Parts of a clock and how a clock mechanism works

Every mechanical clock has five systems: a power source (a falling weight or a coiled mainspring), a gear train that carries the power and counts, an escapement that releases the gears one tooth at a time (the tick), an oscillator that sets the pace (a pendulum or a balance wheel), and the motion work and hands that show the time. A quartz clock replaces most of that with a battery, a quartz crystal vibrating 32,768 times a second, a divider chip and a tiny stepping motor.

Key facts

A clock is a machine for counting. Britannica puts it simply: in every clock, "a device that performs regular movements in equal intervals of time is linked to a counting mechanism". In a grandfather clock the regular movement is a swinging pendulum, and the counting mechanism is a chain of gears that turns the hands. Here is how the pieces fit together.

Clock mechanism diagram

The five systems in a mechanical clock

Power: weights and mainsprings

The oldest clocks were driven by a weight hanging from a cord wrapped around a drum (the barrel). As the weight falls, the barrel turns the first gear. Weights give a steady force, which is why tower clocks, longcase clocks and cuckoo clocks still use them. Winding simply lifts the weight back up, by a crank key or by pulling a chain. A ratchet called the click lets you wind the barrel backwards without turning the gears.

A mainspring is a long ribbon of spring steel coiled inside a barrel. It made clocks portable, so mantel, carriage and alarm clocks are usually spring-driven. A spring pulls harder when fully wound than when nearly run down, so spring clocks are designed to even that out. See how to wind a mantel clock.

The gear train: how clockwork gears count time

The going train is the main set of gears. It carries the force from the weight or spring to the escapement and, at the same time, divides the escape wheel's motion into seconds, minutes and hours. In a typical clock:

• The great wheel (first wheel) sits on the barrel and drives the next wheel's pinion.
• The centre wheel turns once an hour; its arbor carries the minute hand through a friction coupling, which is why you can turn the minute hand to set the time.
• The third wheel and, in many clocks, a fourth wheel step the speed up. In clocks with a small seconds dial, the fourth wheel turns once a minute.
• The escape wheel is the last and fastest wheel. It is released one tooth at a time by the escapement.

Each wheel drives a much smaller gear called a pinion on the next shaft (arbor). Because a large wheel drives a small pinion, each step in the train turns faster than the last. Thin pivots at the ends of each arbor run in holes in the brass plates; worn holes are repaired with bushings.

The escapement: what makes a clock tick

Without something to stop it, the gear train would spin round in seconds and the weight would crash to the floor. The escapement is the device that, in Britannica's words, "permits controlled motion, usually in steps". It does two jobs: it locks the train and lets it advance one tooth at a time, and it gives the pendulum or balance a small push on every swing to keep it going.

• The verge escapement, probably invented in 13th-century Europe, made the first mechanical clocks possible. See who invented the mechanical clock.
• The anchor escapement, an English improvement of the 17th century, has pallets shaped like an upside-down anchor. It works with a pendulum swinging through a small arc, which is what made long pendulums and tall longcase cases practical. The Science Museum in London holds a London longcase clock from about 1680 with this escapement.
• The deadbeat escapement is a refinement used in precision regulators and many modern pendulum movements.

The "tick" and "tock" are the two pallets catching the escape wheel. When they are evenly spaced, the clock is in beat.

The pendulum and the balance wheel

The pendulum is the timekeeper. Its swing time depends almost entirely on its length, not on how wide it swings, which is why Christiaan Huygens's pendulum clock of 1656 was so much more accurate than anything before it. A pendulum that takes one second per swing is about 99.4 cm (39.1 in) long, the length that fits neatly in a grandfather clock. The bob is the heavy weight at the bottom; the rating nut under it raises or lowers the bob to speed the clock up or slow it down. At the top, a thin suspension spring lets the pendulum swing without friction, and the crutch connects the anchor to the pendulum rod.

Portable clocks and watches use a balance wheel instead: a small wheel that twists back and forth on a fine hairspring. It works in any position, which is why carriage clocks and alarm clocks use it.

Motion work and hands

The minute hand is driven by the centre wheel, but the hour hand needs to turn 12 times slower. That job belongs to the motion work, a small set of gears behind the dial. The cannon pinion, a hollow tube that carries the minute hand, drives the minute wheel, whose pinion drives the hour wheel; the hour wheel fits over the cannon pinion and carries the hour hand, turning once for every 12 turns of the minute hand.

Strike and chime trains

A striking clock has a second gear train with its own weight or spring, which lifts a hammer to strike the hours on a bell, gong or rods. A chiming clock adds a third train that plays a tune on the quarter hours. Each train has its own winding point, which is why a grandfather clock has three weights and a chiming mantel clock has three winding holes. The trains are released by levers lifted by the motion work at the right moments. More in clock chimes explained.

How a quartz clock mechanism works

Most clocks sold today are quartz. Quartz is piezoelectric: it bends slightly when a voltage is applied, and produces a voltage when bent, so a tiny crystal can be made to vibrate at a very steady rate. Most clocks and watches use a crystal cut to vibrate at 32,768 times a second, which is 2¹⁵. A chip halves that frequency 15 times to get exactly one pulse per second. In an analog clock, each pulse drives a small stepping motor (a Lavet-type motor), which turns a short gear train and the hands. A typical consumer quartz clock gains or loses about 15 seconds a month, mostly because the crystal's rate changes with temperature. The first quartz clock was built at Bell Telephone Laboratories in 1927.

Clock parts glossary

Sources

• Britannica: Clock
• Britannica: Escapement
• Wikipedia: Wheel train
• Wikipedia: Quartz clock
• Science Museum Group: month-going longcase clock, London, c. 1680
• Science Museum Group: model of the anchor escapement used in longcase clocks

Last updated November 6, 2026. Found an error? Tell us.

More about clocks

• The mechanical clock
• Replace a clock movement
• Watch anatomy
• Atomic clock
• All clock guides
• Types of clocks
• Grandfather clocks
• Winding a mantel clock
• Clock troubleshooting