Patent ID: 8146725

Claim:
A wet-type multi-plate clutch comprising: a clutch case; a hub arranged coaxially with the clutch case and in an interior of the clutch case, the hub including at least one lubricating oil supply port for supplying oil from an inner diameter side toward an outer diameter side of the clutch; a plurality of annular separator plates engaged with a spline member on one of an inner periphery of the clutch case and an outer periphery of the hub; a plurality of annular friction plates engaged with a spline member on the other of the inner periphery of the clutch case and the outer periphery of the hub, the friction plates being arranged alternately with the separator plates; and a piston that urges the friction plates and the separator plates into contact with each other so as to engage the clutch, wherein each of the friction plates has a friction surface formed by attaching a unitary, annular friction material to a first surface of a substantially annular core plate, said friction surface includes a plurality of first oil grooves and a plurality of second oil grooves formed in said annular friction material, each first oil groove has an opening portion opened at an inner peripheral edge of said friction material and an end portion terminating at a point between the inner peripheral edge and an outer peripheral edge of said friction material, each second oil groove has an opening portion opened at the outer peripheral edge of said friction material and an end portion terminating at a point between the inner peripheral edge and the outer peripheral edge of said friction material, said first oil grooves and said second oil grooves are alternately arranged in a circumferential direction equidistantly, a circumferential width of each first oil groove is the same from the inner peripheral edge of said friction material to the corresponding end portion, wherein the first oil grooves act to separate the respective friction plate from an adjacent separator plate during idle rotation, and the second oil grooves act to discharge oil drawn onto the friction surface during idle rotation toward an outer diameter side of the respective friction plate.