Understanding The Pin Tumbler Mechanism

i. Components of a pin tumbler lock

It may surprise you to learn that the common pin tumbler lock is a very old design. Its roots are, in fact, over 4000 years old. Early Egyptian and Greek models, though very large and crude, laid the way for Linus Yale Jr. to produce the very first modern pin tumbler lock in 1861. This design was, surprisingly, not very different from many of the locks we are familiar with today. Perhaps equally surprising is that the Yale company continues to manufacture many locks to this day.

Any lock has two basic functions. The first is to prevent the lock from opening without the correct key. The second is to open when operated with the correct key. If it all sounds overly simple it is because it is. In our descriptions we will be considering a common household deadbolt cylinder, though most locks work in a very similar manner with only slight variations.

Following is a brief introduction to the components of a pin tumbler lock. Later you will see how they work together.

The Shell – This is the body of the cylinder itself. It has a large hole to accommodate the plug and several chambers – known as the “bible” – to accommodate the pin stacks.

The Plug – This is the cylindrical portion of the lock that sits in the shell and accommodates the key. The plug has chambers which correspond with those in the bible of the lock.

Pin Stack – Each chamber in the lock contains one pin stack. A pin stack usually consists of one spring, one driver pin, and one key pin. In the case of a master keyed system, additional master pins may be added to the pin stack.

Spring – It will likely come as no surprise that each pin stack in the bible contains a spring. This spring applies pressure on the rest of the pins stack pushing the pins across the shear line and into the keyway to block rotation of the plug (more on this later).

Key Pins – Commonly referred to as the “bottom pins”, this manual will exclusively use the term “key pins”. These are the pins which actually contact the key when it is inserted in the lock. They are often called bottom pins because it is common, though not necessary, in North America to install the lock with the pins pointed downward. In this configuration the key pins would be on the bottom of the pin stack. Since a lock can actually be installed with the pins pointing upward – as is common in Europe – it can get confusing to refer to the key pins as bottom pins.

Driver Pins – Often referred to as “top pins” (see above), these pins are the pins adjacent to the springs. It is these pins that actually cross the shear line when the pins are at rest, preventing the rotation of the plug.

Master Pins – These are small, wafer like pins that are added to a pin stack for master keying system. We will touch on master keyed locks and the effects on picking later on in the manual.

Shear Line – The shear line is an imaginary line located at the point where the plug meets the shell (illus. 1.4). When the pins are at rest, the driver pins cross the shear line preventing the rotation of the plug.

Keyway – The keyway is the slot through into which you insert the key.

Wards – When peering into the keyway of a lock, it is easy to observe a series of grooves and rails running the length of the keyway. These are referred to as wards. Their primary purpose is to restrict which keys can be inserted into the lock to only the correct model of key. This is why one key will often not even fit into a lock it was not designed for.

Bitting – This refers to the particular combination of pin heights in a lock. Though each manufacturer is different, it is common for there to be as many as 10 possible heights of key pins (numbered 0 through 9). The combination of pin heights is referred to as the bitting.

ii. How does a lock stay locked?

As mentioned in the glossary of terms above, a lock remains locked because of the position of the pins while at rest. The springs keep the pin stacks pushed into the keyway with the driver pins crossing the shear line and blocking the rotation of the plug. In order to unlock the lock the plug must be rotated to activate the cam and retract the bolt. Since the plug cannot be rotated with the pins at rest the cam cannot be activated and the bolt cannot be retracted.

iii. How does a lock unlock?

In order to rotate the plug, each individual pin stack must be lifted to the precise position where the separation between the key pin and driver pin is in line with the shear line (illus.). When the correct key is inserted, each of the pin stacks is lifted to the correct height and there is no longer anything blocking the rotation of the plug, thus allowing the plug to turn, activate the cam, and retract the bolt.

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