DIY stirplate magnet orientation

**Homercidal** · 02-03-2012, 03:30 PM

I'm planning on testing a couple of different magnet orientations using a SS washer with small, round rare earth magnets from EBay mounted on it. Not sure which way would work the best, what do you think will work best?

First one is an Inline orientation, with two magnets of each polarity on each side of the washer. Having two magnets will hopefully increase the amount of pull they have on the stirbar, preventing it being thrown. Having the magnets inline should hopefully allow it to self-center the bar.

The other is radially oriented magnets. I wonder if having each end arranged this way will help "trap" the stirbar between them. Also I wonder if the stirbar will self center, or simply grab onto whatever field is closest...

**OpenContainer** · 02-03-2012, 03:54 PM

My guess would be inline, but since you're running the experiment we'll know soon enough. Good Luck, we're all counting on you.

**Huaco** · 02-03-2012, 04:01 PM

inline would allow for better use of stir bars of different lengths...
Also, if you go radial and the bar grabs to the magnets on the same side of the axis of rotation, you will be rotating your bar off center of the rotation axis of the motor. That is bound to throw the bar...

**Homercidal** · 02-03-2012, 04:59 PM

That seems to be my line of reasoning too.

**Homercidal** · 02-09-2012, 12:50 PM

I got the radial orientation done last night. However, the Inline orientation will be done tonight. I accidentally glued one of the magnets in upside down, against my best attempt at making sure it was correct. So I had to remove it and re-glue it.

I'll see if I can find time to throw a test together tonight and see what happens. Gut instinct is that the radial arrangement will not self-align, and probably won't hold the stirbar any better than the inline.

Sardoman · 02-09-2012, 01:00 PM

What about a stacked orientation. In this arrangement you would have a single magnet's footprint on either side of the axis, and on either side you would have two magnets stacked, one on the other. When I tried this, my magnetic field seemed to be stronger. This is essentially the same thing as your inline orientation, but with the benefit of a stronger field.
Oh, and if you were to use a nonferrous metal or a plastic disk instead of your ss washer you might boost your field with that as well.

**Homercidal** · 02-09-2012, 01:11 PM

I am currently using a stacked arrangement on my stirplate and it's ok, but I'm not sure it's noticeably better than it was with a single magnet.

The SS washer barely attracts the rare earth magnet. I mean, if you touched it to the magnet, you can pick up the magnet, but not by much. It rather seems to act like a shield for the fan motor.

Part of the experiment was to see how well a precisely machined arrangement would help. It seems a great part of the problem with thrown bars is not having a perfectly centered spin. I hope to cure this by placing the magnets with the help of a CNC mill to a tolerance of .001 of an inch from center. And the hole in the center of the washer will help align it to the center of the fan.