Know Your Locks
You see combination locks every day, but have you ever stopped
to think what is inside? In this edition of How Stuff Works
we'll unlock the secrets of a combination lock! We will be exploring the lock in diagram 1A.
Getting inside a lock is not always easy. If it were easy,
it would defeat the purpose of using the lock in the first
place! But once you do get inside you find a collection
of parts like shown in diagram 1B.
There are three cams in a typical combination lock. In
this lock one of the cams is metal and is bonded directly
to the turning face of the lock. The other two cams are
plastic. There are two plastic spacers that fit between
the cams. (diagram 1C)
They fit on a shaft molded into the back of the lock with
a spring pressing the stack of cams together when the lock
is assembled. The spring provides friction between the cams
to hold them in place. (diagram 1D)
The cams each have one tooth on each side, and these teeth
engage as the cams rotate (diagram 2A)
The purpose of the cams is to control a latch that engages
the end of the lock's clasp. The cams each have an indentation
in them. When the indentations align properly the latch
is able to fit into the indentations and release the clasp. (diagram 2B)
Otherwise the latch engages the clasp like this:
If you have used a combination lock before you know the
drill: "Turn the dial clockwise two full rotations
to the first number of the combination. Then turn it counter-clockwise
past the first number to the second number. Then turn the
dial clockwise to the third number and the lock will open."
You can now see why you have to do that. Turning the dial
2 full revolutions gets the teeth of all three cams engaged,
so the three cams are turning in unison. Now when you turn
counter-clockwise only the top cam is rotating. As you go
past the first number, the first cam's tooth engages the
second cam, so now the two cams are moving. The friction
provided by the spring, however, keeps the third cam in
its position. When you turn the dial clockwise again, only
the top cam rotates. Once all three indentations are aligned
properly by this process, the lock opens.
You could, in theory, stack up 4 (or 10) cams if you wanted
to. It would make for a tedious session opening the lock,
but it would certainly be secure!
--www.howstuffworks.com--
