CLIFTRONICS, INC.
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SOLENOID DESIGN CONSIDERATIONS

The following solenoid design considerations will need to be evaluated when you discuss your application with us.
The initial information you will need for selecting a solenoid is a reasonable approximation of your required stroke (or rotation), force (or torque), and duty cycle. Cliftronics engineers will help you fine tune these requirements as the design develops and prototypes are tested.

1. STANDARD OPERATING CONDITIONS

For uniformity of data, the standard operating condition for solenoids in our catalog is defined as a 20°C ambient temperature with the solenoid mounted on the standard heat sink which is listed for each solenoid size in our catalog.

2. STROKE OR ROTATION

The stroke (linear solenoids) or rotation (rotary solenoids) is the total armature travel or rotary shaft motion expected when electrical power is applied to the solenoid. For greatest efficiency and smallest size, you should design for the shortest stroke or rotation possible.
The unit value of linear stroke is inches and the unit value of rotary stroke is degrees of rotation.

3. FORCE OR TORQUE

The force (linear solenoids) or torque (rotary solenoids) is the load that the solenoid is capable of pulling, pushing, holding or rotating, at the start of a specified motion, when energized under a specific set of operating conditions (input voltage, ambient temperature and duty cycle).
The unit value of force is pounds (lbs.) and the unit value of torque is inch-pounds (in.lbs.). Note: One pound of force applied on a one-inch radius arm is one in.lb. of torque.

4. FORCE OR TORQUE VS. TEMPERATURE

For a specific solenoid stroke or rotation, the solenoid force or torque will decrease as the solenoid coil temperature increases. Conversely, the solenoid force or torque will increase as the coil temperature is lowered. This is a result of proportional changes in the coil wire resistance with changes in wire temperature (see 16: COIL TEMPERATURE AND PERFORMANCE / RESISTANCE FACTOR). The higher the wire temperature, the higher the coil resistance will be. Since essentially all power input to the coil is converted to heat, a more efficient heat sink, greater airflow and a cooler environment will cause the solenoid to produce more force or torque.
The force and torque values shown in our catalog are the results of actual tests conducted with the solenoid coil temperature at room ambient. In actual service, the solenoid forces and torques will vary from the catalog values depending on actual coil temperatures. In your preliminary solenoid selection, we recommend allowing a 50% safety margin in your force or torque calculations to compensate for this coil heating.

5. COIL TEMPERATURE RATINGS

The standard coils listed in our catalog are designed to achieve a stabilized coil temperature of 125°C when operated at the rated power and duty cycle, under Standard Operating Conditions. We design our coils in this manner to allow moderate temperature increases in your application environment without exceeding the 175°C rating of the standard coil insulation components. In higher temperature applications, custom coil windings can be designed which utilize higher temperature wire and insulation systems.
To insure that the coil selected for your application does not overheat, our engineers will calculate the expected maximum stabilized coil temperature for your application if you provide us with the power input, duty cycle, ambient temperature and heat sink information. These calculations will also help you confirm that you are not under powering the solenoid in your application.

6. MAXIMUM STABILIZED COIL TEMPERATURE

This is the temperature at which a solenoid coil will stabilize when energized with the maximum power input and subjected to the maximum ambient temperature of the application. Cliftronics standard coils are designed to withstand a 175°C maximum stabilized coil temperature (abbreviated MSCT).

7. HEAT SINK

The standard heat sink is designed to be approximately twenty times the surface area of the solenoid. It stabilizes the solenoid temperature by dissipating the coil-generated heat at the same rate as power is being applied to the solenoid coil. While your application heat sink (mounting bracket or flange) may be a different shape than the standard recommended heat sink, the overall surface area should be approximately the same. Use of smaller than recommended heat sinks will cause the stabilized coil temperature to increase unless compensation is made by reducing the power input, reducing the "ON" time for each cycle, increasing the "OFF" time between cycles, increasing the air flow around the solenoid, or by reducing the ambient temperature.

