How Key Changes Are Provided
There are three ways in which keys are made individual to the locks they fit.
a. By changing the shape of the keyhole. This may be done shorter or longer, wide or narrow, straight or tapering and with projections on the sides which the key must fit, making it difficult or impossible for keys of a different class to enter the lock. In the lock shown, a projection on the keyhole will be noted, fitting a groove in the bit of the key.
b. By wards attached to the lock-case. The two crescent-shaped wards seen near the key in figure 2 illustrate this feature. Similar wards are placed on the lock cover. These fit into the two notches shown on the key bit in figure 4, and their shape and position are varied at will.
c. By changes in the tumblers. There are five flat tumblers in the lock shown, and their lower edges fit into the end of the key bit. By varying their height, changes in the cutting of the key are made necessary.
The security of a lock depends very largely upon its being so made that no key will operate it except the one which belongs to it, and this is obtained by guarding the keyhole by means of a, by preventing the wrong key from turning by means of b, and by still further limitations by means of c.
HOW A CYLINDER LOCK WORKS
FACE OF CYLINDER LOCK.
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.
INTERIOR OF CYLINDER LOCK WITHOUT MASTER KEY.
INTERIOR OF MASTER-KEYED CYLINDER LOCK.