US Patent 7677375 Drive force transmission device

A drive force transmission device has an outer clutch plate and an inner clutch plate, which are rotatable relative to each other about a common rotational axis. The inner and outer clutch plates become frictionally engaged with each other with lubricant oil provided in between. The outer and inner clutch plates each have a sliding surface. The sliding surfaces face each other. A diamond-like carbon film is formed on the sliding surface of the outer clutch plate. Fine grooves are formed in the sliding surface of the inner clutch plate. The proportion of the sliding area to the sliding surface of the inner clutch plate at the initial stage of use is 55 to 90%.