Directional valves are used to control the movement of hydraulic actuators, primarily cylinders or motors. In there simplest form this may be a single on/off switch, opening and closing the hydraulic flow to rest of the system.
Probably the most widely used format is the 4 way industrial valve. These have a pressure and return line and switch the flow between an A and B port. Allowing us to extend and retract a cylinder for example.
There are many different switching configurations of directional valves. These are arranged to control all type of hydraulic switching requirements.
Directional valves direct the hydraulic flow along different pipe lines depending on the valve's position.
Look at the different directional valve symbols to see which ports they switch from, to or between.
Valves are typically two way (P to A), three way (P to A then P to B) or (P to A then A to T) or four way (P, T, A, B connections)
Valves can also have either two or three positions and are switched between the different positions by either, hand, electrical solenoid, air or hydraulic pilot.
Some valves have springs to return them to their fall-back condition, some have detents that hold them in position until they are reset.
The force required to switch the larger valve spools is generally higher than can be achieved by electrical solenoids so often have a smaller hydraulic pilot valve to switch them or internal pilot supply.
There are three main types of directional valves.
Cartridge directional valves
Industrial CETOP platen mounted directional valves
Mobile mono-block slice directional valves
Directional valves can be operated by hand, electrical, pneumatic, mechanical roller or hydraulically.
Select your directional valve based on your application. Mostly it's clear whether it is industrial or mobile etc. but if not, there is probably some similar equipment you can replicate. The main factors will probably be the environment, duty, maintainability, weight or cost. If you are building one machine and you want it to work first time then it's probably a CETOP platen mounted valve, but if you have time for some development then you can probably get a smaller and cheaper design from cartridge technology.
Look at the component's datasheet to select a valve with the necessary flow rate. Make sure you know how much load your actuator needs under the worse conditions and what pressure loss you will get from the rest of the system. Then select a valve with a low enough pressure drop for your supply pressure.
Check the valves' switching limit. Some valves can lock if you try to put too much flow through them
Select an appropriate spool centre position. Consider what you want to happen to the load in the standby condition. Do you want its movement to be restricted with an A, B closed spool or free to move with an A, B to Tank spool etc.
Finally consider the switching conditions. Some datasheet show the spool crossover arrangement as it switches between each position. This can be important if you don't want to lose control of your load as you switch. Also if you have a fixed displacement pump and closed centre spool then you may get excessive pressure peaks as it switches.
Remember you will always get leakage between the port lands on a spool valve. Some valves more than others. Make sure you cleanly vent unwanted trapped in pressure away. Also be careful that your load does not move due to the leakage when the system is turned off. Although be thankful that spool leakage removes the risk of trapped in pressure exceeding the maximum equipment pressure due to thermal changes. You get a lot less trouble with spool valves than poppet valves.
Be careful that the valve does not need a low tank return line pressure on one port. Pressurising the end caps on some valves is not permitted and risks blowing the ends of the valve off.
Make sure you have a stable reference pressure for switching. Some valves may switch at different pressure signals or not switch at all.
Design calcs TBC