What Happens When the Knob is Turned?
All of that portion of the lock which is shown above the round central
post is operated by the knob, the spindle of which passes through the
square hole. Before the knob is turned, the parts are in the position
shown in figure 2, with the latch bolt protruding. Turning the knob to
the left gives the position shown in figure 1, the upper lever in the
hub pushing back the yoke, which in turn pushes back the latch bolt.
When the hand is removed, the springs cause the parts to return to the
position shown in figure 2. Turning the knob to the right also retracts
the latch bolt, as shown in figure 3, by means of the lower lever on
the hub.
The spiral spring on the latch bolt is lighter than the one above
it. This gives an easy, lively action to the bolt, with very little
friction when the door is closed, while the heavier spring above gives
a quick and positive action of the knobs.
What Happens When the Key is Turned?
All of that portion of the lock which is shown below the round central
post is operated by the key. The square stud is attached to the bolt,
and in figure 1, it is seen that the projections on the flat tumblers
prevent the stud from moving forward, holding the bolt in retracted
position. When the key is turned as shown in figure 2, it raises the
tumblers releasing the stud, and then pushes the bolt out, the tumblers
falling into position as shown in figure 3, with the projections
again engaging the stud and preventing the bolt from moving until the
key is turned backward, again raising the tumblers and releasing and
retracting the bolt.
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_.
[Illustration: HOW A CYLINDER LOCK WORKS]
[Illustration: FIGURE 1. PARTS OF CYLINDER LOCK.]
[Illustration: FIGURE 2.