders, prows, radioactivity

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Frontiers in Mechanical Engineering

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Frontiers Media SA


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