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Suitable Metal for Electro-magnet Levers

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James Alford28/12/2021 10:34:46
463 forum posts
79 photos

Hello.

I wish to make some levers to be operated by electro-magnets. The electro-magnets are a small version of the type that are used to hold open doors. What would be best metal to use for the pad that will be attracted to the magnet? I know that some types of metal retain less magnetism than others and are more suited to this use, but I cannot remember what. It is somewhat annoying, really, as I used to work with solenoids and electro-magnets all day long at one time and simply cannot remember.

Thank you.

James.

Tim Stevens28/12/2021 10:55:17
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1584 forum posts

I would start by experimenting with mild steel - as it seems you are not sure of proportions etc yet. And - it seems that your application is not critical about size or weight, so although MS is not the most magnetic material, it has a major advantage as it is easy to machine, and some of the 'clever' magnetic materials can be difficult.

When you know more about the factors applying to your application, such as the degree of magnetism left in the material when switched off, which may be critical, you will find it useful to look at a reference such as Machinery's Handbook, which list material with special magnetic properties. Then, of course, you will need to take advice on where to get such material without having to buy 30 tonnes at a time ...

Regards, Tim

Robert Atkinson 228/12/2021 11:12:50
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1195 forum posts
20 photos

Steel. Mild is OK but when doing this sort of application in the day job we used gauge plate. We had it electroplated but the environment was fairly agressive. Don't go too thin, the force will be reduced.
Iron will get magnetised.

Robert G8RPI.

martin haysom28/12/2021 11:41:02
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88 forum posts

soft iron as used in commercial solenoids and transformers

Brian Wood28/12/2021 12:37:27
2549 forum posts
39 photos

As you say the " keeper" plate will be a pad of some kind, I suggest cast iron might be suitable. It will still be attracted by the electro magnet but it will not become permanently magnetised in use which makes a clean release when the electromagnet is switched off.

Robert Atkinson 228/12/2021 13:09:46
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1195 forum posts
20 photos

Note that iron is more easily magetised than steel. It is also more easily de-magetised which is why it is used for transformers. As the holding electromagnets are DC they will not demagetise the armature (moving part). This will eventually result it it sticking even with the electromaget de-energised.

The armature should be STEEL not iron.

Robert G8RPI.

Robert Atkinson 228/12/2021 13:28:15
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1195 forum posts
20 photos

Some supporting data from Eclipse

" The armature plate may also be known as a keeper plate. An electromagnet or electro-permanent magnet always needs a ferromagnetic surface to clamp onto – the ferromagnetic surface required is generally a mild steel or ferromagnetic stainless steel. "

https://www.eclipsemagnetics.com/products/magnetic-tools-and-standard-magnets/armature-plates-for-electromagnets/

Robert G8RPI.

Bazyle28/12/2021 13:53:18
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6295 forum posts
222 photos

The harder the steel the better it is at retaining magnetism. The best iron for the job is pure iron, like genuine wrought or puddled iron from days gone by. (not the modern 'wrought' iron from blacksmiths which is just mild steel). You might find a bit in an architectural recycling salvage yard where they have a bit of Victorian ironwork that is going to be cut off and is too small for anything else.

With lots of old electronic items being scrapped there are transformers in them that might be useable. Transformer laminations are specially made from pure iron but with silicon added to increase its resistance to heat generating eddy currents in AC applications. Unfortunately for the same reason (eddy currents) that is why they are made from lots of thin bits insulated from one another.

Hey didn't all you guys do this in 'O' level physics like I did?.

John Haine28/12/2021 14:19:37
4622 forum posts
273 photos

Mild steel should be fine unless the application is demanding. You can reduce the propensity for the lever sticking "closed" due to remanent magnetism by arranging that the air gap between the polepieces is kept very slightly open when energised, for example by a small non-magnetic rivet. This will reduce the holding force somewhat but greatly reduce the flux and so the possible permanent magnetisation.

James Alford28/12/2021 20:43:21
463 forum posts
79 photos

Thank you for all of the suggestions. I should have given more details in my original request.

On and off, over a couple of years or so, I have been playing around with plans for a clock which uses solenoids to pull levers which will, in turn, pull the the clock wheels around. There are multiple solenoids, one each for the hours, seconds and minutes hand. The solenoids are controlled by an Arduino Uno.

I plan to make the mechanism from brass, but with magnetic pads for the solenoids to attract. I shall need to keep the weight to a minimum as the solenoids are quite low power and will need to overcome the weight of the mechanism and pawls. Maintaining an air gap is a good idea and should be easy enough to include in the design.

Regards,

James.

