is to be retained depending on not only a shearing stress of the oil but also a direct contact between the protruded portions of a rough surface in such a case the traction is determined in accordance with an unstable factor termed a projection to projection interference depending on variations in the lubricating state hence the traction coefficient u03bc does not necessarily increase even when the formation of the oil film changes in degree due to the increment in pmax in any case as shown above the traction coefficient u03bc does not simply increase with the increment in pmax rather the traction coefficient u03bc reaches the maximum value when pmax reaches a certain value the traction coefficient u03bc is reduced with a further increment in pmax it will thus be appreciated that in designing a power transmission device for transmitting large power if the maximum contact surface pressure pmax is increased based on the conventionally known theory a result is obtained which is actually contrary to that theory that is the traction coefficient u03bc is reduced whereby a large power cannot be transmitted furthermore it is known that the life span of rolling fatigue is inversely proportional to substantially the 7th through 9th powers of pmax accordingly increasing.