Fluid pressure control device

A fluid pressure control device for controlling an extending and contracting operation of a cylinder that drives a load, the fluid pressure control device comprising:

• a supply control valve configured to control supply of a working fluid from a fluid pressure supply source to the cylinder;
• a pilot control valve configured to control pilot pressure led from a pilot pressure supply source to the supply control valve;
• a main passage configured to connect the supply control valve and a load-side pressure chamber of the cylinder on which load pressure of the load acts when the supply control valve is at a neutral position;
• and
• a load holding mechanism provided in the main passage, wherein
• the load holding mechanism includes:
• an operation check valve configured to allow a flow of the working fluid from the supply control valve to the load-side pressure chamber and to allow a flow of the working fluid from the load-side pressure chamber to the supply control valve in accordance with back pressure,
• a switching valve configured to operate in conjunction with the supply control valve by the pilot pressure led through the pilot control valve to switch an operation of the operation check valve, and
• a relief valve configured to open when pressure in the load-side pressure chamber reaches predetermined pressure, and
• the switching valve includes:
• a pilot chamber configured to receive the pilot pressure through the pilot control valve,
• a spool configured to move in accordance with the pilot pressure in the pilot chamber,
• a spring chamber configured to accommodate a biasing member that biases the spool in a valve closing direction,
• a drain chamber provided on an opposite side of the spring chamber with the spool interposed therebetween,
• a drain passage connected to the relief valve, and connecting the drain chamber to the spring chamber,
• a pressure leading passage configured to connect the drain passage and a downstream side of the spool, and
• a check valve provided in the pressure leading passage and configured to allow only a flow of the working fluid from the drain passage to the downstream side of the spool,
• the pilot chamber and the drain chamber are provided as a common space, and
• the drain passage includes a throttle at a portion of the drain passage that connects the drain chamber to the spring chamber, the throttle giving a resistance to the working fluid passing therethrough.