Pin tumbler

The pin tumbler is the cylinder mechanism inside virtually every modern commercial and residential lock. It uses a row of spring-loaded pin stacks — typically five or six chambers, sometimes seven — that a correct key lifts to a precise height. When all pins are at the right height simultaneously, a horizontal boundary called the shear line is clear and the cylinder plug rotates.

Each chamber holds a pin stack of (at minimum) two pins:

• A bottom pin (sometimes called the key pin) that the key directly contacts. Different lengths of bottom pin map to different cut depths on the key.
• A driver pin (top pin) above it, pushed down by a spring.

The plug — the rotating cylinder containing the bottom pins — sits inside a housing containing the driver pins and springs. Without a key, the springs push the driver pins down so they protrude into the plug’s chamber, blocking rotation.

How the key lifts the pins

When a key is inserted, the bottom pins ride on the cuts of the key’s blade. Each cut depth lifts a specific bottom pin to a specific height. If every bottom pin is lifted to exactly the position where the joint between the bottom pin and the driver pin sits at the shear line, the plug is free to rotate. Anywhere even one pin is off — too high or too low — the joint blocks rotation.

Master keying pin tumblers

Adding a master pin between the bottom pin and the driver pin gives the chamber two valid splits — both the change-key depth and the master-key depth produce a clean shear line. This is the basis of every pin-tumbler master key system.

Disc-detainer mechanisms (ABLOY) and dimple locks (Kaba) use different physical principles but solve the same access-control problem.

Related

→ Shear line — the boundary the pins must clear → Bottom pin — the pin the key directly lifts → Driver pin — the pin the spring pushes down → Master pin — the addition that makes pin-tumbler master keying possible