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Elgin Grade 7, Counting the Train

This watch has an interesting repair in the unjeweled lower pivot hole for the 3rd wheel.
There is a divot right next to the hole. A punch has been used to displace a little brass and close the hole some.

When a watch is run without getting it cleaned, the grit that is dust and old oil grinds away at pivots, making them egg-shaped. Eventually, the wheel will no longer sit straight, and the watch will stop. That must have happened here. Some repairer of old has punched the brass to close the hole, then reshaped it to be more round.


This is the dial side. This watch is key-wind, from the back. The ratchet is on the other side, here.

Here we can see the secondary serial number on the underside of the hour wheel.
These are three freshly made pins for the dial feet. Older American watches have posts on the dials (feet) that stuck through the base plate. These pins get pushed into holes in the feet on the other side of the plate, and thus hold the dial on.
The balance for this watch had *a lot* of extra weight on it. The reason soon became clear.
The hairspring needed a little work. This tool holds the spring by the stud. It's ideal for certain manipulations of the spring.
This watch timed perfectly on the timing machine after removing all that extra balance weight, and fixing the hairspring. But the hands traveled much too fast, which is why all that weight had been added to the balance.

Why?

The rate the watch beats is one thing. How far the hands move with each beat is a function of normal gear ratios. The calculation is called "counting the train". We count the teeth on each wheel, and the leaves on each pinion, and plug the numbers into a formula that tell us how the watch will work.

Skipping the algebra, the formula we're interested in here takes this form...

(el * bph) / (ft * et * 2) = [4th wheel revolutions per hour]

Where...
the 4th wheel carries the second hand. So 4th wheel revolutions per hour should be 60, we hope.
el is escape wheel pinion leaves
bph is the beats per hour
ft is 4th wheel teeth
et is escape wheel teeth
and 2 is....  well 2. This is actually the "teeth" on the next thing in the train, which is the 2 pallet stones.

The watch we have is running at 18000 bph.
el is 7
et is 15
ft is 13

(7 * 18000) / (13 * 15 * 2) is 66.67
That's pretty fast, it works out to 160 minutes per 24 hours fast! But this is a grade 7 Elgin. The records say this design runs at 16200 bph, here:
http://home.elgintime.com/elgintime/GnumLookup/7.html

And...
(7 * 16200) / (13 * 15 * 2) is 60!

Conclusion: This watch has an incorrect hairspring, which acts as a simple pendulum, causing the escapement to cycle at 18000 beats per hour rather than 16200, which the train is geared for. This simply runs the hands too fast and the watch reads incorrectly, even though it is not actually running fast.


Replacing the hairspring is difficult. These parts are rare and very often not identified, or incorrectly labeled (even in factory packages).

In this instance we opted to replace the selected train wheels with Elgin parts for 18,000 bph. The hands then tracked correct time.

This watch is an Elgin grade 7, 18 size, 7 jewels, made about 1883





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