Patent ID: 7614225

Claim:
A straight traveling hydraulic circuit comprising: first and second hydraulic pumps; a first traveling control valve and control valves for a working device which are installed in a first center bypass passage of the first hydraulic pump; a first center bypass shifting valve installed on the lowest downstream side of the first center bypass passage and shifted when a signal pressure is applied from an exterior; a second traveling control valve and control valves for the working device which are installed in a second bypass passage of the second hydraulic pump; a straight traveling valve installed on an upstream side of the second bypass passage and shifted to distribute a hydraulic fluid fed from the first and second hydraulic pumps to the working device and a traveling device in response to a signal pressure applied from the exterior; a variable orifice disposed in a confluent passage which connects a passage branched from the upstream side of the second center bypass passage with the second center bypass passage in parallel, the orifice being switched in response to the signal pressure applied from the exterior, and interrupting a traveling of the hydraulic fluid from the working device to a traveling side when the signal pressure is applied from the exterior; and a second center bypass shifting valve installed on the lowest downstream of the second center bypass passage and shifted when the signal pressure is applied from the exterior; wherein, in case of performing a combined operation in which the working device and the traveling device are simultaneously required, the variable orifice and the first and second center bypass shifting valves are shifted to decrease a degree of opening of the first and second center bypass passages in response to the signal pressure shifting the straight traveling valve, thereby driving the working device and the traveling device independently.