How do locks work? A primer

We encounter locks every day, almost everywhere we go. You lock the door behind you as you go into the bathroom in the morning, then lock yourself out of your house. You unlock your car or bike before heading to work, then swipe your fob on the scanner at work to get into the building. Have you ever stopped and wondered how the mechanisms work?

This primer takes a quick look at the main types of lock mechanism, and explains how.

Pin tumbler lock

These locks have been around for a long time – versions employing the basic principles were used by the Ancient Egyptians over 4000 years before the first patent was granted to Mr Abraham Stansbury in 1805. The modern version was invented by Linus Yale, Sr in 1848.

The plug is unable to rotate when no key is in the lock, as the pins are blocking it from doing so.

To be able to rotate, the top of the driver pins need to be above the shear line, and the key pins below it. When the pins are correctly aligned with the shear line, the plug is able to rotate and unlock the lock.

When a key is inserted, the pins rise and fall according to the teeth of the key.

If the incorrect key is inserted, the pins will not align with the shear line, and the lock will not open.

The plug is able to rotate when the pins are aligned with the shear line. Only the correct key is able to manipulate all of the pins in such a way that alignment is achieved.

Tubular pin tumbler lock

The theory behind this lock is similar to a pin tumbler lock, and the lock is opened in the same way. The circular arrangement of the pins though, is thought to make the lock harder to pick.

There was controversy recently when information was circulated via Youtube showing how to quickly open a tubular pin tumbler lock on a bike lock with a biro pen.

Combination lock

This type of lock removes the need for a key, replacing it with a sequence of letters or numbers known only to the owner of the lock. They are popularly used in bike locks, although cheaply made combination locks are known to not offer much protection. Combination locks are documented as early as 1206, by Al-Jazari in his book The Book of Knowledge of Ingenious Mechanical Device.

One side of the lock will feature a number of dials, each of which will have a number of faces. One of the faces will be above a notch – this lock will only unlock when all notches are aligned, allowing the pins on the other side of the lock to pass through.In the image to the left, the correct combination to open the lock (the one that aligns all the notches) is 924.

The two sides of the lock can be seen here. The yellow side contains the dials, and the green side contains a number of pins.

When the green side is inserted into the yellow side, and the dials are rotated, the green pins are blocked from exiting the yellow side of the lock.

All images designed by Wapcaplet and used under the Creative Commons Attribution-Share Alike 3.0 Unported license.