Open frame vs. tubular solenoid: when each actually wins
For the same copper volume an open frame usually shows the higher peak force, because its flat pole face can close to almost zero gap. A tubular solenoid shows a lower peak but holds more of it over a longer stroke, and its closed shell returns its own flux so less leaks away. At equal copper the two curves typically cross between 1 mm and 3 mm of stroke.
Why this happens
The two topologies obey the same equations. What changes is where the flux goes and what shape the working gap takes, and those two differences decide almost every selection that is close.
Start with the gap shape, because that is the real distinction. An open frame, in its classic form, brings a flat-faced armature down onto a flat pole face. The gap area stays essentially constant as the armature travels, so the only thing changing is g, and force varies as 1/g². That is why the curve is steep: going from 0.2 mm to 2.0 mm is a tenfold gap increase and, in the pure inverse-square regime, a hundredfold force reduction. Saturation and fringing soften that considerably in practice, to something nearer a factor of ten to twenty, but the shape of the curve is the same.
A tubular solenoid, by contrast, commonly works between a tapered or chamfered plunger and the edge of the bore. The flux crosses a slanted path rather than a flat one, so as the plunger withdraws, the working area does not collapse the way a flat face does. Peak force is lower, because part of the magnetomotive force is spent across the taper rather than across a single narrow gap. Force at long stroke is higher, and the curve is far flatter.
Dimensional thinking makes this concrete. Fringing extends the effective pole area by roughly one gap length around the perimeter. On a 6 mm diameter face the area is 28.3 mm² and the perimeter is about 18.9 mm, so a 0.30 mm gap adds about 5.7 mm², or 20 %, while a 3 mm gap adds about 56.5 mm², which is twice the face itself. Once the fringing term is larger than the face, the simple geometry is no longer describing the device. A useful rule of thumb is that a flat-face design is comfortable to a stroke of around the face radius, which for a 6 mm face means about 3 mm.
Two curves of equal copper make the crossover visible. The frame gives its peak near zero gap and loses roughly an order of magnitude over the first 2 mm of travel. The tubular starts at around two thirds of that peak but loses only a factor of two over the same 2 mm. They cross somewhere inside the first 3 mm, and everything to the right of the crossing belongs to the tubular.
Leakage then separates them further, in the tubular’s favour. The shell of a tubular solenoid is part of the magnetic circuit: it carries the return flux around the coil and closes the loop, so typical leakage coefficients sit near 1.1 to 1.3. On an open frame the return path is a formed bracket sitting in free air, and coefficients of 1.5 to 2.5 are ordinary. Since force varies with the square of flux density, a coefficient of 2 does not cost half the force, it costs three quarters. That is why a tubular unit with visibly less copper can beat a frame unit at long stroke: less of its magnetomotive force is being spent on flux that never reaches the gap.
Two more consequences fall out of the same structure. The closed shell also acts as a magnetic shield, so the stray field a short distance from the body is commonly an order of magnitude lower than from an open frame of similar rating, which matters when a hall sensor sits nearby. And the same shell protects the coil and the gap from dust and moisture, while an open frame leaves both exposed. Both differences show up in solenoids for agricultural equipment, where the failure mode is contamination of the guided gap rather than the armature losing force.
Check these in order
1. Decide the stroke first, in absolute millimetres. This is the single input that most often decides the answer. Below about 1 mm the frame is usually ahead on force per unit cost; above about 5 mm the tubular usually is.
2. Find the force at the worst point of the stroke, not the peak. The load normally has to be moved at the start of pull-in, when the gap is largest and force is lowest. A peak figure at near-zero gap describes a state the device passes through once.
3. Characterise the load properly. A compression spring is stiffest at the end of stroke and therefore fights the solenoid exactly where the flat-face curve has already collapsed. A detent, gravity or friction load behaves differently. Compare the two curves, not the two numbers.
4. Check the envelope and the mounting interface. A tubular unit needs a round bore and usually a clamp; a frame unit needs a flat mounting face and room for the return bracket. The mechanical interface often costs more engineering time than the magnetic choice.
5. Work out the thermal path. A frame coil radiates from exposed surfaces. A tubular coil is enclosed, so its heat leaves mainly by conduction along the shell into the mounting. Class B insulation permits an 80 K rise above ambient, so an enclosed coil reaching 100 °C inside a 25 °C panel has already spent that allowance. If the mount is plastic or thermally isolated, the enclosed design can run hotter at the same power, and the resistance rise then eats force as described in how to calculate holding force.
