Understanding Tension

i. The binding effect

Tension can be described simply as the application of turning pressure on the plug. Trying to rotate the plug will apply tension as both the shell and the plug press against the driver pins. Tension is applied to a lock using a tension wrench (discussed later).

When tension is applied to the plug the driver pins get “pinched” at the shear line between the plug and shell. Since, as we know, there are imperfections in the manufacturing process, only one pin will actually make contact with both the shell and the plug. This will cause that pin to “bind” at the shear line. If very strong tension were applied we would be unable to lift this pin at all. However, if very light tension is applied, we will be able to lift the pin with enough force to overcome the friction, or “binding”, of the pin. This is referred to as “the binding effect”. You will get to experience this binding effect first hand in an upcoming exercise, but for now it is only important that you understand why it happens.

When a pin binds due to tension, the friction of the pin is greater than the combined forces of the spring in the pin stack and gravity, thus the pin will not be able to drop back into the keyway. While picking a lock, we only wish to bind the driver pins. To bind a key pin at the shear line would mean than the key pin has crossed the shear line and has been lifted too far. This position will be discussed in detail in the section on false sets later in the manual.

It is the binding effect that will allow us to lift and set each individual pin.

ii. Understanding pin setting

Because of a combination of size variations between the inner diameter of the chambers and the outer diameter of the pins, as well as the outer diameter of the plug and the inner diameter of the shell, when tension is applied the plug will rotate very slightly even with the driver pins blocking the shear line. Some very inexpensive locks are quite “sloppy”, while more expensive locks tend to be much tighter. Regardless of the precision of the lock, these tolerances are present in all locks to some degree. This very slight rotation creates a slight ledge at the shear line (illus. blah). When the binding pin is lifted to a point where it crosses the shear line, the plug will no longer be in contact with this pin and the plug will be allowed to rotate ever so slightly, increasing the size of this shear line ledge (illus. blah). At this point, even without pressing on the pin stack, the driver pin will be unable to be pushed by the spring back into the keyway and across the shear line because it will get stuck on the ledge. This pin is now “set”, and this principle is the fundamental key to lock picking. If tension is maintained, the driver pin will be trapped in the bible and the binding force will be transferred to another pin. This pin will now be the binding pin, and it can now be lifted up past the shear line. This, in a nutshell, is how lock picking works. In an upcoming exercise you will get the opportunity to try this for yourself.

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