Hydraulic logic valves can be arranged in a wide variety of combinations. This allows a great deal of flexibility with circuit layouts and the possibility of intricate functional design. Another reason logic valves are used is that they are capable of much higher flow ratings that standard directional valves. They can replicate many standard hydraulic valves functions and can provide virtually leak-free sealing of key ports.
The disadvantage with using logic valves is that they are big and expensive in comparison with standard spool valves and leak free design, that traps in pressure is not always a benefit and can create significant problems.
This is an example of a 4 way - 2 position directional valve using logic elements. A small, pilot supply spool valve is used to switch all four logic valves at the same time and controlling the flow into and out of the cylinder.
This example shows an isolating slice that is used to isolate the flow to a directional valve, typically for safety or emergency operations. Isolating slices often need to be leak free and have low internal pressure drops, so that they do not restrict the normal circuit operation. Logic valves are well suited to this and similar applications.
In this example, we see a freewheeling or crossover valve. This valve layout is used to release the actuator and let it float to whatever position the load requires. Typical applications may be lock gates that move with the water flow or wheel motors during over-speed or towing etc.
This example shows a regeneration circuit which is used to feed the flow out of the annulus side of a cylinder, back into the bore side. Regeneration circuits are used to achieve higher cylinder speeds with lower flows. The force from the bore side of the cylinder is always higher than the annulus side and in regeneration, it will be lower than with full-bore only, but should be similar to the annulus side force, which can be another advantage. When retracting the valves allow the cylinder to operate normally.
In some situations pressure relieving, reducing valves can be used to provide directional control. If you can regulate the pilot pressure signal then a simple proportional control system is possible.
Pilot operated pressure relief valves can also be used to provide a simple directional control system. Pilot response rates can also be controlled to facilitate ramped acceleration and deceleration.