The
movement–the mechanism that measures the passage of time and
displays the current time–is the heart of any clock. Most clocks
have either a mechanical or a battery-operated quartz movement.
While most of today’s clocks have battery-operated movements, older
antique clocks have mechanical ones.
Compared to electronic movements, mechanical clock movements
are less accurate, often with errors of seconds per day, and they’re
suffer from sensitivity to position and temperature, requiring
regular maintenance and adjustment.
Mechanical clock movements use an escapement mechanism to
control and limit the unwinding of the spring, converting what
would otherwise be a simple unwinding, into a controlled and
periodic energy release. This is accomplished through the use of
gears and a lever attached to a pendulum. The most common form in
use today is powered by key- wound springs.
The
verge escapement, also known as the crown-wheel-and-verge
escapement, developed in 1275 C.E., predates the pendulum and
works like a teeter- totter. By attaching the verge escapement to
a pendulum, clockmakers were able to increase the accuracy of
their clocks tremendously, from about 15 minutes per day to 15
seconds per day. Soon clockmakers began retrofitting their clocks
with pendulums. But for this to work the axis of the verge had to
be horizontal. And though this design was good at keeping time,
the pendulum had to swing up to 100 degrees. So to avoid the need
for a very large case, clockmakers used a short pendulum.
Clocks from the late 17th century used the anchor
escapement, also known as recoil escapement, invented about 1670.
Before then, pendulum clocks had used the older verge escapement,
which required very wide pendulum swings of about 100 degrees. To
avoid the need for a very large case, most clocks using the verge
escapement had a short pendulum.
The
teeth of an anchor escapement wheel project radially from
the edge of the wheel, much like an upside down anchor. This
escapement reduced the pendulum's swing to between four and six
degrees, allowing clockmakers to use longer pendulums with slower
beats. These required less power to move, caused less friction and
wear, and were more accurate than their shorter predecessors. Most
longcase clocks use a pendulum about 39 inches long to the center
of the bob, with each swing taking one second. This requirement
for height, along with the need for a long drop space for the
weights that power the clock, gave rise to the tall, narrow case
of grandfather clocks, the first made by William Clement
about 1680. With the increased accuracy that resulted from these
developments, clockmakers began adding minute hands to their
clocks around 1690.
When the pendulum swings the lever locks in the tooth of the
gear, it produces the tick. The back swing of the pendulum
the lever releases the gear produces the tock. The process
is repeated over an over until the clock needs winding. Moving the
pendulum bob up or down adjusts the speed and accuracy of the
clock. Adjusting the bob upward speeds up the clock, adjusting the
bob downward slows down the clock.
By 1715, George Graham introduced the dead beat escapement which
then led to the development of the extremely accurate lever
escapement. Clockmakers rounded the edge of the anchor so as
not to lock the gear. This allowed the pendulum to operate the
clock on both back and forth swings which greatly reduced wear on
the escapement. In most wall clocks that use a pendulum, the
pendulum swings once per second. In small cuckoo clocks the
pendulum might swing twice a second. In large grandfather clocks,
the pendulum swings once every two seconds.
