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How Zippers Work
by Tom Harris

The zipper is one of the simplest machines of modern times and arguably one of the least essential, but it is an immeasurably useful device in our everyday lives. Think how much easier it is to close a pants fly, a suitcase, the back of a dress, a sleeping bag or a tent flap with a zipper than with buttons or cords. The zipper is so effective and reliable that in less than a hundred years, it has become the de facto fastener for thousands of different products.

In this edition of HowStuffWorks, we'll examine the various parts that make up a zipper and see how these components lock together so easily and securely. The system is ingenious in its simplicity.

Hook and Wedge
Zippers can only be manufactured using modern machines, but they are built around two of the oldest and simplest tools in the history of civilization: the wedge and the hook.

A wedge is just an object with a slanted (inclined) surface. If you push a wedge forward against an object, it will push the object to the left or right. In other words, the force exerted by a wedge is always perpendicular to the direction in which the wedge is moving. A door stop is a good example of this principle: When you shove it under a door, it applies an upward force on the bottom of the door. A plow is another common wedge -- when you drive it forward, it pushes dirt or snow to the sides.

A hook, of course, is a curved piece of material that can be used to grab onto another piece of material. Hooks have been used as fastening devices for thousands of years because they are simple and generally sturdy. When used as a fastening device, a hook is generally coupled with a loop, eye or hollow area, which receives the hook.

The Track
A zipper track is made up of dozens of teeth, each of which combines a hook and a hollow. The idea is to latch every hook on each of the two tracks into a hollow on the opposite track. The latching mechanism, called the slide, is just a collection of wedges. You can see how this system works in the diagram below.

As the slide moves up the zipper, the two teeth strips must enter at a specific angle. As the strips move through the slide, the slide's inclined edges push the teeth toward each other. The strips are offset from each other, so each hollow settles onto a hook in sequence. For this to work properly, each tooth must be exactly the same size and shape, and they all must be perfectly positioned on the track. This would be all but impossible without modern manufacturing technology.

In a well-made zipper, the interlocking teeth form an incredibly secure bond -- it is very difficult to separate the teeth by pulling the two strips apart. But the slide can easily separate the teeth, using a simple plow-shaped wedge. When the slide is pulled down, the wedge pushes against the slanted edges of the hooks, pivoting each tooth off of the tooth below it. Just like that, the zipper tracks are detached.

For more information on zippers, including the story of their invention, check out the links in the next section.

Lots More Information!

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