By Josh Cosford, Contributing Editor
Solenoid valves are electrically activated valves, typically used to control the flow or direction of air or liquid in fluid power systems. Used in both pneumatic and hydraulic fluid power functions, the spool or poppet design of most solenoid valves makes them perfect for various functions and applications.
The spool or poppet of the valve connects to a ferrous metal plunger, which is typically spring centered or spring offset, but may be detented instead. The plunger slides within a core tube of non-ferrous metal, itself surrounded by a coil of electrical windings. The coil exists with any range of voltage from 12-48 Vdc to 110-220 Vac. When power is sent through the coil, a magnetic field is created, which pushes or pulls the plunger, shifting the valve.
The most basic solenoid valves are two-way, two-position poppet valves, which simply open and close, modifying their flow path when their coil is energized. They are available as “normally-open” and “normally-closed” versions, which means normally-flowing and normally-blocked, respectively. Normally-open in fluid power contradicts normally-open in electronics, which stands for the switch or contact open and not flowing electrons.
Three-way, two position poppet valves are also common, diverting flow from one channel to another. Two 3/2 valves in parallel can be used to control a cylinder bidirectionally. Although construction varies depending on the use, this type of valve can be used for either pneumatics or hydraulics, but is more common to pneumatic systems.
Spool solenoid valves consist of a machined spool which slides within a machined valve body. One or both ends of the spool are acted upon by a plunger, and when activated by either coil, pushes the spool one way or the other, allowing three positional envelopes. The 4/3 hydraulic solenoid valve is one of the most popular, allowing for bidirectional control of a cylinder or motor from a single valve body. The “ways” of a solenoid valve refer to how many ports it contains, and the “positions” of a solenoid valve refer to how many discrete states in which it operates. A three-position valve employs a spring centered neutral state along with two actuated positions.
For bidirectional motor or cylinder control, pneumatic valves are machined with five ports, and are referred to as 5/3 valves as common practice. The “ways” of a pneumatic valve also include its exhaust ports, to which there are usually two. Occasionally these same valves are described as 4-way, 3-position valves, even though close inspection reveals the two exhaust ports bisecting the pressure port.
Solenoid valves for either hydraulic or pneumatic applications are available as manifold-mounted modular units, such as the pneumatic or hydraulic ISO valves. These valves contain standard mounting and porting patterns, permitting valves from any manufacturer to be installed upon the same manifold. Most often, these valves are also quite economical, and readily available “off-the-shelf.”
The electrical coils of a solenoid valve are optioned with either DIN connectors, lead wires, Deutsch connectors, central connection or any other popular form of electrical connection used in fluid power and automation. Most solenoid valve coils are field replaceable, making repair and maintenance easy for technicians. Coils also have a wide range of application and purpose. Some are intended for the industrial environment, where atmospheric conditions are consistent. Mobile environments are much more demanding and command coils to handle both extreme temperature ranges and exposure to road film and salt, for example.