The zipper
How do two rows of teeth lock with one pull?
The question
Pull a zipper up and two loose rows of teeth lock into one strip. Pull it down and they come apart. What is the slider doing?
Have a guess, then show the answer
The slider hides a Y-shaped tunnel. Pulled up, its narrowing arms squeeze the two rows together so each tooth's bump nests in the next tooth's hollow. Pulled down, the wedge in the middle splits them apart again.
Play
Drag the slider, and watch the tunnel
This is a zipper seen from above, with the top of the slider cut away. Drag the slider right to zip it, or left to unzip it, or use the buttons. Watch the two arms push the rows together and the orange wedge push them apart. Then make one tooth a little off and zip again.
Open. Drag the slider to the right, or press Zip up. Its two arms narrow and squeeze the rows together.
Drag the slider, use the buttons, or focus the slider and press the arrow keys.
What's happening
- 1
Every tooth has a rounded bump on one side and a matching hollow on the other.
- 2
Inside the slider is a Y-shaped tunnel. Two channels come in at one end and join into one at the other, with a wedge between them.
- 3
Pull the slider up and the tunnel narrows around the two rows, squeezing them together. Each bump nests in the hollow of the tooth beside it, locking the rows like gears.
- 4
Pull it down and the wedge in the middle pushes the rows apart, widening the gap until the teeth let go.
- 5
That only works if the teeth are exact. They're made to a hundredth of a millimetre. A little off, and a closed zip can split in the middle.
Simplified, and sources
The drawing is simplified and seen from above, so the bump and hollow on each tooth are shown as a dot on its head; they really sit on its top and underside. There are fewer, bigger teeth than on a real zip, the slider is drawn with its top removed, and the gaps are exaggerated so you can see them. The off tooth is a simple way to show what imprecise teeth can do. It is not a model of exactly how a real zip fails.