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Taking The Mystery Out Of Hydraulic Valves - Spool-Type Hydraulic Directional Valves

Spool-Type Hydraulic Directional Valves

At the most fundamental level, there are two kinds of hydraulic directional valves, those that control hydraulic fluid flow by moving a spool within a bore to selectively connect different ports, and those that control hydraulic fluid flow with a poppet or poppets. Part 1 of this series discusses spool-type hydraulic valves. Don't miss Part 2 of the series, which discusses poppet-type valves.
Taking The Mystery Out Of Hydraulic Valves

The simplest possible spool-type hydraulic valve has an inlet and an outlet port and a cylindrical, grooved, linearly movable spool within the bore to control the flow between them. The spool is usually biased to one position by a spring, and displaced to the other position by a lever or actuator of some kind.

When the hydraulic valve is actuated, the spool is moved, compressing the spring. Depending on body design and initial spool position, either the grooves open a path between ports in the valve body, or the raised sections between the grooves close the ports. When the lever is released, the spring forces the spool back into the initial position.

This type of hydraulic valve would typically be used to turn a system device such as a uni-directional hydraulic motor, on and off. It could also be used to connect or disconnect supply pressure to a portion of the hydraulic system.

Increasing the number of ports from two to four increases the hydraulic valve’s control capabilities. A four-port hydraulic valve, for example, can automatically apply pressure to one side of a cylinder while simultaneously opening a drain path back to the reservoir on the other side, or control rotation of a bi-directional hydraulic motor that does not require an unpowered position.

A double acting hydraulic cylinder normally requires a three position spool valve to provide the ability to hold it stationary as well as to control its direction and speed. There are three common configurations:

  • A closed center circuit design in which the inlet pressure region of the valve spool blocks pump flow in neutral and directs it to the appropriate work port when the spool is displaced. 
  • An open center circuit allows the use of an economical fixed displacement pump, but it means that the control valve must also incorporate additional metering edges to dump the pump flow to tank at low pressure drop in neutral and restrict the pump’s flow path to the reservoir when the spool is displaced to build enough pressure to move the cylinder.
  • A load sensing circuit adds the requirement to drain a signal port in the neutral position and connect it to load pressure as the spool is displaced.

The fundamental spool valve operating principle can be extended almost indefinitely by adding ports, orifices, and grooves to the hydraulic valve. But, regardless of how complex the hydraulic valve becomes the basic operation remains the same.

Don't forget to read Part 2 of the series, which discusses poppet-type valves.

Managing Editor: Doug Drummond
Story Filed: 11/29/2010

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