I started off considering how best to stick a repeater on to an existing structure with magnets, went down the rabbit hole, wrote it all down, and now here it is for posterity.
- TL;DR
- Considerations
- Not Considered
- Technical Matters
- Testing Magnetic pull force
- Fastening magnets to repeater
This post may be helpful for folks who need a way to attach a non-permanent repeater to an existing steel structure; you could be renting, for example, or maybe there are other reasons. You do you, I'm not your Mum.
Along those lines, it's important to have a disclaimer:
This is one person's half-arsed digging over a couple of afternoons. You should always test, test, test before deploying. The risks of a failed attachment run from inconvenience, through loss of equipment, up to property damage and personal injury (eg from falling hardware). It is your responsibility to identify and implement appropriate safety margins.
TL;DR
- You want your magnet to be as close to the actual metal you're sticking it to as possible – thick coatings like lots of paint will reduce the magnetic pull force.
- The magnet surface must be completely in contact with the target steel; attaching a 25mm disc magnet to the edge of a piece of 5mm-thick steel like a “T” shape will fail as soon as you turn your back on it, accelerating your repeater at 9.8m/s^2 towards the ground
- Use reasonable grade magnets (the floppy dark brown sheets that get put through the letterbox with a plumber's name on them for you to stick on your fridge will not cut it unless you have a truly nano-weight repeater)
- Consider side loading from wind, highest and lowest possible temperatures and corrosion risk when choosing your materials
- Magnetic pull force can be “tuned” by playing with multiple magnets, sandwiching magnets between steel plates, varying the thickness of the steeel, alternating poles, moving magnets closer to each other and further away; so you must do some real-world tests!
- Build in a safety margin. It's better to have a repeater that won't come off without effort than one that takes to the skies and hangs in the air in much the same way that bricks don't when it gets a bit warm and windy.
- Test, test, test. Test your setup against steel of the same thickness as your intended destination; eg against the structure itself at ground level, before you put it in its final location – it might not hold properly, or you may not be able to get it off again!
I have a followup post which actually tests some of these assertions.
Considerations
- Needs to support the repeater securely
- side loads from wind
- vertical loads from gravity
- vibration resistance from foot and vehicle traffic
- Needs to withstand the elements
- UV resistance
- moisture resistance both from ambient humidity and direct rainfall
- high and low temperatures
- Needs to be removable in optimal holding conditions
- either without specialised tooling (ie by hand); or
- with minimal/simple specialised tooling (eg aluminium, plastic or wooden lever)
- Needs to handle common enclosure types
- ABS plastic (eg project box)
- don't think anyone uses aluminium project boxes
- PP, HDPE (?)
- Needs to be affordable
- no requirement for specialised tooling to apply
- Needs to be safe
- no PPE required to apply
- minimal radioactivity
Not Considered
- fastening to non-magnetic materials eg aluminium, stainless steel, plastic, wood
- resistance to impacts eg power-washing, thrown objects
Technical Matters
You might want to mount your repeater onto metal that's got some kind of coating on it; it could be paint, or powder coating, or something else. This will affect the holding force of the magnet.
For some typical thicknessess:
- a single coat of standard paint is 25-50 microns
- a single coat of thicker paint could be 75-100 microns
- powder coating can range from 50 microns up to 250 microns for industrial materials
Magnetic field strength drops off as the inverse square of the distance to the metal to which it's fastened; for example, if a 25mm x 5mm N42 disc magnet directly touching a piece of steel (so 0cm away) has a holding force of 84N (8.57kg), then at a distance of 1mm (eg a steel beam that, over its life has had 10 coats of some industrial paint slapped on it), that force is reduced to 53N (5.4kg) — and that's the force perpendicular to the surface.
References
Magnet grade
From magnet.com.au:
A Neodymium (Rare Earth) magnets Grade (eg:N42) is an accurate guide to the strength of a magnet.
Generally speaking, higher grade numbers indicate a stronger or more powerful magnet. The number
comes from an actual material property, the Maximum Energy Product of the magnet material,
expressed in MGOe (Mega Gauss Oersteds). It represents the strongest point on the magnet’s
Demagnetization Curve, or BH Curve. The pull force of a magnet varies with the grade or ‘N’
number. As the Grade increases, so does the magnets pull force. However shape, thickness,
operating temperature and proximity to other magnetic material will also impact on the magnets
performance. Higher Grade magnets can be applied where space is restricted and the use of a larger
more powerful magnet is not possible. Letters after the Grade (N42SH) for instance, refer to heat
tolerance, up to 150°C and the temperature at which the magnet will become unoperational. N30EH
have the highest heat rating and remain effective at temperatures up to 200°C. AMF Magnets have a
range of Neodymium Magnets in various grades from N30 to N52 and with heat tolerances from 80° to
200°C.
References
Twisting force
This could come from off-axis wind, or from wind against a solar panel or antenna.
