Best Magnetic Hooks for Vertical Sheer

September 16, 2018 7:57 AM

What are the best magnetic hooks I can buy on the internet that withstand vertical sheering? i.e. Putting the hooks on the side of something vertical and hanging something heavy from them. Looking to hang a shelving unit that will be about 15-20 lbs. Science questions inside.

Bonus questions: I've read that vertical sheer strength of magnets is approximately 1/3 the strength of horizontal hold. True, or myth? Also, does doubling up the hooks increase the vertical sheer strength, or no? (e.g. if the hooks say 40 lbs, will hanging from two hooks be 40 lbs. x 2, and hanging from four hooks across the shelf hold 40 lbs. x 4?)

posted by juniperesque to Science & Nature (7 answers total)

Science answers: your "1/3 the strength" rule might be a good rule of thumb for some circumstances, but while the vertical hold strength is related to the horizontal hold strength, it also largely depends on the materials on the surface of the magnet and the vertical surface. Maybe for typical refrigerators and typical iron magnets, 1/3 is approximately the ratio (but I don't know). Different surface types (stickiness, roughness, etc) could yield widely varying ratios.

If you want the algebra, an approximate formula for the vertical holding force is be Fv = u * Fh

Where Fv is the vertical force, Fh is the horizontal force, and u is the "coefficient of friction", which, for different pairs of surfaces, can vary from close to zero up to 1, or in some circumstances, even higher. Here is a table to give you a sense of the variation in coefficient of friction.

Doubling up on the hooks would increase the holding strength, yes, with a caveat: when two surfaces are stationary (non-sliding) with respect to each other, one type of friction, with one coefficient of friction, is active - this is the mode you want to be in, since you don't want your hooks to slide. However, this static friction has a maximum amount of holding force, and when you exceed that force, the surfaces start to slide. When two surfaces are sliding, another coefficient of friction, which is smaller, comes into play. That means if your two hooks are equally loaded, the force on each hook could be smaller than the maximum force of static friction. But if they are unequally loaded, one hook might max out on static friction and start to slide, which might result in jerking the other hook into motion, which would mean they both start sliding down.
posted by Salvor Hardin at 8:48 AM on September 16, 2018 [1 favorite]

Oh, and I can point you to one possible magnet solution - these magnets are pretty much the strongest around - they are neodymium magnets, which are crazy strong. This company does sell magnetic hooks, but I don't think they're neodymium, so they are probably not super strong. I'd buy one of the big thick neodymium magnets, and glue something to it to make it into a hook.
posted by Salvor Hardin at 8:51 AM on September 16, 2018 [1 favorite]

The thickness of the steel your sticking to comes into play... too thin and it'll be saturated. A crude check is to see if a paperclip a attracted to far side.
posted by tinker at 9:20 AM on September 16, 2018 [1 favorite]

Nth-ng that neodymium magnets will give you the strongest readily available magnets for the size.
For the shelving unit weight, don't forget to calculate in/or test supporting the weight you want to support at the edge furthest away from the supporting magnet.

The company Salvor Hardin linked to has a blog which gives guidance for Loading in Shear - Friction .

They also invite questions. (e-mail link at bottom of page.)

Cup based or channel based magnets (another company) will give you a more concentrated holding flux and are available in many different sizes. (including up to
405 lb. pull from a 3" diameter magnet!

I have ordered from both of these companies and have received fine products and service from each of them.
posted by tronec at 10:16 AM on September 16, 2018

Minor safety note: if you buy neodymium magnets, especially bigger ones, be aware that they can and will crush things to pull together, up to and including your fingers. You’re probably not in “safety hazard” territory at the sizes you’re looking at, but it bears consideration when you’re handling them.
posted by Alterscape at 10:28 AM on September 16, 2018 [1 favorite]

You can get magnetic hooks from Lee Valley, a company I’ve always found to stock good quality stuff. The strongest ones are rated for 26 lbs straight pull on 3/8” steel and you get 3, so if the 1/3 rule is correct, they should do the trick. You could also contact them - they may well be willing to get out a set of hooks and try hanging something to see whether it seems sturdy.
posted by pocams at 2:08 PM on September 16, 2018 [1 favorite]

If the shear holding force is a result of the friction coefficient, perhaps adhering a sticky/tacky surface to the magnet may increase holding force.
posted by suedehead at 9:43 PM on September 16, 2018