The Cylinder Lock.
Door locks of the highest grade of security are made with a locking
cylinder, which contains tumblers in the form of miniature bolts which
make it impossible to operate the lock except with the key to which it
is fitted. This is screwed into the lock-case through the side of the
door, with the lever on the inner end engaging the end of the bolt in
the lock, so that as it is moved it either retracts or “throws” the
bolt as desired.
Figure 1 shows all the parts of a modern master-keyed lock. Figure
4 shows a broken view of the cylinder with all parts in position.
Figure 3 shows a simpler form used when the master key is not desired.
Figure 2 shows the front, the only part which is visible when the lock
is in use, with its keyway of tortuous shape which will not admit
flat-picking tools.
When the lock is assembled, the pin tumblers project through the shell,
the master cylinder and the key plug holding all parts firmly bolted or
fastened together. When the proper key is inserted, the tumblers are
raised until the “breaks” in all of them coincide with the surface of
the key plug, releasing it and permitting the key to turn it. If any
one of the five tumblers is .002 inch too high or too low, the key will
not turn; so that no key except the one made for the lock can be used.
In the master-keyed lock, the master key causes the breaks to coincide
with the outer surface of the master ring. It is thus possible to
have a master key which will fit any desired number of locks with the
individual or change keys all different from each other and from the
master key.
The balls reduce friction to such an extent that a key has been
inserted and withdrawn for a million times without affecting the
accuracy of the lock.
[Illustration: FIGURE 3.