Free-sprung balance
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The free-sprung balance (also: variable inertia balance) is the most precise regulating organ used in mechanical luxury watches. Unlike a traditional index-regulated system—which adjusts the effective length of the hairspring—the hairspring here remains completely free. The watch’s rate (frequency) is adjusted by altering the moment of inertia of the balance wheel itself.
- How it works: Tiny adjustable weight screws or rotating weights (called masselots) are mounted directly on the balance wheel. Turning these weights inward reduces the inertia and makes the watch run faster (similar to an ice skater pulling in their arms during a pirouette). Moving them outward slows the watch down.
- Famous proprietary systems: Gyromax (Patek Philippe) and Microstella (Rolex).
- Key advantages: Exceptional shock resistance (no regulator index that can shift during impacts) and superb long-term rate stability, allowing the hairspring to breathe perfectly concentric.