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Horology 101 - mainspring barrels

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Published on 11-08-2008 10:40 PM

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Mainsprings were originally just connected to the inner barrel wall on one end and to a center post on the other. This created a few problems. Since the barrel was rotating one way while the watch was ticking, but in the opposite direction while it was being wound, the watch lost time during winding. Also, if the mainspring broke it caused a violent snap and a surge in the opposite direction which would break other parts like jewels and gear teeth. Lastly, the watch was not isochronistic since a fully wound mainspring pushes the gears with much more force than a nearly exhausted spring. The first solution was to add a fusee between the barrel and the gears. The fusee is a conical piece that had a chain wrapped around it which wound up to the point of the cone as the mainspring was wound.

A fusee from a Hamilton marine chronometer:



As the watch ticks the chain unwinds gradually down the cone to the larger diameter portion. The thicker the part of the cone, the more rotational torque it generates thus working in a reciprocal fashion to the power of the mainspring, which therefore offsets the change in force as the mainspring loses energy during unwinding. This also confines the damage of a broken mainspring to the inside of the barrel drum since a chain can only be pulled and can't be pushed. The fusee has it's drawbacks. It never properly solved the issue of losing time during winding, and it took up a huge amount of space so it was impractical for a wristwatch. It was also a very expensive. After 1900 it was rarely seen except in marine chronometers.

The most common type of barrel is called a going barrel. While found as early as the 1700's, it's been employed in most watches since the mid 1800's. It gets it's name because of the result of it's design. The center post is replaced with an arbor that rotates during winding causing the mainspring to coil around it. The arbor spins independently of the barrel which allows the spring to continue unwinding even as the arbor spins during winding, so the watch keeps "going".

top and bottom of a going barrel:


internal view of the same barrel:

To prevent extensive damage to other parts when the mainspring breaks, a safety pinion was added. The gear that's pushed by the barrel is threaded to it's arbor. It gets screwed down tighter as it is driven by the mainspring in the normal direction. When the mainspring breaks and spins the gear in the opposite direction, it simply comes unscrewed and disconnects the barrel from the rest of the movement thereby protecting the other internal parts of the watch.

Another option that solved the loss of time while winding is called the motor barrel. It's essentially the same principle as the going barrel but in reverse: the arbor drives the gears and the barrel rotates during winding. Jewels were sometimes used at the ends of the arbor. While they were technically functional, they served little purpose towards reducing friction due to the slow speed the arbor rotates at. The drawbacks to a motor barrel are the significant amount of extra parts compared to a going barrel (increasing cost) and a broken mainspring will put undue stress on the winding gears. The biggest advantage to a motor barrel is it can utilize a weaker mainspring which is a bit less susceptible to breaking.


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Horology 101 - mainspring barrels
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