COMPONENT OPERTATION / Control Valve MAIN RELIEF VALVE The main relief valve serves so that pressure in the main circuit does not exceed the set pressure when the actuator such as motor or cylinder is in operation. Thus, oil leak from hose and pipe joints and breakage of the actuator are prevented. Relief Operation 1. Pressure in port HP (main circuit) acts on pilot poppet (6) through orifice A (1) in main poppet (3) and orifice B (4) in seat (5). 2. When pressure in port HP reaches the set pres- sure of spring (7), pilot poppet (6) opens and pressure oil flows to port LP (hydraulic oil tank) through passage (9). 3.
At this time, a pressure difference is caused be- tween port HP and spring chamber (10) due to orifice A (1). 4. When this pressure difference reaches the set pressure of spring (10), main poppet (3) opens and pressure oil from port HP flows to port LP. 5. As a result, pressure in the main circuit de- creases. 6. When pressure in the main circuit decreases to the fixed level, main poppet (3) is closed by force of spring (10). Set Pressure Increasing Operation 1. When pilot pressure from solenoid valve unit (SG) acts on port PF, spring (7) is compressed by pis- ton (8). 2. Therefore, force of spring (7) becomes stronger. 3.
Consequently, as pressure required in order to open pilot poppet (6) is increased, the relief set pressure is increased. T3-3-20