| Size | Straight pull | Sideways | Screw alone breaks at | Limited by |
|---|
| Screwed into | Grip | Holds |
|---|
A screw through a clearance hole doesn't hold sideways by being strong — it holds because it squeezes the parts together and friction does the work.
That's why the sideways number is so much smaller than the straight-pull one. Once it slips, the screw shifts until it bears on the hole, and only then does its own strength matter.
In steel the screw is the weak point. In aluminium, plastic or a printed part, the thread in the hole gives out first, often at a small fraction of what the screw could take.
Deeper engagement is the cheapest fix. Doubling the thread depth roughly doubles the pull-out. Going up a screw size does much less.
Two M4s beat one M6, and they stop the thing rotating. Spread them as far apart as the part allows.
But don't assume the load splits evenly — with more than two screws, hole tolerance means some carry more than others. Size for the worst case.