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Linear
Solenoids Technical Information
Solenoids
convert electrical energy into force and motion. When the
coil is energized with electric current an electro-magnetic force
is created around the coil. Enclosed solenoids such as the Lisk tubular
solenoids are designed to direct that magnetic force through the steel
housing and into the stop and armature (plunger). The stop becomes
a north or south pole face depending on the polarity of the coil.
The armature becomes the opposite pole face. These opposite poles
are attracted to one another and this creates the force and motion
in the armature.
The
amount of force
created is related to the amount of electrical current applied. Other
factors such as the number of turns of wire in the coil, the size
of the solenoid, and the magnetic character of the steel used will
affect the amount of force developed.
The
force is also dependent on the air gap or stroke of the
solenoid. The force is lowest at the maximum air gap and highest when
the pole faces are fully seated. In general the force is inversely
proportional to the square of the distance (gap) between pole faces.
Cross
Section of Solenoids
1. Lead Wires - 600 volt Teflon insulated 392ºF (200ºC) rating.
2. Lead Wire Exit
- Standard exit is from end opposite the working end.
3. Stroke -
The distance the plunger will move when energized.
4. Tape Insulation -
Mylar tape insulates outer coil windings from metal housing and also
holds coil wires in place.
5. Threaded Plunger
Connection -
other options also available.
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6. Plunger -
High quality, magnetic steel, nickel plated.
7. Boss
- Additional end plate material provides improved magnetic flux path
as well as increased plunger support.
8. Crimped Housing Joint
- Tight metal-to-metal
joint.
9. Tubular Housing -
High quality magnetic steel with corrosion protected surface not only
completely protects coil but also serves as magnetic flux path for
maximum magnetic efficiency in smallest possible package. Open frame
"C" and "D" units do not offer this protection
nor the magnetic efficiency.
10. Molded Bobbin -
Glass filled molded nylon bobbin, or equivalent.
11. Magnet Wire -
Insulated wire, 392ºF (200ºC) rating, automatically dry wound directly
on bobbin.
12. Stop -
High quality magnetic steel, corrosion protected surface.
No Mount Style -
All standard mounts are dimensioned from or located at the working
end.
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1. End Piece -
High quality magnetic steel, corrosion protected surface.
2. Plunger External
- Face - Plain flat face, standard pull connections available.
3. Stroke - Distance
plunger will move when energized.
4. Plunger Bore Sleeve
- Corrosion resistant, non-magnetic metal sleeve.
5. Panel Nut Mount -
Other mounts are also available.
6. Stop - High quality
magnetic steel, corrosion protected surface.
7. Push Rod - Corrosion
resistant, non-magnetic metal.
General Design Considerations
Before
deciding on a solenoid for a particular application, there are a number
of points that should be considered to assure that the smallest, most
efficient, and proper type is selected. These points all influence
the proper selection and some thought should be given to each.
Pull / Push / Hold
The basic function of the solenoid
is to perform:
Pull Force - When energized, the plunger
retracts pulling the load. De-energized, the plunger is extended to
specified stroke distance.
Push Force - When energized, the push
rod extends pushing the load. A push solenoid is actually a pull solenoid
with the addition of a push rod that rides against the internal face
of the plunger extending through the fixed pole piece.
Hold Force - Plunger is pulled in either
electrically or by external force and contacts the opposite pole piece.
With power applied, the plunger remains fixed resisting the external
pull or push load.
Combination - Two or all three of these
functions can be combined into one solenoid.
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Stroke
The total movement expected
of the solenoid when electrical power is applied. It is also often
defined as the air gap between moveable plunger and the fixed pole
piece in the de-energized condition. For greatest efficiency and smallest
size, design for the shortest stroke possible. When a given amount
of "work" is required, try to use a short stroke / high
force combination rather than longer stroke and lower force.
Force
The load the solenoid is capable
of pulling, pushing, or holding at the start of specified stroke when
energized under a specific set of conditions: of voltage, temperature,
and duty cycle. As the force / stroke charts indicate, the force rapidly
increases as the pull or push stroke length decreases. This again
points up the importance of designing for the shortest possible stroke.
