What if??

Discussion in 'Clock Construction' started by dickstorer, Feb 2, 2015.

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  1. dickstorer

    dickstorer Registered User

    Oct 19, 2010
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    I have built a clock case using a repro ST No. 2 movement. I have modified the movement to be a 72 beat instead of a 80 beat and it works great. My problem is that when the weight bottoms out there is still about 5 inches of space left that I would like to utilize. Could the diameter of the cable drum be increased enough to give me a 10 day run time instead of 8 days? The cable drum is .638" in diameter and there are 16 grooves. If I made a wood sleeve to increase the diameter to .938". That .938 makes the drum diameter just a cables width below the ratchet teeth. Would that work of should I leave well enough alone? When I do the math it gives me about 4 more inches of weight drop.

    I need some expert opinions.

    Dick
     
  2. harold bain

    harold bain Registered User
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    Hi, Dick. Does that 5 inches of extra space indicate you need a longer cable? You could easily experiment with a temporary cardboard sleeve to see how it does. It may need more weight to run it with a thicker drum.
     
  3. dickstorer

    dickstorer Registered User

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    #3 dickstorer, Feb 2, 2015
    Last edited: Feb 2, 2015
    Hello to Harold, thanks for the reply. Yes, it needs more cable. Right now the cable drum us full , if the drum were not grooved I would not have any problem. In my thinking a thicker drum would equate to less driving weight because you are actually increasing the leverage.
    Dick
     
  4. dickstorer

    dickstorer Registered User

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  5. harold bain

    harold bain Registered User
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    Dick, I recall having (still have it) a Vienna regulator that would only run about 6 days before the weights bottomed out. When I did a general clean and recondition on the movement I replaced the cable with a much thinner cable, 50 pound Stren Sonic Braid fishing line. I was totally surprised that this change resulted in a 7 day run time, with about half a day extra if I was late winding it. You could try this on your clock just to see if it makes the difference.
     
  6. R. Croswell

    R. Croswell Registered User

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    I think the thinking here is backwards. To get a longer run time you need the main wheel to turn more times - the clock advances the same amount for each revolution. Making a LARGER drun will require more drop, not less (but a lighter weight because of the mechanical advantage). To get more days run with the same drop you will need a SMALLER drum, and a heavier weight. If the drum is completely full when the weight is all the way up I would leave well enough alone. The only way to get longer run time without changing the weight would be a wider drum which is probably not practical. Now if you got rid of the grooves in the drum perhaps you could get a few extra turns with a smaller cable.

    RC
     
  7. dickstorer

    dickstorer Registered User

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    Mr. Croswell, to quote just part "Making a larger drum will require more drop" more drop is what I am seeking. I am getting my week run but when the weight is all the way down there is 5 inches of space, I was sort of wanting to use that space. And if I get rid of the grooves I can get more turns even with the same cable and same drum diameter. If I make a sleeve for the drum I might as well make it as large as possible.

    DS
     
  8. R. Croswell

    R. Croswell Registered User

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    Sorry I guess I still do not understand your objective. Your question was. "Could the diameter of the cable drum be increased enough to give me a 10 day run time instead of 8 days?". The answer is no. Increasing the diameter of the drum will unspool more cord each time the drum turns one revolution (and the weight will drop further per revolution) but it will not increase the number of times the drum turns. If you simply want the weight to travel (drop) further in 7 days to consume the available space then a larger drum will accomplish that, (and also allow a somewhat lighter weight). If you want the clock to run three more days you will need to have the main wheel turn 30% more revolutions before the weight bottoms or all the cord is unspooled. To accomplish that you will need to get 30% more cord wraps (not inches) around the drum so it will turn 30% more revolutions before running out of cord or bottoming the weight. Getting rid of the grooves and/or using a thinner cable will move toward that objective. Increasing the drum diameter will move away from that objective by increasing the rate of fall. Asmaller drum will decrease the rate of fall and allow the main wheel to turn more times (days) before the weight bottoms but will not in itself increase run time unless you manage to get more wraps on the drum, (or change some gear ratios so the escape wheel turns more revolutions per revolution of the main wheel). In other words, the number of coils on the drum will determine the run time before the string is all unwound. The diameter of the drum will determine how far the weight will need to drop for each revolution of the main wheel.

