Best method to permanently fit fragile magnet-ring into female ring slot? 2

[jimmyjoeyphilp]

Interference fit? Industrial glues? Other methods?

Somewhat unusual situation. Let's say the magnet-rings have a diameter of 75 mm and a vertical and horizontal thickness of 2 mm. These magnet rings will be brought together in a handheld fashion on a regular basis and it would be unconscionable if they came loose or fell out. I'm wondering the best way to keep them in place.

The female "shell" each magnet will fit into could be (depending on model type) stainless steel, aluminum, brass or copper.

Here's an example of a "shell" half:

One half magnetic N pole, other half magnetic S pole.

 

[MikeHalloran]

Given the requisite cleanliness and undisturbed cure time, RTV silicone deserves consideration.


Mike Halloran
Pembroke Pines, FL, USA

 

[jimmyjoeyphilp]

Thanks Mike. Very interesting choice me certainly may consider. Care to elaborate on it? Have you experience with this? How did it go for you? Thanks in advance.

 

[MikeHalloran]

Allow a few thousandths of clearance so you have a finite glue line.
Clean both parts reasonably well, so they are free of particulates and especially oil.
Apply RTV to the groove and drop in the ring, press lightly to distribute the RTV
Squeezing it all out doesn't help.
Now, leave it undisturbed for 24 hours.
Temporary fixturing with blue tape or duct tape will be almost as good, if left undisturbed for an hour.

I have used RTV in production of electromechanical stuff, and of course around the house as caulking and adhesive.
The cheaper construction grades are filled, and not as strong as the unfilled grades.
If you have corrosion-prone (copper) parts nearby, you can get 'electronic grade' RTV that does not release acetic acid while curing.
There are also two-part catalyzed grades that are very strong.
For making boots/strain relief or appearance issues, use self-leveling grades that will run/pour for a bit, or until you breathe on them.

I have also tried to use RTV as Form In Place gaskets for engines, but stopped doing it after a couple incidents of globs of cured RTV getting into the oil sump, fouling the relief valve, and causing an inflation failure of the oil filter on a cold start. (When the relief valve sticks closed, the pump pressure will distort the filter's base plate severely enough so the gasket blows out, and your entire engine oil supply ends up on the ground in just a few seconds.)




Mike Halloran
Pembroke Pines, FL, USA

 

[jimmyjoeyphilp]

Thanks for this Mike!

Would you reckon this magnet ring would be too delicate for a press fit application?

 

[MikeHalloran]

That depends on what you mean by delicate, the mechanical properties of the magnet ring, and the precision with which you can hold dimensions of the magnet ring, and the housing, none of which have so far been quantified.


Mike Halloran
Pembroke Pines, FL, USA

 

[jimmyjoeyphilp]

Neodymium ring magnet, outer ⌀ 77, inner ⌀ 71, thickness 1.5 mm.

Female slot cut to suit required press-fit.

My thinking is this ring is too delicate for a press-fit. And a press-fit certainly limits the materials we can affix the magnet too and stainless would be out of the question I'm sure.

Requiring the ring have holes for fixtures reduces attractiveness of the end product somewhat and certainly complicates the magnet ring manufacture.

Which is what got us to the point where we are considering adhesives such as you suggest and while RTV certainly sounds like a solution the curing time (24 hours!?) would be an issue we'd love to avoid if possible. Hence the question about press-fitting.

 

[KENAT]

I'd be skeptical of press fitting a magnet such as you describe. Even nickel plated they are pretty fragile. Had a bunch of .063" thick (or maybe .031) .5 dia disc nickel plated neo magnets that tended to fracture.

If you want fast cure adhesives you could look at some of the newer cyanoacrylates (super glues) - maybe contact loctite. You will need to treat passive metals to get good bonding.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Compositepro]

A two-part epoxy may be more appropriate for this application. I hope this thread does not grow to 100 replies it is already getting tedious.

 

[MikeHalloran]

Downsides of two-part resins, epoxies, silicones, whatever, include:
- required precision of mixing, and associated measuring, dispensing, and mixing equipment.
- limited 'pot life' after mixing.
- waste of the occasional batch that kicks before it can be used.
- cleaning and/or disposal of the mixing containers and such.

... balanced against temporary fixturing, perhaps a wrap of tape, and enforced immobility for 24 hours or so for RTV. It may be sufficient to just maintain a rotated inventory of a day's product in a quiet place.

;...

My experience with Loctite, over decades, has been mostly positive. A phone call is usually enough to get you onsite help with any given problem, including training if you like.





Mike Halloran
Pembroke Pines, FL, USA