Hydraulic engineering is a transmission type which is very important as alternative to pneumatic, electric and mechanical transmission types. The advantages of hydraulic engineering are well known:
- flexible connection between input and output
- step less adjustment possibility in both directions
- transmission of high forces and torques
- and many more
To realize the different hydraulic applications hydraulic circuits are necessary. Therefore, valves are used as main control modules. Either blocking of the different flow directions or limiting the maximum allowed pressure in the system, valves are fundamental components of hydraulic engineering.
Types of hydraulic valves
A check valve allows the medium flow in only one direction. The opposite direction is blocked with the help of different construction possibilities. In addition to hydraulic oil as main used fluid also systems with gases and other fluids are possible. With other fluids than hydraulic oil or high operating temperatures stainless steel valves are maybe necessary (in consideration of corrosion problems). To realize the closeness of the valves in blocking direction normally ball valves, membrane valves, flap valves or poppet valves are used. Often supported by a valve spring the check valve closeness without necessary oil pressure. The force choice of the spring also defines the opening pressure of the valve in flow direction. The dimensions of the valve define the flow rate. A popular method of construction are cartridge valves. There the valves are brought in a bore-hole. The assembly is realized with a screw-in, press-in or plug-in connection. Especially in the product range of screw-in check valves there are many different versions. Various thread versions and sizes in two different installation directions are possible. Normally the valve is sealed by an axial or radial mounted o-ring depending on the pressure of the system. For high pressure systems a special o-ring sealing are necessary. Other seal possibilities for example cutting edges are as well solutions. Depending on the system temperature special o-ring materials like FKM 80 are used.
Ball check valves are a special type of check valves. With a precise guiding of a ball and the functional stability even when the ball twists in the housing, the ball check valve is one of the most important check valves. Spring-loaded the ball pushes in the valve seat and blocks the flow in one direction. With a precise processing of the valve seat in proportion to the ball the valve gets hermetically leak proof. In flow direction first, the spring force has to be overcome with enough pressure to open the valve. The used medium now can flow, depending on the design type of the valve, axial through the valve or radial with a bore hole just after the valve seat. The hermetically leak proof of a ball check valve offers versatile applications. Valves which are usually designed for hydraulic systems also can be used under consideration of corrosion and temperature with other mediums. Even a usage in a pneumatic system is possible.
Pilot-Operated Check Valve
A pilot-operated check valve is a valve where the block direction can be manually unlocked with an additional control. Thus, with this valve a flow in both directions is possible. The precise leak proof in block direction and simultaneously the possibility to open the block direction are very popular. E.g. in clamping devices or lifting devices these valves are often used. There the components are hold in their final position until the unlocking of e.g. a used hydraulic cylinder is done with the help of the pilot-operated check valve.
Pressure Relief Valve
The pressure relief valve should be considered as an extension design of a check valve. Next to the necessary blocking of the flow in one direction the pressure relief valve also controls the flow in flow direction. This can be done e.g. with a spring. With the help of different spring forces, the opening pressure of the valve can be set exactly (pressure difference ∆p = p1 (valve entrance) – p2 (valve exit). Thus, the pressure relief valve offers to limit the maximum pressure of a system exactly to avoid system failures or consequential damages. In contrast to check valves where high flow rates are welcome, with pressure relief valves often low flow rates are enough to reduce the system pressure in a controlled way. There e.g. pressure relief valves equipped with throttle function (small bore holes) are often used.