Choosing the correct pendulum bob - Practical advice needed

[Michael20]

This post relates to the clock I mentioned in my Ansonia Dial Clock post last Monday - I have started a new topic because the subject matter is nothing to do with the previous post, it just happens to be about the same clock. hope that's the correct way to do this.

The clock in question is I think a Anglo-American dial clock, that is an Ansonia movement in an English case. 

When I acquired the clock the pendulum bob was missing, I took a few measurements and found one that would fit without fouling the bottom of the case - not very scientific I know.

The movement has been cleaned and I now have it on test, it runs well but gains, my concern is I am running out of adjustment on the pendulum bob. I realise there may be other reasons for the clock gaining, maybe a weak mainspring or something binding somewhere that reduces the power, but my first thought is the pendulum bob, as that's not original.

So to my question, I seem to remember being told in a previous post that in theory the weight of a pendulum bob shouldn't make any difference, but then I think OH said in practice it can 

(hope I haven't mis-quoted you there OH)  I will attach pics showing the movement and the pendulum bob and would appreciate any advice about whether this bob is suitable or not.

Thanks in advance.

[AndyGSi]

Have you tried lowering the bob and retiming?

[nickelsilver]

How much is it gaining? 1m/day, 30m/day? If you run out of adjustment, then you need a longer bob. The theory of weight not affecting the rate is true, but the bottom line is the center of gravity of the pendulum*. A lighter pendulum is likely to have a higher center of gravity for a given length than a heavier one.

 

* Was talking about this with a watchmaker friend years ago, he absolutely didn't believe it. I had a Favag master clock with a fairly heavy 1.5kg 50cm pendulum. I added 1.5kg to it, and, while the rate changed a bit, it actually increased the rate, haha. (The extra weight was literally another Favag bob I put on top of the original, effectively raising the center of gravity)

Edited July 20, 2024 by nickelsilver

[Michael20]

20 minutes ago, AndyGSi said:

Have you tried lowering the bob and retiming?

I have, and that's my concern there is very little adjustment left

14 minutes ago, nickelsilver said:

How much is it gaining? 1m/day, 30m/day? If you run out of adjustment, then you need a longer bob. The theory of weight not affecting the rate is true, but the bottom line is the center of gravity of the pendulum*. A lighter pendulum is likely to have a higher center of gravity for a given length than a heavier one.

 

* Was talking about this with a watchmaker friend years ago, he absolutely didn't believe it. I had a Favag master clock with a fairly heavy 1.5kg 50cm pendulum. I added 1.5kg to it, and, while the rate changed a bit, it actually increased the rate, haha. (The extra weight was literally another Favag bob I put on top of the original, effectively raising the center of gravity)

At the moment it is more like 1 minute per hour. Can't use a longer bob with adjustment as the bottom of the thread on this one only just clears the case... heavier bob maybe???

[AndyGSi]

I think you're best trying to add a weight to the bottom of the bob and test again.

I expect the solution would be a bob of approx. the same weight but smaller diameter to lower the centre of gravity.

[Michael20]

48 minutes ago, AndyGSi said:

I think you're best trying to add a weight to the bottom of the bob and test again.

I expect the solution would be a bob of approx. the same weight but smaller diameter to lower the centre of gravity.

Thank you, makes sense I'll try that and report back

[AndyGSi]

Can you confirm what size bob you've got?

Edit

Just realised it's in your photos above.

I think you're probably going to need the slightly lighter 35mm one but on the same wire as your 43mm.

Edited July 20, 2024 by AndyGSi

[Michael20]

5 minutes ago, AndyGSi said:

Can you confirm what size bob you've got?

Hi, its in the pics, 71 gms and about 43 wide x 73 including the wire part

3 minutes ago, Michael20 said:

Hi, its in the pics, 71 gms and about 43 wide x 73 including the wire part

Sorry just seen your previous reply. I can try making one out of parts I have and see what that does. Thanks again for your help.