The mechanical watch
How does a spring keep time for two days?
The question
A wound spring wants to let go all at once. Inside a mechanical watch, it takes two days to unwind, and the second hand ticks on at a steady beat. What holds it back?
Have a guess, then show the answer
The escapement. A balance wheel swings back and forth four times a second, and each swing rocks a little fork that lets the escape wheel slip on by half a tooth, then locks it again. So the spring's energy leaks out in 28,800 tiny steps an hour, and the gears count them.
Play
Wind it, then slow it down
This is a watch movement with the dial taken off. Drag the crown up, or press Wind, and watch the balance swing. Real time is a blur, so turn the speed down to see each tick. Then take the escapement out and watch the spring run down at once.
Stopped. Drag the crown up, or press Wind the crown.
Drag the crown up, use the buttons, or focus the crown and press the up arrow.
What's happening
- 1
Turning the crown winds a long flat spring, the mainspring, coiled inside a drum called the barrel.
- 2
The barrel drives a row of gears, the gear train, that ends at the escape wheel, a wheel with hooked teeth.
- 3
A forked lever, the pallet fork, sits in the escape wheel's teeth and locks it. Its other end is touched by the balance wheel, which swings back and forth on a hair-thin spring.
- 4
On every swing the balance knocks the fork across. The escape wheel slips forward half a tooth and gives the fork a small push, which keeps the balance swinging. That is the tick.
- 5
A modern balance swings 4 times a second, so 8 ticks a second. Take the escapement out and nothing holds the gears back. The spring empties in moments.
The maths
A beat is one swing of the balance, one way. At 4 swings back and forth a second
With a 15-tooth escape wheel moving half a tooth per beat, it turns 16 times a minute while the seconds wheel turns once.
Simplified, and sources
The drawing is simplified. The gears are flat discs in a row, not stacked as in a real movement, and apart from the escape wheel to the seconds wheel (16 to 1), their ratios are shortened so you can see the slow ones move. The 40-hour reserve is a typical figure, not a particular watch, and it is not drained while you watch. The balance spring is left out, and the balance swings at a steady 280 degrees, where a real one loses swing as the spring runs down. With the escapement out, how fast the spring runs down is made up; a real one would be over in a blur, and can damage the watch.