Because it's not perpendicular to the mounting surface, not all magnets experience the same load from the twisting force. Magnets closest to the source direction of the wind will release first, therefore reducing the overall holding power of all magnets, compounded by the twisting motion increasing the gap between the remaining magnets and the mounting surface.
In practice, as soon as the first magnet lets go, the whole thing is toast and will fall.
Side loading
This could be from wind where the direction is parallel to the surface that you're attaching to. It applies a shear force to the contact between the repeater and the mounting surface, attempting to slide the magnets.
One site rates a given configuration's strength against side force as 25%-40% of the direct force, however friction will also be a significant factor: smooth magnets against a polished surface are much more likely to slide than against a rough surface.
Temperature
Most magnets lose force as temperature increases; if heated above the operating temperature, the loss is irreversible. If heated to its Curie temperature, the material becomes completely non-magnetic. Thankfully, the Curie temperature of most magnets we will consider is well above even an outdoor day in the Australian summer, but it's not just the air temperature; steel in direct sunlight has been shown to reach 88C. Understanding the environment you'll be deploying in
is important.
Neodymium magnets have excellent performance up to 150C and down to -138C, but depending on their grade and formulation, they can still lose 0.08%-0.12% per degree C; from the example above, our 84N (8.57kg) of force at 25C would reduce to 77.69N (7.92kg) at 88C; while this temperature range is perhaps an outlier, and the reduction isn't enormous (only 7.5% overall), it should be taken into consideration if the build is a little borderline.
References
Corrosion
Most magnets contain some iron, and are therefore at risk of corrosion if exposed to both water and oxygen (eg outdoors). As a result, magnets are usually coated; check the coating on your magnets before using them; if there are cracks, or chips exposing underlying material, consider replacing them.
References
Magnetic strength between plastic and steel vs between steel and steel
Magnets can be MUCH more performant when sandwiched between two pieces of metal, instead of just between plastic (eg ABS, like most enclosures are made of) and steel.
For example, the 84N (8.57kg) of force from above becomes 302N (30.8kg) when sandwiched between steel plates of ideal thickness; that's a 3.5x increase!
This is because magnets have two sides; if one is up against a non-ferrous (eg ABS plastic, or air) materal then the field lines on that side can't couple in to anything; but if you sandwich a magnet between two steel plates (of sufficient thickness) then the field lines can couple on both sides.
Distance between multiple magnets, and magnet polarity
Let's say you have two square magnets. They're not “pot” magnets, so you can orient them either with both poles the same (eg both “N” against the steel) or with the poles different (one “N”, one “S”). It makes a difference!
Zero separation
Two magnets with zero separation with the same poles against the steel provide LESS force than the same two magnets with zero separation with their poles opposite to each other.
This is because when the magnetic fields enter the steel, the field lines from the two identical poles repel each other within the steel; while the field lines from the two opposing poles couple and increase the attractive force.
Some separation
The effect reduces with distance between magnets, and eventually evens out to effectively nothing. The exact distance required between magnets will depend on the individual magnets, primarily their shape and individual strength.
But consider if you're layering a surface with magnets, you could potentially tune the holding force by alternating some or all of the poles.
References
Testing Magnetic pull force
All this is so complex that the best way to check it is probably to slap some things together and give it a try.
You want to test 3 things:
- what pull force you get perpendicular to the target surface
- what side loads the configuration can handle
- can you get it off again!
Perpendicular pull force
- Set up your configuration of magnets, plates, etc
- Attach a digital “hanging” scale to the thing so that you can pull the thing directly “away” from the surface
- Pull on the scale, watching the “weight”, until your setup comes apart
- note: if the scale comes apart, you have exceeded the measurement capacity of the device
Side load pull force
- Set up your configuration
- Attach the digital “hanging” scale to the thing so you can pull it “across” the surface, parallel to the surface
- Pull on the scale until your thing moves, even a little bit, and note the “weight”
Twisting load pull force
- Set up as before
- Attach something of a “standard” length (it doesn't matter much what that is so long as you use the same something in all your tests, and attach it the same way) to the thing; one example might be a pair of vice grips clamped to the side wall of the open enclosure
- Attach your digital hanging scale to the same point of the something
- Pull on the scale until your thing twists off
- if you break the side of the enclosure, your magnets are stronger than your project box
Important: removal
- Set up as before
- Try and take it off by hand. Consider that you might be wearing gloves (if it's cold), or be in an awkward position (if you're reaching over the side of something high up).
- it's lovely to have a setup that gives 500kg of pull force but, once it's in place, it's not coming off again without significant effort and possibly destruction of components
Fastening magnets to repeater
Option 1 – mechanical attachment
This includes screws and bolts through holes molded into the magnet, the use of molded or 3d-printed “shrouds” which capture the magnet against the enclosure.
