A question for those that do no pot their rotors

[mattg]

Hi
I have seen many examples of magnet rotors where the magnets are not 'potted' in resin, for example in Chris Olsons recent project:
http://fieldlines.com/board/index.php/topic,144577.36.html
In these situations, JB weld or similar has been used, often with mechanical holding methods as well- a machine screw or pin.

My question is, with the large attraction force between the magnet discs, has anyone experienced a failure of the adhesive?
Is any surface preparation of the magnets required (I understand they are generally coated to prevent corrosion)?

For a new project I plan on using an adhesive and a protective layer of paint rather than resin as I have done in the past. I will not be able to (easily) use a mechanical method- the specific magnets are not available pre drilled. I can change the design, but would rather stick (haha) with a simple adhesive if possible.
All advice appreciated
Thanks
Matt

[Fused]

On a 9" dual rotor I JB welded my 1x2x1/2" mags. Painted with a rust preventing paint.
They been up over a year without any problems. Ive seen over 700 rpm without a mag coming off yet.
(potting the mags is probably a better way to do it though)
Just what Ive found.

Fused

[mattg]

Thanks fused.
What thickness stator are you using?
Thanks
Matt

[Fused]

Hi
My stator was 7/16" thick, as I remember. Seems it came out of the mold quite close to that.
I used 1/4" plate for rotors and hung em on 3/4" dia shaft with pillow block bearings.
More than you asked for.....





Can we make this pimp my mill night? 
Have fun!

Fused

[Flux]

If the magnets are well stuck with a good epoxy and the surface is clean when you done it there is little chance of them moving. Most of the shed magnets have been with demagnetised ones that have rubbed the stator.

If you are doubtful then anything that stops them moving radially outwards will give you reassurance. If you must put the magnets right at the edge of the disc then the stainless band is probably best. If you can put then a little bit in from the edge you can drill holes and fit roll pins, Two for each magnet to sit on will make sure they can't move.

If things are right you shouldn't have any problems if you just stick them on. If you get a bad stator rub then heat will demagnetise them and the force and heat may also damage the bond so then they may move but this is a fault condition to be avoided at all cost and the magnets damaged in this way will be of little use even if you keep them on.

I have also found that when cast in polyester the rusting in our climate damages the nickel coating so even if the coating is bonded properly to the disc the magnets are still only held by the mass of resin when the coating lets go.


I suspect it is a case of worrying about failures that have occurred for various reasons other than normal operating conditions. I have run machines on test with the magnets just held by their attraction and they haven't moved so with a good epoxy bond you should have lots in hand but there is no harm in using a band or roll pins or similar to give total Peace of mind.

I have had no luck potting in polyester, I seriously doubt that vinyl ester will be satisfactory in our climate. I have one potted in epoxy and only time will tell if that holds up so I think you may be on the right track not to pot if you can find a suitable protective coating. If your climate doesn't encourage rust then you are definitely on the right track.

Flux

Flux

[mattg]

Thanks chaps.
Fused- thanks for the info and pictures, always nice to see variations on the basic design.

Flux- I think we may be in the same rust inducing climate! So far I have potted in polyester....so time will tell.

Again, thanks for your advice.
Matt