Hydraulic coupler

The invention relates to a hydraulic coupler 4 having an actuating piston 7 cooperating with an actuator 3 and an actuating piston 10 operatively connected to the actuating piston 7 via a fluid-filled coupler chamber 9 arranged in a coupler housing 8 with an adjustable transmission ratio. According to the invention, a hydraulic coupler 4 is provided which is improved with regard to its function. This is achieved in that the transmission ratio between two limit values is variably adjustable in a stepless manner. For this purpose, the actuating piston 7 and/or the actuating piston 10 has a circumferentially variable area 26 cooperating with the coupler chamber 9, and the circumferentially variable area 26 cooperates with a circumferentially varied mating area 27 of the coupler housing 8.