Non-return valves or check valves
Simple non-return valve
Check valves have the task to shut off the flow in one direction and allow free flow in the opposite direction within a hydraulic circuit. They are also referred to as non-return valves:
The functioning is quite simple:
The sectional view shows a check valve with a cone as a closing element which is pressed by the spring onto the seat of the housing. The pressure required to move the closure member against the spring can be referred to as the opening pressure. This depends on the chosen spring and the area of conical surface and is usually between 0.5 and 3 bar.
Application hydraulic non-return valve
Mounting within the pump line, to protect the pump from pressure surges or to avoid oil drain due to external load on the pump.
Install parallel to a filter. Upon reaching a certain pressure (e.g. 3 bar), the oil is diverted to the filter. So the system can still work despite of a clogged filter.
Mounted parallel to the throttle point: only in the direction A to B you have a throttling. In the reverse direction the check valve opens.
Unlockable non-return valve
This shut-off valve can be unlocked via a pilot line in the reverse direction. The functioning is as follows:
If the port A has high pressure and lthe port B low pressure, so is the valve is in the forward direction. If the pressure conditions changes (port A on low pressure), the main poppet is held by the spring and the system pressure on its seat.
By pressurizing the control port X the pilot piston pushes the main poppet off its seat. Now, the oil can also flow from B to A. They say, the valve is now "unlocked".
Application: The main application is to secure and shut-off under load (for example the lifting platforms). For maximum safety and in consideration of the leakage these type of valves should be mounted close to the cylinder.