How to Specify a Custom Solenoid

Solenoid force is not a single number. In a typical linear solenoid, force rises steeply as the air gap closes, so it is lowest at the start of the stroke and highest when the plunger is seated. A specification that gives one force value without saying where in the stroke it applies can produce a solenoid that holds well but cannot start the load moving.

Give the force needed at the start of the stroke, at the end, and anywhere in between where the load peaks. Include friction, external springs in the mechanism, and the weight of the load if it acts against the solenoid, then allow margin for wear and tolerances. For a rotary solenoid, give the same information as torque across the rotation angle.

A solenoid's force depends on coil current, and coil current depends on the voltage and the coil's resistance. Copper's resistance rises about 0.4% per °C, so a coil that warms by 80 °C has roughly 30% more resistance and draws about a quarter less current from the same voltage. Because force in an unsaturated magnetic circuit rises roughly with the square of current, that costs a large share of the force.

The worst case is the lowest supply voltage with the coil at its highest operating temperature, and that is the condition the force requirement has to be met in. Give the nominal voltage, its tolerance, the source (battery, regulated DC bus, or AC), and any limit on the current or energy available per actuation.

Duty cycle is the share of time the coil is energized: on-time divided by on-time plus off-time. It sets how hot the coil runs, which in turn sets how much power, and so how much force, a given size can deliver. Specify the on-time, the off-time, and the longest time the solenoid will ever be held on continuously, because a 25% duty cycle with a ten-minute on-time is a very different problem from one with a one-second on-time.

If the solenoid must hold for long periods, say so early. Peak-and-hold drive, which reduces current once the plunger is seated, or a latching design that holds with no power, is often a better answer than a larger coil.

Decide what the mechanism must do when power fails: hold its position, return to rest, or move to a defined safe state. That one answer often decides between a monostable and a bistable solenoid. If the solenoid must hold against an external load, shock, or vibration, give that holding force separately from the actuation force, and say whether it applies with power on, off, or both.

Specify how quickly the stroke must complete after the drive switches on, and how quickly the solenoid must release after it switches off. Actuation time depends on coil inductance, drive voltage, and the mass of the load; release time is often dominated by the suppression circuit across the coil. Both can be engineered, but only when they are stated as requirements.

The envelope caps the size of the magnetic circuit and coil, and so the force available, so give the maximum dimensions, the mounting, and how the load attaches. For the environment, give the temperature range, shock and vibration levels, and any exposure to moisture, dust, or cleaning chemicals. For life, give the number of cycles the solenoid must survive; it decides plunger guidance, wear surfaces, end-stop design, and spring fatigue.

• Giving force only at the seated position, where a linear solenoid is strongest.
• Specifying force at nominal voltage and room temperature instead of the worst case.
• Stating a duty cycle without the longest continuous on-time.
• Leaving out what must happen on power loss.
• Forgetting external loads during holding: shock, vibration, or a spring in the mechanism.
• Fixing the envelope before checking that it can physically deliver the force.

We review the requirements with you and flag any that conflict, such as a force that cannot fit the envelope at the required duty cycle, then model the magnetic circuit. Prototypes are built in-house in two weeks and characterized on the bench: force versus stroke, response time, temperature rise, and holding force. Production units are built in our own shop under our ISO 9001:2015 quality system. Rough numbers are enough to start; we help refine the rest.