Hydraulics > Control Valves

Control valves fall into two main groups: spool and rotary.  

To deal with rotary valves first, these are manually operated valves and are un-balanced, what this means is that the pressure within the system is acting on the controlling surfaces so more physical effort may be required to operate.  We generally only use this type of valve in simple low cost single function systems.

Spool type control valves are available with all manor of control: electric, pneumatic, hydraulic or manual. The basic principle that they all operate on is the same, these are balanced valves, the hydraulic pressure in the system does not affect the force required to move the spool and re-direct the oil flow.

In the example below we have a representation of a spool valve in its neutral position showing its outlet, tank and pressure ports blocked.

Should you pull on the black handle then the pressure port (red)will be opened to A And the return oil (blue) to B, push the handle and the opposite is so.

The reason that these are called balanced spool valves is that the hydraulic pressure is acting on the same surface area in both directions, so one is cancelling the other out, if the spools were as the diagram below this would not be the case.

Counter balance valve

The valve shown below is a counterbalance valve; they do not allow oil to return from a function unless there is oil being supplied to the said function. V1 and V2 are connected to the control valve and C1 C2 are connected to the function. C2 will not allow oil back to the control valve unless the pressure in V1/C1 is high enough to push the pale purple piston so as to open the dark purple valve; if the function tries to run away with itself then the pressure in C1/V1 will decrease and the spring will force the valve back into its seat. Should you put pressure into V2 then the yellow ball we be pushed against its spring and allow the oil straight through to C2. Over centre valves are available in double configuration if required

If an over centre valve was fitted to the example below with A connected to C1 and B connected to C2, the weight would only be able to drop at a rate controlled by the amount of oil being passed through V1, irrespective of load.