Pressure-limiting valves limit the inlet pressure by opening the outlet to the tank via the function element (which is closed at rest) on reaching the set pressure. This action occurs by opening against a closing force, which is normally applied by a spring. Pressure-limiting valves are always in the parallel circuit. If the pressure-limiting valve is used to protect a circuit or a system against overpressure it is one of the most important elements in a system. It is set to the nominal pressure and should always be placed in proximity to the pump and protected against unauthorised adjustment. As the pressure-limiting valves are always under pressure, they should be hermetically sealed at rest. Valves that provide protection for overpressure of a Hydraulic accumulator must be designed as safety valves with unit approval.
Directly controlled pressure-limiting valves:
The opening pressure of the valve is determined by the valve spring, which has a direct effect on the ball/cone. If the pressure P exceeds the force of the valve spring, the valve cone/ball opens. However, the actual maximum pressure during operation is dependent on the respective flow (flow rate from P to T) and the oil viscosity that is present during this. In the case of higher flow rates, this may mean that the pressure settingis exceeded (high flow resistance). For this reason directly controlled pressure-limiting valves are mostly only used for small/medium-sized flow rates. Pilot-controlled pressure-limiting valves are mostly used for larger flow rates, starting from approx. 80 lpm.
Pilot-controlled pressure-limiting valves:
The closing element is subjected to the pressure of the medium, which is controlled by a pilot valve . As a consequence, the pressure difference between the start of opening and end of opening is very small. Pilot-controlled pressure-limiting valves are therefore the most common design for the protection of systems.
π/Q2 behaviour of a pressure-limiting valve; 1. directly controlled, 2. pilot-controlled
Pilot-controlled pressure-limiting valve