Keep in mind that the solenoid force will decrease as coil temperature
increases. For any given stroke, the force will reduce to about 65%
of the chart values when the coil reaches the 350° F (177°
C) stabilized temperature.
Pole Face Configuration
Flat pole face (S-short
stroke) - The plunger face (internal end) and face of the fixed pole
piece are flat. A flat face construction should be selected for relatively
short strokes or where maximum hold forces are required.
Conical Pole faces (L-long stroke)
- Plunger face and face of fixed pole piece are matching 60°
concave and convex cones. For relatively long strokes with lower holding
forces, a conical pole face version should be selected.
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Duty Cycle
A comparison of the time a solenoid is
energized to the time it is de-energized, expressed as a percentage:
Duty Cycle % = [(ON time) / (ON
time + OFF time)] x 100.
For a given force / stroke requirement,
the smaller the duty cycle percentage, the smaller the solenoid.
Continuous duty (C) - solenoid
On continuously without interruption for periods of about 30 minutes
or longer.
With no OFF time to allow the coil to
cool, continuous duty solenoids must have coils wound to limit the
current drain and must be large enough to provide for adequate heat
radiation to prevent coil burn-out. This means that a continuous duty
solenoid will be considerably larger than an intermittent design to
produce the same force / stroke.
One way to get around the continuous
duty unit size problem is to go to a two-coil work / hold design.
Intermittent Duty (I) - Solenoid
ON for only a short time, usually not more than 2 or 3 minutes, then
followed by an OFF time which is normally at least as long as the
ON time. This ON time and OFF cycling can be repeated continuously
over total life of the solenoid. As the ON time decreases and the
OFF time increases, more current can be applied to the solenoid without
causing coil overheating. By being able to use larger current drain
(increased power), a smaller size solenoid can be selected to produce
the same force / stroke output.
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Pulse Duty (P) - An intermittent
duty unit with extremely short ON time. A duty cycle normally in the
10% to 25% range. This is maximum input power for that size solenoid.
Special Power (X) - Applications which
require different power levels than those shown for standard C, I,
or P duty cycles. This covers applications at ambient temperature
significantly above or below 76° F (25° C), or a desired
current drain, below standard value, or a special duty cycle, or other
design requirement affecting power.
All coils for our standard solenoids
are wound to order on demand. Therefore it is no problem to accommodate
a special winding for "in between" duty cycles or special
voltages or temperature conditions in our standard solenoids.
Power / Voltage / Resistance
All units are rated on the basis of maximum
allowable input wattage the solenoid can draw without exceeding the
350° F (177° C) stabilized coil temperature when operated
at its rated duty cycle in a 76° F (25° C) ambient atmosphere.
So that the solenoid operates at its
designed power level; coils are wound to various resistance values
depending on specified voltage and duty cycle.
Coils can be wound for any DC voltage.
Those most commonly specified are 6, 12, 24, 28, and 120 VDC. For
the very small size units operating at the higher voltage levels,
special coils may be required.
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Nominal Coil Resistance
The nominal coil resistance at 76°
F (25° C) can be determined as follows:
Resistance = Voltage2
(as specified) / Wattage (as specified)
In operation, the coil resistance will
increase due to the heating of the coil wire.
Current Draw
For any catalog item, the current draw
at 76° F (25° C) can be determined as follows:
Current = Wattage / Voltage.
Coil Temperature
The actual temperature of wire in the
coil winding. It is the combination of ambient coil temperature plus
heating due to current flow through the coil.
Ambient Temperature - The stable
coil wire temperature with no electrical power to the solenoid.
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Heat Rise - The stable increase
in coil wire temperature during solenoid operation at rated voltage
and duty cycle in designed ambient conditions. Standard units are
designed for a 274° F (134° C) maximum rise in a constant
76° F (25° C) ambient environment.
Stabilized Temperature - The final
stable temperature the coil wire reaches during operation - ambient
temperature plus coil heat rise. Standard units are designed for 350°
F (177° C) maximum stabilized temperature.
Heat Sink
- It is important to consider the size of the heat sink when selecting
the solenoid size and applicable wattage.
