SOLENOID ACTUATED CONTROL VALVES
Solenoid Actuator
In a solenoid actuator, an electro-magnetic field moves an armature which moves a push pin. The push pin moves the valve spool.
The two most popular solenoid actuators are the air gap and the wet armature.
Air Gap Solenoid
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| Illustration 1 | g01063222 |
An air gap solenoid is shown in illustration 1. When the coil is energized, an electro-magnetic field is created. Such a field develops whenever electricity flows through a wire. When the wire is straight, the field is relatively weak. When the wire is wound into a coil, the electro-magnetic field becomes much stronger. The field takes a circular shape around the coil. The higher the number of turns are in the coil, the stronger will be the field.
When the flow of electricity through the coil remains constant, the electro-magnetic field acts very much like the field of a permanent bar magnet. The electro-magnetic field attracts the armature. The armature moves a push pin and the push pin moves the valve spool in the control valve.
The air gap solenoid is protected by a cover. The air gap solenoid also has a manual override feature. The manual override allows the valve to be activated when the solenoid is disabled. A small metal pin is located in the cover. The pin is positioned directly in line with the armature. When the pin is pushed into the cover, the pin mechanically moves the armature. The armature moves the push pin which shifts the spool.
Wet Armature Solenoid
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| Illustration 2 | g01063224 |
The wet armature solenoid (Illustration 2) is a relatively new arrival on the electro-hydraulic scene.
The wet armature solenoid consist of a rectangular frame, coil, tube, armature, push pin, and manual override. The coil and the rectangular frame is encapsulated in plastic. The tube fits into a hole that runs through the coil center and two sides of the frame. The armature is housed within the tube and is bathed with hydraulic fluid from the directional valve. The hydraulic fluid is a better conductor of the electro-magnetic field than air. Therefore, the wet armature solenoid works with greater force than the air gap solenoid.
When the coil is energized, an electro-magnetic field is created. The electro-magnetic field moves the armature. The armature moves a push pin and the push pin moves the valve spool in the control valve.
In the wet armature solenoid, the manual override is located on the end of the tube. This tube houses the armature and the push pin. The manual override is used to check movement of the directional valve spool. If the solenoid fails because the spool is jammed, check the spool movement by pushing in the manual override. The manual override may also be used to cycle the actuator without energizing the complete electrical control system.
Solenoid Controlled, Spring Offset, Pilot Operated, Two-position, Four-way Directional Control Valve
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| Illustration 3 | g01063225 |
Illustration 3, shows a Solenoid Controlled, Spring Offset, Pilot Operated, Two-position, Four-way Directional Control Valve.
The solenoid controlled, spring offset, pilot operated, two-position, directional control valve is not frequently equipped with two solenoids. The second solenoid is considered an unnecessary expense and an additional solenoid to worry about in the system.
The solenoid is used to shift the pilot valve spool. The pilot valve spool is returned to its original position by a spring. When a system is designed for large oil flow, a large directional valve is required. A substantial force is needed to shift the large valve spool. The solenoid that is needed to generate that amount of force would be very large. In very large solenoid valves, a small solenoid controlled pilot valve is positioned on top of the larger main valve spool. When shifting is required, pressurized oil flows from the small solenoid controlled pilot valve to either side of the larger valve spool.
As shown in the deenergized position, pilot pump oil is being sent through pilot oil line (A) to the left of the main valve. The main valve is shifted to the right connecting the main valve pump oil to main valve line (B). Main line (A) and pilot line (B) are connected to tank lines (T) .
When the solenoid is energized, the pilot spool shifts to the right. Pilot pump oil is then connected to pilot line (B) and the right end of the main valve. The main valve shifts to the left connecting main pump oil to line (A) and line (B) to tank line (T) .
Pilot line (A) is also connected to tank line (T) .
Solenoid Controlled, Pilot Operated, Three-position, Four-way Directional Control Valve
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| Illustration 4 | g01063226 |
Illustration 4 shows a Solenoid Controlled, Pilot Operated, Three-position, Four-way Directional Control Valve.
The pilot valve is controlled by two solenoid valves. The pilot valve also has a spring that is located at both ends of the valve spool. When neither solenoid is energized, the pilot valve spool springs hold the pilot valve spool in the CENTER position. When the pilot valve is in the CENTER position, pilot oil flow to the larger control valve is blocked and both pilot lines are open to the tank. The springs in the three-position directional control valve return the control spool to the CENTER (HOLD) position.
Spring centering is the most common means of centering a directional control valve spool. The spring is located at each end of the valve spool. When pilot oil pressure is applied to either end of the directional valve spool, the valve spool moves and the valve spool compresses the spring on the opposite end. When the pilot oil pressure is removed, the spring returns the directional control spool to the center position.
Solenoid Failure
Most solenoid actuator failures occur when valves are stuck. The stuck valve spool prevents the armature from closing properly. The most likely cause of a stuck valve spool is contamination. Contaminants such as silt, metal chips, and other particles may become lodged between the spool and the bore. The contaminants will cause the spool to stick. Also, oxidized oil particles can create a gooey varnish which clogs the clearance between the spool and bore walls and cause the spool to stick to the bore.
Silt, metal chips, and other contaminating particles can be removed when a filter is installed. The varnish buildup can be removed by washing the valve in lacquer thinner. The proper oil change and filter change can help eliminate most of these type problems.
When the valve is stuck and the solenoid is energized, the solenoid coil receives a constant high flow of current that generates excessive heat. The solenoid is not designed to dissipate the excessive heat and the coil burns out. Overheating conditions most often occur during the periods of high ambient temperatures or system low voltage.
Problems with solenoid failure due to high ambient temperatures may be controlled by increasing the air flow across the solenoid. The temperature of the hydraulic oil can be lowered in order to allow more heat to be drawn from the solenoid through the hydraulic system. Sometimes, a different valve design may be required for operating during very hot weather. Some arrangement must be made to allow the system to operate at a lower temperature.
When the voltage to the coil is too low, the electro-magnetic field is not sufficiently strong to attract the armature. Just as when the spool is stuck, the current continues flowing through the coil. The constant flow of current generates the excessive heat.
Other factors also affect the proper operation and life expectancy of the solenoid actuator.
The solenoid actuator may fail in the following conditions:
- When cycled excessively
- When short circuited
- When operated with an incorrect electrical supply (wrong frequency, wrong voltage)
Spring Offset, Solenoid Controlled, Two-position, Four-way Pilot Valve
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| Illustration 5 | g01063227 |
In the ISO symbol in illustration 5, the spring offset pilot valve is shown in its normal position. The pump oil flows to (A) and the oil in (B) flows to the tank.
When the solenoid is energized, the solenoid moves the valve against the spring. The pump oil then flows to (B) and the oil in (A) flows to the tank.
Solenoid Controlled Pilot Operated, Spring-centered, Three-position, Four-way, Closed-center Control Valve
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| Illustration 6 | g01063228 |
In the ISO symbol in illustration 6, the solenoid controlled pilot operated, spring-centered, three-position, four-way, closed-center control valve is shown in the normal position. All four ways are blocked at the valve. When the solenoid on the right is energized pilot oil is directed to shift the spool to the left. The pump oil flows to (B ) and the oil in (A) flows to the tank. When the solenoid on the left is energized pilot oil is directed to shift the spool to the right. The pump oil flows to (A) and the oil in (B) flows to the tank.