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Magnetic Crane

Magnetic Crane

Magnetic Crane


I am attempting to help a friend build a magnetic crane demonstation... My background is not in magnetism so I need some advice...

All we want the crane to do is lift a object off the floor and move it to another part of the room and drop it.  We are limited on power supply (do not know where we will show and tell this thing), so we are trying the following;

The crane will have a DC electromagnet on it.  The object will have a permanent magnet on it, this way to lift we will not need power.  To release we will just pulse the electromagnet and repel the PM.

My Questions are as follows:

1.  Is there a possibility we will demagnetize the PM?  If so how can we avoid this?

2. How do you guess-ta-mate the required size of the EM and PM?

3.  What material can we use for the core of the EM and the PM itself to insure they do not break, yet keep strength up and hopefully costs down?

Any suggestions are always appreciated!

RE: Magnetic Crane

Q1.  Is there a possibility we will demagnetize the PM?  If so how can we avoid this?
A1: If you use a rare-earth permanent magnet, you will not have to worry about demagnetizing  Hard ferrite (ceramic) are much cheaper, but there is a higher risk of demagnetization.  Avoid using Alnico

Q2. How do you guess-ta-mate the required size of the EM and PM?
A2: Is isn't easy.  I'd suggest getting the smallest rare-earth disc magnet you can find that'll hold what you want and work up from that.  You'll need _lots_ of turns of wire on the electromagnet to counteract the field from the permanent magnet.

Q3.  What material can we use for the core of the EM and the PM itself to insure they do not break, yet keep strength up and hopefully costs down?
A3: The rare-earth magnets are less prone to breaking than hard-ferrites, but they cost more.  One cheap & practical (from a machining standpoint) material for the EM is low carbon steel.

Good luck

RE: Magnetic Crane

Yes, you can do it.  Yes, you will be slightly demaging the PM.  This should not be fatal.  You will need to adjust the pulse to just be enough for release.
The core of your EM will be some mild steel.  As low of C as you can find and well annealed.  If this will be small you can  probably make it solid and not have any trouble.
How big of an object do you want to lift?  I have some hold-down magnet assemblies that are 2.5" diameter and 2" tall that will lift ~50 pounds.  They are an Alnico magnet with steel pole pieces.  Nothing brittle or fragile is exposed.
I can help you with a PM magnet assembly.  Heck you build a good one by tearing apart an old loud speaker and using the ferrite ring magnet in it.  Or any nice sized retangular magnet can be used.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

RE: Magnetic Crane

You're making it way to complicated using the PM. Just do an electromagnet and solve your power supply issues. it will be easier.

Yes you can demag the PM.

You need a lot more details for any useful answer to your question.

You use low carbon steel for the core.

RE: Magnetic Crane

I'd try it without the magnet too.  It is true that with any electromagnet with a ferromagnetic core (eg mild steel) there will be some remanent flux density but provided your load object is heavy enought it will fall away when you shut off the current.

If your electromagnet is an inverted U and the object presents a flat surface covering both poles of the electromagnet, you will get a closed magnetic surface without airgaps.  This will maximize the magnetic flux and therefore the lifting force (force is proportional to flux density squared).  Assuming you aren't trying to lift a very large mass and hold it for a long time, you should be able to get away with battery power although you will probably need to go for a lot of fine turns on your electromagnet.

(you get the most power out of a battery when the load resistance equals the internal resistance of the battery; you can find the latter by simply shorting the battery, the internal resistance = open circuit voltage divided by short circuit current.  I wouldn't use this method on a car battery though!)

RE: Magnetic Crane

- thinking about it a bit more, if the load does stick to the electromagnet due to magnetic remanence when you switch off the current, you can put a small non-magnetic strip between them (e.g. a plastic strip either on the pole faces of the electromagnet or on the top of the load).  This is similar to what they do in electrical relays to stop them sticking when de-energizing, they use a small brass stud on the end of the electomagnet.

RE: Magnetic Crane

Thanks for the help guys.  It has helped me move in the right direction.

To explain more, my associate is a electronics/wireless guy and  he will be using some simple LV wireless IC's to control this thing, so consumption must be kept low.

Not quite sure how we will attack it yet.  Never thought doing a project like this after working 8 - 10 hours a day could be so taxing!

Anyways, has anyone heard of a Electroperm magnet?  How small can you get these things?

Also, does anyone have any real good sites on magnets, so I can brush up on my theory?


RE: Magnetic Crane

My earlier suggestion was to go all electric. If you would provide load information we could make a better choice of electromagnets for you. That will tell us power demands. It certainly could be run from a 12v battery if the load is small.

However. A permanent magnet solution is another possibility. There are three types of magnets you might consider.

One rotates part on the circuit internally canceling the external field. The smallest one I know of has a footprint of 4.25x2 inches weighs 5 lbs and lifts 200 pounds on a 2 inch thick plate. It can be equiped with a motor to rotate the internals.

Another as you mention uses a coil as part of the permanent magnetic circuit. This is much more difficult to design and are not available in a small cheap package.

The third and cheapest solution is to look for a tramp metal pick up magnet. McMaster Carr might have them, or even Home Depot or Lowes. They are used to pick up nails and such. You'd get some ideas from one of these anyway.

I understand your project stresses. I just finished helping a buddy deliver a gold panning pilot plant for school kids last night. Got home about midnight after finishing assembly by headlamp. And that was after spending 6 hours at 4H livestock weigh in's. Oh, don't put a 10' pipe in your small station wagon when it's only 9'11" between the hatch and windshield.

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