A free-sprung balance is a balance with no regulator acting on its balance spring. Instead, the rate is set with screws or weights on the balance itself. The hairspring is held at each end and can expand and contract with no curb pins touching it. Rolex and Patek Philippe both use the design.
How does a free-sprung balance work?
A free-sprung system runs the balance wheel without any index or regulator arm. The balance spring is held at both ends, and no curb pins touch it. To change the rate, the watchmaker moves mass on the balance itself, using meantime screws or small weights mounted on the wheel. Moving mass towards the rim raises the balance’s inertia, so it swings more slowly. Moving mass away from the rim has the opposite effect.
How is it different from a regulated balance?
In a regulated balance, a regulator arm sits over the balance spring. Moving the arm changes how much of the spring is effectively active, and so changes the rate. A free-sprung balance has no such arm. Its spring is left alone, free to expand and contract. The trade-off is practical. Adjusting a free-sprung balance is more complicated, and it takes more labour to produce.
What is the Patek Philippe Gyromax?
The Gyromax is Patek Philippe’s free-sprung balance. It was granted Swiss patents in 1949 and 1951, and first appeared in watches in 1952. Six to eight small rotatable weights, called collets, sit on pins in recesses in the top of the rim. Turning a single collet corrects the wheel’s poise. Turning opposing pairs changes its rate. Rolex’s Microstella balance is built along similar lines.
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