Page 11 - Peter Paul 2019 Catalog

VALVE ACTUATORS
Valve actuators are the parts that apply force to shift a valve’s flow directing elements, such as a poppet or plunger. The speed and frequency of valve shifting are key factors in fluid power system performance. As long as the actuator produces enough force to shift the valve, the system designer can select any appropriate actuator for the conditions and type of con- trol under which the system will operate.
Actuators for directional-control valves are either mechanical, pilot, electrical, or a combination of these. Different types
of actuators can be installed on the same basic valve design. A common valve body often is used that accepts a variety of different actuators.
With a mechanical actuator, a machine element or person pushes, pulls, or turns the valve’s flow-directing element to shift it to another position. Manual actuators include levers, palm buttons, push buttons, pedals, cams, rollers, stems, and screws.
Springs are used in most valves to hold the flow-directing element in an un-energized position. A spring holds the non- actuated valve in one position until an actuating force great enough to compress the spring shifts the valve either open or closed. When the actuating force is removed, the spring returns the valve to its original position.
Pilot-actuated valves are shifted by pressurized fluid (air or oil) that applies force to a piston that shifts the valve’s flow- directing element. Pilot-actuated valves can be mounted in any convenient or remote location to which pressurized fluid
A 2-way normally closed valve, top, is held closed by a spring, and opens when its solenoid is energized. A 2-way normally open valve, bottom, is held open by a spring an blocks flow when its solenoid is energized.
Single-acting air cylinders are a common application for 3-way valves.
In the example above, left, a normally open valve routes air to the cylinder to extend its piston. Energizing the solenoid shifts the valve and allows the cylinder spring to push air out of the cylinder through the valve.
At right, a normally closed valve blocks compressed air flow to the cylinder, so the cylinder’s spring keeps the piston retracted. Energizing the valve’s solenoid shifts the valve to route compressed air into the cylinder, extending its piston.
can be piped. The absence of sparks and heat buildup makes pilot-actuated valves attractive for applications in flammable or explosive environments.
However, solenoids are the most common method of actuation. Simply put, a solenoid is a coil of wire wrapped around a moveable metal core with a small space between them.
Running an electric current through the coil creates a magnet- ic field that moves the metal core. The core is attached to the valve’s flow directing element, so the valve shifts as a result of the electrical power. When the current is cut off, a spring returns the valve to its un-energized state.
Solenoid valves are especially popular for industrial machines because of the wide availability of electric power in facilities. However, mobile equipment makes extensive use of solenoid operated valves as well. Solenoids operate from either AC or DC power, and the selection of AC or DC depends on the form of electrical power available.
There is a practical limit to the force that solenoids can generate. This means they cannot directly operate valves requiring high shifting forces. Furthermore, valves using large solenoids can require substantial electrical power when valves must remain actuated for long intervals. Heat buildup can also pose problems in these situations. The solution is to use small, low-power solenoids in combination with pilot pressure. The solenoid starts and stops pilot flow, and pilot pressure provides the high force to shift the valve’s flow-directing mechanism.
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