Force vs. Coil Temperature
For any given stroke, solenoid force
decreases with coil temperature increase and, conversely, increases
when coil temperature is lowered. This is the result of changes in
coil wire resistance with changes in wire temperature. The higher
the wire temperature, the higher the resistance will be. For estimating
purposes, for every 7° F (3.9° C) change in coil temperature
from ambient output force changes about 1%. A solenoid operated in
a 76° F (25° C) environment at rated voltage and duty cycle
long enough for the coil to reach the 350° F (177° C) rated
stabilized temperature will have an output force reduction of approximately
65%.
Loose or Fixed Push Rod
Standard construction for the high performance
solenoids provides options for both loose and fixed push rods on push
style units. Standard heavy-duty construction provides only a loose
push rod. Heavy-duty designs that require a push rod attached to the
plunger can be easily accommodated upon special request.
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Removable / retained plunger / push
rod
With standard construction the plunger
is removable in all styles except the heavy-duty push type. On push
units the push rod is also removable, either as a separate piece or
attached to the plunger, depending upon the option selected.
Either the plunger, or the push rod,
or both can be configured to be retained in the solenoid if required.
Return Springs
Return springs can be designed into standard
units.
Environmental Protection
Standard units are designed to meet normal
environmental and operating conditions encountered in conventional
industrial applications.
Humidity / water splash - Coil area is
protected and exposed surfaces plated to withstand occasional water
splash and normal in-plant ambient humidity conditions. For continuous
high humidity exposure or water immersion, special encapsulated or
molded coil constructions are available. For extreme environmental
conditions, hermetically sealed designs are also available.
Sand / Dust / Dirt - Under normal in-plant
ambient conditions, standard designs should perform satisfactorily
over expected life. Unusual conditions of airborne contamination may
require protective boots to seal off the plunger cavity. Additional
protection of exposed surfaces may be required. Special sand and dust
sealed designs are available.
Temperature - Since solenoid output force
will continue to improve (though current draw will increase) with
ambient temperature decrease, operation of standard designs in an
ambient temperature as low as -65° F (-53° C) presents no
problem. It is when solenoids will be operated in an ambient environment
above 76° F (25° C) that some caution must be taken. If
the temperature is significantly above 76° F (25° C), coil
burning may occur even if solenoid is operated at its rated duty cycle
and voltage. Special high temperature coil designs are available.
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Potted Coil
A common option for extended environmental
protection is a Potted or encapsulated coil. The usual procedure is
to flood the gap between the coil and the housing with epoxy. Another
less costly encapsulation that would be applicable to high volume
applications would be overmolding the coil.
Potting or overmolding help in making
the coil humidity / splash resistant in applications where this is
common. If more severe conditions exist further sealing of the coil
cavity can be done with o-rings and special connectors. The coil will
also be very shock and vibration resistant.
Another benefit potting / overmolding
provides is the ability to conduct the heat, generated when operating,
more efficiently to the housing where it can be dissipated easily.
This will allow more power to be applied to a given size solenoid.
Service cycle life
Standard construction nominally rated
for 1,000,000 cycles. In actual service, cycle life exceeding this
figure is constantly being experienced. Periodic cleaning and lubrication
will help in extending life. Severe operating conditions - a heavy
side load on the plunger, for example may shorten cycle life. Since
many factors other than the solenoid construction itself have their
effect, the rated life expectancy is valid only for the laboratory
conditions under which life tests were run. Special long life designs
are available.
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Response Time
Response time, usually measured in milliseconds,
is from the point at which power is applied, until the plunger reaches
the end of its design stroke. Two main factors contribute to the overall
response time. One is time it takes the current to overcome coil inductance
and develop the required magnetic flux field. The other is the time
it takes for the plunger to actually travel the stroke distance. Flux
build-up normally takes more than half of the total response time.
Generally speaking, the response time varies between 5 milliseconds
for small size units at short strokes up to 250 milliseconds for long
stroke, larger sizes.
To achieve faster response times the
solenoids must be overpowered. A solenoid size must be selected which
will produce a force at the start of the required stroke several times
as large as would be needed under normal speed operation. Stroke should
be as short as possible to keep plunger travel time at minimum. Special
high-speed designs are available.
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