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# Thread: Urgos Pendulum Length - I want to Change it

1. ## Re: Urgos Pendulum Length - I want to Change it (By: Tinker Dwight)

Originally Posted by Tinker Dwight
I should note that adding the weight at the center ( 1/2 way ) is
the most effective way to speed up a pendulum.
Other locations take larger amounts of weight.
This still assumes a simple pendulum. A pendulum with other weight
distributions may have other maximal effect points.
I could show the math if anyone is interested.
Tinker Dwight
I tried adding weight to the center of the pendulum and there was no discernable difference in tempo.
The 10.5” bob weighs 28 oz. so I attached 2 oz of steel shot to the center of the pendulum, halfway between the suspension spring and the bottom of the pendulum. After 24 hrs. the clock was exactly the same as before I added the weight (i.e 1 hr. 48 mins slow). I moved the weight up a bit thinking the “center” should be to the center of the bob. This too made no difference. Maybe a minute or two after 24 hrs.
Thanks for the tip but it does not work. At least not for me. This is a lyre pendulum so I’m sure the weight distribution is not as simple as a stick.

2. ## Re: Urgos Pendulum Length - I want to Change it

Originally Posted by Torre
I tried adding weight to the center of the pendulum and there was no discernable difference in tempo.
The 10.5” bob weighs 28 oz. so I attached 2 oz of steel shot to the center of the pendulum, halfway between the suspension spring and the bottom of the pendulum. After 24 hrs. the clock was exactly the same as before I added the weight (i.e 1 hr. 48 mins slow). I moved the weight up a bit thinking the “center” should be to the center of the bob. This too made no difference. Maybe a minute or two after 24 hrs.
Thanks for the tip but it does not work. At least not for me. This is a lyre pendulum so I’m sure the weight distribution is not as simple as a stick.
The calculation was for a simple bob pendulum. The mass distribution
of a lyre pendulum is quite different.
Can you send me a rough measurement of the grid iron frame
and weight as well as the location and weight of the bob part.
I can do another calculation to see about where and what
can be done.
We are not beat yet.
Tinker Dwight

3. ## Re: Urgos Pendulum Length - I want to Change it (By: Tinker Dwight)

Tinker,
I truly appreciate you helping me out with this.
The new photo I have attached shows the longer pendulum so you may compare it to the first photo of the clock I posted.

The longer pendulum’s geometry is as follows:

Overall length (not including the adjustment knob) = 42.5”
Overall weight = 59.0 oz.
Dia. of Bob = 10.5”
Weight of Bob = 28.0 oz.
Center Mass = 30” from top/12.5” from bottom
The leader is 7” long so you can add another 7” to the suspension spring.

Thanks again!
Mark

4. ## Re: Urgos Pendulum Length - I want to Change it (By: Torre)

I did some calculations but I'm still not sure about the
grid iron geometry and its CG. I estimated it from the overall
CG.
Anyway, a 1 to 2 ounce weight mounted about 2 inches below
the joint between the leader and the pendulum rod.
This was using your 5 seconds per hour number.
Tinker Dwight

5. ## Re: Urgos Pendulum Length - I want to Change it (By: Tinker Dwight)

Thanks Tinker.
That is quite a bit higher up!
I'll give this a try tomorrow and report back.
And I think you meant to say 5 seconds slow per "minute" not per "hour".
Per hour I could live with.
Thanks again,
Mark

6. ## Re: Urgos Pendulum Length - I want to Change it

Originally Posted by Torre
Thanks Tinker.
That is quite a bit higher up!
I'll give this a try tomorrow and report back.
And I think you meant to say 5 seconds slow per "minute" not per "hour".
Per hour I could live with.
Thanks again,
Mark

Don't bother. I don't know why I thought it was per hour but that was
my calculation. 5 seconds per minute will be quite different.
I won't be able to look at the calculation again until Monday.
If while waiting, can you get me the CG of the grid iron, without the bob
and the measurments to the ends of the grid iron from some reference
I'll need a better number than "about" 5 seconds per minute. Can you measure
over a longer period?
Tinker Dwight

7. ## Re: Urgos Pendulum Length - I want to Change it (By: Tinker Dwight)

Ha. No problem. Yeah 5 sec. per hour could be adjusted by turning the adjustment knob on the bottom a half turn.
I have measured the slowness over a 48 hr. period and it was 214 min. (and some seconds) which equates to 4.45 mins./hr. and 4.45 sec./min.
I have removed the bob and took measurements of only the “grid iron” section with the neck piece.
This center of gravity is 22.5” from the top (add 7” for the leader). The dimensions of the grid iron section is 18.875” long x 3.5” wide.
Another thought I had was to try and duplicate the center of gravity of the shorter pendulum which came with the movement. Both pendulums have virtually the same overall weight at 59.0 oz as they are identical with the exception of an extended neck piece.
The comparison of center mass is as follows:

Longer Pendulum = 30” from top/12.5” from bottom
Shorter = 23.25” from top/13.25” from bottom

I would think a good experiment would be to try and raise the center mas of the longer pendulum by 6.75” so that is duplicates the shorter one which works. I will try this over the weekend and let you know what happens.

