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Shearing pin tumbler lock

11 Jun 2009
Progress: Design Stages

I've never picked or bumped a lock, but from what I gather the basic principle is that you bump, pick or rake each pin to the shear line while applying a turning force, so that once in the correct place, the drive pin and key pin separate. The turning force keeps them in the correct position until all the pins are aligned, and the plug is free to rotate.

I wish to build a lock where instead of the pins being solid cylinders, they are a stack of metal washers. The top and bottom part of each pin would be connected through the centre holes by a small metal chain (possibly only with one or two links).

In normal operation the pins should slide nicely in their tubes, and as they're under compression between the springs and the key, vertically they should be identical to normal pins.

When a wrong key is inserted, the lock would twist about five degrees before stopping, either way. The pins would shear, then the linkage between top and bottom disks in the stack would be clamped at the normal shear line.

When someone attempts to pick or bump the lock, instead of a single point in the drive/key pin column being able to shear, it is able to shear at all the gaps between washers. You would only know if that particular pin is in the correct place once all the other pins are in the correct place. To adjust the vertical alignment of a pin, the lock must be twisted back to vertical and it would be hard to stop all of the pins returning to their default positions.

Essentially this turns the lock into a combination lock. To the user it would be a normal key, but to the lock picker there would be thousands of combinations of pin positions where the lock would turn the first few degrees. Even if all but one of the pins was in the correct position, you would have no idea* if the lock was any closer to being open.

The obvious downside is that the whole assembly would have to be slightly larger, so the linkage between disks is strong enough. That's a small price to pay though for exponentially increasing the time a lock takes to pick. Also, bumping would be almost impossible, since there are so many shear points the pins could stop at, not a single point like in a conventional lock.

Plus the clear advantage that regardless of all this, the pins will not move vertically unless no turning force is applied, which is the key part to any lock picking method.

(*If you had a special tool for it, you could probably measure the amount of play the pin has vertically, which would be much more if you'd managed to separate the drive pin from the key pin. Especially if the key pin was very short.)