A Breguet overcoil is a hairspring whose outermost turn is partly lifted into a different plane from the other coils. It is the most widely used way to make a balance spring isochronous. The raised coil helps the balance keep the same rate whether the mainspring is fully wound or running down.
How does a Breguet overcoil work?
In a flat spring, every coil lies in one plane. In an overcoil, the outer turn is raised above the rest. That change lets the spring open and close more evenly and symmetrically as the balance wheel swings back and forth. In theory, it expands and contracts more concentrically than a flat spring, and more efficiently. With the overcoil, the balance spring took on a concentric form.
Where it comes from
Abraham-Louis Breguet devised the overcoil in 1795. He lifted the spring’s last coil and made its curve less tight. Before that, balance springs were flat. They were made of copper or iron and had few coils. The overcoil improved on that earlier design. The Breguet firm moved to silicon hairsprings in 2006. It has since made silicon springs that have its signature terminal curve.
What kinds of overcoil are there?
There are two kinds: the gradual overcoil and the Z-bend. The Z-bend lifts the coil into the second plane with two kinks at complementary 45-degree angles. It is the harder of the two to make. Overcoils are hard to form, so many modern watches use a ‘dogleg’ instead. A dogleg moves the outer coil aside with sharp bends but keeps it in the same plane. It works slightly less well than a true overcoil.
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