Weight of bob in relation to getting a mechanical clock accurate

dorsetman

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I have cobbled together a skeleton clock using bits of other clocks. It all works well except it runs too fast despite the pendulum and bob at its maximum length. Also the pallets are at their maximum penetration into the escapement. Any further penetration and clock will not run. My question: Is the weight of the bob a factor and in which case can adding or reducing the weight affect the timing?
 

Bernhard J.

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Hi,

Theoretically the weight of the bob is almost irrelevant (assuming that the weight of the bob is a lot higher than the weight of the pendulum rod). The center of gravity of the pendulum is decisive. This center of gravity is typically located at the bob. To lower the center of gravity further (thereby increasing the effective length of the pendulum), the pendulum rod would have to be lightened. However, since the pendulum rod is usually considerably lighter than the bob, the effect should be minimal. Making the bob heavier in its lower region will have the same effect, but also be minimal.

To have an effect, depleted uranium could be placed at the bottom of the bob. ;)

Best regards, Bernhard
 

Bernhard J.

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Seriously, the only measure will be to replace the presumably disc-shaped bob by a bob of another form with lower center of gravity. Or if the bob is disc-shaped, large and rather light, to replace it by a smaller bob, but of similar weight. This will lower the center of gravity of the pendulum by a couple of mm and, accordingly, let the clock run slower.
 

dorsetman

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Hi,

Theoretically the weight of the bob is almost irrelevant (assuming that the weight of the bob is a lot higher than the weight of the pendulum rod). The center of gravity of the pendulum is decisive. This center of gravity is typically located at the bob. To lower the center of gravity further (thereby increasing the effective length of the pendulum), the pendulum rod would have to be lightened. However, since the pendulum rod is usually considerably lighter than the bob, the effect should be minimal. Making the bob heavier in its lower region will have the same effect, but also be minimal.

To have an effect, depleted uranium could be placed at the bottom of the bob. ;)

Best regards, Bernhard
Ok..thanks for that. I am puzzled however as the entire gear train including the escapement, pendulum and bob came out of a single clock movement I purchased online. So if it worked then (as per the original manufacturer) then why does it not work for me. My only avenue left is that after all the pendulum and bob was not part of the original movement. I liked your depleted uranium bit though.. very droll. :) P
 

dorsetman

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Seriously, the only measure will be to replace the presumably disc-shaped bob by a bob of another form with lower center of gravity. Or if the bob is disc-shaped, large and rather light, to replace it by a smaller bob, but of similar weight. This will lower the center of gravity of the pendulum by a couple of mm and, accordingly, let the clock run slower.
Now that's an idea. I will try that. thanks. P
 

Chris.K

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As stated above 99% of the time weight is not the factor but center of gravity (mass), I was building a New Haven round top long drop and ran into the 1%. Of the Constence in the equation I could not lengthen the pendulum rod, nor did I wish to change escapements so I had to go with a heavy bob to slow the clock down enough so I could adjust it for time. After using multiple different bobs I wound up using a 12oz. bob if I remember right, and got it adjusted to keep accurate time. Chris,
 
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lpbp

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A thinner suspension spring would help.
 
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Willie X

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There is a wide range of 'things to do' with the pendulum assembly but changing the weight alone is usually not a factor.

Accuracy is due to the design of the clock and any cheap quartz clock (or watch) will outperform any mechanical clock.

Some folks think the bob is supposed to be dead centered in the lower glass or bullseye, when most of the time it is not.
More often it is low, near (or below) the bottom of the door frame. It can also be well above center. There are/were no rules in this department. :)

Willie X
 
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demoman3955

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There is a wide range of 'things to do' with the pendulum assembly but changing the weight alone is usually not a factor.

Accuracy is due to the design of the clock and any cheap quartz clock (or watch) will outperform any mechanical clock.

Some folks think the bob is supposed to be dead centered in the lower glass or bullseye, when most of the time it is not.
More often it is low, near (or below) the bottom of the door frame. It can also be well above center. There are/were no rules in this department. :)

Willie X
I just recently had the same problem i posted about here, and had to unbend some of the bend in the suspension spring and rod assembly to drop it about 3/8s if an inch. worked like a charm, and really didnt drop it too much visually either.
 

Bernhard J.

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I just recently had the same problem i posted about here, and had to unbend some of the bend in the suspension spring and rod assembly to drop it about 3/8s if an inch. worked like a charm, and really didnt drop it too much visually either.
How does bending of the spring and rod assembly allow lowing the bob further without touching the bottom of the case?

Cheers, Bernhard
 

R. Croswell

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I have cobbled together a skeleton clock using bits of other clocks. It all works well except it runs too fast despite the pendulum and bob at its maximum length. Also the pallets are at their maximum penetration into the escapement. Any further penetration and clock will not run. My question: Is the weight of the bob a factor and in which case can adding or reducing the weight affect the timing?
First, the escapement (now that it has been messed with) should be adjusted for proper escapement operation and that depends on the type of escapement and may not be with the verge in as far as possible. You want equal drops off of each pallet. We do not know from your description if this escapement is a recoil or deadbeat. Then make sure the clock "is in beat".

