How does a nail hold two things together? How doesn’t it not fall back out of the hole it created over time?
In: Engineering
It is just friction. Often on things that get a lot of movement (decks are a good example of this) the nails do slowly push themselves out of the holes. You’ve got to tap them back in.
Screws aren’t really any different except they have larger contact areas and the movement needs to be directional.
The nail rams the wood out of the way, but the surrounding wood tries to push the displaced wood back into place, the force from this results in significant friction that holds the nail in place.
This is also why nailing into the end grain (into the end of a plank) is much weaker than nailing into the side of a plank, as the wood fibers are much better able to try squash back together when you nail into the side.
Friction. The nail is being squeezed on all sides very tightly because it was forced into the wall. But if there’s enough vibration, over the course of time it could fall out. But it’s still unlikely.
Nails can work their way out over time, but it will not simply fall out because you are compressing the material when driving it. So there is pressure acting against the nail to keep it in place. However the pressure depends on the material, its density, composition, and integrity. (Imagine nailing two pieces of cheese together, or nailing drywall / particleboard.) How quickly would those nails “fall back out?”
However, you will notice that even in an ideal medium like a pine 2×4, screws, or glue along with nails are recommended over nails alone for reasons of durability.
Nails generally should be used with sheer force applications . So if you look at a wood deck. You nail the joists together sideways. Nails are very strong with holding a sheer force. The decking boards should be screwed. Screws have a great pull force resistance.
If you screw a screw in half way and hit it with a hammer it breaks very easy because it has poor sheer resistance. Also if you ever see a deck with the decking boards nailed in a few years they will most likely be lifted.
Here is a good style explaining:
[nails vs screws](https://www.artofmanliness.com/articles/nails-or-screws/)
That is a cool manufacturing question!
It’s known as an interference fit – sometimes known as force fit or friction fit. Essentially in intense friction between the material and the nail holds the piece in place to the nail.
Other forms of fits are Clearance & Transition. Clearance being minimal friction while Transition being the goldilocks of not too much but not too little friction.
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