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 has been employed in most watches since the mid 1800's. It gets its name because of the result of its 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 its 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.

 

I notice that you use the contraction of it is (it's) rather than the possessive form of the neuter pronoun 'it', although it doesn't take away from the tutorial.

 

For the sake of all the pedants, corrections have been made.

 

Greetings; new forum member here. I have vastly enjoyed this forum in the background for almost one full year after being intrigued by watch collecting. I certainly enjoy the wisdom of the crowd and, in particular, enjoyed this educational post. These types of posts are my favourite as they are truly a contribution to the great community I see here. I hope to be able to offer the same.... after a few years (or decades) of further learning. Sincere thanks from a newbie.

 

Thanks for coming out of the shadows to comment.

I have to do posts like this as penance for all the ball busting I do otherwise.

 

Here's some more penance for you for the time you gave me my tag .

A mainspring barrel with an unusual feature, please explain.

 

Who are you and what have you done with @ulackfocus?

 

Geneva stopworks - allows the middle of the mainspring to be used, excluding the areas where the torque curve is less flat - provides better timekeeping. Often these parts are removed from the barrels by people who don't know what they are for...

 

Again truly fascinating. Thanks so much. Like Rob.... Said above I too hope to contribute something useful here someday.

 

GIYF

BTW not to be confused with the Geneva drive, which is used to translate rotary motion into an indexing motion. Commonly used on industrial machinery - I've rebuilt a few of these back in my engineering days...we used them on machines that would assemble bearing races, rollers, and cages together, indexing the table of a press.

In effect the stopworks is modified from a traditional Geneva drive design to allow only a specific amount of unwinding. Similar design, different goals.

 

Hey now, I'm not just a one trick pony who doles out sarcasm.

Okay, gotta run - I smell a n00b that needs hazing.