What is the hydraulic check valve?

Check valve is a one-way directional valve. Its function is to allow the flow in one direction only. It does not permit the flow in the opposite direction. (Refer to Figure 7.31.) For this reason, they are also known as nonreturn valves. In order to avoid any leakage, these valves are always of poppet type construction. They consist of a ball or a poppet, which is kept in its normally closed position by a spring. When the fluid pressure overcomes the spring force, the poppet is forced off its seat, allowing flow in that direction. Flow is not possible in the opposite direction. This valve is used frequently in hydraulic and pneumatic circuits because of its number of applications

Applications of Check Valve

1. To prevent return flow in hydraulics: Check valves are used to prevent return flow of fluid from the hydraulic system. Its application is shown in the basic hydraulic circuit diagram for control of single acting cylinder. Like the pressure relief valve, a check valve is required in every hydraulic system.

2. Speed control: These valves can also be used to control the speed of a pneumatic or hydraulic cylinder. This application is shown in speed control in circuits 

Speed in pneumatic and hydraulic circuits can also be controlled by using a check valve rectifier. Working of check valve rectifiers can be understood by studying the example given in Figure 7.32. This circuit is drawn to obtain the slow forward and backward motion to the ram. When pressurized fluid (either hydraulics or pneumatics) is fed to pressure port P, it escapes out of the DC valve via working port A and go the rectifier. In this, check valves are so arranged that fluid have to pass through

variable speed control when flowing from the pump/compressor to the cylinder. As a result the speed is reduced, i.e., the piston moves slow during forward stroke.

When the position of the DC valve is changed, the pressurized fluid starts flowing through the rod side of the cylinder and is discharged from the other side of the cylinder. The pressurized fluid coming out of the cylinder has to pass through check valves and variable restrictors. This results in the lowering of speed of the piston during the backward stroke. Hence, speed of the cylinder can be reduced by the use of flow control valves

3. To bypass the blocked lines in hydraulics: Sometimes check valves are also used to bypass the flow from blocked lines as shown in Figure 7.33.