8. COIL HEAT DISSIPATION

Cliftronics standard coil/housing construction allows effective transfer of heat from the coil winding through the iron housing to the heat sink. To reduce stabilized coil temperatures, or to allow increased power input to the coil, thermal transfer between the coil and housing can be further improved by utilizing epoxy potted or impregnated coils. Coil potting also serves to improve environmental properties such as shock and vibration resistance.

9. DUTY CYCLE

The timing of a solenoid cycle or duty cycle is normally defined by the application. It is the percentage of coil "ON" time to the total actuation cycle time as shown in the following equation:

10. CONTINUOUS DUTY

Cliftronics standard solenoids are designed to achieve a stabilized coil temperature of 125°C when operated continuously at the listed continuous duty rated power input under standard operating conditions. Our standard unit coils are designed in this manner to allow moderate temperature increases in your application environment without exceeding the 175°C rating of the standard coil insulation components.

11. INTERMITTENT DUTY

Higher forces than available at continuous duty operation can be achieved by applying higher power levels to the solenoid. However, increasing the power input will result in higher coil temperatures, which will exceed the coil temperature rating unless the power is applied intermittently. The maximum "ON" time must be controlled and must be short compared to the "OFF" time between power applications, allowing the solenoid coil sufficient time to cool between actuation's.
Cliftronics standard solenoids are designed to achieve a stabilized coil temperature of 125°C when cycled at the intermittent rated power input, observing both the maximum "ON" time listed in each data table and the standard operating conditions (20°C ambient temperature and standard heat sink). The percent duty cycle must be maintained ("ON" time vs total cycle time) to insure that the maximum design coil temperature is not exceeded.

COIL WINDING DESIGN

12. OHM'S LAW

Voltage, current, and coil resistance relationships are defined by Ohm's Law as follows:

Voltage (E) = Current (I) x Resistance (R)

Other helpful relationships are shown in this chart:

VOLTAGE = E (Electromotive Force); Volts

CURRENT = I; Amperes

POWER = W; Watts

RESISTANCE = R; Ohms

13. VOLTAGE / RESISTANCE

Coils may be wound and resistance adjusted for any DC voltage. Those most commonly specified voltages are 6, 12, 24, 48, and 100 VDC. Standard coil windings, available for all standard Cliftronics solenoid configurations are listed in our catalog.

14. COIL CURRENT

For any catalog item, once you determine the power required to generate the necessary force, the coil current may be calculated as follows:

15. COIL RESISTANCE

After you determine the required coil current for your application, the approximate coil resistance value may be calculated as follows:

Cliftronics standard coil resistance values for each solenoid size are listed in our catalog. The coil winding with the coil resistance closest to your calculated value should be the best initial choice for your prototype testing. Our engineers will help you compensate for temperature, duty cycle and other application variables to insure selection of the most appropriate coil winding for your solenoid.

16. COIL TEMPERATURE AND PERFORMANCE / RESISTANCE FACTOR

It is known that the resistance of copper wire will change by .393 percent for each degree the wire temperature varies from 20°C. This proportional change in resistance is called the resistance factor and can be utilized to predict how a solenoid will perform at high or low temperatures. (resistance factors are listed on page 47 in our catalog) If we call t2 any other temperature, the proportion is defined as:

For example, a solenoid with a 100 ohm coil at 20°C is designed to stabilize at 125°C with a 24 VDC continuous duty input. The resistance factor for the coil at 125°C may be calculated as follows:

Rf = 1 + .00393 (125°C - 20°C) = 1.4126

The Resistance of the coil can be calculated:

R20°C x Rf = Rt2

100 ohms (@ 20°C) x 1.4126 = 141.26 ohms

Using Ohm's Law, E = IR, the current for this coil at 125°C will decrease in the same proportion that the resistance increases. The current for this coil at 20°C is:


Therefore, the current for this coil at 125°C will be:

