While servicing a 98 year old Elgin recently, I had a need to obtain a donor movement. Unfortunately it didn't yield all of the parts needed, so another watch was snagged to act as a donor.

The second watch was actually in better condition so the first watch was completed "as is" and I stripped, cleaned and inspected the second and then re-assembled the train.

After ensuring that all wheels were fine and there was decent backlash I fitted the anchor and balance and gave it a wind so I could get some movement to oil the pallets.

Winding felt nice and smooth and I was starting to feel that glow of satisfaction for a job well done, when there was a sudden "whizzzz" noise and the balance stopped.

DANG!

I checked for power in the barrel (it was dead) and then took the barrel bridge off to see what had happened.
It was then that I saw the benefit of the Elgin Safety Pinion.

The guilty items laid out.



The problem? As I didn't have a replacement mainspring, I re-used the old blued steel mainspring and it just had enough and decided to snap.

Energy stored in a mainspring is considerable, especially in pocket watches with large springs. If the spring breaks, there is nothing to retain the barrel under tension and it usually spins at high velocity in the direction opposite to normal rotation. If this sudden reversal of energy is transmitted through the gear train it can strip teeth on the wheels and cause other damage.

To prevent this, (referring to the lower row of items from left to right in the photo below) the centre wheel shaft is keyed to the centre wheel and has a pinion that's screwed on to it (protected by a washer) with a left hand thread. (Correction, this one has a RH thread)



Under normal tension, the barrel is keeping the pinion "wound tight" on the threaded shaft. If there is a sudden change in direction from the barrel, the pinion is rapidly unscrewed from the shaft due to the direction of the thread. As soon as that happens, power from the barrel is disengaged from the centre wheel so no damage can be transmitted through the train.

So I can re-assemble the safety pinion, then use a donor spring I have (an old blued steel one), or I can order a new mainspring from the US.

Decisions decisions decisions.

Luckily I don't have to be the final arbiter, I'll leave that to the owner to do make the call (over to you Geoff).

Until the next exciting episode............

 

Nice bit of design engineering there Jim
Go with your spare blue spring, we don't need to spend too much on the donor

Geoff

 

So, acting under instructions from the owner, a donor mainspring was cleaned, oiled and installed in the barrel.
All put back together and found I had a beat error of about 8ms .
At least that was better than the snowstorm that greeted me on the screen of the time grapher when it was delivered.

Four or five tries to reduce beat error resulted in this. I know the amplitude will be lower if I set the lift angle to the correct value (about 44º?).



So now it's finish cleaning the case, polish the crystal, clean the dial and hands and re-assemble it ready to go back to its new owner.

 

Hi Jim,

IMO blued steel mainsprings, whether used or NOS, simply should not be used if at all possible. Your example shows the two main reasons, and the first is breakage. As you say the mainspring in a watch can hold a lot of energy and if released all at once it can do significant damage. I've posted these before but they are worth showing again. Here's a Panerai that came to me with a broken mainspring:



Note the area in the red box:



Closer look reveals teeth missing off the barrel, and one laying on the jewel:



Closer look at the barrel drum:



It doesn't have to be a pocket watch sized spring to do damage, as this is the barrel from a Nomos Club that I serviced recently with a broken mainspring, and you can see the teeth are deformed:



This is a smaller 10 1/2 ligne movement with a fairly light mainspring. The shock was enough to deform the teeth in the center wheel as well:



The second is shown in your timing results. If the lift angle is actually 44 degrees for this movement, then the actual amplitude you have in a horizontal position is likely around 190 degrees or so, and when the watch is vertical in a carrying position it will drop even more. These old steel mainspring are not only brittle, but lose their strength over time much more than a modern white alloy spring would (they become "set"). I don't know what the availability is like for alloy mainsprings for Elgin movements, but if you could find one that would obviously help the amplitude (and timekeeping) a great deal.

I know that some of the US makers did make the switch to alloy springs before they stopped making pocket watches - for example I know Hamilton used them (this is the brand I'm most familiar with). No idea if Elgin did though.

With the advent of modern alloy springs, the frequency of mainsprings breaking dropped, and the need for things like the safety pinion mostly disappeared, as a spring breaking and causing damage is actually fairly uncommon on wrist watches in my experience, despite what I've shown here. I have two Speedmasters in the shop right now that came in with broken mainsprings, and no other damage was done.

Cheers, Al

 

This was going to be a donor but ended up being saved and now ticking like a Swi.............

Well it's ticking nicely, and I'm reluctant to let it go, but it's off to a new home.