Locks

Last updated: July 15, 2008.

Dangerous criminals, gold bars, glittering jewels—what do they have in common? All are kept secure by locks and keys. These ingenious forms of protection use intricate metal mechanisms to prevent you getting in or getting out, depending on where they're used. There are many different kinds of lock and they often work in very different ways, but one of the most common is the cylinder pin-tumbler lock used in padlocks and Yale door locks. Based on a mechanism invented in ancient Egypt, it became a winning invention in the 1860s thanks to the efforts of American inventor Linus Yale, Jr. (1821–1868) and the Yale company named for him.

Photo: A secure padlock like this has a cylinder, pin-tumbler lock inside it. The hardened metal loop at the top is designed to stop you sawing through it.

How locks work

Do you have a Yale-type lock on your door at home? Maybe you have a padlock you use to chain up your bicycle? The heart of a lock like this is a sturdy metal cylinder that can swivel inside an equally sturdy metal housing. When the correct key is in place, you can turn the cylinder freely and open the lock; without the key (or with the wrong key inserted), the cylinder refuses to turn and the lock stays shut.

Photo: Left: Turn the padlock over and you'll see the cylinder underneath. Held in place by metal pins inside, the cylinder will rotate only when push the correct key inside it. Right: Keys that fit this kind of lock have a jagged profile.

If you could open up a lock like this—not an easy job, by any means!—you'd see that the secret is a series of thin metal pins that run down from the housing into the cylinder (1), locking it in place. In fact, there are two separate sets of pins, an upper set (3, colored red here) and a lower set (4, colored blue). A set of small springs (2) just above the pins keeps them in place. How does it all work?

Without a key in the lock, the upper pins drop down from the housing into the cylinder, locking it in place, as shown in the first picture below.

How does it open? Every key has a slightly different profile of raised areas so it fits only the lock it's intended to. The wrong key won't fit in the lock at all. When you push the jagged edge of the correct key (5) into the lock, it pushes the pins upward against the force of the springs (6). The further in you push the key, the more pins it lifts. With the right key in place, the upper pins are all pushed just above the edge of the cylinder so they no longer lock it to its housing. When you turn the key, there's nothing to stop the cylinder rotating, so the lock opens.

Further reading

• Yale: Yale locks, their history, and a handy glossary of lock terms.
• Pin tumbler lock: There are some really nice lock illustrations in this Wikipedia article.