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# Solenoid, armature length

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#### ccurtis

##### Well-Known Member
It is well established that the axial force on a solenoid armature is an inverse, exponential function of the distance that the armature protrudes from the coil. This is because of the air gap that exists and changes with that distance.

So, I'm wondering if I make the armature longer than the coil length, protruding out both ends of the coil, but offset from center, does that helps to increase the initial force on the armature since the armature always fills the hole in the coil (no air gap) and the force seeks to center the armature?

Would an armature like that even move? An armature that is all ready in the coil would only become magnetized wouldn't it?

I think the armature will tend to centralise itself in the energised coil, so that the push-pull effects of the protruding parts will cancel each other out.

I think the armature will tend to centralise itself in the energised coil, so that the push-pull effects of the protruding parts will cancel each other out.

I may have misunderstood the description, but I took it as the armature was already centralized in the coil, with equal lengths extending out of the coil.

The OP says "offset from center".

The OP says "offset from center".

But in that same sentence he says, "So, I'm wondering if I make the armature longer than the coil length, protruding out both ends of the coil"

I read the OP to mean it doesn't necessarily protrude by equal amounts at the two ends.

I read the OP to mean it doesn't necessarily protrude by equal amounts at the two ends.

Yes, that is correct. I envision the armature protruding out one end more than the other end initially. I do expect the armature will center itself. My question is about the force of the pull, whether is be greater with an armature so designed compared to typical armature that does not fill the core of the coil, initially at least. I tend to expect the force will be greater, but am somewhat doubtful since I don't know of any solenoids constructed with over length armatures.

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Yes, that is correct. I envision the armature protruding out one end more than the other end initially. I do expect the armature will center itself. My question is about the force of the pull, whether is be greater with an armature so designed compared to typical armature that does not fill the core of the coil, initially at least. I tend to expect the force will be greater, but am somewhat doubtful since I don't know of any solenoids constructed with over length armatures.

Guess I'm dense. How can an armature stick out of both ends of a coil without going completely through the coil? An armature that is already through the coil is not going to move.

Guess I'm dense. How can an armature stick out of both ends of a coil without going completely through the coil? An armature that is already through the coil is not going to move.

Maybe I am dense and the armature will not center itself because it will not move if it already fills the hole in the coil?

If the armature is fully under the coil it will not move. regardless if it is partially centered or not.
Max.

Maybe I am dense and the armature will not center itself because it will not move if it already fills the hole in the coil?

Exactly. If the armature is already through the coil, what will cause it to move? It will just make the armature magnetic, as long as the coil is energized. There needs to be a part of the armature that isn't within the coil, to cause movement.

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