US Patent 7596893 Work machine

A work machine enabling a hybrid drive system to be directly driven by energy contained in a return fluid discharged from a hydraulic actuator. The work machine includes a hydraulic actuator control circuit and a swing control circuit. The hydraulic actuator control circuit serves to control hydraulic fluid supplied from pumps of a hybrid drive system to travel motors and work actuators. The swing control circuit serves to control a swing motor generator, which functions as an electric motor and, during braking of rotating motion of the upper structure, functions as a generator. The hydraulic actuator control circuit includes an energy recovery motor provided in a return passage, though which return fluid recovered from a work actuator flows. The energy recovery motor is adapted to be driven by return fluid and thereby drive a motor generator of the hybrid drive system.