Why 400 Day and 1000 Day Clocks Run So Long

Most clocks run one day or eight days on one winding. A 400 day clock has a torsion pendulum to make it run longer. A torsion pendulum hangs from a thin flat wire (called the suspension spring) and rotates in one direction and then the other, instead of swinging back and forth as a standard pendulum does.

Standard 400 Day Clock (about 12 inches tall)

The torsion pendulum of a Standard or full-sized 400 day clock rotates in 7.5 seconds, or 8 rotations each minute (four rotations in each direction). The clock makes one “tick” near the end of each rotation. Preceding each tick, a gear with pointed teeth called the escape wheel gives an impulse to the anchor. During impulse, the vertical pin in the anchor pushes the fork to the left or right. The fork applies this energy to the suspension spring (the thin wire holding the pendulum), and the suspension spring transfers this energy to the pendulum.

Escapement of Schatz Standard 400 Day Anniversary Clock, observed from front:

The clock movement supplies one unit of energy to the pendulum for each oscillation. A torsion pendulum oscillates about 8 to 20 times more slowly than a swinging pendulum. This represents a major savings in energy, enough to increase an 8-day running time to 64 to 160 days. Further energy savings comes from using smaller pivots (load bearing areas on the ends of gear arbors or shafts) and by using small gears. By having more gears than an 8 day clock, the mainspring unwinds slowly enough for the clock to run about 400 days on one winding.

Miniature 400 Day Clock (about 8 – 9 inches tall)

Miniature 400 day clocks use the same principles as the standard models, but have smaller gears and pivots, making them more energy-efficient. On the most popular models, those by Schatz and Kundo, the pendulum makes 10 rotations per minute. Other miniature 400 day clocks have pendulum rotation rates of 6 to 10 rotations per minute.

Why 1000 Day Clocks Run So Long

The Schatz 1000 day clock, based on Schatz’s miniature 400 day clock movement, has a larger mainspring with about 2.5 times the amount of energy storage. The pendulum makes 10 rotations per minute.


Escapement of Schatz standard 400 day clock, observed through the peep holes in the back plate:


Video showing 400 day and 1000 day clocks in operation (note – 400 day and 1000 day clocks do not strike or chime, the sounds you hear are in the background):

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  1. The ratchet keeps the mainspring from unwinding, so that it can operate the clock. You turn the winding square counter-clockwise (as you face the back of the clock) to wind the clock. If it won’t go any further, it is wound all the way up. Do not try to unwind it, you may damage the clock or hurt your hand or fingers. If the clock is not running, it may need to be overhauled. The following web page gives information about my repair service: https://billsclockworks.com/repair/400Day.htm

  2. I have a 400 day clock that has a rachet on the mainspring and I’m not sure I understand its purpose. Doesn’t it prevent the mainspring from moving at all? If so, how is the mainspring able to power the clock and escapement at all? Do I need to remove the rachet to wind it, and are there risks involved with the loaded energy?

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