Advantages
- magnets aren't required to be attracted to the enclosure (it's not sticking to the enclosure magnetically)
- faces of magnets which will contact the location surface are open to the air – there's nothing “extra” in between the magnet and the metal
- it can be combined with other methods to strengthen the connection between the magnet and the enclosure
- doesn't need to be UV-stable
- works just the same in all temperatures
- can be robust against vibrations, with some caveats
Challenges
- you can't drill holes in magnets (they break up), so you have to source magnets that are manufactured specifically for the purpose
- the need to have fasteners pass through magnets reduces the magnetic field area, which makes magnets weaker than whole ones of the same size and shape
- fastening through faces of the enclosure introduces penetrations which then must be sealed against moisture ingress (“rain”)
- vibrations can cause fasteners to work loose; this can be addressed through the use of:
- locking compounds (eg Loctite) – these need to handle environmental challenges too
- lock nuts (eg Nyloc)
- locking washers (risky if not chosen and torqued properly)
Examples of magnets manufactured in this way include
https://www.aliexpress.com/item/1005009717220817.html
Option 2 – glues
Glues, putties, or other adhesives which are added by the maker of the repeater to stick the magnets to the enclosure.
Advantages
- can be combined with mechanical attachment to increase connection to the enclosure
- allows use of magnets molded without holes, which gives full magnetic field strength for the surface area
- no need to make holes in the enclosure
- glues with a level of flexibility can be used which may be required for environments where there's vibration
Challenges
- glues must adhere well both to the magnets and the enclosure
- glues need to be UV-stable
- glues need to handle the complete range of temperatures without softening, becoming brittle, or otherwise failing
Option 3 – potting
Using a compound which initially flows and then sets into a solid (flexible or otherwise), encasing the magnets and all or part of the surface of the enclosure. This differs from glues in that it surrounds the magnets, providing some level of mechanical connection as well as the adhesive connection.
Advantages
- magnets without holes
- no holes in the enclosure
Challenges
- potting compounds are generally expensive
- setting up an enclosure for potting a surface requires a bunch of pre-work (building raised edging which won't leak, etc)
- finding a potting compound which will bond to both the enclosure and the magnets can be difficult
- potting the magnets deeper into the compound to increase mechanical strength reduces the field strength attaching the repeater to the metal, per the inverse-square law
Option 4 – self-adhesive films or double-sided tape
Many magnets come with an adhesive film already applied, and a peel-off strip so you can stick them to a surface; or you could use double-sided tapes of various formulations and thicknesses to create your own sticky surface.
Because the risks of this approach far outweigh the convenience, it is NOT RECOMMENDED without extensive testing over a long period in target conditions. Most people don't want to do this, so just avoid this one.
Advantages
- pre-made “self-adhesive” backings on magnets can be effective when bonding to a variety of materials
- double-sided tapes let you play with different adhesion characteristics and thicker ones could help insulate the enclosure from vibration
- there's not much for you to do – just peel and stick
Challenges
- likely to be affected by environmental conditions, especially over time – UV exposure, temperature, and rain
- formulations are hard to come by, so their adhesive properties to different materials is as much guesswork as anything: testing is recommended
- if one formulation is tested and works well, the manufacturer may change materials or formulations without notification, affecting subsequent batches of material
- you're likely to drop a repeater on someone's head after a week or two
This approach fixes a steel plate to the enclosure, and sandwiches magnet(s) betweeen it and the destination surface. Because of the increased magnetic pull force from having both sides of the magnet(s) engaged, this can really improve how well your repeater sticks to the target.
You can do your own fun magnet calculations at https://www.kjmagnetics.com/magnet-strength-calculator.asp?calcType=disc or https://magnetengineer.com/playground/index.html?view=calc
Advantages
- Enormously improved holding power over all the other methods
- Can “tune” by using multiple magnets, flipping their polarities, adjusting the spacing between them, or changing the thickness of the intermediate steel
- Can combine with other options to ensure the magnets stay with the intermediate plate instead of staying on the target surface when removing the repeater
Challenges
- Thicker steel on the enclosure makes it heavier, which requires stronger magnets
- Holding force will vary with the thickness of both steel plates; if the holding force between the target surface and the magnets is greater than the holding force between your intermediate plate and the magnets, they'll stay behind when you remove the repeater; test on similar thickness steel first and consider mechanical or glue reinforcement between the magnets and your intermediate
- Won't do anything for “pot” magnets (eg those with a stainless external “cover” so that they're only magnetic on one side, which is how a lot of the magnets with mounting holes are made
- fastening the intermediate plate to the enclosure can be its own barrel of laughs, but using an enclosure with mounting tabs and popping some bolts through is probably easiest — just consider the thickness of the bolt heads so that they don't stop the magnets making contact!
- steel plates that are full of holes (I'm looking at you, various “make a bracket” products and nailing plates) really mess with the magnetic flux; avoid, avoid, avoid.
Option 6 – electromagnets
All the above options talk about permanent magnets. Given the low power systems we're talkingabout here, battery life, solar panel power output, risk of interference, electromagnets aren'tworth considering. This option only exists because some smartarse will bring it up at some point if it's not here.
