Electromagnet assembly questions regarding friction, attractive force, and application design.

[Witrebel]

Hey guys,

I have what I assume for you will be a rather simple question. I am a summer intern at a semiconductor manufacturer and I am building a drop tester for the reliability department. I want to catch and release the free fall mass using an electromagnet.

The free fall mass is approx 2lbs and I was planning on having it machined from A2 Tool steel.
The guide rails are 1/2 inch dia 1556 steel rods.
The magnet I planned to use was a DC electromagnet from Mcmaster having what I believe is a radial pole arrangement. It would be 1.25 inch dia.

The idea is that the electromagnet rides on a linear actuator, so that when it engages the free fall mass, the mass can be elevated, and then when the magnet is turned off, the mass is released into free fall, subsequently generating an impact.

I am worried about the interaction of the guide rail with the free fall mass and the electromagnets.
1) Is it likely that the free fall mass and the guide rails will be subjected to an attractive force? This would pose a problem as an attractive force between them would result in increased friction and thus increased wear. Not to mention the extra strain on my linear actuator.
2) Is it likely that the guide rails themselves would be drawn towards the electromagnet itself, resulting in friction between the rail and the electromagnet?
3) Is it possible that the rail would become a shunt for the magnetic field, and that I would not get any attractive force between the free fall mass and the
4) is the cutout where the rails go a problem for my application as there is going to be very little if any of the free fall mass directly in contact with the north pole of the electromagnet?


I am open to any and all suggestions for this project, bear in mind that the geometry of the block/guide rails must remain virtually unchanged.

 

[EdStainless]

I would make sure that the rails are as far outboard as possible.
Otherwise they might interfere.
The guides that connect the mass to the rails should be non metallic, or at least non-magnetic.
This would minimize any interactions.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[RyreInc]

With the magnet you have selected it looks like the north pole will be directly in front of the guide rail, meaning that it will be part of the magnetic circuit. It also appears that the south pole will be roughly half over the guide rail and half over the free fall mass, meaning that some but not all of the flux will be shunted through the guide rail.

You'd likely be better to have e.g. two smaller magnets on either side of the rail so that the flux is directed through the free fall mass only.

(That's an axial pole magnet BTW.)

 

[Witrebel]

Ed:

The "guides" that connect the mass to the rails are simply the half inch radius'd slots in the A2 free fall mass. We add a thousandth to the spacing of the guide rails to give half a thou tolerance between the block and the rails. But other than that, the free fall mass directly contacts the guide rails. There is no sleeve, bushing, or bearing. What do you mean by as outboard as possible?

Ryre:

Would switching to a magnet with parallel poles remove the guide rails from the magnetic circuit?

E.g.

 

[RyreInc]

Using a single magnet such as you describe would help somewhat, depending on the permeability of the guide rail vs. that of the free fall mass (permeability in tool steel is very hard to nail down unfortunately). With similar permeabilities a portion of the flux will still travel through the guide rail.

Using two such magnets on either side of the of the rail (or one offset to one side) would work better, as there is no (easy) path through the guide rail in that case. You would want to make sure the setup is symmetrical, i.e. norths on outsides, souths on insides.

Will there be a matching magnet(s) on the other side? If so, perhaps you could still use only two magnets, diagonal from each other, for a more balanced support.

 

[Witrebel]

Ryre:

The plan was to use one magnet only, on one side. They make a small 1/2inch axial magnet such that I could use two and have them completely isolated from the guide rail, one on either side of the same face. However these only have 4 lbs of max pull giving me a total of 8lbs. It works in theory but there is very little factory of safety. The single parallel pole magnet offers 80lbs max pull, which provides a much more comfortable factor of safety.

You said that depending on the permeability of the guide rails and the permeability of the free fall mass, some of the flux will go through the guide rails. How will this manifest itself in terms of attractive forces?

For example, If I use an axial magnet that is completely covered by the face of the free fall mass, the magnetic circuit is contained within the tool steel, and as I understand it, there would be no attractive force between the free fall mass and the guide rails.

If I use the parallel poles magnet, you say some of the flux will inevitably travel through the guide rails. Would this result in the free fall mass and the guide rails being attracted to eachother? And the guide rails to the magnet itself?

Furthermore, in the "ideal" case where the free fall mass takes 100% of the flux and none travels through the guide rails, would the guide rail see no attractive forces at all? Or would the free fall mass still become magnetized due to carrying the flux and hence attract the guide rail to itself?

The more I think about this problem the more I realize how little I know about magnets. Time to study!

 

[IRstuff]

There's always leakage, so unless the rail is designed to be nonmagnetizable, there's a possibility of some interaction. An alternative is to make the rails out of aluminum

TTFN
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[RyreInc]

You're on the right track. If flux is passing through the the rail then there will be attraction to both magnet and freefall mass (and as IRstuff points out, there will inevitably be some leakage regardless of placement--the question is how much is acceptable?). And residual magnetization will mean continued attraction after the magnet is disengaged, however this number may be quite low, maybe even negligible (again, I'm not sure of the properties of tool steels; I recall seeing another thread on that subject here).

In the ideal case where no flux passes through the rail then there will be no attraction, and any residual magnetization will also not pass through the rail--however that is ideal and not real world.

I like the aluminum rail idea. There are also non-magnetic stainless steels and other metals.

 

[Witrebel]

I am no materials science expert but it is my understanding that dissimilar metals don't always play nice. I had assumed that tool steel would chew aluminum rails up not to mention the increase in the coefficient of friction between them. The goal here is to be repeatable and as close to free fall as possible.

 

[IRstuff]

If you really want repeatability, then you may want to consider making the rail an air bearing. This could allow for a slightly looser spacing, but the air pressure would keep the payload centered on the rail and almost completely eliminate any friction. To further stabilize the payload, you could possibly have a trailing "pontoon" which would have the same air bearing structure, but would be deployed a few inches behind the payload, which would reduce the potential angular DOF of the payload.

TTFN
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[Witrebel]

I am basically automating an existing manual process. The current process is steel rails and tool steel free fall mass. The repeatability is good enough as is. I just think aluminum would wear out faster leading to drift over the lifetime of the system. If you can point me in the direction of 3xx series stainless linear shafts that could work. So far all I've found are 4xx series which I understand to be magnetic.

 

[IRstuff]

Does the electromagnet have to be on that side? Seems to me that using either of the adjacent faces by turning the structure would minimize the any magnetization issues with the rail, and potentially minimize any issues with angular misalignments as well. Both the increased bulk of material to trap the field and the increased distance to the rails would alleviate the attraction to the rails.

TTFN
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