While a number of factors can have some effect on how fast the clock runs, the controlling variable is the length of the entire pendulum assembly (suspension spring, pendulum leader, pendulum bob) from the point where the suspension spring flexes to the center of mass of the entire pendulum assembly. Now unless you have calculated that length by counting the number of teeth on the wheels and pinions of the time train, that exact number is of little value to you. To get from where you are now to where you want to be focus on the center of the pendulum bob, then lower that point toward the bottom of the clock case.

Do not try to slow the clock by using a very heavy pendulum bob. Use a normal pendulum perhaps 3 oz. give or take a little. The clock may struggle to run if the pendulum is too heavy and thereby run more slowly and may become unreliable. A larger diameter bob will affect the center of mass and cause the clock to run slower but may look strange.

We seldom know the true backstory of clocks sold online. This clock may have been sold as "running", I would suggest that it probably never kept time with the pendulum parts that it was sold with.

Again, we need to see pictures of what you are working with. If we are considering what was a typical American mantel clock, I would start with a new suspension spring and leader wire (0.003" thick suspension) and cut the length of the leader wire somewhat longer until the clock keeps time.

RC
 
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Bernhard J.

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A larger diameter bob will affect the center of mass and cause the clock to run slower
I would suppose exactly the other way around, because a larger bob (with uniform mass distribution) will raise the center of gravity and not lower it?

His problem is that the distance between pendulum bridge and bottom of the clock must remain unaltered and the pendulum - running too fast - almost touches the bottom of the case. Anything making the pendulum longer will make the clock run extremely slow (i.e. not at all), because the end of the pendulum then hits the bottom of the case. At least this is my understanding, but this may be wrong.

Cheers, Bernhard
 

demoman3955

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How does bending of the spring and rod assembly allow lowing the bob further without touching the bottom of the case?

Cheers, Bernhard
on mine i had room to lower it. It came with a 90 degree bend then 2 45s to clear the arbor for the hands, so i just took a bit out to lengthen them. Yours is totally different if its already close to the bottom.
 

R. Croswell

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I would suppose exactly the other way around, because a larger bob (with uniform mass distribution) will raise the center of gravity and not lower it?

His problem is that the distance between pendulum bridge and bottom of the clock must remain unaltered and the pendulum - running too fast - almost touches the bottom of the case. Anything making the pendulum longer will make the clock run extremely slow (i.e. not at all), because the end of the pendulum then hits the bottom of the case. At least this is my understanding, but this may be wrong.

Cheers, Bernhard
As stated, we really need to see pictures of this clock. If the pendulum is hung by a small loop at the top, a larger pendulum bob will make it run slower. If the pendulum is supported from the bottom, a larger pendulum bob would make it go faster. Not knowing which this clock has, I was focusing on moving the center of pendulum bob closer to the floor of the clock (regardless of how it is supported) to make the clock go slower.

I would begin with a pendulum bob that is correct for this clock, which may or may not be the one that came with the movement, then adjust the length of the pendulum leader wire until the clock keeps time. If we are talking about a situation where the “normal” pendulum hits the floor before the clock slows enough to keep time, a thinner suspension spring will sometimes help.

This movement is apparently being married to a different case. A more drastic step is to change the escape wheel to one that has more teeth (more ticks per revolution) which will require a shorter pendulum to keep time.

RC
 

ChimeTime

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I am puzzled however as the entire gear train including the escapement, pendulum and bob came out of a single clock movement I purchased online. So if it worked then (as per the original manufacturer) then why does it not work for me?
With all due respect, the part about "coming out of a single clock" may be in direct conflict with the statement "that I purchased online".

Regulars here will remember I had the same issue about a year ago. In that clock it turned out that the dealer had mixed parts simply to make the sale. Luckily, the owner realized it had no value and only wanted it to run. The answer in that instance was to raise the suspension point AND cut a hole in the bottom to elongate the pendulum.

My point is, you may need to factor in the character of the seller. Regards.

IMG_20210827_170753489.jpg


IMG_20210827_170735010.jpg
 
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dorsetman

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Now that's an idea. I will try that. thanks. P
You were right :) I dug out a old fishing weight which happened to be the same weight as my bob and battered it into a shape which lowered the gravity (see picture) and bingo..the clock ran a lot slower
Now that's an idea. I will try that. thanks. P
It worked :) I used a old lead fishing weight which happened to be the same weight, battered it into a new shape (see picture) and bingo the clock slowed right down and into the right ballpark so thanks for that. Now all I have to do is to either modify my existing bob or make or obtain a new one. This has proved to me that my original purchase included the wrong bob. We live and learn. Thanks again P

bob 29 Sep 2022 at 10-28.jpg
 
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