Why Doesn’t a Mainspring Break?

February 25th, 2010 | 3 Comments

Given that watches are often worn for decades, it is easy to wonder how the mainspring keeps accurate time and doesn’t break from being wound and rewound hundreds and thousands of times. This article will explore how the mainspring stays strong in the face of continuous use.

The Mainspring’s Function in a Watch Movement

The mainspring is wound around an axle (the arbor), with its inner end hooked to the axle. The typical, nearly universal way the inner end is attached is with a hole, or eyelet, in the spring, and a finger on the arbor that clicks into the hole. The outer end of the mainspring is attached to a cylindrical box, called the barrel, by small hooks or tabs.

The spring winds by turning the arbor, and after winding its force turns the arbor, and therefore the barrel, the opposite way to run the watch. To prevent the spring from unwinding in a matter of seconds, the winding mechanism must have a ratchet and pawl (or click).

To see how a mainspring is assembled, check out this great video:

This complex part has a rich history behind its modern functionality.

Mainspring History

While mainsprings are now a reliable form of power for mechanical and automatic watches, it wasn’t always that way. Mainsprings first arrived on the time-telling scene in Europe during the 15th century, resulting in smaller, more portable clocks. Previously, clocks had been driven by weights, which aren’t conducive to wrist or pocket wear. The oldest surviving mainspring-powered clock is the Burgundy Clock, which currently resides at the Germanisches National Museum in Nurnberg.

These early mainsprings were fashioned from steel, without tempering (which hardens or toughens the metal), and broke easily. During their lifespans, the mainsprings did not run for long and had to be wound twice each day.

Until the 1960s, because mainsprings are constantly stressed, they generally broke from metal fatigue long before other parts in the watch. Mainsprings were considered to be expendable and were the largest cause of watch repairs.

Another problem with early mainspring-driven clocks and watches was non-constant force; it diminished as the spring unwound, which is problematic because timepieces have to run at a constant rate to keep accurate time. Therefore, early clocks and watches slowed down as the mainspring ran down!

Read on to learn how innovations turned the early, breakable mainsprings into today’s stalwarts.

Innovating the Mainspring

Since the mainspring is a crucial piece of a watch’s movement, its feeble nature was troubling. So how did we reach today’s durable and dependable mainsprings?

An early solution to the diminishing force problem was the Stackfreed, a cam mounted on the arbor. Early in the mainspring’s unwinding, when the mainspring was pushing strongly, the stackfreed provided an opposing force, and later, when the mainspring was almost run down and pushed weakly, it provided a helping force. While the idea sounds great, the stackfreed actually added too much friction and substantially reduced a timepiece’s lifespan. It was abandoned after about a century.

The Fusee, a cone-shaped pulley turned by a chain wrapped around the barrel, was an innovation that lasted much longer. Its curved shape continuously changed the mechanical advantage to even out the force as the mainspring ran down. Fusees were used until the 1800s, when the going barrel gained popularity.

The going barrel was invented in 1760 by Jean-Antoine L├ępine. It produces a constant force by using a longer than necessary mainspring. The mainspring is coiled under tension in the barrel and only the inner turns of the spring are used. The result is that the barrel provides nearly constant force over the watch’s design running period.

The appearance of automatic watches has further eradicated this problem. Since automatic watches keep the mainspring fully wound at all times (when worn), the mainspring never gets to the point where force diminishes.

In addition to the evolved function of the mainspring for improved performance, metallurgy improvements in the last century have made breaks rare. Most modern mainsprings uses an alloy of cobalt, nickel and chrome (“White Alloy”). Additionally, the modern mainspring has and a distinctive “S” shape that helps deliver a more constant force as the spring unwinds.

Next time you contemplate the durability and accuracy of your mechanical or automatic watch, remember that the way to today’s ultra-reliable modern mainspring has been a long road of mechanical and metallurgical innovations.

Cassie Wallace

  1. 3 Comments | Tell us what you think!

  2. By Chris on Feb 25, 2010

    Thank you for that very informative write up. Its amazing how much design and technology goes into just one little piece of a watch.

  3. By FernandoC on Feb 25, 2010

    Well… Actually, as every wathcmaker knows even modern mainsprings break. They are metal after all. Good modern lubrication makes them last much more but even a vintage, well mantained spring lasts for decades. Couriously enough, the way to assure a broken mainspring is to manipulate it like in the vide shown… The hunmidity in the hands should never touch the mainspring or anything inside the watch. They rust and break in a couple of years…

  4. By Frank Granados on Feb 25, 2010

    The mainspring is the weakest link of mechanical watch movements. It ancient technology (Some 200+ years) that still works well but has its obvious limitations.

    I resolve the problem by purchasing a new Seiko Ananta Spring Drive GMT. Bye, bye mainsprings and hello perfect accuracy and glide motion seconds hand. :)

3 comments | Tell us what you think!