Hydraulic drive system for work machine

A hydraulic drive system for a work machine, comprising:

• a prime mover;
• a variable displacement hydraulic pump driven by the prime mover;
• one or more actuators that are driven by a hydraulic fluid delivered from the hydraulic pump and includes a hydraulic cylinder for moving a work device upwardly and downwardly;
• one or more flow control valves that control a flow of hydraulic fluid to be supplied from the hydraulic pump to the one or more actuators;
• a regulator that performs load sensing control for controlling a delivery flow rate of the hydraulic pump such that when a difference between a pressure of a hydraulic fluid supply line of the hydraulic pump and a maximum load pressure is greater than a target LS differential pressure dependent on a rotational speed of the prime mover, the delivery flow rate of the hydraulic pump is decreased, and when the difference between the pressure of the hydraulic fluid supply line of the hydraulic pump and the maximum load pressure is smaller than a target LS differential pressure, the delivery flow rate of the hydraulic pump is increased;
• an unloading valve that opens and returns a hydraulic fluid of the hydraulic fluid supply line of the hydraulic pump to a reservoir when the pressure of the hydraulic fluid supply line becomes equal to or higher by a predetermined value than the maximum load pressure of the one or more actuators, the predetermined value being equal to or larger than the target LS differential pressure;
• and
• a hydraulic energy recovery device that includes an accumulator connected to the hydraulic cylinder and the hydraulic fluid supply line of the hydraulic pump and accumulates a hydraulic fluid returned from the hydraulic cylinder into the accumulator when an operation of lowering the work device is performed, and supplies and regenerates at least a part of the hydraulic fluid accumulated in the accumulator to the hydraulic fluid supply line of the hydraulic pump when an operation other than the operation of lowering the work device is performed;
• wherein the hydraulic energy recovery device includes a regeneration selector valve device that controls a regeneration flow rate of a hydraulic fluid to be supplied from the accumulator to the hydraulic fluid supply line of the hydraulic pump;
• and
• the regeneration selector valve device is configured to control a communication between the accumulator and the hydraulic fluid supply line of the hydraulic pump such that, when the difference between the pressure of the hydraulic fluid supply line of the hydraulic pump and the maximum load pressure is greater than the target LS differential pressure, supply of the hydraulic fluid from the accumulator to the hydraulic fluid supply line of the hydraulic pump is limited, and when the difference between the pressure of the hydraulic fluid supply line of the hydraulic pump and the maximum load pressure is smaller than the target LS differential pressure, supply of the hydraulic fluid from the accumulator to the hydraulic fluid supply line of the hydraulic pump is permitted.