will
use
this
mo del
to
explain
complicated
mechanical
defects.
The
" atland"
mo del
of
a
lo
c
k
is
shown
in
Figure
3.1.
This
is
not
a
cross
section
of
a
real
lo ck.
It
is
a
cross
section
of
a
very
simple
kind
of
lo ck.
The
purp
ose
of
this
lo
c
k
is
to
keep
tw
o
plates
of
metal
from
sliding
ov
er
eac
h
other
unless
the
prop
er
key
is
presen
t.
The
lo ck
is
constructed
by
placing
the
tw
o
plates
ov
er
each
other
and
drilling
holes
whic
h
pass
through
b
oth
plates.
The
gure
sho
ws
a
t
w
o
hole
lo
c
k.
Two
pins
are
placed
in
eac
h
hole
such
that
the
gap
b etw
een
the
pins
do
es
not
line
up
with
the
gap
b etw
een
the
plates.
The
b ottom
pin
is
called
the
key
pin
b ecause
it
touches
the
k
ey
.
The
top
pin
is
called
the
driver
pin.
Often
the
driver
and
k
ey
pins
are
just
called
the
driver
and
the
pin.
A
protrusion
on
the
underside
of
the
b ottom
plate
keeps
the
pins
from
falling
out,
and
a
spring
ab
o
v
e
the
top
plate
pushes
do
wn
on
the
driver
pin.
If
the
key
is
absen
t,
the
plates
cannot
slide
o
v
er
eac
h
other
b
ecause
the
driv
er
pins
pass
through
b oth
plates.
The
correct
key
lifts
the
pin
pairs
to
align
the
gap
b etw
een
the
pins
with
the
gap
b etw
een
the
plates.
See
Figure
3.3.
That
is,
the
key
lifts
the
key
pin
un
til
its
top
reaches
the
lo
c
k's
sheer
line.
In
this
conguration,
the
plates
can
slide
past
eac
h
other.
Figure
3.3
also
illustrates
one
of
the
imp
ortan
t
features
of
real
lo cks.
There
is
alwa
ys
a
sliding
allow
ance.
That
is,
any
parts
whic
h
slide
past
eac
h
other
must
b e
separated
b
y
a
gap.
The
gap
b etw
een
the
top
and
b ottom
plates
allows
a
range
of
keys
to
op en
the
lo ck.
Notice
that
the
righ
t
k
ey
pin
in
Figure
3.3
is
not
raised
as
high
as
the
left
pin,
y
et
the
lo
c
k
will
still
op en.
7
Figure
3.1:
Flatland
mo del
of
a
lo ck
Figure
3.2:
(a)
Flatland
k
ey
raises
pins
Figure
3.3:
(b)
Prop
er
key
allo
ws
plates
to
slide
8