How Do Hydraulic Spool Valves Work?

Hydraulic Spool Valve Basics

A hydraulic spool valve is a switching device used to control hydraulic devices. Spool valves are used in everything from automotive power-steering systems to cranes. A spool valve can turn the flow of hydraulic fluid from a hydraulic pump to an actuator on and off by blocking off the route the fluid takes.

Hydraulic Spool Valve Structure

A hydraulic spool valve is a cylinder inside a sealed case. It usually has valves leading to the pump and the tank on one side, and valves leading to one or more hydraulic devices on the other side. Pressure can flow into the valve from the pump into the hydraulic devices, or drain out of them back into a hydraulic storage tank. A controller moves the valve back and forth in its case to slide the spools into different positions. The position of the rotor will only allow the hydraulic fluid to flow in one direction to perform a specific task.

Moving a Hydraulic Piston

Hydraulic spool valves can be used in many different ways to perform many different functions. One of the most common uses is to drive a pressurized hydraulic piston. The piston is sealed in a cylinder with a valve on either end, both leading to the the spool valve. When the spool valve is set in the "push" position, fluid flows from the pump into the bottom of the cylinder while the fluid in the top of the cylinder is allowed to flow out into the hydraulic tank. This pushes the piston out. When the valve is set in the "pull" position, fluid flows into the top of the cylinder and out of the bottom, causing the piston to pull back into cylinder. Check out the link below to see a spool valve moving a piston.