Hydraulic circuit for construction machine

A hydraulic circuit installed in a construction machine to drive a hydraulic cylinder provided in the construction machine, the hydraulic cylinder including a bottom side fluid chamber and a rod side fluid chamber and adapted to be operated in an expansion direction by a supply of hydraulic fluid to the bottom side fluid chamber while discharging hydraulic fluid from the rod side fluid chamber and to be operated in a contraction direction by a supply of hydraulic fluid to the rod side fluid chamber while discharging hydraulic fluid from the bottom side fluid chamber, the hydraulic circuit comprising:

• a hydraulic pump for supplying the hydraulic fluid to the hydraulic cylinder;
• a tank for receiving the hydraulic fluid discharged from the hydraulic cylinder;
• a control valve operable to be switched between an expansion driving position for leading the hydraulic fluid discharged from the hydraulic pump to the bottom side fluid chamber and leading the hydraulic fluid discharged from the rod side fluid chamber to the tank to thereby operate the hydraulic cylinder in the expansion direction and a contraction driving position for leading the hydraulic fluid discharged from the hydraulic pump to the rod side fluid chamber and leading the hydraulic fluid discharged from the bottom side fluid chamber to the tank to thereby operate the hydraulic cylinder in the contraction direction;
• a bottom side line connecting the bottom side fluid chamber of the hydraulic cylinder to the control valve;
• a rod side line connecting the rod side fluid chamber of the hydraulic cylinder to the control valve;
• a quick return circuit having a branch line branched off from the bottom side line and led to the tank so as to return the hydraulic fluid discharged from the bottom side fluid chamber directly to the tank while bypassing the control valve when the hydraulic cylinder is operated in the contraction direction, and a quick return valve provided in the branch line to allow the hydraulic fluid to be flowed through the branch line only when the control valve is switched to the contraction driving position;
• and
• a recycling circuit which supplies a part of the return fluid discharged from the bottom side fluid chamber to the rod side fluid chamber of the hydraulic cylinder, the recycling circuit allowing the hydraulic fluid to be flowed only in one direction from the bottom side fluid chamber to the rod side fluid chamber only when the control valve is switched to the contraction driving position,
• wherein the recycling circuit comprises:
• a bypass line connecting the bottom side line to the rod side line;
• and a recycling selector valve provided in the bypass line and adapted to be switched between a closing position for closing the bypass line and an opening position for performing a function as a check valve which allows the hydraulic fluid to be flowed only in one direction from the bottom side line into the rod side line only when operating the hydraulic cylinder in the contraction direction,
• wherein the bypass line is connected to a portion of the bottom side line, the portion located on an upstream side of a branching point at which the branch line is branched off from the bottom side line in a flow direction of the return fluid discharged from the bottom side fluid chamber.