Hydraulic actuator circuit

摘要

This invention provides with a hydraulic actuator circuit comprising a holding valve (17) disposed in a boom extension-side pipeline (13) in the circuit an opening of which is controlled by a control valve (15) and an assist cylinder (20) for controlling an opening of the holding valve in such a way that, in a condition wherein a holding pressure drops to a valve of not larger than a given value, the opening of the holding valve is reduced, whereby the circuit can prevent an unusual sudden lowering of a boom with a load.