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Salient. Victoria University Student Newspaper. Volume 36, Number 10. 23rd May 1973

Pin & Tumbler — Common Locks

Pin & Tumbler — Common Locks

The plug in a pin tumbler cylinder will not turn in the shell unless the correct key is inserted. To understand why, let us look into a cylinder. In the following cross-section view we see a section of a key in the key way. The key is supporting two small pins while a small coil spring is pressing down upon them from the top. In this position the plug cannot turn because the lower pin is half-way between the plug and the shell.

In the next illustration, however, the section of the key is much shorter & now it is the upper pin that is preventing the plug from turning.

But the following illustration shows that the key has raised the pins just high enough so that the lower pin can separate from the upper pin and permit the plug to turn.

Drawing of locks

The following side view of the lock without the key inserted shows how all of the upper pins have been pushed into the plug, thereby locking it.

Engineering diagram

This illustration shows how the correct key "lines up" the lower pins so that all of the upper and lower pins meet at the top of the plug, which is known as the shear line.

Engineering diagram

Here we see how all of the lower pins are locking the plug to the shell when a plain key is inserted in the lock.

Key in a lock diagram