A ramp wrapped round a stick
Why Is a Screw So Hard to Pull Out?
A screw thread is a very long, very gentle slope rolled into a spiral, and long gentle slopes trade distance for force.
-
Step 1 of 5
Unroll the thread and you have a slope.
Cut a long thin triangle of paper and wind it round a pencil. The sloping edge spirals up the pencil exactly like a thread — because that is what a thread is.
-
Step 2 of 5
A long slope needs less push.
Getting to the same height by a gentle path takes a smaller push for longer. The screw hides that long path inside a short stick.
-
Step 3 of 5
The screwdriver multiplies it again.
Your hand goes right round the wide handle while the tip barely moves. That is a second machine stacked on the first one.
-
Step 4 of 5
Coming out has to undo all of it.
Pulling straight up fights every millimetre of that spiral ramp at once. Turning is the only cheap way back, which is why a screw holds and a nail lets go.
-
Step 5 of 5
The wood grips back.
As the thread cuts in, fibres are pushed aside and press against the metal for the screw's whole length. The friction from that grip is what actually holds.
A single 40 mm woodscrew can hold well over 100 kg in solid timber — far more than a nail of the same size.
The short version
A screw is a ramp rolled into a spiral, so many easy turns become one huge squeeze — and pulling straight out has to fight the whole spiral at once.
Try it yourself
Cut a long right-angled triangle of paper and roll it round a pencil, sloping edge outward. You have just drawn a thread.
Topics