Shut-off Valve

A check valve is a hydraulic component that completely blocks the flow of the operating medium in one direction and allows it to flow in the opposite direction. These valves usually function as seat valves, which means that they seal without leaking oil when closed. In industrial hydraulics, they secure loads against unintentional lowering and control complex movement sequences in machines.

Functional principle and design of check valves

Check valves typically use the force of a spring or the existing system pressure to press a closing element against a valve seat. Hardened balls or cones serve as closing elements, ensuring a metallic seal. As soon as the pressure in the direction of flow exceeds the spring force, the closing element lifts off the seat and opens the cross-section. In the opposite direction, the medium presses the element further into the seat, which increases the sealing effect.

In practice, a distinction is made between simple check valves and controlled variants. While the simple check valve reacts purely mechanically to pressure differences, unlockable models allow the locking state to be specifically influenced by an external control pressure. This flexibility makes them an indispensable component in modern hydraulic circuits.

The most important types of check valves

In hydraulics, there are various designs, each of which performs specific tasks within a system. The selection of the appropriate valve type depends largely on the required safety function and the circuit design.

Simple check valves

These valves represent the basic form. They allow the volume flow in only one direction. Engineers often use them to protect hydraulic pumps from pressure surges from the system or to provide hydraulic filters and coolers with a bypass. If the filter is dirty and the back pressure increases, the check valve opens and directs the oil past the filter to prevent system failure.

Unlockable check valves

A releasable check valve has an additional control piston. When pressure is applied to a control port, this piston mechanically pushes the closing element away from the seat. This allows the medium to flow back in the direction that is normally blocked. This function is essential for keeping hydraulic cylinders safe under load and retracting them in a controlled manner when necessary.

Changeover valves

The changeover valve has two inlets and one outlet. A freely movable ball automatically closes the inlet at which the lower pressure is applied. The higher pressure is switched through to the outlet. These valves are often used in control technology to process signals from two different sources without them influencing each other.

Twin check valves

These components, also known as lock blocks, combine two unlockable check valves in one housing. They secure double-acting cylinders in both directions of movement. As soon as pressure is applied to one side of the cylinder, this pressure simultaneously unlocks the opposite side so that the oil can flow out. If the pressure drops, both sides close immediately and fix the piston in its current position.

Technical data and selection criteria

When designing a lock valve, engineers must take various parameters into account to guarantee the durability and functional reliability of the system. The following table provides an overview of the relevant technical parameters.

Areas of application in B2B hydraulics

Lock valves are found in almost every hydraulic application where safety and precision are paramount. In mechanical engineering, they serve as load support valves. They prevent heavy press punches or tool slides from falling uncontrollably in the event of a hose rupture or pump failure.

In mobile hydraulics, such as cranes or work platforms, they secure the support cylinders. Here, it is crucial that the seat valves are completely leak-free, as even the smallest leaks over long periods of time would lead to a dangerous drop in the load. In the clamping hydraulics of machine tools, shut-off valves also maintain a constant clamping pressure, even when the main unit is switched off.

Maintenance and fault diagnosis for check valves

Although check valves are considered very robust due to their simple design, contamination in the hydraulic oil can impair their function. Particles between the closing element and the seat lead to internal leaks. The valve no longer “holds” properly.

Technicians usually check the function by measuring the pressure behind the locked valve. If the pressure drops quickly when the pump is stationary, this indicates wear on the valve seat or a broken spring. Regular oil analyses and the use of high-quality filters significantly minimize this risk. HK Hydraulik recommends that shut-off valves always be checked for leaks during system maintenance, as they perform a critical safety function.

Standards and symbols according to ISO 1219

The representation of check valves in circuit diagrams follows the international standard ISO 1219. A simple check valve is represented by a circle (the ball) lying at an angle (the seat). An arrow indicates the direction of flow. Unlockable valves are given an additional dashed line for the control oil connection.

Compliance with these standards is essential for documentation and maintenance in the B2B sector. It enables service technicians worldwide to immediately understand the logic of a hydraulic control system and efficiently locate sources of error. When designing new systems, the correct symbols form the basis for error-free production and commissioning.