HYDRAULIC DRIVE DEVICE FOR CONSTRUCTION MACHINE

摘要

A main relief valve 13 includes a biasing force altering unit 60. The biasing force altering unit 60 constitutes, in combination with a gate lock valve 23 and a gate lock lever 24, relief pressure altering means for manually switching the relief pressure of the main relief valve 13 between a first pressure (a standard pressure of 25 MPa, for example) and a second pressure for engine start-up (e.g., 3.0 MPa) lower than the first pressure and allows the main relief valve 13 to return the hydraulic fluid discharged from a hydraulic pump 2 to a tank T in conjunction with an unloading valve 9 when multiple actuators 5a, 5b, ... are not in operation under the ambient temperature below freezing point. As a result, it is possible to reduce the load on the hydraulic pump during cold engine,start-up,without compromising the anti-hunting characteristics of the unloading valve, thereby improving the engine start-up performance.