| 摘要 |
<p>PROBLEM TO BE SOLVED: To obtain operability of high gain and fine inching performance by providing a cutoff valve in an oil passage for recovering discharge oil of a first hydraulic pump into an operating tank in a neutral psoition of a spool valve for a boom, and constituting the cutoff valve in such a way as to be closed in proportion to boom lifting pilot pressure at the time of simultaneous operation to boom lifting side and arm lead-in side. SOLUTION: A cutoff valve 19R is provided in a by-pass passage 18R for recovering discharge pressure oil of a first pump 22 of a boom spool valve 14 corresponding to the neutral position of a boom operating lever 29, into a hydraulic tank 41. When boom and arm operating levers are simultaneously operated to the boom lifting side (a) and arm lead-in side (b), the cutoff valve 19R is closed in proportion to pilot pressure led out of the boom hydraulic remotecontrolled valve 27. The cutoff valve 19R is controlled with differential pressureΔP between arm pulling and boom lifting pilot pressure as a parameter. A by-pass passage 18R is throttled to build up pressure so as to obtain operability of high gain in order to increase pump discharge.</p> |
