FHF 175 Mainspring

[mcoulton]

Hello

I don't have very much experience with mainsprings and I've just removed one from a FHF 175 movement. As you can see from the picture it's still coiled up. Any video's I've seen always show the mainspring almost unwound. Is there an issue with mine and should I see if I can order a replacement? 

Thanks in advance for any help that can be provided. 

Michael

[rogart63]

Looks a little tired. You can buy a new spring and put in pretty easy. http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&0&2uswk&FHF_175 

Here you find find the size of the spring. I thought cousinsuk had it but it was obsolete. You can maybe find a newer FHF 175 movement pretty cheap. And use that mainspring. 

[KOwatch]

As you noticed on that tired mainspring, it should have an "S" shape.

This one at Cousins will do the job, it's a little shorter than what's listed on Ranfft but that won't make much of a difference

1.45 x 0.10 x 300 x 9 Non-Automatic

GR3953

If you get another FHF175 donor movement it might have a similar old mainspring, and you'd need a mainspring winder to put in into your barrel unless you just swapped the whole barrel over, unserviced. As you've probably seen, the new ones come in a package so you don't need a winder. New spring is much cheaper than a winder, until you're replacing lots of mainsprings.

You can fiddle with some of the other dimensions when finding replacement springs, if you can't find new or NOS fits, but the next shortest spring of a given size is usually the best bet.

 

 

[Nucejoe]

FHF 175 came in several versions with subsecond and sweep second, Dr ranfft lists a big family of variant with no known difference, so you face different hieghts.

Are hieght of MS the same for all? 

[mcoulton]

Can some one kindly shed some light what these mainspring numbers mean:

1.45 x 0.10 x 300 x 9

Thanks

Michael

Edited December 29, 2020 by mcoulton

[Michael1962]

Width x thickness x length x ‘I don’t know’ but I think it might refer to the length at the barrel end of the spring that is doubled back on itself. The bridle. 

[nickelsilver]

4 hours ago, Michael1962 said:

Width x thickness x length x ‘I don’t know’ but I think it might refer to the length at the barrel end of the spring that is doubled back on itself. The bridle. 

Almost there- height (most used term, width is probably ok but might get confused with thickness) x thickness x length. The final number isn't always given but it refers to the inside diameter of the barrel.

[KOwatch]

Relevant to choosing another size, these are generalities:

the mainspring barrel size (last number, 9mm ) is the size that the new mainspring is packaged. You press it into your barrel. Must be equal or smaller than the ID of the barrel you are fitting a mainspring into. Smaller is OK, it will expand to fill the barrel when inserted.

Length (3rd number, 300 mm) should be close to the number specified. Shorter is OK, probably just a little longer is OK also. Corresponds to the power reserve, how many hours the watch will run on a full wind.

Thickness/Strength (.10mm) is the thickness of the metal spring. This represents how much power or torque the mainspring will deliver to your movement, if the width, material and age of the spring are the same. Thicker will deliver more torque, eventually resulting in a higher amplitude of your balance, eventually leading to overbanking. Thinner, low amplitude, poor performance. Condition of the movement, the gear train, number of complications, oil condition, will all effect how this effects your movement.

Width/Height (1.45mm) this is the height of the coiled mainspring so it must be smaller than the inside height of your barrel. Next size smaller (-0.05mm) here shouldn't be  a problem, the spring will be marginally looser in the barrel, otherwise it should be fine. Technically you would want to compensate for the reduction with a thicker spring, to keep the torque the same (essentially keeping the cross-sectional area of the spring the same), and there is a formula to calculate all this, although you'd probably be fine without engineering the replacement, depends on how fussy you like to get with <1% differences.

You shouldn't have to change more than 1 of the length/width/height values to find a replacement for an unavailable spring, usually the length being the easiest and least consequential in terms of effect on the operation of the movement.

You may also find an older Dennision/White-A-loy numbering system for mainsprings in the Bestfit catalog and elsewhere, you can use to find NOS mainsprings on ebay. There's  lookup tables to translate it to metric. Your target would be "511M" or  "511K"  I believe (where the letter is the length, M=317mm and K=292mm ). I'm not sure about those, there's usually no cost savings, and who knows how old they are or how they have been stored, but they are available.

 

 

 

 

 

 

 

[Poljot]

On 12/27/2020 at 2:11 PM, mcoulton said:

Hello

I don't have very much experience with mainsprings and I've just removed one from a FHF 175 movement. As you can see from the picture it's still coiled up. Any video's I've seen always show the mainspring almost unwound. Is there an issue with mine and should I see if I can order a replacement? 

Thanks in advance for any help that can be provided. 

Michael

 

Hi there,

This is how almost every old carbon steel mainspring looks like. These are pre-1960-ish production mainsprings and they never looked as S-shaped, not even 60 years ago, not 150 years ago or ever. The newer alloy springs do look like "S". I would not recommend getting NOS carbon steel mainspring. It may be waste of money and it can break at any time.

[mcoulton]

Thanks to everyone for their responses. It's been quite an education! My plan is to try and use the existing mainspring and see what the response is. I've take a time-graph of the movement before it was dissembled and cleaned. Once I've reassembled it and put it back on the time-graph, I'll have a better understanding of where the project is. 

Michael

[mcoulton]

If your interested in reading more about the subject, here's a link to an article I found:

 

https://www.vintagewatchstraps.com/mainsprings.php

 

[KOwatch]

Quote

The logical implication of the BHI statement about the potential damage that a broken mainspring can cause is that if the spring hasn't already broken you are lucky, and that old carbon steel springs should always be replaced with a modern 'unbreakable' type.

 

Edited December 31, 2020 by KOwatch