# Seth Thomas1801 - 1900AmericanI thought pendulum bob size didn't matter when it came to making a clock run properly?

#### captainclock

##### Registered User
Hello Everyone, a couple of years ago I had gotten from my Great-Grandparents House when we were cleaning it out so we could sell it an 1858 Seth Thomas Column and Cornice Clock (which most of you guys are probably familiar with my never ending fights I had with getting that clock to run and stay running correctly.)

Anyways I did finally get it working correctly (or as correctly as a 160+ y/o clock can work) and the odd thing is that everyone on here was insisting that the size of the pendulum bob didn't matter when it came to how the clock ran, but apparently that wasn't the case with my Seth Thomas clock, because I had previously had the clock "running" (it was running but not correctly) with a larger diameter pendulum bob because someone else on here suggested that my clock should of had a larger diameter bob because that's the way their's was.

But after I finally made the necessary repairs to the movement to get it to run like it was supposed to (including getting the correct verge assembly) the clock was having trouble losing time with the larger bob (no matter how high the bob was raised) and so I tried a smaller diameter bob that I had ordered along with the larger diameter bob ( from when I was trying to figure out which size I needed to use with this clock) and sure enough the clock has been spot on accurate with the smaller bob adjusted to the center of its adjustment plane, so it seems that in this case size does matter in terms of the bob and how the clock runs.

Is there anything to what I've just experienced here or is this just some weird fluke?

#### bruce linde

NAWCC Member
Donor
the rate of a clock is determined by the length of the pendulum. the 'length' of the pendulum is the distance from the flex point of the suspension spring (at the top) and the center of mass (COM) of the pendulum bob. your larger bob's COM was functionally lower, making the clock run slower. the smaller bob's COM is a bit higher, effectively shortening the pendulum and making the clock run faster. make sense?

#### captainclock

##### Registered User
the rate of a clock is determined by the length of the pendulum. the 'length' of the pendulum is the distance from the flex point of the suspension spring (at the top) and the center of mass (COM) of the pendulum bob. your larger bob's COM was functionally lower, making the clock run slower. the smaller bob's COM is a bit higher, effectively shortening the pendulum and making the clock run faster. make sense?
Yes It does, but I wonder why my clock ran fine and kept time with the large bob with the wrong type of verge installed (half-dead beat verge is what I had installed after the original verge broke on me because I didn't realize that was the wrong type of verge at the time) but then with the correct type of verge installed (recoil verge) it now will only run correctly with the smaller bob? Does the type of verge and escapement that the clock uses also affect the pendulum and how it makes a clock run?

#### leeinv66

##### Moderator
Staff member
NAWCC Member
A recoil escapement takes more energy to run than a half or full deadbeat escapement. Your bigger bob has more mass and the gearing of your movement is struggling to apply enough force to keep that mass in motion. It is not the physical size of the pendulums that is making the difference, it is the mass of the bobs.

#### JeffG

NAWCC Member
make sense?

I'm not sure that your example makes sense to me.
If the two bobs are the same weight, but with different diameters, wouldn't the regulating nut position the COM of the larger bob a little higher up on the pendulum, therefore decreasing the effective length?
Doh! Unless we're talking about hanging bobs. Then yes, larger = longer (assuming the same weight).
Makes sense.
-Jeff

#### novicetimekeeper

##### Registered User
I'm not sure that your example makes sense to me.
If the two bobs are the same weight, but with different diameters, wouldn't the regulating nut position the COM of the larger bob a little higher up on the pendulum, therefore decreasing the effective length?
Doh! Unless we're talking about hanging bobs. Then yes, larger = longer (assuming the same weight).
Makes sense.
-Jeff

I agree that it seems that way, however I think that with the larger bob the rod becomes less significant. When you fit a smaller bob the mass of the rod as a proportion goes up raising the CoM of the assembly.

#### Chris.K

NAWCC Member
99% of the time as stated above length and COM will correct most things fast or slow. I had a problem with a New Haven long drop time only 'regulator' that kept running fast. I reached out here and other sites and everyone said make it longer. I did not have that option as I would have to chop off the bottom of the clock. Within the 'equation you have constants and variables and when the constants are satisfied it comes down to weight. I had a repro Munger 6oz and it still ran fast so I bought a 12 and 14 oz bobs. Now the clock runs great with the 14oz bob and is tuned in to about a minute a month. You cant use a feather nor a brick for a bob. It's the 1% that gits ya...

#### bruce linde

NAWCC Member
Donor
I'm not sure that your example makes sense to me.
If the two bobs are the same weight, but with different diameters, wouldn't the regulating nut position the COM of the larger bob a little higher up on the pendulum, therefore decreasing the effective length?

yes. you're using the same regulating nut, so it really comes back down to the COM of the bob. with your larger bob the diameter prevented you from getting the COM high enough... and the movement ran slow. the smaller diameter bob could go higher and give you the higher COM required.

#### michael isaacs

##### Registered User
I'm with Jeff, not following that logic. But that's were I live most of my life

#### michael isaacs

##### Registered User
OK, got it now. Takes longer the older I get.

#### tok-tokkie

##### Registered User
Remember that it is the distance from the flex point of the suspension to the COM of the bob + rod. It is not just the bob alone.
Switching to a lighter bob at the same position (position=flex point to center of bob) results in a higher COM because the contribution of the rod now has a greater effect thus raising the combined COM.
A conventional regulating nut moves the entire bob (the bob rests on the regulating nut). So if the lighter bob is a different diameter to the heavy one then it needs to be moved so that the position COM of the lighter bob matches where the heavy bob COM was.

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