Why the Test for the First Number Sometimes Works and Sometimes Doesn't

Before reading this page, if you haven't already, please read the pages on how a combination lock works.

When you turn the dial twice around to the right and dial the first number, the notch on the back cam lines up under the pawl. When you turn the dial left almost a full turn, at approximately 3 marks before the first number, the peg on the front cam starts to push the peg on the middle cam and both move. Just before the correct third number is reached, the deep notch on the first cam lines up under the pawl. Now, when the shackle is pulled, the middle cam is the only cam holding back the pawl. The full weight of the pawl bears on the middle cam. Turning the dial moves the front cam and the middle cam. The pawl is dragging on the surface of the middle cam, so it provides resistance, and the dial is harder to turn. However, when the dial turns right, only the front cam turns, the middle cam bears the full weight of the pawl, and the dial turns freely. If you turn the dial left again, this time only the front cam turns, and the dial turns freely.

Now, repeat the test, but do not dial the correct first number. When you turn the dial twice around to the right and dial an incorrect first number, the notch on the back cam does not line up under the pawl.When you turn the dial left to the correct third number and pull on the shackle, the pawl is held back by both the middle and back cams. If the cams are equally sharing the pressure from the pawl, the middle cam is bearing the half the force, so it doesn't provide as much resistance. In theory, it should provide half the resistance. The dial should be a little more than half as hard to turn.

However, the two cams are unlikely to share the pressure equally, especially if they have slightly different diameters. If the back cam gets more than half the weight, the dial should be less than half as hard to turn, and the difference is more noticeable. If the middle cam gets more than half, the dial may seem to be just as hard to turn as when the correct first number had been dialed. It will be difficult to tell any difference.

Besides being different diameters, the cams may be slightly out of round, and the holes in the centers of the cams may be slightly off center. These factors may help or hinder us in locating the first number.

If the test is performed at a fake sticking place, the pawl may drag on the surface of the first cam, at the bottom of the shallow notch, as well as one or both of the other cams. When that happens, it is easy to detect, since the dial does not turn freely. On some locks, at some of the fake sticking places, the back cam, when its notch is not lined up, takes nearly all the load of the pawl. When the notch on the back cam lines up, the load transfers to the bottom of the shallow notch. The dial becomes harder to turn and we know that we have located the correct first number.

Click here to return to the previous page.