Mainspring

[BritAsianinJap]

I'm really interested in finding out what main characteristics in a mechanical watch decide how much power reserve one can achieve. Can an existing watch be retro-fitted to enhance power reserve and is it recommended? I'm thinking does it impact on the other components in a watch?
 

[clockboy]

That is an interesting question. The length of the spring determines how long the watch will run (power reserve). I have fitted springs that are just a bit longer than the original because the original size was not available with no issues. The strength is a more complex matter. In my opinion for hand wind watches having a slightly stronger strength spring than the original should not cause any issues. However for auto wind watches this could cause issues as the barrel/spring is just being wound via an oscillating weight.
Fitting weaker springs could cause lower amplitude. This is certainly an issue with clocks. I have replaced many tired clock springs that have just lost their effectiveness,so I dare say the same applies to watches.

[oldhippy]

The watch movement is designed in a way to run and work with a certain type of mainspring which is height width and strength. Any other mainspring fitted could cause all sorts of damage and effect to parts that make up the movement and the time keeping.   

[BritAsianinJap]

Thank you for the replies, yes I fully understand that it may be possible to make some minor refinements (especially on hand wound watches) however one probably cannot get away with making drastic changes to for example doubling power reserve from 40hrs to 90hrs simply by retro-fitting a mainspring and a new barrel even if it was possible to obtain one with equivalent dimensions. I was intrigued by the recent purchase of a system 51 Swatch automatic with a power reserve of 90 hrs compared to one of my Omegas. The Swatch barrel does however look somewhat larger.

[szbalogh]

3 hours ago, BritAsianinJap said:

I'm really interested in finding out what main characteristics in a mechanical watch decide how much power reserve one can achieve. Can an existing watch be retro-fitted to enhance power reserve and is it recommended? I'm thinking does it impact on the other components in a watch?
 

First, one has to consider the characteristic of the spring. The next diagram shows the spring force compared to winding angle (red lines, original spring). The new spring should have the same peak force (blue line) as the old one not to overstrain the gears. 

The geometry of the barrel hovewer determines the spring length. 

General rule according to DeCarle: Practical watch adjusting is to have 1/3 space for the spring couils, 1/3 open and 1/3 for the barrel arbor. 

Coil number of the spring (possible winding angle) can be increased only if the spring material is thinner. 

I can imagine that there are a newer and more modern spring steels those have the same strength but are thinner than old ones. But the problem is that we have even trouble to find original or similar springs to our watches. To find the required one for this problem seems impossible. And even if there is a source one need to shape the spring to fit in the barrel (make both end hooks). I dont think it worth the time to have +10-30 % more power reserve. 

On the other hand, manual wound watches are designed to wound every day at the same time. And by knowing your watch and with a daily routine they can be made incredibly accurate since the spring force changing every day the same amount. 

[Tmuir]

You beat me to it szbalogh I was just going through this last night at my watch repair course so its all thresh in my mind

But to just add a bit more there are formulas that go into it deeper

For example to work out the correct length of the spring is

length = Pi(inner radius of barrel squared - radius of the barrel arbor) / 2 * thickness of mainspring.

On average the thickness of the spring is around 1/100th of the inner barrel diameter.

 

A very simple check you can do to see if the spring in the barrel is the correct one and not just one some hack found and stuck in is check the k factor.

The k factor is the ratio between he radius of the barrel arbor core and the thickness of the mainspring. This usually falls between 10 and 14

k = radius of barrel arbor / thickness of the mainspring

Bottom line, the spring in the barrel will be the best spring for the watch unless some hack has fitted an incorrect spring at some point.

[oldhippy]

1 hour ago, szbalogh said:

First, one has to consider the characteristic of the spring. The next diagram shows the spring force compared to winding angle (red lines, original spring). The new spring should have the same peak force (blue line) as the old one not to overstrain the gears. 

The geometry of the barrel hovewer determines the spring length. 

General rule according to DeCarle: Practical watch adjusting is to have 1/3 space for the spring couils, 1/3 open and 1/3 for the barrel arbor. 

Coil number of the spring (possible winding angle) can be increased only if the spring material is thinner. 

I can imagine that there are a newer and more modern spring steels those have the same strength but are thinner than old ones. But the problem is that we have even trouble to find original or similar springs to our watches. To find the required one for this problem seems impossible. And even if there is a source one need to shape the spring to fit in the barrel (make both end hooks). I dont think it worth the time to have +10-30 % more power reserve. 

On the other hand, manual wound watches are designed to wound every day at the same time. And by knowing your watch and with a daily routine they can be made incredibly accurate since the spring force changing every day the same amount. 

That's what I was told many years ago 1 third for spring, 1 third for arbor and 1 third for space.

[bobm12]

Hi guys, very nice topic and information here. I moved the post to a more appropriate section.

Cheers,

Bob

[Tmuir]

4 hours ago, Tmuir said:

You beat me to it szbalogh I was just going through this last night at my watch repair course so its all thresh in my mind

But to just add a bit more there are formulas that go into it deeper

For example to work out the correct length of the spring is

length = Pi(inner radius of barrel squared - radius of the barrel arbor) / 2 * thickness of mainspring.

On average the thickness of the spring is around 1/100th of the inner barrel diameter.

 

A very simple check you can do to see if the spring in the barrel is the correct one and not just one some hack found and stuck in is check the k factor.

The k factor is the ratio between he radius of the barrel arbor core and the thickness of the mainspring. This usually falls between 10 and 14

k = radius of barrel arbor / thickness of the mainspring

Bottom line, the spring in the barrel will be the best spring for the watch unless some hack has fitted an incorrect spring at some point.

Just noticed a mistake in my formula, corrected formula below

length = Pi(inner radius of barrel squared - radius of the barrel arbor squared) / 2 * thickness of mainspring.

 

[jguitron]

I'm just now going though the section on mainsprings on Daniels's book. Fascinating stuff. Despite all the calculations there's no good way to determine which spring the watch will need when designing one other than some rough rules of thumb.

It's way more technical than I need but fascinating for sure and highly recommended.




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