I have a watch with an Omega 601 movement that has a strange fault.

When the watch is fully wound the balance speeds up to about 43200 BPH for a couple of minutes before slowing back down to the normal 19800 BPH. Apart from that the movement seems to be running well with good amplitude and accuracy.

Could anyone explain what the problem is likely to be? The watch has just been serviced and was fitted with a new GR 25351 mainspring (Non-Automatic 1.05 x .12 x 360 x 9.5). Could this be the source of the problem and is the fault likely to damage the movement?

I will obviously take the watch back to the watchmaker when I next visit him but in the meantime I'd like to get an idea of what the problem is likely to be.

 

Your watch isn't really running at 43200. Your timegrapher listens to the various escapement noises and tries to make sense of them to calculate rate/amplitude/beat error. If you have a cheap timegrapher then it does it's best and says the rate is 43200.

The issue that you're almost certain to have is that you have too much amplitude (more than about 325 degrees for this calibre) and the impulse pin is hitting the back of the fork and making an extra noise that the timegrapher is trying to read as part of the escapement sounds. As this is happening when swinging both ways, the rate is coming out to be more or less double the actual rate. It's really not a good thing and, I think, Al posted a video on here where you can hear it. Try searching for gallop or galloping posted by Al.

Your mainspring is too strong. When the mainspring is coming down a little from "full wind", the torque drops a little and the amplitude drops a little and your timegrapher is happy as the extra noise has disappeared.

I don't have a 601 mainspring here to measure but my mainspring book shows a thickness of 0.11 mm. The GR steel will be similar to the Omega steel (as some Omega mainsprings for these calibres are made by GR) so, with a thickness of 0.12, you have some 30% extra strength leading to an amplitude that's too high. It may be that this GR spring is the closest generic but, it's always best to go down in strength, unless you are trying to cover up some problem. And yes, I know that Cousins list this GR 25351 for the cal 601.

Out of interest, for the 550 series, the Omega spring is 0.11 mm thick and there are two GRs: one is 0.105 and the other is 0.12. I use the 0.105 spring if my customer wants a cheaper generic but, I am sometimes chasing amplitude so might drop this option. I don't like the thicker GR as you can get the issues that you are seeing.

Hope this helps.

Cheers, Chris

 

So roughly 10% more thickness gets you 30% more force? Is that a typo or for real, I wouldn't think that little differance would have that much effect. I'm not trying to be my usual smart ass self, I'm genuinely curious. Educate me... please!

 

Mainspring strength is proportional to the cube of it's thickness, all else being equal. So if you double the thickness, the spring becomes 8 times as strong.

Hooke's law need not apply...

 

It's a cubed effect as it's the moment of inertia of a rectangular section so, yes it is about 30%. Basically, the moment of inertia for a mainspring (or any beam of rectangular section) is:

I = height * thickness*thickness*thickness/12

I can't do a cubed symbol in that equation, sorry...

Cheers, Chris

 

Cross section! Makes sense. All is now clear! Cool, thank you Gentlemen

 

If you start from a resting position with the watch stopped, and the roller jewel resting in the pallet fork, consider this. When the watch is wound fully and with it running, the balance wheel wil travel anywhere from about 270 degrees to 315 degrees from the resting position, with each beat of the wheel. That means on one return trip of the wheel, it travels somewhere between 540 to 630 degrees. At the end of each swing of the wheel, the roller jewel travels far enough that it can approach the back side of the pallet fork at the end of each swing. A mainspring that is even slightly too strong for the watch indeed can cause the balance wheel to over-rotate, and the roller jewel to travel sufficiently far at the end of its swing that it can hit the back side of the pallet horn. One one-hundredth of a mm too much strength giving 30% too much power? I have never heard that, but using too strong a spring is a no no. As to hurting the watch? You could end up with a broken roller jewel.

 

Many thanks for your replies, I really appreciate them.

I obviously want to resolve the issue as I don't want to risk damaging the movement. I wouldn't normally risk using generic parts but I when I purchased the mainspring from Cousins I was under the impression that it was fairly common to use this particular mainspring in a 601.

@Archer - I found an old post where you measured the 6001208 Omega spring at 1.05 x .12 (i.e. the same size as the GR spring I used) so I am nervous that if the mainspring is changed to the original Omega part I will still end up with the same problem https://forums.watchuseek.com/f6/mainspring-dilemma-1788506.html

Is it worth asking my watchmaker to try adjusting anything else before resorting to swapping the mainspring? When the knocking stopped, two to three minutes after the movement was fully wound, the amplitude didn't seem particularly high - well below 300 degrees dial up according to my 'cheap' timegrapher (I don't want to risk fully winding it again to get an exact figure!).

@ChrisN - I'm glad you also mentioned the 550 series as I also purchased a couple of GR25331X springs, which Cousins describe as Omega 550 Automatic "GR" Mainspring 1.05 x .11 x 360 x 9.5, for when my 565 movements need a service. Is there a reason you haven't tried this GR model as it seems the same size as the Omega spring and do you think I'm best not to use it in a 565?

 

All I can tell you is that I use genuine Omega springs, and I've never had a 600 series Omega with rebanking (a.k.a. knocking, galloping).

I don't have a Cal. 600 mainspring in stock, so I can't check who makes the spring for Omega (they use springs made by several makers) so it's possible that the alloy being used in the spring from Cousins is different than the one used by Omega, and this is resulting in a stronger spring in the end, even with the same dimensions. Note that even the same maker can use different alloys for springs used in the same movement - for example some ETA movements have two different grades of alloy used in the springs, depending on the grade of the movement.

Also note that some movements will rebank at lower amplitudes than what Chris has mentioned. The amplitude that will cause rebanking is not universal across movements, as it depends a lot on the design of the roller jewel and pallet fork.

