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# Pure axial force. Electromagnet?

## Pure axial force. Electromagnet?

(OP)
Hello all,
I have an application where I need to apply a "pure" axial force on a shaft, that means without any sort of reaction moments that may appear on the shaft. So I was thinking to use a electromagnet and a permanent magnet. As shown in the picture, the permanent magnet (2) is rotating, while the electromagnet (1) is stationary. I know there is no such thing like "pure" axial force in practical applications, so my questions are:
-The reaction moment is small enough to be ignored?
-If it can't be ignored, can it be masured?

### RE: Pure axial force. Electromagnet?

Without any return path and with the large separation between the two, you are going to generate very little axial force. The side loading will depend on how closely you can align the axis of the two. Of course you can measure it. You could mount one or the other on springs and see how much it deflects or you could use force transducers. You could model it in FEA. Without specific details it is impossible to give specific answers.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: Pure axial force. Electromagnet?

(OP)
Ok, so let's change the problem a bit.
If I have two coaxial disc shaped neodymium magnets, one mounted on the shaft and the other one always stationary. The gap between them, let's say, is 0.5 mm, and they repel each other. My shaft is also mounted in a bearing so the axial force between the magnets is transfered to the shaft and to the bearing. Now I can vary the axial force by moving the stationaty magnet along his axis, that means I can vary that axial force on the bearing, that's good. And now I want to put the shaft in a rotational motion untit it reaches a specific speed, after that I decouple the power from the shaft and let it to slow down. The shaft (I also use a flywheel to increase moment of inertia) will slow down due to the moment of friction from bearing (predominant), by knowing the mass of my shaft and the flywheel and also measuring the deceleration, I can calculate this moment of friction from the bearing. The big question is: My measurements will be influenced by the magnets, except for the axial force?
So I want to use magnets to put an axial force on my bearing, and calculate the moment of friction.
Sorry for my english, obviously it's not my first language.

### RE: Pure axial force. Electromagnet?

There will always be some side load, you will not get the symmetry good enough to avoid it.
But it should be very small compared to the axial force.
Why not one magnet on the rotating shaft and a mated electromagnet, then just change the current in the electromagnet. You know that he magnets will get warm, and that their strength will change as you run this test.
Depending on the accuracy that you need you might need different magnets.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Pure axial force. Electromagnet?

(OP)

#### Quote (Why not one magnet on the rotating shaft and a mated electromagnet, then just change the current in the electromagnet.)

I guess it is more hearder to build?

#### Quote (You know that he magnets will get warm, and that their strength will change as you run this test.)

Why the magnets will get warm? That's not good, that means a part from the kinetic energy of my flywheel will be transformed into heat.

### RE: Pure axial force. Electromagnet?

Because of nonuniform magnetic fields (even within the magnets) there will be eddy currents.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Pure axial force. Electromagnet?

(OP)
How can I vary the current in my electromagnet? What component I need? A rheostat?
The maximum current in my electromagnet is 1.5A.

### RE: Pure axial force. Electromagnet?

Ho about a variable voltage power supply?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

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