How a Pin Tumbler Lock Works: Pins, Plug, Shear Line

Most door locks in the world use the pin tumbler design. A key with the right pattern of cuts lifts a set of small pins to exactly the right heights, which frees a rotating part so it can turn. Without the correct key, at least one pin blocks the turn. This article explains the parts, shows how the key works step by step, and covers the history and the known weaknesses.

The parts

• Shell (housing or cylinder): the fixed outer part with a round hole in it.
• Plug: the inner cylinder that rotates when the lock is opened. The key slot, called the keyway, is in the plug. A cam or lever at the back moves the bolt.
• Chambers: small vertical holes drilled through both shell and plug.
• Key pins: pins of differing lengths that sit at the bottom and touch the key.
• Driver pins: pins above the key pins, usually of uniform length.
• Springs: push the pin stacks down toward the key.

Wikipedia says most cylinders have five or six pin stacks. The thin gap between the plug and the shell is the shear line.

Locked: pins straddle the gap

With no key in, the springs push each stack down. The driver pins cross the shear line, so they sit partly in the shell and partly in the plug. The plug cannot turn because the pins are in the way, like a bolt through two hinged parts.

Unlocked: the key lifts each pin to the line

Each cut on the key has a depth. As the key slides in, every cut raises its key pin by a set amount. When the right key is fully in, the joint between each key pin and its driver pin sits exactly at the shear line. Nothing crosses the gap, so the plug turns freely and the cam pulls back the bolt. A wrong key leaves at least one pin straddling the shear line, and the plug stays locked. Because every pin must be correct, the number of possible key patterns grows quickly with the number of pins.

Master keys

Some buildings need one key for a single door and another key that opens many doors. Wikipedia explains that master-keyed locks usually add a spacer pin to each stack, giving driver, spacer and key pins. That creates two shear points per stack, so two different key heights will open the lock.

A short history

Locks are very old. Wikipedia's article on lock and key says they have been used for over 6,000 years, with an early example found at Nineveh in ancient Assyria. The Egyptian wooden pin lock followed, in which a key lifted pins out of holes in the bolt. The article on the pin tumbler lock says the basic principle may date to about 2000 BC in Egypt.

The modern form came much later. In 1805 Abraham Stansbury received an English patent for a double-acting design, where lifting pins too far or not far enough prevents opening. Linus Yale Sr. created the modern pin tumbler lock in 1848, according to Wikipedia. In 1861 his son, Linus Yale Jr., patented a smaller flat key with serrated edges, and pins of varying lengths inside the lock, which is the basis of today's design.

Why picking works

No lock is made perfectly. Science ABC explains that manufacturing tolerances mean the chambers never line up exactly, so when someone applies light turning force, one pin binds before the others. This weakness is why the design can be opened without the key by someone with the right tools and skill. Security therefore depends on factors such as the number of pins and how accurately the lock is made.

Wikipedia lists other known attacks, including lock bumping and snap guns, and a method called lock snapping, which breaks the cylinder by force and which it says can take about 50 seconds to 2 minutes. Security pins such as spool and serrated pins can mislead or delay a picker but may not stop an experienced one. Cylinders certified to standards such as Sold Secure SS312 Diamond or TS007 3-Star are described as resisting drilling, picking, bumping, snapping and plug extraction.

Sources

• Pin tumbler lock, Wikipedia
• Lock and key, Wikipedia
• How Do Pin Tumbler Locks Work? Science ABC