6. Check side load and guidance. A flat armature with no external guide tolerates very little side load before friction starts consuming force. A bore-guided plunger carries a moment better. As an order of magnitude, a 10 mm bore running a plunger at 0.05 mm radial clearance has 0.1 mm of diametral play, which is enough to matter once the linkage pushes off-axis. If the linkage applies an off-axis load, this check outranks the force comparison.
7. Check interference and environment together. Nearby sensors, magnetically sensitive components and unshielded cables favour the tubular. Dust, condensation and washdown also favour it.
8. Cost it at volume, not as a prototype. A formed frame with a bobbin is cheap to tool. A drawn or machined shell with a precision bore is not. At 100 pieces the tooling is the whole story; at 100,000 the per-piece difference is, and it is measured in tens of percent rather than in cents.
What actually to change
| Situation | Choose | Why not the alternative |
|---|---|---|
| Stroke below roughly 1 mm, cost sensitive | Open frame | A tubular shell adds cost and buys stroke you are not using |
| Stroke of 5 mm or more | Tubular | A flat face has run out of effective area well before this |
| Load is a stiff spring compressed at the end of stroke | Tubular or a tapered frame | The flat-face curve is at its weakest exactly where the spring is strongest |
| Side load from the linkage | Guided or bore-guided design | Friction will consume the margin regardless of topology |
| Hall sensor or reed switch nearby | Tubular | The shell returns its own flux and shields the external field |
| Dust, condensation or washdown | Tubular | An open frame leaves the guided gap exposed |
| Very high duty in a confined space | Either, but verify the thermal path | An enclosed coil sheds heat mainly by conduction |
| High volume, simple function | Open frame | Tooling cost per unit is far lower |
When it IS the harder problem
You need both short-stroke peak force and long-stroke reach. Then neither topology as drawn will do it, and the answer is a hybrid: a flat-face frame driving a lever, or a tubular with a two-stage plunger. That is a mechanism problem before it is a magnetic one.
The load is genuinely non-linear in position. A detent, a bistable mechanism or a linkage with a toggle point means the requirement is a force profile, not a force. Overlay the load curve on both candidate curves before choosing.
You are replacing an existing unit and the envelope cannot change. Then the choice is often already made, and the useful work goes into matching the force curve rather than the peak, and into checking that the end stop and the return spring still do what they did. Geometry-driven force loss of this kind is the subject of latch geometry and lock solenoid force.
Both designs meet the force requirement and the decision is now thermal or acoustic. At that point the tie-breakers are heat path, noise and stray field rather than force, and those need to be measured rather than argued. Judge by the curve, not by the peak: the topology that looks weaker on a datasheet is frequently the one that holds its force over the stroke you actually need.
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Frequently asked
- Which gives more force, an open frame or a tubular solenoid?
- At short stroke, generally the open frame, because its flat pole face can close to almost zero gap and the force rises steeply as the gap shrinks. At longer stroke the tubular design usually wins, because the working gap is spread over a tapered or annular path rather than concentrated in a flat face, so its force falls more slowly. For comparable copper volume the two curves often cross somewhere between 1 mm and 3 mm of stroke.
- Why does a tubular solenoid leak less flux?
- Because its steel shell is part of the magnetic circuit. The shell carries the return flux around the coil and closes the path, so the leakage coefficient typically sits around 1.1 to 1.3, against 1.5 to 2.5 for an open frame where the return path is a formed bracket in free air. Force varies with the square of flux density, so a leakage coefficient of 2 does not lose half the force, it loses three quarters of it.
- How do I decide how much stroke a design can usefully deliver?
- Start from the pole face dimension. Fringing extends the effective area by roughly one gap length around the perimeter, so once the gap approaches the face radius the fringing term is adding as much area as the face itself and the simple geometry has stopped describing what is happening. A typical flat-face design is therefore comfortable to a stroke of around the face radius, and beyond that needs to be checked by analysis rather than assumed.
- Does the tubular design have any disadvantage?
- Several. It costs more, because the shell and the bore need machining or drawing to closer tolerance than a formed frame. It radiates heat less freely because the coil is enclosed. And it is less well suited to a load that applies side load to the plunger, since the bore cannot carry the moment as well as an externally guided flat armature. It also has a lower peak force, which matters when the load is a stiff spring or a firm detent.