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The mainspring is the primary power source for a watch movement.

The mainspring is the main driving force and directly manipulates the train of wheels causing them to turn.

A watch mainspring is located within a container called the barrel. The barrel can also known as the great wheel.

The Mainspring is hooked on both ends. When under tension the mainspring applies pressure to a corresponding hook in the barrel wall and causes it to turn.

It is a coiled spiral made up of an alloy of metals in order to provide a specific amount of force or strength.

Main springs are covered in several lessons within our Watch Repair Course. These topics include a basic overview in Level 1. Determining how to measure up and choose the correct replacement for a broken main spring is covered in level 2. Learning how to remove and refit a main spring is also covered in level 2. Learning how to troubleshoot issues with the power source in level 3

Modern main springs are usually provided under the GR (General Resorts) code system. They are also commonly sold by their dimensions.

The important dimensions to note are the height, thickness (or strength), length – and the inner diameter of the watch mainspring barrel.

If the main spring height is incorrect, it may foul the barrel lid and cause performance issues. It may also not sit correctly within the barrel causing too much stress on the spring and shortening it’s life.

The thickness directly affects the performance or amplitude of the watch.

The length of the main spring determines how long a watch will run for, but the length is limited by the inside diameter of the mainspring barrel.

Mainspring (Wikipedia)
An uncoiled modern watch mainspring.
Clock mainspring
Keys of various sizes for winding up mainsprings on clocks.

A mainspring is a spiral torsion spring of metal ribbon—commonly spring steel—used as a power source in mechanical watches, some clocks, and other clockwork mechanisms. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it unwinds, until the next winding is needed. The adjectives wind-up and spring-powered refer to mechanisms powered by mainsprings, which also include kitchen timers, music boxes, wind-up toys and clockwork radios.

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