COMPONENT OPERTATION / Control Valve OVERLOAD RELIEF VALVE (WITH MAKE-UP FUNCTION) The overload relief valves are located in the boom, arm, bucket and auxiliary circuits. The overload relief valve prevents each actuator circuit pressure from rising excessively when the actuators are moved by external force. In addition, when the actuator circuit pressure is re- duced, the overload relief valve draws hydraulic oil from the hydraulic oil tank and prevents the occur- rence of cavitation. (make-up function) Relief Operation 1. Pressure in port HP (actuator circuit) acts on pilot poppet (6) through orifice (1) in piston (9). 2.
When pressure in port HP reaches the set pres- sure by spring (7), pilot poppet (6) opens, and pressure oil from passage (5) flows along the ex- ternal circumference of main poppet (2), and flows to port LP (hydraulic oil tank). 3. At this time, a pressure difference is caused be- tween port HP and spring chamber (8) due to ori- fice (1). 4. When this pressure difference reaches the set pressure of spring (8), piston (9) and main poppet (2) open and pressure oil from port HP flows to port LP. 5. Thereby, the actuator circuit pressure decreases. 6. When pressure in the actuator circuit decreases to the fixed level, piston (9) and main poppet (2) are closed by force of spring (8).
Make-Up Operation 1. When pressure in port HP (actuator circuit) de- creases lower than pressure in port LP (hydraulic oil tank), main poppet (2) moves to the right. 2. Hydraulic oil flows to port HP from port LP and cavitation is prevented. 3. When pressure in port HP rises to the fixed level, main poppet (2) is closed by force of spring (4). T3-3-22