Force or torque may be predicted for this solenoid at 125°C by applying a current of .17 amps to the unit at 20°C.
The actual coil stabilized temperature may be determined by measuring the proportional change in resistance with temperature. After applying the 24 VDC application voltage to the example solenoid for a sufficient period of time to allow the coil temperature to stabilize (approximately 1 hour), the coil current or resistance could be measured. In this case, the actual current would be .17 amps and the resistance would be about 141 ohms.
Setting up the proportion:

Rearranging the Resistance Factor formula to solve for temperature t2:

Once a stabilized coil temperature has been determined for a specific application, we can predict the performance of that unit for any set of application variables such as voltage, current and temperature.

17. SPECIAL COILS

Cliftronics standard coils are wound to stabilize at 125°C when operated at the rated power input and duty cycle under standard operating conditions. Since many applications require different power input and/or operate under different ambient temperature conditions, our engineers will design special coil windings to meet your special needs. Please discuss your special requirements with us.

18. TWO COIL WORK/HOLD

Two coil units offer ideal solutions for applications where solenoids must be energized for long periods and where physical size and weight of the continuous duty solenoids are a problem. The two coil designs combine the smaller size advantage of the intermittent solenoid with the lower power consumption of the continuous duty design. The work or pull-in coil operates at the high power levels of the intermittent duty solenoid and moves the load through the complete stroke. At the end of the stroke, the second or hold coil is added to the circuit. The two coils together, in series, hold the load continuously at a power level which, in most cases, is substantially below that of the conventional single coil continuous duty solenoid. The "adding" of the second coil can be accomplished by an external mechanical switch or with a timed electronic driver circuit.

19. DRIVER CIRCUITS

Electronic driver circuits are available for various voltages and all solenoid sizes. Designs are available for switching pulse and hold, two coil solenoids and for Pulse Width Modulation of power to single coil solenoids. Circuits will be custom designed to meet your application and timing requirements. Please call our engineers to discuss your Driver Circuit application.

20. RETURN SPRING

In many linear and rotary solenoid designs, it is possible to incorporate armature or shaft return springs, often eliminating the need for external springs in your device. Please discuss your spring requirements with our engineers so we can help you evaluate your return motion load requirements and to allow us to recommend a spring design to best suit your application.

21. RETURN SPRINGS FOR ROTARY SOLENOIDS

Cliftronics standard rotary solenoids include clock type return springs installed externally on the solenoid shaft. Standard spring torque settings for each size rotary solenoid are listed in our catalog. These springs are adjustable, normally to ± 30% of the nominal value. Other springs are also available to provide higher or lower nominal torque values. The standard spring torque tolerance is ± 20% from nominal value.

22. RETURN SPRINGS FOR HINGED CLAPPER SOLENOIDS

Standard hinged clapper solenoids are supplied with a non-adjustable internal return spring designed to counter-balance armature weight and the armature hinge spring. Additional springs and optional spring adjustment features are also available. Consult our engineers to discuss your return spring requirements.

23. RETURN SPRINGS FOR CLAPPER AND HOLDING SOLENOIDS

These standard solenoids are not supplied with return springs. Because of the small space typically available in these low profile units, we recommend that external springs be used whenever possible. In some cases, however, it may be possible to design low profile springs for internal installation. Our engineers are ready to help you with your return spring requirements.

24. RESPONSE TIME

The response time, or "pull in time", of a solenoid is the time, usually in milliseconds (ms), from the point at which power is applied to the coil, until the plunger, armature or shaft has reached the end of it's design stroke or rotation.
Two main events contribute to overall response time. One, (1) is the time it takes for the current to overcome coil inductance and develop the magnetic flux field required to generate the required force. The other event, (2) is the time it takes for the plunger or shaft to actually travel the stroke distance. Flux field build-up normally takes more than half of the total response time. This graph illustrates these events.

