In analytical instruments the forces are small and easy to meet. Heat, vibration, stray magnetic field and particle generation are what decide the selection, because each of them perturbs the measurement itself.
A solenoid moves its own plunger over a stroke; a holding electromagnet has no moving part and holds an external ferrous workpiece at zero gap. Ask whether something travels or stays put, and the choice settles.
Two positions means a solenoid; repeatable intermediate positions mean a servo, and no solenoid selection changes that. What the servo option really costs only becomes clear once the drive, feedback, transmission and commissioning are counted.
In elevator and rail locking the first decision is which state the mechanism reaches on power loss, not how much force it needs. Fail direction sets duty, thermal budget, and whether residual magnetism becomes a safety defect.
In physics a solenoid is a coil; in industry it is a complete actuator of which the coil is one part. A bare winding makes a field, not force — force needs the iron circuit and a moving plunger.
IP65 certifies water jets, IP67 certifies immersion, and the second digit is not cumulative. Neither covers detergent, hot water or thermal-cycling ingress — the three things that actually cause field failures.
Duty cycle is only meaningful with the cycle period stated. 25% over five minutes and 25% over twenty seconds produce completely different coil temperatures — state on-time and off-time in seconds instead.
A relay's armature carries electrical contacts, so its output is a switched circuit. A solenoid's armature is a plunger, so its output is force over a stroke. Identify the load and the choice settles in one step.
In packaging lines the binding constraint is cycles per minute and the thermal budget it creates, not peak force. Four constraints specific to packaging — cycle rate, settling time, washdown or dust, and fail direction — decide the selection.
Insulation class states the coil's temperature ceiling, not its running temperature. Class B is 130 °C, F is 155 °C, H is 180 °C — and the number that decides survival is ambient plus temperature rise, set by duty cycle.