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The Mainspring

by Jordan Ficklin

One of the reasons I’m talking about mainsprings is because they have been on my mind a lot lately. The word on the street is that many mainsprings for older watches are no longer being produced. I have noticed the price increasing and I’m sure my customers have as well. The price of many mainsprings have doubled, tripled, or more in the past few months. There are very few companies making mainsprings. It is my understanding that even the great and powerful dragon, Swatch Group, doesn’t have a mainspring producer. The main company is Generale Ressorts. Rolex is believed to produce their own mainsprings, but beyond that I don’t know of any other companies making mainsprings. We will have a mainspring crisis on our hands in short order.

The mainspring is the driving force of a mechanical watch. The mainspring is housed inside of the barrel which is considered the watch’s “motor”. I like to think of a mechanical watch as a delicate game of tug of war with the mainspring and barrel on one side and the balance and hairspring on the other. They constantly push and pull on each other via the escapement with the balance ultimately winning. The balance and hairspring are considered the heart of the watch as they control the timekeeping but the mainspring is still very important.

The mainspring is a thin metal spring which gets wound tight inside of the barrel via the arbor which is attached to the center of the spring. In a hand wound watch the owner winds up the spring every morning by turning the crown. In an automatic watch the spring is constantly wound by the motion of the owner transfered to the oscillating weight via the wonders of inertia. The click allows the arbor to turn in only one direction when winding the watch and prevents the mainspring from uncoiling from the center out. The outside end of the spring is attached to the barrel which is a geared drum which acts as the first gear in the watches train.

Inner coils of a pocket watch mainspringThe typical (and almost universal) from of attaching the arbor to the spring at the center is via a hole or “eyelet” in the spring and a finger on the arbor which locks into the hole. There are however many methods for attaching the external coil to the barrel. One common method is a tongue which engages a slot in the outside wall of the barrel. Another common method is a T shaped end which engages a hole in the barrel lid and floor. Combinations of these are also common. For automatic watches the mainspring is not connected to the barrel at all. Because these watches constantly wind there needs to be a safety mechanism to prevent the mainspring from becoming overwound, therefore delivering too much force, or causing damage to occur (including the spring possibly breaking). For this reason the spring is constructed so that it is forced against the outside wall by a Y shaped bridal and a special grease is used which allows the spring to slip only when the spring is almost completely wound. In this fashion the spring can still drive the barrel but can also slip if wound too tight instead of breaking.

Here are some common mainspring ends and their names and abbreviations from the Jules Borel catalog:

FuseeTraditionally mainsprings were made from high carbon steel tempered to spring hardness. The problem with these springs is they lose their “springiness” over time, they break, and they don’t deliver a consistent force as they unwind. Over time there have been several solutions to solve this problem. One early solution was the fusee. The fusee incorporates a conical shaped gear and a small chain which drives the rest of the watch. The shape of the gear changes the torque applied from the barrel as the mainspring winds down, compensating for the change in force applied by the spring. The stackfreed was another solution using a spring and an eccentric cam to apply varied resistance to the barrel as it winds down in order to achieve the same compensation effect.

The modern mainspring solves these problems by using new alloys and a very distinctive shape. Most modern springs are made from an alloy of cobalt, nickel, & chrome commonly referred to as “White Alloy.” The S shape of the mainsprings pictured at the top of the article actually helps to deliver a more constant force as the spring unwinds. The change in torque delivered over the first 24 hours is very small when compared to blue steel springs. The automatic watch also combats this problem by keeping the mainspring almost fully wound at all times. If the mainspring is at approximately the same wind it will deliver the same force.

It would appear the industry has determined that this problem is adequately solved. Any attention given to the barrel in modern watches is focused on increasing the power reserve of the watch, which does in its own right improve the consistency of the force delivered, especially if the watch is still wound frequently. Other focus has focused on the escapement. With a constant force escapement, like this one by G.P. change in torque delivered by the mainspring does not affect timekeeping abilities.

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2 Comments

  1. Stephen D. Robbins
    Posted October 14, 2008 at 10:38 am | Permalink

    Very informative. A big help for us who are not watchmakers to understand this crucial part of the movement.

  2. J.Peter
    Posted October 15, 2008 at 7:54 am | Permalink

    Stephen, thank you for the encouraging words. I’m hoping to produce more articles like this in the coming weeks.

3 Trackbacks/Pingbacks

  1. […] In a continuation of posts on the fundamentals of watches here comes talk of the gear train. The first post discussed the mainspring and barrel. […]

  2. […] J.Peter alluded to in his post on mainsprings, replacements for many older mainsprings are no longer being produced and the reserve stock […]

  3. […] game of tug of war: a battle between two resisting sides connected by one common thread, a rope. Some relate mechanical watches to a constant game of tug of war: the mainspring and barrel vs. the balance and hairspring. All of these parts are important in […]

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