Testing Magnetic Meshcore Mounting
Based on the previous post, I decided to do some testing — because without testing it's just some rando asserting things.
Also by testing I can identify other assumptions and note them here.
Because I'm writing things down, this is science not just farting about.
Test Setup
I wanted to ensure that tests were repeatable and that, if I was asserting that pull force was measured perpendicular to the surface, it really was perpendicular; so I made a jig:
The main elements are:
- A piece of 2x8 pine, about a metre long
- A chunk of 2x4
- A couple of old shelf brackets from the random-screws tin
- Some random screws from the random-screws tin
- a bit of old curtain rod
The measurements come from a cheap digital “fishing scale” from Amazon; it can weigh up to 50kg, it claims, but I'm not sure I want to take it that far.
Measurements are captured by taking a phone video of the face of the fishing scale, and reviewing later.
Method
- Get some bits of various thickness steel, eg “heavy brackets” from Hammer Barn; “nailing plates”; whatever you have lying around
- Fasten to the base with ... fasteners. I used screws from the random-screws tin
- Attach your magnets to your thing, attach the thing to the base steel, hook the fishing scale between the curtain rod lever and the thing
- Point your phone with one hand at the display of the fishing scale, put your boot on the end of the 2x6 so it doesn't lift up, press RECORD and haul up slowly on the end of the curtain rod.
- Realise that the fishing scale slips along the rod, and drill some holes in it so that you can drop in a different random screw from the random-screws tin to stop it sliding
- Repeat with different magnets, materials, thicknesses, etc until it gets dark.
Herewith, some photos for your amusement:
Materials tested
Bases
- 6mm thick galvanised steel “plate” (the long part of one of these
- 1mm thick galvanised steel “nail plate” one of these
Magnets
- 25cm dia “pot” magnets with centre screw hole aliexpress example
- 18mm x 3mm disc magnets like these
Lifting/twisting element
- A 115 x 65 x 40mm polycarbonate enclosure like this one
- A cable tie
- Some galvanised tie wire (when the cable tie broke)
Results
| Enclosure side | 25mm pot magnets screwed to box 1 end lifted | 25mm pot magnets screwed to box centre twisted | 25mm pot magnets screwed to box centre lifted | 18mm neodymium discs 1mm nail plate on box centre lift | 18x3 mm neodymium discs 3mm plate same polarisation centre lift | 18x3 mm neodymium discs 3mm plate alternating polarisation centre lift |
|---|---|---|---|---|---|---|
| Base material | ||||||
| 6mm thick steel | 11.38kg | 8.08kg | 21.33kg | - | - | 17.4kg |
| 1mm thick steel (many small holes) | 3.91kg | 6.63kg | 16.86kg | 3.7kg | 12.34kg | - |
That's a horror-show, of a table. Sorry.
You can clearly see that I didn't test all possible combinations. It was getting dark.
But the tl;dr is:
- 21kg of pull force onto a 6mm thick steel beam makes it Very Unlikely to come off by accident, and Quite Tricky to take off on purpose by hand
- Sandwiching magnets between steel plates greatly increases the pull force; 1. It's much easier to lift just one end than the whole thing at once
Other things discovered:
- holes in either side (eg the nail plate was full of holes, for the nails) really mucks with the magnetic flux (duh). Don't do this unless it's for really light loads, or in situations where it's protected from winds and thrown objects.
- keeping magnets in place between plates is the tricky part because they want to slide all over as soon as you reduce the clamping force; this results in a dramatic, sudden, and catastrophic failure.
Conclusion
The repeater I'm building will end up weighing about 500 grams, with the steel plate, antenna, solar panel, etc all on it.
A 17.5kg pull force gives a 35x safety margin for holding it in place, which I'm very happy with, and which also won't easily budge if someone pokes at it or a really, really strong wind hits it.
If I need more “stick”, I can always pop a couple more magnets on.
What's next
I'll pop another post up showing the build of the next repeater, taking the above into account. This page will be edited to pop a link —> here <— once that's done.