Watch Technology: Constant Force

As we all know “precision” is the holy grail of watchmakers from the inception of the concept of time. And one of the main obstacles in a reliable mechanical timekeeping is the unstable delivery of power to the movement regulator (escapement). The term “constant force” is often heard in the world of horology.

As we can see on this graph. The x-axe is the torque/power and the y-axe is the number of turns mainspring completes inside its barrel.

To understand constant force, we need to go back to mainspring and barrel. As we all know that mainspring is a powerful ribbon spring which delivers to the watch. Because of the nature of spring, in any mechanical device, the force delivered by a spring is not uniform as it unwinds due to the dampening forces acting on it. Therefore, the delivered force is maximum at the beginning which decreases when spring is almost unwound.

The result of this fluctuating energy is the diminished precision. Since the escapement of the watch works on the varying driving force. Making it difficult to regulate the watch since balance wheel is not isochronous – meaning the time of oscillation is independent of the size of oscillation or swing. Therefore, a watch does not run at the same rate throughout its duration of the run. A problem which was an issue for many master watchmaker’s such as Lepine, Breguet and Berthoud was first acknowledged by a watchmaker called Peter Heinlein of Nuremberg in the 16th century with a compensatory possibility known as Stack Freed. This intervening device placed pressure on a cam attached to the barrel to equalize the force delivered – a counter-intuitive device given that watchmakers always try to reduce the friction.

Another old solution dating back to 15th century is fusée and gut cord (later a chain) which consists of a cone-shaped pulley linked to a chain coiled around the the mainspring unwinds the chain rolls on the barrel off the fusée (very much like gears of a bicycle) the increasing leverage of the fusée compensates the waning torque of the barrel.
Fusée and chain mechanisms are extremely complex to manufacture, difficult to adjust making them very rare and expensive component. Also, they require a lot of space to give an idea Zenith’s Academy George Favre-Jacot has 18mm chain consists of 575 parts.

As an evidence of superb craftsmanship and manufacturing skills, many watchmakers and manufacturers have used them in recent years like Breguet, Lange and Sohne, Zenith, Cabestan and Ferdinand Berthoud.

With exception to Romain Gauthier, who also chose to improve this device by using short ruby linked chain together with snail cam instead of a conical fusée sapphire lined mainspring barrel. All components are positioned on the same plane so the force is transmitted in a straight line. In his superb timepiece called Logical-One. The basic principle is same but his execution will theoretically be more efficient and stable.

 Another barrel/spring improvement is Stop-works. It prevents the over winding of the mainspring. By stopping the winding before the mainspring is completely coiled around the arbor.

The most famous style of stop-woks is Maltese cross mechanism also known as Geneva Stop works
It is important to note that while all these mechanisms are used to ensure some degree of constant force, but they are generally not referred as constant force mechanism or Remontoir d’egalite which are found down the gear-train either just before the escapement.
With improvements of escapements and barrels, modern watches deliver consistent power. Making these devices unnecessary and are seldom used today.