Mainspring
The mainspring is the coiled steel ribbon that stores a mechanical watch's energy. Winding tensions it, and as it unwinds it slowly releases that energy to the movement. The total energy the spring holds sets how long the watch runs unworn, known as its power reserve.
At a glance
- Function
- Stores mechanical energy
- Location
- Inside the barrel
- Determines
- Power reserve duration
The mainspring is the power source at the heart of the movement. It sits inside a cylindrical housing called the barrel, with one end hooked to the barrel wall and the other to a central arbor. Winding turns that arbor and tightens the spring.
How energy is stored and released
The tensioned spring drives the movement through the gear train as it unwinds, but it does so in a controlled way rather than all at once:
- Winding: turns the arbor, tightens the spring, and loads energy
- Unwinding: the spring slowly opens, feeding the gears and escapement
- Power reserve: how long a full spring keeps the watch running
In an automatic watch the spring is wound by the motion of your wrist, while a manual-winding watch relies on you turning the crown by hand.
What sets the running time
The spring's length, thickness, and alloy decide how much energy it stores, and therefore the power reserve. For more theory, browse our movement terms; for a real-world example, read our Orient Mako 3 review.
Examples
In an automatic diver the mainspring is wound through the day by the motion of your wrist; set the watch down and it keeps running for the length of its power reserve.
View this watchOn a hand-wound watch you fill the spring yourself with a few turns of the crown each day, since no rotor winds it for you.
Comparison
How the spring gets wound depends on the watch type.
| Option A | Option B | Notes |
|---|---|---|
| Automatic winding | Manual winding | An automatic uses a rotor to wind the spring from wrist motion; a manual watch is wound by hand at the crown. |
Related terms
Watches that show this
Frequently asked questions
What does the mainspring do?
The mainspring is a mechanical watch's energy store. Winding tensions it, and as it unwinds it slowly feeds that energy to the gear train and movement, keeping the hands turning at a steady rate.
How does the mainspring set the power reserve?
The total energy the spring can hold decides how long the watch runs on a full wind. A longer or more efficient spring stores more energy, so the power reserve is longer.
Can you overwind the mainspring?
On most modern watches the spring stops accepting more once full, so true overwinding is prevented. Automatics use a slipping bridle in the barrel; on a manual watch you simply stop when the crown meets firm resistance.