mainspring replacement

[lad111]

I recently had a main spring break (striking) for an A12 Junghans movement so now I need to replace it. Couple of questions: The spring dimensions are 5/8″ x 58″ x .017″. Does the replacement have to be these exact dimensions and if not how much leeway do I have. I have not been able to locate an exact fit trying Merritts, Timesavers and R&M imports. They all have similar springs but none match the 58″ length, closest is 69″. If I cannot use the 69″ length does anyone know of another supply house that may carry such a spring. If I can use the longer length should I replace the timing side as well? Thanks for any input.

[arutha]

When replacing mainsprings the main thing to remember is that new steel is always a bit stronger than the old steel used in mainspring manufacture so if you are replacing a spring with one of equal thickness it will be slightly stronger. In most cases this is fine. If you can see one that is the correct dimensions but is just a fraction thinner you will end up with the right sort of power. If you can find nothing else that is close to your original spring I am sure this longer one can be shortened. Hopefully Bob or someone has a technique for this?

[willofiam]

Hey, just like Paul mentioned, to error on the lighter side is better and probably ends up just as strong with a new one. as for the time side just make sure you give it a good check for nicks or tears or if it may be set, you do not necessarily have to replace the time side if all is well but if there is any concern you may as well replace it while its apart and your ordering one anyway, the cost is minimal considering what can happen or the time involved doing it later. hope this all helps, William

[lad111]

I’v gone ahead and ordered a slightly lighter spring (in fact I ordered two, in case I have to replace the timing side) but the length is 11″ longer than the original. Will this cause a problem?

[arutha]

If it is a riveted loop end mainspring you will have a bit of a job to shorten it and then re-rivet the loop if you have not done this before. I would think 11″ might be too long but what I would suggest, if it is not a customers clock, is to try it as it is, worse case scenario is that the extra 11″ of spring will not allow it to unwind quite as far so instead of running for 8 days it might only run for 6 or 7, unless it is a 30 hour clock and then it might only run for 24 hours or so, these are just estimates, until you try it you wont know but you will not do any harm to the clock installing them as they are. If you find the clock is not running for as long as you would wish then you will need to look at shortening them.
Paul.

[lad111]

It is not a customer clock and if the only damage will be fewer days of running that will not be a problem since I wind clocks every day. Thanks for your response.

[digitaltripper]

@Arutha wrote:

If it is a riveted loop end mainspring you will have a bit of a job to shorten it and then re-rivet the loop if you have not done this before. I would think 11″ might be too long but what I would suggest, if it is not a customers clock, is to try it as it is, worse case scenario is that the extra 11″ of spring will not allow it to unwind quite as far so instead of running for 8 days it might only run for 6 or 7, unless it is a 30 hour clock and then it might only run for 24 hours or so, these are just estimates, until you try it you wont know but you will not do any harm to the clock installing them as they are. If you find the clock is not running for as long as you would wish then you will need to look at shortening them.
Paul.

Now see, I would have thought that the extra 11″s would have let it run longer….But when you pointed out that it may not unwind enough, it made sense !! So does this mean that if it were 11″s shorter that it may run longer as the spring could unwind a bit further, or is this the wrong way of looking at it?

Jim

[arutha]

Hi Jim,
my thinking would be if there is less spring to wind then there is less spring to unwind. I am sure they would have worked out the optimum length when designing the clock but you just never know. One other thing to watch out for is certain loop end mainsprings can be fitted in the wrong position which means as it unwinds it can stop on the centre arbor etc, I think I did this on an Ansonia ships clock once, If you dont get the loop on the correct post the spring unwinds into the movement which only allows it to run for a much shorter period.
You are quite right about a longer mainspring running for longer, as long as it had room around it to open out as far as it could but you usually find small posts in the movement somewhere to stop the spring interfering with the other wheels which in turn only allows them to open out so far.
Paul.

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