Ian Marr·May 22, 2020Architect
As far as the actual withdrawal force, we look that up in the NDS.
Let’s go with a 16d nail in douglas fir framing. Let’s also assume that it has annular rings as many framing nails do.
We get 32 lb/in. At 3.5″ we get 112lb required for withdrawal.
Now let’s not forget that this assumes perfect, straight out withdrawal not levering out like with a claw hammer.
There are very much magnets capable of supplying 112lb of force, or hold per se. But if all you got to work with was the head of the nail:
16d nail have a head of roughly 11/32″ across. Let’s assume that this is a regular nail and not a pinhead, moon head, or other type of pneumatic nail. That gets us 0.0928…, call it 0.09 in2. That’s 1,244.44 lb/in2.
So, if you have a magnet with a field capable of grabbing the whole nail you get 112lb. If your magnet can only grab the nail head you need a magnate capable of providing 1,244lb/in2 to pull it out.
As to which of those two numbers better reflects the realities of magnetic flux, that’s for somebody else to figure out, but here, at least, are some numbers to go off of.
I like your numbers. Practically speaking, it seems unlikely that one could easily provide 1245 lb force to the magnet to separate it and the nail from the wood. If the nail was in a loose peice of lumber, the magnet has just become a handle for lifting the piece of wood. And if the wood was part of a wall of the building the magnet has become a handle on the side of the building that nearly half a dozen people could hang off of. The cartoons are definitely fiction, as far as I’m concerned.
MKDesign 