Generally speaking, solenoid response times vary between 5 ms to 250 ms. Faster times are possible for small size units with small armature mass and short strokes. Longer times will be required for long stroke, larger size units.
To achieve faster response times, the solenoids must be over-powered. A solenoid size must be selected which will produce a force at the start of the required stroke, which is several times larger than would be required under normal speed operations. Also, the stroke should be as short as possible to keep the plunger or shaft travel time at a minimum. Special high-speed designs are available. Please call our engineers for specific information.

25. DROP OUT TIME

Drop Out Time, or "release time", is the time, normally in milliseconds (ms), from the point where the power is removed from the solenoid coil to the time the plunger or shaft returns to it's normally de-energized position.
Two main events contribute to the overall drop out time of a solenoid. One is the time it takes the magnetic flux field to collapse when the power is removed. The other event is the time it takes for the plunger or shaft to actually travel to its de-energized position as driven by gravity or spring return.
Drop out time can be increased by arc suppression diodes in the solenoid circuit, by residual magnetism in the iron circuit and by having closely matched or ground pole faces. Conversely, drop out time can be reduced by removing diodes from solenoid circuits and by inserting non-magnetic shims between the pole faces to insure their separation.

26. ANTI-RESIDUAL MAGNETISM STOPS

To achieve the maximum holding, output forces and torques, Cliftronics standard solenoids do not include any anti-residual magnetism shims or stops. Such devices are often used to provide a quicker drop out time. These shims, fabricated from .002 to .010 thick stainless steel, brass or plastic, keep the pole faces from meeting and effectively increase the working air gap. While these shims usually do not change the solenoid stroke, the greater air gap results in lower working and holding forces. Our engineers will work with you to determine if your application requires drop out shims and what effect their use will have on the overall solenoid performance.

27. MOUNTING

Cliftronics standard solenoids are provided with threaded mounting studs, which are rugged and easily adaptable to most applications. Standard lengths and tolerances are primarily determined by the manufacturer of the threaded studs. However, custom lengths can be provided to meet your application requirements.
Custom mounting features may be necessary in some installations. Our engineers will be happy to work with you to develop the most functional and cost-effective mounting features such as flanges or brackets for your application.

28. DESIGN SERVICE LIFE

Standard solenoid construction is normally designed and rated for 1,000,000 operating cycles. We are continually receiving laboratory and field reports of cycle life exceeding the standard design life. Actual cycle life will depend heavily on the application use and environment. Periodic cleaning and lubrication will help to extend solenoid life. Operating conditions such as excessive side loads or inertial loads on the armature or shaft may shorten solenoid life. Since many factors, other than the solenoid construction itself, have an effect on solenoid life, the rated solenoid life expectancy is valid only for the laboratory conditions under which the life tests were conducted. Our engineers will be happy to discuss solenoid life cycle enhancing features and options, such as special bearings, lubricants and heat treatment that could enhance solenoid life expectancy.

29. PROTOTYPE

Standard solenoids can be assembled for prototype testing purposes and for generating design ideas. We maintain a stock of many common standard solenoids for prototype evaluation. These units can normally be delivered very quickly. If the prototype solenoids must be fabricated and assembled, delivery may take a little longer. Testing the right unit the first time may save you valuable time in the development time cycle of your product. Our sales staff and engineers are ready to work with you to get the best prototype solenoid into your hands in the shortest possible time.

30. STANDARD VS CUSTOM SOLENOIDS

Standard solenoids will often meet the requirements of your application. Standard solenoids can usually be produced with reduced delivery times since we maintain an inventory of many of the standard detail components.
Cliftronics custom-engineered products are cost effective for production volumes. Often, modifications to our standard solenoid components will meet your application requirements. Delivery would depend primarily on availability of any special components or tooling. Cliftronics Sales and Applications Engineers will guide you in making your selection of both standard and custom features to best meet your form, fit and function requirements.

We are committed to our goal of:

· Continuous Improvement

· Problem Prevention

· Total Customer Satisfaction