Help Help finding modern replacement hole end mainspring for old German clock

DannyBoy2k

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I'm searching for replacement mainspring for an old German time and strike that recently had the time spring snap close to the arbor. The measurements, as best I can get them are:

Thickness: 0.1525" (0.39mm)
Width: 0.8375" (21.3mm)
Length (guestimate based on trying to wrap a string around the coils: 53-ish"

I'm not really finding anything in the Timesavers/Merrits catalogues in these ranges and wondering what dimensions can be manipulated to find a suitable modern spring. Any advice is greatly appreciated.

EDIT: More information

Barrel Diameter: 40.53 mm
Arbor Diameter: 12.02mm

I measure that I probably have 22.5 mm of height from the bottom of the barrel to the space the cap would hit.

EDIT EDIT:

Found a better way to measure the spring and my actual length is probably closer to 66-67" for the broken spring including the coil that was on the arbor.

Cheers,
Dan

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DannyBoy2k

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If sacrificing 1 mm of width isn't too detrimental, this spring from Merrits looks really good:
Ms220 -----------------25/32 --------20.0-----------.0155 ---------.40 -------------53 -------------38 mm

I'm just not sure how much of a role the width of the spring plays in overall strength. The thickness and length would almost be dead on.

EDIT: In the third post, based on the barrel diameter and arbor diameter, the calculated length of the spring should be ~60". So maybe not so dead on.

~Dan
 
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DannyBoy2k

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More information:

Barrel Diameter: 40.53 mm
Arbor Diameter: 12.02mm

With the thickness I measured on the old spring, the calculated thickness (by the formula .3927(D*D - d*d)/T) would be 59.4". My length measurement was as best as I good get. Could be closer to 60" all said and told. Merrits has this spring at 61", but only 0.012":
Ms227 -----------------53/64 --------21.0-----------.012 -----------.30 -------------61 -------------35 mm

~Dan
 
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DannyBoy2k

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Just talking to myself here, but hoping the additional information will help people make me a suggestion. I found a better way to measure the spring (flat on bench and slowly wrap thread around the coils) and my initial length estimate was pretty off. My actual length was probably closer to 66-67" for the whole spring.

Given that, my leading contender is this Merritt's spring:
Ms228 -----------------53/64 --------21.0-----------.0125 ---------.32 -------------67 -------------38 mm

~Dan
 

shutterbug

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You could consider a longer spring, as long as the width and thickness was pretty close. You can always cut the spring shorter at the outside end, anneal about 2 inches of it and make a new hole.
 

DannyBoy2k

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Thanks, Shutterbug. I'm hoping not to have to do that as I don't have all the tooling I would need for it, but I will if all else fails.

I was attempting to use the "Estimate Mainspring Thickness and Length Calculator" and getting some really weird numbers. (http://theindex.nawcc.org/CalcMainspringEstSize.php)

5 gear train
Barrel - 68 teeth
2nd wheel - 70 teeth, 12 pinion leaves
Center pinion - 10 leaves

I counted all these twice to be extra sure. With my barrel measurements of 40.3 mm inside diameter and 12.03 mm arbor diameter, this gives me an estimated thickness of 0.7474 mm and length of 787.2 mm (30.99"). This is way off of the spring that I know was working at 0.39 mm thick and ~1676 mm (~66"). Looking at the train picture I posted above, I can only assume this is because my center pinion is also on my 3rd train wheel? I was just surprised the calculator was so far off and wondered if there was something obvious I was missing.

To get anything I can currently find at Merritts and Timesavers with the same width (21 mm) and length (67"), I'd have to go with a thickness of 0.32 mm (0.0125"). I'm just wondering if this is just too much of a change from 0.39 mm (0.0153"). I guess I'm just going to have to give it a try, and, if that fails, try and make a custom spring from a longer 0.40 mm one.

~Dan

EDIT: I just did some basic gear train math and, if the center shaft has to spin for 8 days * 24 hrs = 192 turns:
192 / (68 / 12 * 70 / 10) = 4.84 turns of the main wheel

Anecdotally, it probably did take around 9 half-turns of the key each week to wind this clock (it's my parents' clock). So, I'm confused on how such a long, thin spring was able to work so well for several decades if the estimated mainspring thickness and length calculator is to be believed.
 
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Allan Wolff

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These German movements usually have fairly strong mainsprings which is determined mostly by the thickness. Your original thickness measurement of .01525" sounds about right, although I have found some with thicker springs. Width affects the the strength somewhat, but not as much as thickness. I would be afraid that the .0125" thick springs will be underpowered. Don't get too hung up on the length. You can vary 6 to 8 inches either way without making a significant impact on the run time. As you found, these movements will typically run 10 days or more on a wind, so if you wind it every week, you can tolerate quite a bit of difference on the length of the spring.
Given your most recent set of measurements of .8375" X .0153" X 66", I would use Timesavers 15930 which is .827" X .0157" X 61"
Good luck,
Allan
 

DannyBoy2k

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Thanks, Allan! I stumbled across that same spring last night and thought it looked like a candidate. I'll give it a go.

Still wondering if someone has an explanation about why the "Estimated Mainspring Thickness and Length" calculator is so far off. Recommending a spring that is nearly 3x the thickness of my current one. Would be (0.7474 / 0.39) ^ 3 = 704% the strength of the old spring.

Cheers,
Dan
 

RJSoftware

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its easy to make new hole. Heat with propane torch, fold over end when red hot. cut notch with dremel wheel., unfold, makes diamond shape cut. Shape it around similar sized arbor.
 

shutterbug

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The calculator requires you to know the thickness of spring you'll need. Then it can do a pretty good job of calculating length. Alternately you can provide length and it will find thickness for you.
 

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