DIY stirplate magnet orientation

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...

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.

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...

That seems to be my line of reasoning too.

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.

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.

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.

It's a bit old, but I'm curious.. Which magnet setup worked best?

I've been having a lot of trouble with my stir bars being thrown lately so I have been researching methods to update my stir plate to function better.

I can't remember if I actually got to try the inline arragement. I had a hard time putting these together, btw. Those magnets are a PITA to work with! I used a CNC at work to cut the recesses and had to fight to get the glue, magnet, and clamp in place all at once. Plus, the JB Weld Quick has a lot of ferrous metal of some sort and it wanted to ooze out and around the magnets.

IIRC RL got in the way and never got to fire up the second unit. Still need to finish painting the first one. Maybe if I get time today I'll clean my desk and try to find the parts I need for the second unit.

Any update on this? I'm going to be putting together a stir plate tonight.

I have been meaning to dig this stuff out and finish it for a while, but it's one of those things that never seems to pop into my head when I'm at home with nothing to do.

I'll try and dig it out and have a look tonight. I did get one made, but off the top of my head I can't remember which one I put together. It worked fine, but the stirplate is all apart as I planned to install different switches and paint the housing for a friend.

I distinctly remember the JB Weld kind of sucked because it has metal in it, and when I put the magnet on the SS washer the JB Weld oozed itself all over the magnet due to magnetic field. It also didn't seem to hold as well I expected, and my manhandling the magnets caused some others to pull free. I'll have to look at a different kind of adhesive. The magnets and washer are fairly smooth.

max384 said:

Any update on this? I'm going to be putting together a stir plate tonight.

Click to expand...

I have been using the inline arrangement for years with the 1" & 2" bars, go for it.


Cheers,
ClaudiusB

I just finished building a stir plate and my logic/concept is a little different.

Rather than use the standard hard drive magnet, or circular magnet. I used a bar magnet. I turned the bar magnet 90 degrees so the N and S poles are facing toward the outside of each end of the stir bar, instead of using two magnets one N side up and one S side up. The longest length of the magnet is closest to the stir bar.

This forces the stir bar to face perpendicular to the magnet, and when turning the bar magnet it turns the stir bar by both pushing and pulling the stir bar ends through the magnetic fields. Its only a minor variation, but rather than utilize only the pull/attraction of a magnet I am utilizing both push and pull. Major success for me. I hope I explained myself without pictures.

I'm playing around with some magnets and stir bars right now and what I'm finding is that you do not want the magnets on opposite end or your cross bar opposing. What seems to work best is to have just a continuous magnet. It doesn't matter if + or - is up, but it has to be one or the other. Mixed does not work, and on it's side does not work.

Brewmenn said:

I'm playing around with some magnets and stir bars right now and what I'm finding is that you do not want the magnets on opposite end or your cross bar opposing. What seems to work best is to have just a continuous magnet. It doesn't matter if + or - is up, but it has to be one or the other. Mixed does not work, and on it's side does not work.

Click to expand...

I've got 2 huge rare earth magnets on my site plate. .75 X .25 thich. I have the poles opposing each other. If not, the 2 inch sir bar really didn't want to stay hooked up while spinning liquid.

Inline. At least for me, the other way that you're showing is nearly what I tried first and it threw the bar off. Could have been user error, but it did.

I had a round magnet with a hole in the center from a refrigerator magnet. It's not a rare earth magnet, but it's stronger than your typical ceramic magnet. I dremeled it down the center and then flipped one side. It works great.

Huaco said:

I've got 2 huge rare earth magnets on my site plate. .75 X .25 thich. I have the poles opposing each other. If not, the 2 inch sir bar really didn't want to stay hooked up while spinning liquid.

Click to expand...

Yeah, your right. I had it backwards.