How Do Combination Locks Work?

Features of a Combination Lock

Combination locks, whether a Masterlock or the locked door of a safe, all look very much the same if you put them under an X-ray. Inside, combination locks are comprised of a series of plates, or wheels. How many wheels depends on how many numbers are in the combination. There would be three wheels for three numbers, for example. A spindle, connected to the combination dial, goes through the center of the wheels and turns the drive cam at the back of the lock. A pin sticks out from the drive cam. This pen makes contact with a tab on the wheels when the combination dial is turned. Sitting atop the wheels is a bar called a fence.

How the Lock Works

When the combination dial is turned, the drive pin makes contact with the tab on one of the wheels. It then turns that wheel to the point where the wheel's notch faces upward. That would be the first number in the combination. The dial is then turned the opposite direction, eventually picking up another wheel's tab and turning that wheel until its notch faces upward. This pattern continues until all of the wheels' notches face upward, allowing the fence bar to fall down into the open gap. This in turn unlatches the lock and the combination is complete.

How Locks Are Cracked

It might seem with this complicated system that it would be impossible for a safecracker to "crack" the code of a combination. And, despite what you see in the movies, doing it by ear is not a particularly viable plan. The most common way safecrackers are able to pick combination locks is by knowing the combination to begin with. Many safes and locks come with a preset combination. The intent is that the buyer change this combination when the safe is purchased. Many do not. These preset combinations are well known to professional criminals, thus making their safecracking quite easy. This is all the more reason to think up your own combination when buying a safe.