Automatic mainspring slipping all the way

[ColinC]

I have a put a new automatic mainspring into a barrel and when I wind it I can see (through the small hole in the barrel lid) that the mainspring just slips and unwinds.  The watch now has about 20 seconds of reserve!

I am not sure what could be wrong - I did exactly what I have done before, I applied a very thin layer of Mobius 8213 grease to the barrel wall and installed the mainspring.

The watch is an AS1700 and the spring is a new GR2937X (should be the right one!).  The watch originally had a 2 part spring with a regular spring inside small spring. That used to work OK  (with the wrong non-auto mainspring installed).

I am going to take it apart again and look - but any ideas what issues I might be looking for?

[nevenbekriev]

Sounds like the new spring doesn't want to unwind and it's coils stay stuck together. May be it has stayed on stock several decades and the lubrication has hardened.

[AndyGSi]

5 hours ago, ColinC said:

The watch originally had a 2 part spring with a regular spring inside small spring. That used to work OK  (with the wrong non-auto mainspring installed).

So what was wrong with the original spring if it worked OK?

It should have a 2 part spring with separate brake spring.

Edited September 7, 2024 by AndyGSi

[Kalanag]

Given the extremely low power reserve, the cause is neither the braking grease nor the barrel. Even though the number of the spring does fit the caliber, there is something wrong with the spring (bridle).

By the way: Are you 100% sure that the spring slips? Could it be the arbor not hooked correctly?

Edited September 8, 2024 by Kalanag

[ColinC]

10 hours ago, AndyGSi said:

So what was wrong with the original spring if it worked OK?

The original spring was of the old blued type and the eye broke as I wound it into the winder. I temporarily replaced it with an alloy non auto spring inside the old brake spring. That spring wasn’t the right spec and I had low amplitude. 

I revisited the watch and got the train of wheels running much better (jewelling tool and fixing end shake) At the same time I also put in the new correct mainspring  - brand new from Cousins

Additional question… The braking grease - what does it do? What happens if you put too much or too little?

It has been my understanding that it lets the spring slip in a controlled manner and that without it the spring would over wind and then slip violently.

I took it apart again and inspected the new spring and the barrel. The barrel wall has no ridges and has some wear where the spring has been sliding (nothing terrible).  I took some advice I found here somewhere and roughed up the barrel wall a bit and was a little more generous with the grease. I had a very small improvement but it still just winds to a certain point and then starts to slip and slowly slips away all the power

Edited September 8, 2024 by ColinC

[AndyGSi]

Not sure if you've got the service manual but I've attached it along with the section regarding the barrel & mainspring.

What's the length on the brake spring on the GR2937X in relation to what's shown below?

Edit

I should have also reiterated the comment from @Kalanag

Are you 100% sure that the spring slips? Could it be the arbor not hooked correctly?

 

AS 1700 1701 Service Manual.pdf

Edited September 8, 2024 by AndyGSi

[ColinC]

5 hours ago, AndyGSi said:

What's the length on the brake spring on the GR2937X in relation to what's shown below?

Thanks for your reply - the GR2937X is an automatic mainspring with a bridle.  I took out the old brake spring and used just the automatic spring on its own (not using a brake spring)  - I have done this several times before and it has just worked!

Interestingly I went back to look at the watch again today and it looks like it is behaving better.  Perhaps the grease has "set" a bit?  I can now wind the watch and watch the barrel go around 5 times before it starts to slip - but unlike yesterday, once it starts slipping it only slips a bit and then stops.

The service manual says it should be 6 turns of wind and I am only getting 5 but I will be happy if I can get 5/6ths of 40 hours reserve (33 hours). 

Edited September 8, 2024 by ColinC

[caseback]

The 2937x matches the dimensions given by ranfft.

Strange.

Long shot: have you measured the dimensions of the new spring to verify they're correct?

[AndyGSi]

12 minutes ago, ColinC said:

Thanks for your reply - the GR2937X is an automatic mainspring with a bridle.  I took out the old brake spring and used just the automatic spring on its own (not using a brake spring)  - I have done this several times before and it has just worked!

The bridle operates as the brake spring and a shorter one could explain the early slippage.

[Kalanag]

11 hours ago, ColinC said:

… Additional question… The braking grease - what does it do? What happens if you put too much or too little?

It has been my understanding that it lets the spring slip in a controlled manner and that without it the spring would over wind and then slip violently…

The spring will never slip violently, even if you use regular watch oil at the barrel wall. The main purpose of braking grease is preventing the barrel wall from wearing rapidly. It ist therefore filled with MoS2, PTFE or Graphite.

The key for a decent power reserve and slipping behavior is the spring in combination with the bridle. A slightly bent bridle for instance will ruin the correct braking function.

EDIT: I just saw your comment that the spring behaves better after sitting a while. That could be because you used too much grease which can not creep out of the way and prevents the bridle from contacting the barrel wall correctly.

Edited September 8, 2024 by Kalanag