There are two primary methods used within hydraulic valves. Some control hydraulic fluid flow by using a spool, bore and different ports, while others use one or more poppet valves. Both of these types of hydraulic valves are designed to control or change the direction of hydraulic fluid flow.
There are a number of different hydraulic valves that use a spool and bore to control flow. These valves also have two or four ports to facilitate the movement of the hydraulic fluid. When these types of valves are activated, the spool compresses an internal spring. This opens up a pathway between the ports. When the hydraulic valve is deactivated, the spring decompresses, forcing the spool back into its original position and closing the pathways again.
There are also hydraulic poppet valves, which perform the same function in a different way. In this model, the flow of hydraulic fluid is controlled by the opening or closing of the valve's poppet. The poppet is held shut by the force of a spring or sheer pressure. When an external force, such as pressure, venting or mechanical energy, is applied to the poppet, it opens and allows the hydraulic fluid to flow. When this force is removed, the poppet closes again. Poppet valves can be modified so that their default position is either open or closed.