Basic Hydraulics - Check Valves

CHAPTER 9 - Check Valves

Figure 9.1 - schematic illustrations of check valves

Check valves are a simple but important part of a hydraulic system. Simply stated, these valves are used to maintain the direction that fluid flows through a system. And since check valves are zero leakage devices we can use them to lock hydraulic fluid from the cylinders. This section has been designed to help you understand the function of different valves and the strategy of where they are used in the system.

In-line check valves are classified as directional control valves because they dictate the direction flow can travel in a portion of the circuit. Because of their sealing capability many designs are considered to have zero leakage. The simplest check valve allows free flow in one direction and blocks flow from the opposite direction. This style of check valve is used when flow needs to bypass a pressure valve during return flow, as a bypass around a filter when a filter becomes clogged, or to keep flow from entering a portion of a circuit at an undesirable time.
Figure 9.2 - silt that has formed between hydraulic components

Pilot Operated
Because of slight spool leakage on standard directional control valves, a check valve must be added to the circuit in order to hydraulically lock a cylinder. The check valve in Figure 9.2 is referred to as a pilot operated check valve.

Unlike a simple check valve, reverse flow is required through the valve to extend or retract the cylinder. This is accomplished by allowing pilot pressure to act on a pilot piston, thus opening the check valve and retracting the cylinder. To extend the cylinder, the check valve allows fluid to flow freely in one direction and blocks flow in the opposite direction.

Check valves have zero leakage and are used to maintain the direction that fluid flow through a system. 

In-line check valves are classified as directional control valves.

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