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Pendulum Length

Reuven Gruber

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Jun 2, 2020
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What exactly does "Pendulum Length" mean?

If the movement says 114cm, where is this measured?
 

ChimeTime

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It is the "ideal length" of the pendulum leader, that is to say, the length that was calculated during the movement's design. I say leader, because suspension springs vary, temperatures vary, and local gravitational pull can vary, so the location of the bob will vary for each user.
 

Willie X

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Feb 9, 2008
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First, there is no answer to your question.
Lots of folks get 'wrapped around the axle' trying to rationalize these numbers.

You CAN safely assume that a 10 or 15cm marked movement would require a short pendulum, like for a small mantle clock.

A 114 or 116cm marked movement would require a very long pendulum, like for a tall case clock.

But, if you have the case, you would already know that. :rolleyes:

So my advise would be: don't waist any of your time trying to make an arbitrary number meaningful, not going to happen.

Willie X
 

Reuven Gruber

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Jun 2, 2020
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First, there is no answer to your question.
Lots of folks get 'wrapped around the axle' trying to rationalize these numbers.

You CAN safely assume that a 10 or 15cm marked movement would require a short pendulum, like for a small mantle clock.

A 114 or 116cm marked movement would require a very long pendulum, like for a tall case clock.

But, if you have the case, you would already know that. :rolleyes:

So my advise would be: don't waist any of your time trying to make an arbitrary number meaningful, not going to happen.

Willie X
They're must be a specific answer to my question. Since the manufacturer stamped the information on the movement it must have a specific meaning
 

Joe Somebody

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Not to be ignorant, but i think the OP's question was where and how do you measure pendulum length ?

Let's say this is a Hermle 114cm pendulum, is this measured from the suspension spring to the bottom of the bob?
 

NEW65

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technically yes. However there are other factors involved when it comes to accuracy of time keeping and pendulum length. Example the diameter of the disc, weight of disc, lyre or stick , suspension spring , verge - escape wheel distance, general wear in movement, actual weight driving time train. I’ve had movements that has said 94cm pendulum but the original pendulum hasn’t been suitable due to one of the above factors.
 

bruce linde

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Technically it’s going to be from the flex point of the suspension spring to the center of mass of the bob… but all of that is always approximate.
 

Reuven Gruber

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Technically it’s going to be from the flex point of the suspension spring to the center of mass of the bob… but all of that is always approximate.
I think that technically it's the distance from the pivot point (flex point of the suspension spring) to the center of mass of the entire pendulum (which is slightly higher than the center of mass of the bob)
I was recently told that the number stamped on the movement is the distance from the suspension spring attach point to the bottom of the rating screw. I just did an experiment on a problematic movement and it seems to confirm the explanation
 

Jim Hartog

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Hello,

In pure physics, a "simple" pendulum has a mass-less rod, a massive point bob, and no friction. It does not exist in the real world.

In the real world, the pendulum is called a "compound" pendulum. A regular clock pendulum is a compound pendulum (not to be confused with the type that has a bob both above and below the point of rotation like a swinger). The length of the real world pendulum is measured from the point of rotation (which may be indistinct as in a suspension spring) to the center of oscillation (not the center of gravity/mass of the entire pendulum). The center of oscillation and the center of mass are close but not in the same place. Read the section on the compound pendulum from the link (Wikipedia) below.


As Willie says, those numbers that you find on a movement can be mysterious and vary from manufacturer to manufacturer.

Another confusing one is "pendulum drop". Some of the numbers found on movements refer to this distance. This is the one that is easy to measure for the case builder. If you read Tran's section on movement descriptions in his Seth Thomas movement descriptions. you will find "Pendulums, from center of movement to end of rating screw, xxxx inches". I believe Ansonia is also an example of this. For Ansonia, drop is the distance from the minute arbor to the bottom of the rating nut.

Then, there are compound, compound pendulums (two bobs, top and bottom of a rod, like a metronome), torsional pendulum (like an anniversary clock) and the uncommon two bobs set apart from each other on a horizontal rod attached to a vertical rod that swings like a teeter totter. I've only seen pictures of that one, and I don't have a name. I guess a balance wheel is an example of the torsional "pendulum" not highly affected by gravity.

Jim
 
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Willie X

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That double compound one Jim mentioned is something to see. I've only seen two and they were brought to me by the same customer. These were made by H-M about 1990. I have a photo somewhere.
The clock had two large heavy cylinder pendulums about the size of modern GF clock weights. The lower pendulum arm was impulsed by an electronic module in the base and the upper arm just swung from the motion imparted by the top part of the lower pendulum arm which it was pivoted to. The actual clock (quartz battery) was like a lollipop atop the upper arm/pendulum. The clock head was heavy and almost balanced against it's pendulum's weight. And although the lower pendulum's motion was large (about 18 inches) and constant, the upper pendulum waved all over the place. Something that hard to imagine and even harder to descripe.!!!!!!!

Maybe I can find the photos. Willie X
 
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shutterbug

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I'll just add that it also depends on the type of pendulum. If you want a Lyre type pendulum, they will usually be 5 or 6 inches longer than the stated length. You could try googling "pendulum for" followed by your exact movement number.
 
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