Check valve and pilot operated check valve

- Jun 26, 2019-

The one-way valve includes a common one-way valve and a hydraulically controlled one-way valve. When not specified, the one-way valve refers to a common check valve. This article will introduce the structure, principle and application of check valves and pilot operated check valves. The following figure shows the common check valve and pilot operated check valve products.

check valve

1 check valve

The one-way valve is the simplest component of the structure and principle of the hydraulic system, but it can be indispensable in any system. Its role in the hydraulic system can be described as “simple and rough”.

(1) Structure and working principle of check valve

The principle of the check valve is to make the oil forward and reverse (similar to the diode in the circuit). According to the different valve core structure, the one-way valve can be divided into two types of structure: ball valve and cone valve. The ball valve has simple structure and convenient manufacture, but has no guiding property and poor sealing performance, and is rarely used. The process of the cone valve is strict, but the sealing performance is good. The one-way valve used in the engineering is mostly a cone valve structure. The one-way valve has two structures of straight-through and right-angle according to whether the direction of the inlet and outlet ports is the same. The following figure shows the schematic diagram of the cone-type straight-through check valve.

check valve-1

The valve is mainly composed of a valve core, a valve body, a valve seat, a spring and the like. When P1 enters the oil, the pressure oil pushes the valve core against the spring force to make the check valve conduct; when P2 enters the oil, the hydraulic pressure and the spring force press the valve core on the valve seat, and the oil is cut off at this time. . The straight-through check valve has a simple structure and a small volume, but is easy to have noise and vibration, and is inconvenient to replace the spring. The right-angled check valve in the figure below effectively reduces the vibration and makes it easier to replace the spring.

check valve-2

(2) Check valve products

In addition to tubular check valves, check valves have many structural forms, such as superimposed, cartridge, and plate. Here are some check valve products in different configurations.

The following picture shows the Rexroth M-SR cartridge-type check valve, which is used in the inserted valve block and has both straight-through and right-angle configurations.

check valve-3

The figure below shows the Z1S type superimposed check valve and symbol, which are superimposed on the valve block.

Z1S modular check valve

(3) Application of check valve

Generally, the requirements for the check valve are: the opening pressure is small, the reverse sealing is good, the forward flow resistance is small, and the action is sensitive. The check valve is used in the case where the oil is not allowed to flow backward: it is used for the outlet of the hydraulic pump to prevent the hydraulic pump from being damaged due to the excessive pressure of the system; the unnecessary connection of the oil passage is separated; and the good sealing is utilized. It realizes the pressure maintaining function; it is used with other components to form a composite valve, such as one-way throttle valve, one-way sequence valve, etc.; the spring with larger rigidity is used as the back pressure valve.

2 hydraulic control check valve

(1) Structure and working principle of hydraulic control check valve

The pilot operated check valve is a one-way valve that can reverse the flow of oil as needed. The figure below shows the principle of the pilot operated check valve.

pilot operated check valve

Unlike conventional check valves, pilot operated check valves add control pistons and control ports. When the control port has no pressure, it is the function of the ordinary check valve; when the control port passes the pressure oil, the push rod pushes the one-way valve core to realize the oil backflow.

(2) Hydraulic control check valve products

The following figure shows the schematic diagram and symbols of Rexroth SV and SL hydraulic control check valves. The valve adopts a structure with an unloading valve core to avoid excessive control pressure. The SL type hydraulic control check valve increases the external drain port and avoids the influence of the A chamber pressure oil on the control pressure. Please refer to the relevant sample for a detailed description of the valve.

pilot operated check valve-1

Two hydraulic control check valves can form a two-hydraulic check valve, also known as a hydraulic lock. Hydraulic locks are used in mechanical equipment for mechanisms that require locking. The picture below shows the Rexroth Z2S hydraulic lock, which is a modular structure.

modular check valve

(3) Application of hydraulic control check valve 

The hydraulic control check valve is widely used in hydraulic systems, mainly utilizing the good sealing performance of the cone valve core. The figure below shows the locking circuit of the hydraulic lock. The positioning accuracy of the cylinder is only related to the internal leakage of the cylinder, and is not affected by the leakage of the system. In order to reliably close the spool of the hydraulic lock when the lock is required, the center of the reversing valve should be such that both the A and B ports are open to T. Usually the center uses the "Y" or "H" function, and the hydraulic and reversing valves It is not advisable to add other components, such as throttle valves.

pilot operated check valve-2

In addition to the reliable positioning of the cylinder, the pilot operated check valve has many other applications. For example, when the weight is prevented from falling, it is used in conjunction with the accumulator in the case where pressure is required, and when the cylinder requires a large amount of oil, it is used as a liquid filling valve.