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Watch Running Too Fast? Too Slow?

Is your watch running too fast or too slow?  

What determines the rate of a mechanical watch?

Mechanical watches rely on the period of the balance wheel to allow the escapement to advance the mechanism at a regular pace. The balance wheel turns one way, then back the other. With each cycle the mechanism, the thus the hands, are advanced a bit. The period of the balance wheel is the number of these cycles, called beats, per unit of time.


Here we see a balance wheel moving to and fro. 
The most common rate of a vintage mechanical watch is 18,000 beats per hour, although there are other rates. The watch's rate is the ideal period of the balance wheel, which is essentially a pendulum whose period is dictated by the length of the hairspring, and the mass of the balance. In most vintage watches, the effective length of the hairspring can be controlled by a regulator. The regulator turns around the center of the balance and moves two regulator pins along the beginning stretch of the hair spring. The spring passes between the two pins. Moving these pins makes the hairspring effectively longer or shorter and thus changes its period.


In this image, the expansion and contraction of the hairspring are visible as the wheel turns. The outer end of the hair spring is fixed to the arm, the balance cock, that holds the upper pivot of the balance wheel. The inner end of the hairspring's coils are fixed to the balance wheel axis.

In both these images, the regulator arm is visible with the F and S markers for faster and slower, respectively. In most watches, the full range of the regulator will not result in a change of more than three to five minutes a day, give or take.

Mechanical watches do not have perfectly steady rates. The rates are changing throughout the day as a watch runs. If everything else is functioning well, the intent of the regulator is to evenly distribute rate errors over longer periods of time, error created by things like the watches physical orientation throughout the day, or temperature changes, so that on average the watch reads well over this period of time, such as a whole day. But it is a relatively fine adjustment.

What if a watch runs very, very fast? Like gaining a minute or more in an hour?

A very fast rate is well beyond the range of the regulator to control. It is an indication of a serious problem. The most common cause is dirt, oil, magnetism or simple rust, on the hairspring. This can cause the spring's loops to touch at some point as the spring coils move in and out. When a spring has this trouble, one can see this happening with careful observation of the running spring. The hairspring will jump and shutter, rather than smoothly expand in and out, with the motion of the balance wheel. If the coils touch, at all at any point, this simply has the effect of making the spring shorter just as the regulator can, and thus faster - much shorter and much faster.

A hairspring problem is the most common cause of a watch running very much too fast. A watch with such trouble can be as much as 15 minutes fast in an hour. There are other causes too. For example a mainspring that is too strong can drive the escapement too fast. The escapement may be damaged causing it to sometimes slip. Or a wheel may be missing a tooth causing it to jump ahead as the bad spot comes around.

What if a watch is very, very slow, such as losing minutes an hour?

Interestingly, under normal conditions where all the parts are correct, a watch can not physically run very, very slow. Watch problems tend to make a watch run fast, or stop. Again, the period of the balance dictates the frequency at which the escapement allows the mechanism to unwind. If a watch is behind by an hour at the end of a day, it is almost certainly because it is actually stopping completely at some point and restarting without being noticed. A watch might do this once for a long period, or several short times during the day. A huge array of factors play into problems causing a watch to stop, some of them causing just slight pauses each hour, which may be hard to notice.
Sometimes, with watches that seem slow, the trouble is not the watch at all, but the hands. Under the dial is the mechanism that links the hour and minute hands. The hands ride on a part called a cannon pinion that is slipped, friction tight, on to the center wheel shaft. It has to slip because when you set the watch, you force the hands to move ahead or back. The cannon pinion slips so that the hands can move without forcing the train of the watch.
It is common that a cannon pinion fits too loose, and slips all the time. In this state the watch may tick along, at an excellent rate, while the hands stay still.

Conclusions

The long and short of it is that vintage mechanical watches have imperfect rates, rates which also vary slightly as they run. Small rate errors can be accounted for, through the fine adjustment of the regulator. But larger errors indicate a more serious trouble. A very fast watch probably has a hairspring problem. A very slow watch is probably stopping.

Want to find out more? 

There's plenty of information throughout this blog. Checkout the archive links at the right side, and the word links at the bottom of this page - and there's much more at Elgintime.com!


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