The most widely used type of pressure control valve is the pressure-relief valve because it is found in
practically every hydraulic system. Schematic diagram of simple relief valve is shown in Fig. 1.1 and
three-dimensional view is shown in Fig. 1.2. It is normally a closed valve whose function is to limit the
pressure to a specified maximum value by diverting pump flow back to the tank. A poppet is held seated
inside the valve by a heavy spring. When the system pressure reaches a high enough value, the poppet is
forced off its seat. This permits flow through the outlet to the tank as long as this high pressure level is
maintained. Note the external adjusting screw, which varies spring force and, thus, the pressure at which
the valve begins to open (cracking pressure)(Fig. 1.3).
It should be noted that the poppet must open sufficiently to allow full pump flow. The pressure that exists
at full pump flow can be substantially greater than cracking pressure. The pressure at full pump flow is
the pressure level that is specified when referring to the pressure setting of the valve. It is the maximum
pressure level permitted by the relief val