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Author Topic: Broken mainspring?  (Read 2674 times)
eatbrains
Officer
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United Kingdom United Kingdom


« on: December 31, 2010, 01:16:00 pm »

I've got a 1970s-ish ingersoll which, although the crown would wind and it would ratchet, it wouldn't wind and, when I disassembled the mainspring barrel I discovered that the mainspring was broken.  Does anyone know why that would be?
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watch_guy
Deck Hand
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United States United States


« Reply #1 on: January 01, 2011, 07:04:31 am »

Mainsprings are blue spring steel, which is fairly brittle.

Over time, the metal fatigues from repeated flexing. In addition, corrosion often sets in at the very edge of the spring. This corrosion may not be visible to the naked eye, but is present. The combination of the two can lead to stressed areas that will eventually fracture and lead to a broken spring.

Back in the old days, a broken mainspring was the most frequent cause for a visit to the watchmaker. Modern replacement springs are made of an alloy that doesn't corrode and won't break under normal conditions.
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Abslomrob
Deck Hand
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Canada Canada



WWW
« Reply #2 on: January 10, 2011, 09:45:07 pm »

There was a thread on one of the other watch sites that discussed an "Urban Legend" regarding the effect of lighting on mainsprings, suggesting that under the right conditions, tightly wound springs would "shatter" during a thunderstorm.  Dunno if its true though...
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eatbrains
Officer
***
United Kingdom United Kingdom


« Reply #3 on: January 16, 2011, 06:11:21 pm »

What about getting a replacement?  Are they different lengths/strengths etc. and how much would buying one be approximately?
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watch_guy
Deck Hand
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United States United States


« Reply #4 on: January 17, 2011, 06:46:12 am »

Here's a quick primer on replacing mainsprings:

Yes, the dimensions do vary, sometimes even within the same model from one manufacturer. Width is generally constant for all watches of the same model. When replacing springs, it's important to match the width, as a spring that is too wide or not wide enough simply won't work at all in the watch(although it is possible sometimes to reduce the width a little bit if necessary).

Widths are traditionally measured on the Dennison scale, which is a somewhat arbitrary set of numbers that was developed back in the 1850s. A Dennison gauge has a series of numbered slots around its perimeter, and the spring is measured by trial fitting into these slots. In recent years, there has been a switch to metric measurements. Most springs will quote the width in both Dennison and mm.

The next dimension to consider is the thickness. Since the width is taken as a constant, the thickness determines the strength of the spring. When describing a spring, the term strength is used interchangeably with(and often in preference to)thickness. As a rule of thumb, the more jewels a watch has, the weaker a spring it needs in order to run correctly. While a watch will run with springs over a wide range of strength, a spring that is too weak will cause low amplitude on the balance wheel and can result in poor timekeeping. By contrast, a spring that is too strong can cause the balance to swing all the way around-a condition known as overbanking or knocking-which, aside from being really bad for the watch, will also cause poor timekeeping. An Ingersoll, being an unjeweled watch, will require a fairly strong spring. Like width, strength can also be measured using a Dennison gauge, or in millimeters.

Length is probably the least critical dimension, as it mainly effect run time. The maximum theoretical run time of a watch with a given mainspring is dependent on the difference in the number of turns when fully wound and when fully unwound. For a given barrel diameter, winding arbor diameter, and spring thickness there is a length which will maximize run time. If you're handy with calculus, it's possible to calculate this. Personally, I just try and match length as closely as I can, and don't worry if it's a little too long or too short.

Finally, there are about a dozen different ways that the end of the spring is attached to the barrel wall. Probably the two most common are a hole in the end of the spring combined with a hook on the barrel wall, or a T end on the mainspring that fits into slots on the barrel(sometimes you will see a combination of these). I've also seen dollar watches on which the end of the spring is riveted to the barrel wall. If this is not correct, there are ways of altering the end.

The Jules Borel catalog has several pages of springs, all with dimensions listed. If I can't find a replacement by part number(as is possible with many of the watches I work on) I measure the old spring and try and match as closely as I did. Remember, it's most critical to match the width, then strength and finally length.
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