THE load-carrying capacity of a bearing surface is dependent upon the surface temperature. The surface temperature is the temperature of the surface material microscopic in depth and expanse. High surface temperature is the cause of surface failure. Load-carrying capacity is affected by surface finish, coefficient of friction, speed, thermal conductivity, lubrication, material, and so on, but all factors affect surface temperature. Bearing surfaces fail because the areas in contact reach the melting point of the material.
Surface finish affects the load-carrying capacity because it determines the actual area and distribution of that area in contact. It is reasonable to suppose that a bearing surface carrying a given load satisfactorily is dotted with high-pressure areas, microscopic in size and depth, that melt and cool almost instantaneously. The gradual oxidation that takes place could be the process of normal wear. It is only when the high-temperature areas get too large and deep to cool instantaneously that failure occurs.
Rough surfaces have greater load-carrying capacity than smoother surfaces because of their ability to cool. The load-carrying capacity in pounds per square inch of a bearing surface decreases as the contacting area increases. The load-carrying capacity in pounds per square inch of a bearing surface can be increased by dividing the area into small areas. The capacity of such an interrupted surface can be increased further by filling the grooves or cavities with a suitable material.
