Mainspring (Zugfeder)
The coiled spring inside the barrel of a mechanical watch movement that stores the energy used to drive the gear train. Power-reserve duration depends on the mainspring's length, width, and material; service-wear of the mainspring is one of the most common service-replacement items.
The mainspring (German Zugfeder) is the coiled spring inside the barrel of a mechanical watch movement that stores the energy used to drive the gear train. When the watch is wound, the mainspring is coiled tighter around the barrel arbor; as it unwinds, the stored tension drives the gear train through the escapement and ultimately moves the hands.
How the mainspring delivers energy
A wound mainspring at full tension pushes hardest, gradually losing torque as it unwinds. Two strategies handle this:
- Stopwork mechanism. A mechanism that prevents the mainspring from fully unwinding to its weakest tension range and prevents winding past its safest tension. Common in vintage movements; replaced by slipping bridles in modern automatics.
- Slipping bridle (modern automatic). Used in automatic movements; the outer end of the spring slips against the barrel wall when over-wound, preventing damage but providing continuous winding from the rotor.
The "useable" torque range is typically the middle 60 to 80 percent of the mainspring's wound state; the highest and lowest tension ranges produce inconsistent rate.
Power reserve
Power reserve duration depends on:
- Mainspring length. Longer spring → more turns of energy stored.
- Mainspring width and thickness. Bigger spring → more energy per turn.
- Multiple barrels. Some movements (Patek Philippe 240 PS, Lange L901) have two or three mainsprings in parallel barrels for extended reserve.
- Movement efficiency. Lower friction in the gear train and escapement extends usable reserve.
Typical power reserves:
- 38-42 hours — common in basic ETA and Sellita-based movements.
- 48-50 hours — modern Rolex and most contemporary in-house calibers.
- 70-80 hours — modern long-power-reserve movements (Tudor MT calibers, Omega Master Chronometer family).
- 8 days — Panerai P.5000 series, some specialised long-reserve movements.
Material and longevity
Modern mainsprings are made from special alloys:
- Carbon steel (traditional, pre-1970s). Required frequent replacement; susceptible to "setting" (taking a permanent curl that reduces output).
- Modern alloy springs (Nivaflex and similar). Set-resistant, longer service life. Standard in contemporary watches.
A worn mainspring typically presents as significantly reduced power reserve (e.g., a 48-hour movement that now runs only 30 hours) or as rate inconsistency between fully-wound and partially-wound states.
Service implications
Mainspring replacement is a standard part of manufacturer service:
- At every regular service interval (5 to 7 years for traditional movements, 8 to 10 years for modern long-life calibers).
- When power reserve has degraded beyond original specification.
- When the spring has visibly set (the watchmaker inspects the unwound state).
The replacement is straightforward for current production movements where the spring is OEM-stocked. For vintage references where the OEM mainspring is no longer available, a watchmaker selects the closest matching spring from available stock — a process that can introduce subtle changes in rate behaviour.
For dealers, a watch with significantly degraded power reserve is a "needs service" finding rather than a deal-breaker. The cost is included in pre-sale service or disclosed to the buyer.
Related: caliber, escapement, balance-wheel, manufaktur-service.