Thursday, 18 October 2018 16:37

Chopard's Clever Balance Wheel Adjustment

In 2003, Chopard was granted a patent on a clever system for adjusting a balance wheel on a watch. 

The most common ways of adjusting a balance wheel are a bit clunky. There are weights held on by pins. Screws that can be moved in and out, or removed entirely. Monochrome has a great article about this. All of these are to adjust the balance wheel's moment of inertia.

Think of the balance wheel like a figure skater. The skater takes a big push off, and starts spinning. At this point, the skater has some energy in the spin that is not going to increase. But, if the skater brings their arms closer to their body, they will spin faster. That's because by doing this they are reducing their moment of inertia - the energy it takes to spin - and spinner faster with the same amount of energy in the spin.

Similarly, the balance wheel is always getting a set amount of energy from the watch's power train. So to adjust its oscillation, you need to vary the balance wheel's moment of inertia. 

Enter the Chopard patent. It discloses small u-shaped weights that slot into holes in the outer rim of the balance wheel. By turning the weight at particular angles, you can adjust whether there is more weight toward the outside of the balance wheel (extending the skater's arms and increasing the moment of inertia) or closer to the center of the balance wheel (bringing the skater's arms in).

In a seeming rare case for this site, we can actually see that this patent is in use in a current Chopard watch.

Published in Patents