Thanks again,
Mark

8. ## Re: Urgos Pendulum Length - I want to Change it (By: Torre)

Probably be a lot easier, and better, to change the wheels in the movement.
To do what you are trying to do will probably require exchanging the present bob for a dummy bob, just the brass or plated aluminum cap/disk.
Willie X

9. ## Re: Urgos Pendulum Length - I want to Change it (By: Willie X)

Contrary to common thinking, the center of gravity is only
part of the controlling factor of the pendulum.
In fact, we will most likely be lowering the center of gravity to speed
it up, not raising it.
It is the moment of inertia that slows it down. The lower you can get the
center of gravity without increasing the moment of inertia the faster
the pendulum will be.
The equation for a complex pendulum ( any pendulum without a simple
ball at the bottom ) is:

Period = 2*Pi* ]SquareRoot( SumMomentsOfInertia/(SumOfCgFromFulcrum*g) )
The SumOfCgFromFulcrum is the distance from the fulcrum times that mass
at the center of gravity.
So, you can see that lowering the CG by itself actually speeds the pendulum
up.
The problem is that after, around the center of the pendulum, adding mass
increases the SumMomentsOfInertia faster than the SumOfCgFromFulcrum.
Having the grid iron throws this off a little so I need to hunt for the minimum
So, matching CGs may actually make the pendulum slower, depending on the
amount of mass you add and where.
We want to find the most effective point to add mass and how much to add.
Above or below that optimal point will just be wasting weight that could be
more effective elsewhere.
Wait till Monday when I get back to a machine that has a spread sheet.
Tinker Dwight

10. ## Re: Urgos Pendulum Length - I want to Change it (By: Tinker Dwight)

Thanks gentleman,
I am still hopeful there is some way to modify the pendulum to make this work based upon your equation.
While I agree changing the second wheel is the "right" way to go it is suck a PITA to take apart a tubular bell movement.

I cannot change the center of gravity to match the shorter pendulum anyway as it would require nearly 16 oz. of weight added near the top. Or I would have to drastically decrease the weight of the bob. Not a realistic.

Thanks again,
Mark

11. ## Re: Urgos Pendulum Length - I want to Change it (By: Torre)

Adding the weight at the top of the pendulum would
have almost zero effect on the rate of the pendulum.
It is the center of gravity relative to the fulcrum that
counts. Adding weight at the fulcrum has no effect, since
it is at the center of the rotation. It would slow the
clock a little because it would add to the rotational moment
of inertia, because it would have to have some diameter,
larger than zero.
Remember I said, the idea is to lower the center of gravity,
without increasing the rotational moment of inertia. Adding
at the top of the pendulum does neither in the right direction.
Tinker Dwight

12. ## Re: Urgos Pendulum Length - I want to Change it (By: Tinker Dwight)

It's an interesting experiment for sure but no doubt the right way to go is to change out the second wheel as suggested.
I have another Urgos movement from which the longer pendulum actually fitted to. I'm not sure where the second wheel is? I am assuming it is near the back of the movement? Most photos of movements I see show it directly behind the clock hands. I don't think this is the case on these movements.
Thank you.

13. ## Re: Urgos Pendulum Length - I want to Change it (By: Torre)

Is the second wheel the wheel directly behind the winding cable barrel?

14. ## Re: Urgos Pendulum Length - I want to Change it (By: Torre)

It will be the wheel / pinion set that the winding barrel teeth drive (mesh with).

David

15. ## Re: Urgos Pendulum Length - I want to Change it (By: David S)

Ok, I ran the numbers again. I don't have all the rotational inertia but this
is a close ball park number/
The weight should be added at about 10.25 inches down from the hook at the
top end of the pendulum ( don't include the leader and spring. )
The calculated value is 28 oz. or 1.75 lbs. This is most likely a low value
since I didn't include extraneous inertia, like the width of the grid iron or the
bob's rotational inertia. These are usually a small factor. I'd start out with trying
2 lbs of lead. The diameter side to side or up and down should be kept small..
It is heavy but the movement should be able to handle it.
Is the bob hollow or lead filled. Reducing its weight can make a big difference.
grinding some lead out with a bur will help a lot.
Tinker Dwight

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