Elastic Deformation

The interesting events of lock picking happen over distances measured in thousandths of an inch. Over such short distances, metals behave like springs. Very little force is necessary to deflect a piece metal over those distances, and when the force is removed, the metal will spring back to its original position.

Deformation can be used to your advantage if you want to force several pins to bind at once. For example, picking a lock with pins that prefer to be set from front to back is slow because the pins set one at a time. This is particularly true if you only apply pressure as the pick is drawn out of the lock. Each pass of the pick will only set the frontmost pin that is binding. Numerous passes are requred to set all the pins. IF the preference for setting is not very strong(i.e. the axis of the plug holes is only slightly skewed from the plug's center line), then you can cause additional pins to bind by applying extra torque. Basically, the torque puts a twist in the pug that causes the front of the plug to be deflected further than the back of the plug. With light torque, the back of the plug stays in its initial position, but with medium to heavy torque, the front pin columns bend enough to allow the back of the plug to rotate and thus cause the back pins to bind. With the extra torque, a single stroke of the pick can set several pins, and the lock can be opened quickly. Too much torque causes its own problems.

When the torque is large, the front pins and plug holes can be deformed enough to prevent the pins from setting correctly. In particular, the first pin tends to false set low. Figure 9.2 shows how excess torque can deform the bottom of the driver pin and prevent the key pin from reaching the sheer line. This situation can be recognized by the lack of give in the first pin. Correctly set pins feel springy if they are pressed down slightly. A falsely set pin lacks this springiness. The solution is to press down hard on the first pin. You may want to reduce the torque slightly, but if you reduce torque too much then the other pins will unset as the first pin is being depressed.

It is also possible to deform the top of the key pin. The key pin is scissored between the plug and the hull and stays fixed. When this happens, the pin is said to be false set high.

Loose Plug

The plug is held into the hull by being wider at the front and by having a cam on the back that is bigger than the hole drilled into the hull. If the cam is not properly installed, the plug can move in and out of the lock slightly On the outward stroke of the pick, the plug will move forward, and in and out of the lock slightly. On the outward stroke of the pick, the plug will move forward, and if you apply pressure on the inward stroke, the plug will be pushed back.

The problem with a loose plug is that the driver pins tend to set on the back of the plug holes rather than on the sides of the holes. When you push the plug in, the drivers will unset. You can use this defect to your afvantage by only applying pressure on the outward or inward stroke of the pick. Alternatively, you can use your finger or torque wrench to prevent the plug from moving forward.

Figure 9.2: Driver pin false set by elastic deformation

Pin Diameter

When the pair of pins in a particular column have different diameters, that column will react strangely to the pressure of the pick.

The top half of Figure 9.3 shows a pin column with a driver pin that has a larger diameter than the key pin. As the pins are lifted, the picking pressure is resisted by the binding friction and the spring force.

Once the driver clears the sheer line, the plug rotates (until some other pin binds) and the only resistance to motion is the spring force. If the key pin is small enough and the plug did not rotate very far, the key pin can enter the hull without colliding with the edge of the hull. Some other pin is binding, so again the only resistance to motion is the spring force. This relationship is graphed in the bottom half of the Figure. Basically, the pins feel normal at first, but then the lock clicks and the pin becomes springy. The narrow key pin can be pushed all the way into the hull without loosing its springiness, but when the picking pressure is released, the key pin will fall back to its initial position while the large driver catches on the edge of the plug hole.

The problem with a large driver pin is that the key pin tends to get in the hull when some other pin sets. Inmagine that a neighboring pin sets and the plug rotates enough to bind the narrow key pin. If the pick was pressing down on the narrow key pin at the same time as it was pressing down on the pin that set, then the narrow key pin will be in the hull and it will get stuck there when the plug rotates.

The behavior of a large key pin is left as an exercise for the reader.

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