Magnet wont spin on my stirplate~help!

[GmanNJ]

So i got the cigar box, the Hard Drive rare earth magnets,the PC fan, the poteniometer and a switch. When on the fan and control work great but stir bar wont spin. I even took a hole saw to the top so the wood would not effect the magnet to stir bar attraction.

With the flask on top the stir bar is attracted to and follows the magnet but once I turn it on it sits. it does not get thrown as it would if the magnet was spinning too fast. I notice only half the stir bar is on the magnet. I have 2 stir bards one is flat and one is round both are 25mm (1 inch)

any suggestions?
thanks

 

[jwelch1103]

It sounds like you may have mounted the magnets with the same poles facing up. You need one of the magnets North Pole up and one with the North Pole down.

 

[cegan09]

Without seeing it's hard to tell.

however, first guess is that the magnets are not really setup right. Either not spaced correctly, or the polarity is wrong. You need different polarity in each magnet to grab each side of the stir bar. I had a problem recently when one of my bar magnets got flipped so they had the same polarity pointing up.

 

[day_trippr]

How many "hard drive magnets" are being used?
Typically, it's just the one, which makes the whole pole thing simple

Cheers!

 

[GmanNJ]

I am only using 1 magnet so it really should be easy and it looks like so many others in the forum I am at a loss as to why it is not working

 

[Dan65]

I built a stirplate recently and noticed that if i turn the poteniometer up to spin the fan faster that the stirbar will not move(I was using a dremel router bit as my stirbar). It was almost as if the magnet on the fan was moving too fast to even grip the stirbar. I just start with it off and slowly turn it up till it would spin, but that was the problem that I had.

What kind of poteniometer do you have in there. I found out that there is a type A and B.

 

[GmanNJ]

Yes there are two types.
Linear and logarithmic. A linear taper will give 50 % resistance when turned 50% but this is not the one you want (its also called an A type). The logarithmic is also called an audio taper and gives a more natural and expected progression. Linear taper will not move then jump up to almost full speed where an Audio taper is more a gradual increase.

I am using an Audio Taper Pot

I am thinking that maybe the material I used on top of the motor and have glued the magnet to is too thin and I am actually getting some magnetism from the motor. I am going to try a large fender washer instead.

thanks everyone for the suggestions. Keep em coming!

 

[cegan09]

I"m going to say the issue is the magnet. All hard drive magnets are not equal. The one you're using looks smaller and slightly different in shape to the ones I'm used to seeing people use. So if it doesn't have as good of a line between positive and negative, or if that change occurs off center, it's not going to spin the bar very well, or at all. If you place the bar directly on the magnet, does it grab both sides well? Meaning can you push one side of the bar easier than the other?

I used rare earth magnets so that I knew with 100% certainty that I was getting the spin center in the middle of the fan's axis of rotation.

 

[GmanNJ]

that may be it as it only grabs one end of either stir bar!

I do have 2 of these short magnets if i can I will try to increase the overall length by putting them end to end

if not I will look for the rectangular ones as you have

thanks for the suggestion

 

[dyqik]

that may be it as it only grabs one end of either stir bar!

I do have 2 of these short magnets if i can I will try to increase the overall length by putting them end to end

if not I will look for the rectangular ones as you have

thanks for the suggestion

The magnet you are using likely has one pole on top, one on the bottom. One end of the stir bar is attracted, but the other end is repelled, and spinning the magnet doesn't transfer any torque to the stir bar. You need a second magnet placed beside the first one, but the other way up. Testing with the stirbar should tell you which way to mount it - you need them arranged so that one end of the stirbar is attracted to one magnet, and the other end is attracted to the other magnet. You'll get the best coupling if the magnets are spaced about the same as the spacing between the ends/poles of the stirbar.

 

[GmanNJ]

that did it.... the shape of the magnet- who woulda thought!!?

here it is vortexing away (is that even a word?)
thank you again

gman

 

[WhoDatDad78]

Should I use my HDD magnets or a few much stronger and smaller button type magnets? I have everything else I need for the stirplate, less a potentiometer and switch.

 

[cegan09]

I tried button magnets, but found they weren't great for varying sized stir bars. I went with the bars you can see in the picture i shared above. Holds anything from a 1/2" to 2+" bar with no issues. Never tried hdd magnets personally.

 

[GmanNJ]

Button magnets did not work for me either. Bar types will probably work best but if you have a HD magnet I would try that first since you already have it

In my case there was no good delineation north from south so I used the second one. Good thing about HD magnets they are thin and powerful (like any rare earth magnet)

make sure you read my post earlier about potentiometers (linear and logarithmic) and get one that will allow you to get a real variation in speed