Check valve, also known as non-return valve, as the name suggests, allows oil to flow in only one direction. It is generally composed of a valve body, a spring, and a valve spool. It typically has two sets of ports. The oil pressure at both sets of ports and the spring force act on the valve spool. Therefore, what determines the position of the valve spool, and thus the opening and closing of the flow passage, are the oil pressure at these two ports and the spring preload.
According to ISO 1219-1:2006 (GB/T 786.1—2009), the graphical symbol of a check valve can also be drawn without the spring, but that is for the sake of convenience and simplification. True check valves without springs are rarely used.
1. Function
When the pressure difference between port ① and port ② exceeds the spring preload pressure, the valve spool is pushed open, opening the flow passage, and liquid flows through. Otherwise, the valve spool is pressed against the valve seat, closing the flow passage. It belongs to the category of on-off valves.
To minimize leakage when closed, seat valves are generally used, especially poppet valves, while spool valves are less commonly used.
Springs are generally included. The functions of the spring are: 1) To overcome possible resistance and close quickly. For this purpose, harder springs are sometimes selected. 2) To press tightly to reduce leakage. 3) The flow passage opens only after the pressure exceeds a certain value.
The spring preload generally exceeds the weight of the valve spool to ensure that, regardless of the installation orientation of the check valve, the valve spool can return to position.
2. Applications
Check valves are widely used in hydraulic systems.
A check valve is generally installed at the pump outlet to prevent oil in the pipeline from flowing back to the tank through the pump’s flow passages when the pump stops. When used in this application, the hydraulic resistance should be as small as possible, because when the pump is working, the entire pump output flow must pass through this valve, which results in continuous energy consumption. Therefore, if a small amount of reverse leakage can be tolerated, a weak spring can be considered. If the valve can be installed vertically and the weight of the valve spool can help return and close the flow passage, the spring can even be eliminated.
Check valves are also used for pressure holding in accumulators. Oil is supplied to the accumulator from port ①. When the pressure at port ① is low, the check valve can seal this port to prevent backflow. This application requires the leakage amount to be as small as possible.
Check valves are also used to protect radiators. When the pressure at port ① exceeds the allowable pressure of the radiator, it opens, bypassing part of the oil to prevent overpressure damage to the radiator. This application is equivalent to a relief valve with low opening pressure. The opening pressure determined by the spring preload is of primary importance. Second is fast opening speed, which can reduce pressure spikes. Therefore, the valve spool mass should be small, and slight leakage is acceptable. The same applies when used to protect filters.
The check valve in [the system] is used to enable the motor to achieve different speeds in both directions with a fixed pump flow rate. When the directional valve is in the lower position and P connects to B, due to the closing function of the check valve, all oil flows to the motor. If the directional valve is in the upper position and P connects to A, part of the oil bypasses through the two-way flow valve and check valve, and the motor speed in this direction will be lower.
It is also installed at the return port of the directional valve, used as a back pressure valve, to ensure that the pressure in the system circuit never falls below a minimum pressure — back pressure. This can improve oil rigidity and enhance the smoothness of hydraulic cylinder motion when load fluctuates.
From a functional perspective, check valves with higher opening pressure can also be used as relief valves with low opening pressure. There is no essential difference in structure and function between the two. When pressure exceeds a certain value, the check valve opens the flow passage, so there is no essential difference from a sequence valve either.
3. Performance
An excellent check valve should generally have characteristics such as low forward flow resistance, rapid reverse closing, reliable sealing, and long service life.
Opening pressure. The value calculated based on spring preload and effective working area is only the pressure when the valve is about to open. The actual opening pressure generally refers to the pressure in the inlet chamber when the valve spool just opens. What does “just opens” mean? Hydac has provided a testable definition for its products — the pressure value at the inlet when the flow rate reaches 16.4mL/min. It is generally set between 0~2.5MPa.
Pressure drop – flow characteristics. During forward flow, because it is necessary to overcome the spring force and the friction between the valve spool and valve body, the pressure difference between the inlet and outlet is always higher than the opening pressure. If lower flow losses are desired, the spring should be as long and soft as possible. In this way, after opening, when the spring is compressed, the spring force will not increase too much, especially when used at high flow rates.
Internal leakage. Internal leakage refers to the leakage through the sealing surface between the valve spool and valve seat when reverse sealing should occur. For check valves, internal leakage should generally be as small as possible.
4. Some Variants
In addition to spherical valve spools, conical ones are used even more.
Cartridge type. Can be installed inside manifold blocks, with the valve body completely inside the block, with no part exposed outside the block. It is fixed only by its own threads or by other components such as pipe fittings, retaining rings, etc., and the depth is generally arbitrary.
From a structural perspective, there are those with flow entering from the end face and those with flow entering from the side. This can sometimes simplify the design of manifold block flow passages and reduce pressure drop.
With additional functions, such as the type with reverse relief valve, which can maintain pressure but will not cause overpressure due to, for example, thermal expansion of the oil. It can be used for, say, hydraulic cylinders of support legs of fixed platforms on mobile machinery.
Plastic valve spool. Bosch Rexroth launched the Z1S type check valve in 2010, stack type, nominal size 10, rated flow 100L/min, allowable pressure 350bar. The valve spool and spring seat are made of engineering plastic. Operating temperature: -20~+80℃. The cone angle of the valve spool is slightly smaller than the valve body, and it deforms under pressure, providing good sealing at 10% of rated pressure. Service life can reach 20 million cycles. Because the valve body is cast iron and harder than the valve spool, it will not wear. Even if the valve spool wears, it is easy to replace.
