Friction clutch having a hydraulic actuator, and drive unit having at least one such friction clutch

A drive unit for controlling the transfer of drive torque from a drive member to a driven member, comprising:

• a disk pack operably disposed between the drive and driven members;
• an actuator for applying a clutch engagement force on said disk pack, said actuator including a stepped piston having a first smaller piston portion disposed for sliding movement in a first pressure chamber and a second larger piston portion disposed for sliding movement in a second pressure chamber;
• a pressure source in fluid communication with a fluid reservoir and operable for providing a supply of pressurized fluid;
• a hydraulic circuit including a pressure line communicating with said second pressure chamber, a first check valve disposed in said pressure line, a branch line connecting said pressure line to said first pressure chamber upstream of said first check valve, a drain line in fluid communication with said second pressure chamber, a suction line connecting said fluid reservoir to said second pressure chamber, and a second check valve disposed in said suction line;
• and
• a control valve unit operably arranged between said pressure source and said pressure and drain lines;
• wherein movement of said stepped piston from a retracted position toward an extended position due to increased fluid pressure in said first and second pressure chambers causes said clutch engagement force to be exerted on said disk pack, and wherein movement of said stepped piston from its retracted position to an intermediate prestroke position is caused when said first check valve is closed such that pressurized fluid from said pressure source is supplied by said control valve unit through said branch line to said first pressure chamber.