What if you reduce a movement's size?

Discussion in 'Clock Construction' started by NoVa Larry, Nov 11, 2011.

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  1. NoVa Larry

    NoVa Larry Registered User

    Jan 23, 2009
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    I've searched the forum and the net, but didn't find an answer.

    If you want to scale a piece, what happens as you reduce the physical size of a movement and case? The pendulum length would decrease, which would seem to require a different escape wheel. What else would change in the train?

    How do you estimate the weight required to initially power a time-only movement? With a shorter drop and a shorter drum, there would be less total power available. Would this reduce the run time, or would the required drive power scale with the size?

    Thank you for any comments that you can offer.

    Very respecfully,
    Larry
     
  2. tok-tokkie

    tok-tokkie Registered User

    Nov 25, 2010
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    The only bit I can comment on is the shorter pendulum will swing faster. The equation of pendulum length to time is here - use the Rule of Thumb http://en.wikipedia.org/wiki/Pendulum_%28mathematics%29
    So you will have to adjust the escape wheel to have the required number of teeth to give 1 rev per minute only if it has a seconds hand on that arbor.
    If the seconds hand is on another arbor or if it has no seconds hand things are much simpler as you then just need to change the gearing to the minute arbor. Trouble is changine the gearing means the arbor to arbor distance changes so you have to make other adjustments.
    If you scaled the clockworks down but left the pendulum length as original you would not need to make any alterations to the clockwork design.
     
  3. NoVa Larry

    NoVa Larry Registered User

    Jan 23, 2009
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    I've assumed that the tooth counts on the wheels would stay the same, and their diameters and spacings would scale. If that's correct, wouldn't it be easier to alter just the escape wheel?

    Very respectfully,
    Larry
     
  4. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    Hi
    Depending on how much you reduced it,
    you can compensate by just increasing the
    number of teeth on the escapement wheel.
    At some point is may make more sense
    to change one of the ratios in the train
    to keep a practical escapement wheel size.
    Just remember, every tick tock at the escapement
    wheel is one escapement tooth. Take
    that through the ratios to the minute hand
    and it has to be one revolution per hour.
    As for friction loss, there are a lot of
    variables to consider. For the most part,
    a faster swinging pendulum will need slightly
    more energy.
    Tinker Dwight
     
  5. NoVa Larry

    NoVa Larry Registered User

    Jan 23, 2009
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    Thanks Tinker, Tok Tokkie,

    I'm considering trying to scale a banjo to 50-60%.

    The scaled drive weight should reduce by the cube of the scale, so gear and pivot friction would become critical. I have no idea how much that load will scale, but my assumption (that bad word keeps creeping in) is that it relates to torque, which suggests that the load would scale as a factor of the scale rather than as a power, like the weight would.

    So, being at a point that seems to say it wouldn't work, I sought insight from those who know markedly more than I do -- I'm a beginner. Sorry for not putting more detail in my initial post.

    Very respectfully,
    Larry
     
  6. Tom McIntyre

    Tom McIntyre Technical Admin
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    Sounds like a fun project. Keep us posted.
     
  7. Scottie-TX

    Scottie-TX Registered User
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    #7 Scottie-TX, Nov 21, 2011
    Last edited: Nov 21, 2011
    I can address two of your questions to some extent:
    First and foremost, amount of power regardless of amount of space for descent will be most affected by type of escapement. For a short amount of descent and 8 day running, ONLY a deadbeat suffices.
    As you suspect, diameter of driving drum affects necessary weight needed as well as affecting duration. A smaller diameter drum will require more weight but will run longer.
    To estimate approximate amount of weight needed to power a Time Only timepiece you need only to look at those already designed and made. I can tell you that one here, deadbeat of course, accomplishes six days on one pound. It's winding drum is scarcely more than a quarter inch diameter. Small pendulum clocks with one week duration are ALL about efficiency if weight driven.
    In a sense I would say that size is not scalable. That is, not linear scalable. A timepiece of, let's say, eighteen inches tall will NOT require half the power of one, 36" tall. . A smaller amount of descent may indeed require slightly more weight than a larger amount of descent . A taller one can have a much larger winding drum and still run one week. One here, nearly 36" tall is reliable on less than a pound for seven days!
     
  8. NoVa Larry

    NoVa Larry Registered User

    Jan 23, 2009
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    Scottie,

    Thank you for your substantive comments.

    (Kinda on topic here) Which got me thinking about the early banjo movement that I'm working on: Why does that simple of a movement require 7# to drive it?

    I've heard of using a spring scale in series with the weight to find the movement's "stall" (?) weight. Would it creat a problem if I reduced the drive weight?

    I wish you and all a happy Thanksgiving.

    Very respectfully,
    Larry
     
  9. Scottie-TX

    Scottie-TX Registered User
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    If a TO movement requires seven pounds it is recoil. Typically, recoil movements require twice or more the amount of power to sustain.
    A fish weighing scale will work. Suspended between clock and oversize weight will register the last amount of weight available when movement stopped due to lack of power. Add 20% for reliability.
    I use overswing - supplemental arc - to determine power required. Hang four pounds. Observe overswing. 10 to 20 degrees overswing ? Less power required. Hang 2#: Now two to three degrees overswing. Use 2 and 1/2 or 3 pounds. It'll work!
     
  10. NoVa Larry

    NoVa Larry Registered User

    Jan 23, 2009
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    Scottie,

    Thank you. I'd not thought of the difference in the type of movement as effecting the drive weight - I'd thought that the differences were in accuracy. I guess a better statement was that I didn't think, because that difference makes complete sense, and is consistent with regulators needing less power.

    The relationships come out slowly.

    Very respectfully,
    Larry
     

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