John Haine28/12/2021 21:44:06
4622 forum posts
273 photos

Knowing more of the application, have you considered using more of a "stepper motor" approach? There's an interesting book "The Electromechanical Arts of Weston Bye" which is available from Camden in the UK - slightly pricey but has some good ideas. He describes a couple of versions of simple stepper motors that can be driven from an Arduino type processor, which could possibly be better, and quieter, than using solenoids. There's a cool clock that has a single stepper with 3 coils and a magnetic gear train to drive the hands. The basic design of a stepper motor has a more efficient magnetic circuit than a solenoid I think.

A posting here:

https://www.model-engineer.co.uk/forums/postings.asp?th=91571

**LINK**

responded to a similar question to yours.

There is also a discussion on here about Lavet - style stepper motors that could be useful.

https://www.model-engineer.co.uk/forums/postings.asp?th=171950&p=1

**LINK**

I built a clock that I call the Arduinome that is based on the Synchronome. The latter uses an electromagnet to reset the gravity arm that impulses the pendulum every 30s and also to advance the movement by half a minute. Both solenoids are normally very noisy for use in a home. I have replaced both with standard stepper motors partly for that reason.

Michael Gilligan28/12/2021 21:53:26
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20057 forum posts
1040 photos

This is probably a digression too far, James … but; have you seen the Clifford escapement ?

**LINK**

https://worldwide.espacenet.com/patent/search?q=pn%3DCA477160A

MichaelG.

John Olsen28/12/2021 22:37:13
1240 forum posts
94 photos
1 articles

It seems to me that what is wanted is a material with high permeability and low remananent magnetism. In less technical terms, you want a high amount of suck when the electromagnet is on, and a low amount when it is off. If it retains too much magnetism when the electromagnet is off, it will tend to stay in the same position even when released. Some relays had to have a little brass pip that made a gap when the moving part was pulled against the pole piece as otherwise they would pull in and never release despite the springs.

Anyway, soft iron is generally the most commonly available material that will fill the bill. Transformer laminations are also a very suitable material, and readily available from scrapped transformers, although only in thin sheet form. You may want to stack up a few pieces if you use that source.

regards

John

duncan webster28/12/2021 22:50:24
3919 forum posts
61 photos

Sounds like a synchronome or gents slave on steroids. I've wondered whether having 2 off 120 tooth ratchet wheels coaxial , the hour wheel slightly smaller, and the minute wheel having every 10th tooth cut deeper so the pawl engaged both would work. The hour would be advanced by 1/120 revs every 5 minutes. I have a synchronome slave in our living room. The click every 30 seconds is not obtrusive, nothing like the clonk of a master.

Edited By duncan webster on 28/12/2021 22:51:07

duncan webster29/12/2021 00:23:00
3919 forum posts
61 photos

To save you all writing in, I meant every 12th tooth, and evet 6 seconds

James Alford29/12/2021 10:18:26
463 forum posts
79 photos

Thank you, again, for the suggestions and links, which I shall read, especially about the Lavet motor.

I did consider stepper motors at one time, but having written the code for the Uno and built the circuitry, I am inclined to stay with solenoids, unless there is a way to operate the stepper motor with the existing code. The code simply sets an output pin on the Uno to "on" momentarily when something is required to operate.

Duncan: your idea of coaxial gears is not dissimilar to an earlier thought that I had, which used coaxial wheels, one being pulsed each second and pins on the seconds wheel dragging the minute wheel around each 60 seconds, the minute wheel doing the same for the hours and so forth for the days of the week as well.

Regards,

James.

Edited By James Alford on 29/12/2021 10:19:45

Edited By James Alford on 29/12/2021 10:29:48

Michael Gilligan29/12/2021 10:29:04
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20057 forum posts
1040 photos

I would forget the coaxial arrangement, and use a ‘Regulator’ dial layout.

MichaelG.

duncan webster29/12/2021 11:28:42
3919 forum posts
61 photos

So how do you make that work with one solenoid?

James Alford29/12/2021 12:33:43
463 forum posts
79 photos

I decided against the coaxial approach quite some time ago, to be honest. From memory, each wheel was smaller than the one in front of it. Each wheel was in a case with a small slot at the top. The seconds wheel had a short arm protruding backwards. The arm had a pawl hanging down and this engaged with the exposed tooth on the minutes gear and dragged it around. When it reached the case, the case disengaged the pawl until the next rotation. This was repeated for the hours. It was all becoming too complicated so I gave up.

Regards,

James.

Robert Atkinson 229/12/2021 12:54:08
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1195 forum posts
20 photos

Do your "solenoids" have magnetic cores? I read the original post as they had. If they have fixed magnetic cores technically this makes them electromagnets, not a solenoids. A solenoid has no fixed magnetic core and the moving part is normally Iron. This works because there is nothing ferromagnetic for the core to stick to if it retains magnetism. The moving part attracted to an electromagnet, the armature, is normally made from a material thati s harder to magnetise. This is typicall steel.
I think this subtle distinction between soleniods and electromagnets has caused some confusion as to a suitable material for the armatures.

Robert G8RPI

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