Drive system for construction machine

A work machine comprising:

• a work device including a plurality of work members;
• a single rod-type hydraulic cylinder that drives one of the plurality of work members;
• a bottom-side line connected to a bottom chamber of the single rod-type hydraulic cylinder;
• a rod-side line connected to a rod chamber of the single rod-type hydraulic cylinder;
• a bidirectional-type first hydraulic pump of which a delivery port on one side is connected to the bottom-side line through a first control valve and a delivery port on another side is connected to the rod-side line through the first control valve;
• a bidirectional-type second hydraulic pump of which a delivery port on one side is connected to the bottom-side line through a second control valve and a delivery port on another side is connected to the rod-side line through the second control valve;
• an operation lever device having an operation lever for operating the single rod-type hydraulic cylinder to extend and contract;
• a working fluid tank;
• a flushing valve that is connected to the bottom-side line and the rod-side line, and that discharges a surplus flow rate of a lower-pressure side one of the bottom-side line and the rod-side line into the working fluid tank when a differential pressure between the bottom-side line and the rod-side line exceeds a predetermined pressure;
• and
• a controller that controls opening/closing of the first and second control valves and controls tilting amounts of the first and second hydraulic pumps, wherein
• the work machine further includes
• a bottom pressure sensor that detects a pressure in the bottom chamber;
• a rod pressure sensor that detects a pressure in the rod chamber;
• a rod-side discharge line that connects a portion of the rod-side line between the rod chamber and the first control valve and the working fluid tank;
• and
• a rod-side proportional valve provided in the rod-side discharge line, and
• the controller is configured to, when the operation lever is operated to a side of extending the single rod-type hydraulic cylinder in a state in which the pressure in the rod chamber is higher than the pressure in the bottom chamber, open the rod-side proportional valve such that the differential pressure between the pressure in the rod chamber and the pressure in the bottom chamber is reduced below the predetermined pressure.