Patent ID: 7596893

Claim:
A work machine comprising: a lower structure adapted to be driven by a travel motor; an upper structure that is rotatable on the lower structure by a swing motor generator; and a work equipment that is mounted on the upper structure and adapted to be operated by a work actuator; the work machine further including: a hybrid drive system comprising: an engine, a motor generator adapted to be driven by the engine so as to function as a generator as well as receive electric power so as to function as an electric motor, an electric power storage device that serves to store electric power fed from the motor generator functioning as a generator, as well as feed electric power to the motor generator functioning as an electric motor, and a pump adapted to be driven either one of or both the engine and the motor generator; a hydraulic actuator control circuit that serves to control hydraulic fluid fed from the pump of the hybrid drive system to the travel motor and the work actuator; a swing control circuit that serves to: feed electric power from the electric power storage device of the hybrid drive system to the swing motor generator so that the swing motor generator functions as an electric motor, and recover electric power generated by the swing motor generator functioning as a generator to the electric power storage device during braking of rotating motion of the upper structure; wherein the hydraulic actuator control circuit further includes: an energy recovery motor provided in a return fluid passage though which return fluid recovered from the work actuator flows, the energy recovery motor being adapted to be driven by return fluid and thereby drive the motor generator of the hybrid drive system, and a recovery clutch that is provided between the energy recovery motor and the motor generator of the hybrid drive system and serves to enable or interrupt transmission of rotational power.