Relief
valves are the protectors of hydraulic circuits. They not only protect
the pump but also the electric motor and other components of the
circuit. Relief valves may be of pilot-operated type or the direct-acting type. The pilot-operated type employs a small control spring while the direct-acting type uses a large one. Either type usually uses different springs for different pressure ranges. The relief valve is normally closed, i.e., the valve piston keeps the exhaust core or connection closed until such a time as the pressure exceeds the spring setting, when the valve opens and oil is returned to the oil reservoir. After the excess pressure is relieved, the exhaust passage is quickly and positively closed.
A pressure relief valve is a normally closed valve connected between the pressure line and the oil reservoir. Its main purpose is to limit the pressure in a system to a prescribed maximum by diverting some or all of the pump output to the tank, when the designed set pressure is reached.
A simple relief valve may consist of nothing but a ball or poppet held seated in the valve body by the compressive force of a heavy spring. When the pressure at the inlet is insufficient to overcome the force of the spring the valve remains closed and hence it is very often referred as a normally closed valve. When the preset pressure is reached, the ball unseats and allows flow through the outlet to tank. In most of these valves, an adjusting screw is provided to vary the spring force. Thus the valve can be set to open at any pressure within the specified range. The pressure at which the valve first opens is called the cracking pressure.
