Change-over valve for boom cylinder of excavating/slewing work truck

Abstract

In an excavating/slewing work truck, the boom falls freely at the time of lowering operation without requiring any power but the flow rate of a pump increases excessively when the speed is balanced with other acutuators and power loss is inevitable for enhancing the operability. In order to eliminate this inconvenience, a first oil path () connecting a bottom side cylinder port (CB) and a tank port (T), a second oil path () connecting a pump port (P) and a rod side cylinder port (CR), and a third oil path () connecting a pump port (P) and a tank port (T) are provided, respectively, with first, second and third restrictors (), () and () at the boom down position of a change-over valve () for the boom cylinder of an excavating/slewing work truck, wherein the first restrictor () restricts by such an amount as the work machine lowers gravitationally an the second restrictor () restricts by such an amount as the pressure on the boom side is not exceeded.