Rebanking can be caused by other things, such as the depth of the pallet stones being too shallow on the escape wheel teeth (this will increase amplitude), but if that were the case it would not stop the rebanking after a few minutes, and it would be going on for a very long time.
All the advice so far has assumed that the issue is the watch rebanking - I'm not 100% convinced of that, but without the watch in front of me it's difficult to diagnose what's really going on.

Cheers, Al

 

Note that the Cousins people are not watchmakers. For a 565 on their website, I see this:


These are the three I mentioned earlier and the thicknesses are (measured from a selection of actual springs): 0.105; 0.120; 0.110. Not sure why the first one is listed as 0.11. The second one is a bit short and may not give the 38 hours minimum power reserve.

The other one you mentioned is as shown below and it is for a smaller diameter barrel but Cousins have certainly stated it's for a 550 in the title. I'm not saying it wouldn't work but it isn't optimal as these barrels have an ID of 10.5 mm (as I recall).


Much of the train for the 550 series and the 600 series is the same and I'm surprised that the 0.12 thickness is what Al measured for a 601 spring as I am measuring 0.11 for the 550 series. If you take a 552 versus a 601 (as examples), the 2nd wheel, fourth wheel, escape wheel, fork, balance are all the same. So, it surprises me that the spring strength is different (at first glance). Anyway, could be a different modulus steel or some other aspect that I've not looked into - I'm finished for the week.

There are ways to reduce amplitude when you're building the watch so, if it is checked then, it's much easier to resolve. I do think that the issue you've described is just the watch re-banking as it disappears quickly. Al makes a good point that the only way to be sure is to have it on your bench but I can't see what else might cause this very short term effect. I would say that I wouldn't expect to see that result with my tester (or Al's) as they should cope better and just show 19800 but a very high rate.

Cheers, Chris

 

The dimensions given by cousins are sometimes.... weird ou just inacurrate.

 

It's been a while since I started this thread so I thought I would provide an update.
I bought an original Omega 6001208 mainspring and had a watchmaker install it.

I was obviously confident that it would solve the problem. However, when I fully wind the movement it is still overbanking for a short period of around 20 - 30 minutes. If I put the watch in a vertical position it stops overbanking more quickly (as expected), in around 5-10 minutes.

I'm obviously very frustrated so I'd really appreciate your thoughts on what may be causing the fault (I know it's difficult without having it on your bench).

I thought that if there was a problem with the way the escapement was set up then the overbanking would last longer than the short period I'm experiencing of up to 30 minutes.

The sound the movement makes when it is fully wound is like a galloping horse as demonstrated in the video @Archer made, and the amplitude is very high according to my Timegrapher. So I don't really see what else the fault can be other than overbanking.

 

I don't think that anyone can fully determine/resolve this issue without having the watch in hand as using a proper Omega mainspring did not solve it.

Something appears to be wrong and as the watchmaker you used has serviced (first post) and replaced the mainspring, they should look at it to try and solve the issue. If I were you, I'd drop it back in and say that you think there is an issue and could they use their professional tester and confirm, or not. It must have a warranty so they would probably be very happy to look at it. We offer warranties in case anything goes wrong and it does happen occasionally.

I think you're in UK and if you get stuck and want me to put it on a proper tester to check that it really has the issue you think, you can post it to me and I'll do that for the price of the return postage (about a tenner).

Best regards, Chris

 

By the way, if you're very interested in this, have a look here at pages 5 to 11 and 14 to 15. It will give you an idea.

https://www.witschi.com/assets/files/sheets/Witschi Training Course.pdf

Cheers, Chris

 

Thanks Chris for your reply and kind offer.

I'm based in the UK but am overseas this summer. The watchmaker did indeed tell me to send the watch back to him but I thought I would ask on the forum first about what could possibly be the root cause of the problem.

The watchmaker seems pretty competent although he is by no means a specialist of vintage Omega. He has serviced several watches for me and I've been happy with his work so far. He also serviced a 565 movent at the same time as the 601 and it is running well.

I'll let him take another look to see if he can resolve the overbanking issue.

 

I know it's been a while but I thought I would update this thread.

I called in at the watchmaker and he said he was surprised that installing the Omega mainspring hadn't resolved the overbanking problem. I'm not sure why he hadn't noticed this himself before returning the watch to me.

He inspected the movement for several minutes and then said the ratchet wheel click was causing the problem. He adjusted the click and also said the crown needed replacing but I'm not sure how that related to the overbanking. Luckily he had the correct replacement Omega crown.

I'm pleased to say the watch is running well now with acceptable amplitude and timing. Thanks again to @Archer and @ChrisN and others for the advice.

 

What?

Did he explain further?

 

We don't speak the same language so I couldn't get a detailed explanation. He definitely pointed to the click when showing me what was causing the problem.

Does this explain it:
"The click spring maintains tension on the click in the clockwise direction. The very typical two-toothed click design, illustrated, prevents the click from holding the mainspring at absolute full tension. When the crown is released after winding, the click is rocked counter-clockwise (against the click spring) by the large tooth and the small tooth engages and locks the ratchet wheel. This allows the ratchet wheel and arbor to rotate slightly clockwise. This action relieves a bit of tension in the mainspring and prevents excessive tension that might cause the transmission of too much power to the gear train and, thus, knocking of the balance wheel."

 

Understood.

Possible the click had a jammed spring, or dirt etc and wasn't doing the "relax spring" motion it should.
There should be a definite "roll back" on winding as the click relaxes, most notable in movements like ETA 6497/6498.

 

Based on the description, that would be the only thing it could be off the top of my head. So one can conclude that the watchmaker didn't assemble the spring properly under the click, since these don't really go out of adjustment on their own.

By the way the crown wouldn't have anything to do with this issue...