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Pendulum (watchmaking)

The balance is a moving part used to regulate the movement of a clockwork mechanism .

Rocker of a clock

Balance with anchor exhaust
Clock mechanism with rocker, Lichtenwalde Castle Clock Museum

The pendulum of a clock is a moving element that is reciprocated in a reciprocating motion. It is horizontal or circular at the beginning and is called foliot or pendulum in current watches. It may also take the form of a pendulum , consisting of a vertical rod, capable of oscillating about a horizontal axis, and having a weight at its low end. This weight is generally in the form of a domed disc, usually a heavy metal (such as steel), in order to reduce the influence of the air resistance forces .

The movement of the pendulum is regulated and perpetuated by a mechanism called escapement , generally of the “anchor” type; exhaust provides the counting pulses to the escapement wheel, which drives by gear wheel of the second and those of the minutes and hours.

The maintenance of the movement was assured in the old clocks by the descent of a counterweight connected to a drum by means of a chain or a cord. It was periodically back up this counterweight.

Relaxing a spring is currently preferred because of its small footprint; This spring also “comes up” with a key

Causes of Error

The pendulum period is related to the length of the stem. The longer it is, the slower the beat. As a result, during a change in temperature, the period changes, modifying the time measurement. To obviate this drawback, certain improved clocks use a balance with mercury reservoirs, the expansion of which compensates for that of the balance rod while maintaining the center of gravity of the balance substantially in the same place.

Other temperature compensation systems exist. The principle is to realize the balance by means of rods made of two metals having different coefficients of expansion (generally iron and brass). The points of attachment of the rods are arranged in such a way that the dilatations are opposed and compensated. This technique has the advantage of being less expensive (and also less polluting) than the use of mercury.

Clock of a watch

A bimetallic pocket watch pendulum with its spiral spring .

In watches , or chronometers , the balance is a flywheel of inertia , generally circular, which pivots about an axis of rotation. A spiral spring is coupled to it and allows the pendulum to oscillate with a regular movement.

This movement is maintained by the anchor, which imparts to the pendulum a pulse at each of the two alternations of an oscillation. The exhaust acts as a lever arm, and the force of the main spring, transmitted by the cogs, is exerted on the friction surfaces of the anchor. In the first escapements of the Roskoff type, the axis of the recessed balance was used as an escapement .

Each of the alternations of the oscillations of the pendulum produces a slight noise responsible for the “tic-tac” characteristic of watches.

The friction surfaces of the exhaust, or those receiving the balance shaft, are generally made of synthetic ( ruby ) stone to prevent wear.

The majority of watches are called ” anti-shock ” thanks to a system of protection of the axes by means of very thin steel springs (mounted at the end of axes).

In mechanical quartz watches, movement is ensured not by the expansion of a spring but by an electric motor supplied by a battery whose pulses are controlled by an electronic circuit called a quartz oscillator because the accuracy of its Resonance frequency is ensured by a quartz ( electronic component ).


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