A solenoid datasheet usually quotes one force figure — “25 N” — and one stroke figure — “15 mm”. Engineers who size on those two numbers alone regularly end up with a prototype that pulls 18 N at 15 mm, or a coil that cooks itself at 40 °C ambient. The missing piece is the curve behind the numbers, and the thermal budget around it.
1. Force is a curve, not a number
Magnetic force in a solenoid is roughly inversely proportional to the square of the air gap. That single relationship explains most of the behaviour designers find surprising:
- Holding force (armature seated, gap near zero) is the largest number on the sheet — often 3–10× the mid-stroke force.
- Starting force (armature at full stroke) is the smallest. It is the number that decides whether the mechanism will move at all.
- Mid-stroke force is where many mechanisms actually do their work, and it is rarely quoted.
Always ask for the curve, or at minimum three points: force at 0 mm, at 50% stroke and at 100% stroke, at rated voltage and at the intended duty cycle. A supplier who cannot provide three points is guessing.
2. What moves the curve
| Variable | Effect on force | Practical implication |
|---|---|---|
| Coil temperature rise | Copper resistance rises ~0.4% per °C; a 60 °C rise cuts current and force by ~20% | Specify force hot, not cold. A 25 °C lab measurement overstates field performance |
| Supply voltage | Force scales approximately with V² | A 10% low supply costs ~19% of the force |
| Return spring | Subtracts from net force across the stroke | A 3 N spring can erase the margin at full stroke |
| Friction and side load | Adds to the required force, often non-linearly | Add 20–30% margin for unlubricated or dusty mechanisms |
| Air gap contamination | Introduces a parasitic gap | Sealed or filtered actuators hold their curve longer in service |
3. Duty cycle, calculated rather than assumed
Duty cycle (ED, from the German Einschaltdauer) is the fraction of time the coil is energised in a repeating cycle:
ED % = on-time ÷ (on-time + off-time) × 100
A coil rated at 25% ED with a 10 s cycle can be energised for 2.5 s. A coil rated at 100% ED can be held indefinitely at rated voltage — which is why bistable (latching) designs exist: they hold mechanically and draw power only during the transition.
The failure mechanism is thermal, and it is cumulative. Insulation classes set the ceiling: class B (130 °C), class F (155 °C), class H (180 °C). A coil running at 120 °C in a 25 °C lab will exceed a class B limit in a 55 °C cabinet, even at the same duty cycle.
4. A worked sizing example
Requirement: move a lock bolt 8 mm against a 9 N load, DC 24 V, one actuation every 30 s, hold for 2 s, ambient up to 50 °C.
- Required force at 8 mm: 9 N load + 3 N spring + 25% margin ≈ 15 N at full stroke.
- Derate for heat: at 50 °C ambient with a 60 °C rise, expect ~20% loss versus the 25 °C curve — so look for ~19 N at 8 mm on the cold datasheet.
- Duty cycle: 2 s on in a 30 s cycle = 6.7% ED. A 10% ED coil is adequate, and a smaller coil can be used than for continuous duty.
- Voltage check: if the supply can sag to 21 V, add another 23%: target ~23 N cold at 8 mm, or move to a larger frame.
- Thermal check: confirm the coil’s rated ED at 50 °C ambient, not at 20 °C, and confirm the housing has some ventilation path.
That sequence — load, margin, thermal derating, voltage sag, thermal verification — is the whole method. It takes ten minutes and avoids most field failures.
5. When continuous duty is unavoidable
Three options, in order of cost effectiveness:
- Use a bistable (latching) solenoid. Power is applied only to change state; permanent magnets hold position. The best choice for battery-powered and heat-sensitive products — see our bistable solenoid range.
- Reduce holding current after pull-in. A PWM driver or a series resistor cuts coil power by 50–70% once the armature is seated, because holding needs far less force than starting.
- Specify a larger frame at lower power. A 15 W coil in a 25 mm frame runs cooler than an 8 W coil in a 12 mm frame at the same force, because surface area scales faster than losses.
6. Signals that a solenoid is being over-driven
- Force drops noticeably after 30–60 s of continuous operation (thermal derating in progress).
- Coil resistance measured hot is 20–25% above the cold value.
- Insulation smells or the bobbin discolours near the end caps.
- Actuation becomes intermittent at low supply voltage — the margin was never there.
Frequently asked questions
Can I run a 25% duty solenoid continuously at half voltage?
Sometimes, but it is not a design method. Half voltage gives roughly a quarter of the force, which usually fails the load requirement. Derate by duty cycle, not by voltage.
Does a heatsink help?
Yes, meaningfully — clamping the frame to a metal bracket can cut coil temperature by 15–25 °C. It is the cheapest thermal fix available and is often overlooked in compact enclosures.
How much force margin should I design in?
20–30% over the calculated load at full stroke, after thermal and voltage derating. Below 20%, manufacturing tolerance and contamination will find the gap.
Cixin Industrial has manufactured custom solenoids and electromagnets since 2011, with in-house coil winding, machining, assembly and testing. If you are sizing a solenoid for a new design, send us your specification and we will return a feasibility assessment with a recommended model and sample lead time.