Need 1/4" O.D. Magnet with 10 lb. Pull

[brengine]

Has anyone come across any magnets that are 1/4" (max) O.D. and can have around 10 lb. pull force? I've found some Neodyium (sp?) rare earth that seem to be the strongest available in that size. They are cylindrical in shape and are rated to have a 3.5 lb pull force, but it isn't strong enough. None of the electromagnets I found are that small. We took a hardrive apart, and it has a really strong magnet in there, but we didn't know if it would be possible to machine and it not loose it's magnetism while keeping the same strength over that 1/4" diameter.

Someone thought of making our own electromagnet, but I don't know how to calculate the resultant pull force based von the number of windings. So I don't want to put the time into fabricating our own unless I am fairly certain it will work.

I'd prefer a purchased component, but any assistance you can provide would be great.
Thank you,
Ken Bolen

 

[israelkk]

The pull of a magnet is related to its pulling area and the square of its maximum flux density. Therefore, there is a physical limit to the force. An analysis an FEA analysis I made gives the maximum force of 4.1 lb.

 

[swarfsup]

KenBolen,
If polarity is not an issue you can greatly increase permanent magnet strength by placing one pole inside and concentric with an outer reverse pole. Leave a 0.06" space between the two and cap the upper non-working end with a 0.1" iron disk. Another way to accomplish this is to join two half-cylinders with poles reversed and a 0.06" spacer sandwiched between. Again the non-working end should be capped with a 0.1" thick iron disk. We often sandwich a rectangular flat neodymium magnet (poles on the flats) between two 0.1" steel plates that are a 16th inch longer than the magnet along one edge, and use the assembled magnet as a magnetic stop or tool mount.

 

[IRstuff]

According to this website's calculator, it's possible, given the appropriate material, size, etc.:

http://www.magnetsales.com/Design/Calc_filles/PullDisc.asp

TTFN

 

[dgallagher]

What is the application?

 

[hacksaw]

multipole magnets are commonly used to increase pull force as is use of cup shaped pole pieces (with the magnet inside). with properly design assembly, you may find that limiting the force will become your criteria.

 

[brengine]

dgallagher,

The application is this:
-7mm (0.256&quot diameter shaft *with* a flat-tip screwdriver slot in the end
-Shaft is mounted horizontally in a blind bore. The shaft is completely inside the bore, about 0.25" below/behind the surface. The opposite end of the shaft has a spring acting to pull the shaft into the bore.
-We want to pull the shaft out of the bore with a 1 kgf (2 lb) force. Because of the spring at the opposite end, the 1 kgf will position the shaft in the desired position so that a pin can be driven thru a cross-drilled hole in the shaft, locking it in position in the bore housing.

I can't change any of those given requirements.

Our possible solution:
-Attach a magnet to the end of the shaft
-The magnet has a wire attached to it's other end that goes over a pulley, with a hanging 1kg dead weight at the end...resulting in a 1 kgf pull force at the shaft (magnet just makes a solid connection to the shaft with whatever force needed to hold).

Problem:
-The 3.5 lb pull force (rated) neodyium (sp?) rare earth magnet won't stay attached to the shaft once the 1 kgf weight is applied. We're guessing that the lost surface area from the screwdriver slot in the shaft had something to do with it, and that the 3.5 pull force is stated as "under ideal conditions"...which this certainly won't be.
-The 10 lb is just a guess. We don't care what the pull force is as long as the magnet stays connected the shaft.
-The magnet from the hard drive that I mentiond will hold it, but currently that magnet is about 0.125" thick and about 1.5" in diameter. We haven't tried machining it down yet.

Thanks for all the input,
Ken

 

[hacksaw]

forget using a magnet, especially if the hollow shaft is made of steel(it will conduct the flux away from your piece).

suggest that you use a high grade epoxy and glue it to your extractor and pull...

you can also bore a pilot hole in your solid rod and tap for an appropriate screw, and pull...

having said all that, if you up-end the assembly and tap whilst maintaining a slight pressure on the pin, you stand a good chance of solving the problem when the rod is catapulted forward. the alternative is to compress the inner shaft spring and quickly remove the force, the shaft (depending on your spring design) may rebound enough to either catch or engage your pin...oil pressure has also been used in similar assemblies.

if you can do none of the above or any other contrivance works then you need to have your design team include "assembly" as part of the basis for design and save your self the grief...

good luck

 

[MintJulep]

Why do you need the spring? Once the shaft is locked in place in the bore it can't move, right?

 

[brengine]

Hacksaw,

I'm not sure what you mean by hollow shaft. It is a solid shaft with a screwdriver slot in it, mounted inside a blind bore.

This is for an assembly cell, one part every 30 seconds. We are planning on glueing the magnet to our pull tooling, that isn't the problem. The magnet doesn't stay stuck to the shaft we're trying to to pull once the 1 kgf force is applied. Glueing to the shaft we're trying to pull isn't an option.

Adding a hole and/or tap to the shaft isn't an option.

The rebounding spring won't work because we want it held in that position for ~3 sec.

>>>
you need to have your design team include "assembly" as part of the basis for design and save your self the grief...
<<<
I agree. But we were contracted to do the assembly, and the customer is unwilling to change their design. While at the same time they can not offer any other way for us to assemble the system.

So here we set,
Ken

 

[brengine]

MintJulep,

The spring is actually a torsion spring, that returns this shaft to it's home position after it is rotated (~90 degrees). The spring pushing axially on the shaft is more of a side-effect of the assembly. It doesn't really serve a purpose &quot;axially&quot;, but is still required. Yes, the pin will hole it in place.

Ken

 

[zeusfaber]

You don't say much about the size or quality of the slot in the end of the shaft - or what materials are involved (sucking eggs and all that, but I don't suppose that's why the magnets have been a bit disappointing so far?) or whether the slot could tolerate any damage.

I wonder if there's any mileage in creating a tool with a split blade that expands inside the slot to grip it?

A.

 

[hacksaw]

Ken,

mis-understood your assy. if the material surronding the bore is ferromagnetic, then you will get flux closure from the magnet, through your piece, and the side-wall; in addition to the flux closure from the magnet to the side wall.

what you might consider is a small diameter steel tube(0.250 od and a 0.065 wall) with a NdCo of FeB magnet inside the tube and recessed by a few mils.

This creates a fixture with a flux concentration at the end of the steel tube and in the core. epoxy the mag inside the tube. get a magnet with as good of a L/D as you can get. stack them (2-3) for more force.

It will increase the pull force considerably as it is brought into contact with your part.

good luck,

 

[brengine]

zeusfaber ,
It is a steel shaft. I'll have to look up the specifics tomorrow.

The slot is ~1.5mm wide and 1mm deep. As far as quality, nothing that high toleranced/finished.

The expanding split-tip tooling sounds interesting. I'll bring it up tomorrow, but depending on how many more actuators/moving tooling would be needed, the complexity could go a little overboard...but that's what are engineers for!

Ken

 

[brengine]

hacksaw,

The shaft is actually mounted thru a needle bearing, steel needle rollers in a steel cage. That bearing is pressed in an aluminum investment casting.

I'll definitely give your steel tube-magnet assembly a try. It seems pretty feasible (and simple). I just don't have enough experience/knowledge with magnetics to know what else to try.

Thanks,
Ken