Oscilllator
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Advice on centering a magnetic stirrer
So I a trying to make a magnetic stirrer using a cpu fan and a neodymium magnet.
The problem is that I am finding it almost impossible to center the magnet on the fan, so that it does not vibrate while in use.
So far the method I have used is to move around the magnet of the fan, testing to see if its centered every time. I did this for literally 8 hours
with no luck, so I moved on to another idea I had.
I got the magnet almost centered, then placed it on a folded up tea towel. This had the effect of absorbing the vibrations, however the system is
unstable if used for any length of time >30 seconds.
On to method 3! I glued the magnet in its almost centered position, then placed some small screws on it in an effort to even out the weight to stop
vibration. So far I have had no luck, and am all out of ideas.
Does anyone else have an idea on how to center the magnet? I have spent at least 12 hours on this project and would hate to abandon it or have to
spend another 12 hours.
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smaerd
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I built one of these and the problem I had was actually that the neodymium magnets would pull the fan up towards the flask, and interfere with the fan
motor itself. So it couldn't stir through a lot of things. So I went ahead and invested in a real stirrer for I think 30(US) dollars on e-bay. Used
the magnets to build a stir-bar retriever and the fan for something else.
The way I was fixing the magnets though was with super glue. It actually held them pretty well. I ended up mounting the magnets on a 2-liter
pop-bottle lid I hot-glued to the center of the fan to give it more room away from the motor, that seems to be a bit better. I never got it quiet
right, maybe someone else can give better advice about things like vibrations.
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watson.fawkes
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Quote: Originally posted by Oscilllator  | So I a trying to make a magnetic stirrer using a cpu fan and a neodymium magnet.
The problem is that I am finding it almost impossible to center the magnet on the fan, so that it does not vibrate while in use.
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Look up static balancing of rotors. That process is the first step in full dynamic balancing, but should be
adequate for a something like this. It does, however, require taking out the rotor as a separate assembly, which may well not be possible for a little
fan as you describe.
If you'd like to know more of what's going on with this, read up on the moment of inertia and the spinning of free bodies in rotation, precession, and
nutation.
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jock88
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The fan motor is probably going too fast.
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m1tanker78
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You could try experimenting with carefully placing some small blobs of hot glue on various fan blades at various distances from center. I haven't
tried this but I assume you would be able to easily remove the blobs until you get it right. Just make sure the glue won't come off too easily - as in
during operation.
You could also try with pieces of electrical tape to identify the 'problem area' before taking further steps.
Tank
Chemical CURIOSITY KILLED THE CATalyst.
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Oscilllator
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| Quote: Originally posted by m1tanker78 | You could try experimenting with carefully placing some small blobs of hot glue on various fan blades at various distances from center. I haven't
tried this but I assume you would be able to easily remove the blobs until you get it right. Just make sure the glue won't come off too easily - as in
during operation.
You could also try with pieces of electrical tape to identify the 'problem area' before taking further steps.
Tank |
I did that, only instead of using blobs of hot glue I stuck small metal objects to the magnet. Much easier to move around and take off.
also @watson i'll look that up and report back
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Oscilllator
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Looked it up and it seems I wont be able to centre it using static balancing. I cannot take the fan out of the assembly, and it seems the thing your
balancing also needs some kind of axle on both ends. My stirrer does not have either of those requirements.
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watson.fawkes
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| Quote: Originally posted by Oscilllator | | Looked it up and it seems I wont be able to centre it using static balancing. I cannot take the fan out of the assembly, and it seems the thing your
balancing also needs some kind of axle on both ends. My stirrer does not have either of those requirements. |
You can statically balance a wheel by putting a temporary axle on it, FYI.
My guess is that if you spent so much time trying to balance a rotor fixed into the fan and you didn't get anything to work is that it's not going to
work that way. My guess is that the fan itself isn't rigid enough to deal with the vibration. So make it more rigid. You can make your own rotor with
a bar of plastic or metal, affix magnets, and then balance it before trying to mount it. If you have a reasonably balanced rotor add-on the only issue
is getting it concentric with the motor axis.
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Hexavalent
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For Christmas, my brother gave me a novelty 'self-stirring mug'...it's basically a cup with a sealed motor at the base, connected to a shaft with
attached paddles. It is switched on and off by a small switch and actually generates a pretty good vortex in the liquid. Perhaps (maybe with a few
modifications?) this would perform well as an ad-hoc magnetic stirrer for those who are desperate for one.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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Oscilllator
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| Quote: Originally posted by Hexavalent | | For Christmas, my brother gave me a novelty 'self-stirring mug'...it's basically a cup with a sealed motor at the base, connected to a shaft with
attached paddles. It is switched on and off by a small switch and actually generates a pretty good vortex in the liquid. Perhaps (maybe with a few
modifications?) this would perform well as an ad-hoc magnetic stirrer for those who are desperate for one. |
I have rigged up a fan stuck to a rod that you can put on the end of a drill. It works really well for some things, but would fall apart if subjected
to high temps or solvents like acetone (It has a superglue component to it). Still no substitute for a real magnetic stirrer 
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