    I'm not sure why one would want to alter the ST No. 2 design from the original design. If it is simply to have the weight end the week's run at the bottom of the case, which would allow the weight's position to be a better relative indicator of the time remaining, why not just lengthen the cord so the clock stops just before the weight bottoms and not wind it as high?

    RC
     
  9. dickstorer

    dickstorer Registered User

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    My thoughts are: Existing drum diameter is .638, pi x diameter gives circumference, number of grooves x circumference gives total length of cable, since we have a pulley we can only use half of that length, turns out to be approx.. 32 inches. Increase the drum to 2.94 and we will gain almost 6 inches of cable. That is another 3 inches of weight fall.

    My purpose for altering the repro movement--I wanted a clock with a longer pendulum. Changing from a 80 beat to a 72 beat gains nearly 5 1/2 inches. Altering is easy, just make a 36 tooth escape wheel and pinch in the pallets just a little and that's it.

    As it is now, when the weight comes to the end of its fall it appears like it has stopped before it has run down.

    An alternative would to do as you suggest: Get rid of the grooves, might be the simplest.

    DS
     
  10. MartinM

    MartinM Registered User

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    Just taking out the grooves on that one will mean having to move the end of the cable that's tied to the movement pillar, forward and hoping, during the wind, that it spools correctly under a weighted condition. Leaving the string in the middle of the pillar will cause cable crossing, for sure.
     
  11. Tinker Dwight

    Tinker Dwight Registered User

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    As RC points out, a larger drum will take more cable per day.
    It will require less weight.
    The next question is, is there a pulley on the weight.
    If so, you can use a some what larger drum, more
    cable and more weight to get to 8 days.
    It is even possible to add another pulley at the top
    if you already have a pulley at the weight.
    Tinker Dwight
     
  12. dickstorer

    dickstorer Registered User

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    There is a pulley and I am getting 8 days. I just thought it would nice to make the weight use all the weight fall space available.
     
  13. Tinker Dwight

    Tinker Dwight Registered User

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    The run time will still be limited by the diameter
    of the cable and the width of the drum, regardless
    of the length of the cable used.
    If you want it to use the entire fall,
    measure the amount, extra, you want to consume.
    Then multiply by PI. That will be the radius increase
    or then multiply by 2 * PI to get the increase
    in diameter.
    Tinker Dwight
     
  14. MartinM

    MartinM Registered User

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    I think the only way to do what you want is to determine how many turns the spool makes in 8 days and then build up the spool diameter to a size maybe 4mm larger than what it takes to use up the ~84cm of total cable in that amount of turns and then cut new cable landing 'threads' 2mm deep. (the 2mm and 4mm figures should be evaluated against the existing 'threads'.)

    Say it takes 10 turns to run 8 days.
    84cm in 10 turns = 8.4cm per turn
    8.4cm circumference would be a 2.673cm diameter drum across the bottom of the threads or 3.073cm before cutting the threads.

    I suspect in the final analysis, the factor of 90% will play a part in the solution as all you've really done is reduce the 80bpm/cable usage rate to 90% of its original value.
     
  15. R. Croswell

    R. Croswell Registered User

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    Bottom line is if you want more days run time you need more turns on the drum.

    If if you want the weight to drop further (faster) with on additional run time, then make the drum larger.

    RC
     
  16. Tinker Dwight

    Tinker Dwight Registered User

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    For a given clock train gearing, the diameter only effects the length
    of cable used to run xx days.
    The width of the cable and the width of the drum determines
    The maximum run time. The diameter only effects the case size,
    not the maximum run time.
    Run time is how many turns, regardless of cable length.
    My earlier calculation was incorrect because I forgot to divide
    by the number of turns.
    Tinker Dwight
     
  17. dickstorer

    dickstorer Registered User

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    Just as I was thinking.
     
  18. tom427cid

    tom427cid Registered User
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    Interesting topic....................
    If I may: I have changed drum sizes by turning a wooden sleeve on the lathe,splitting it,and notching it for the cord/cable. If I leave one turn on the new drum dia. the pieces seem to want to stay in place.
    My logic re enlarging the drum dia is that all other things being equal the drum is going to rotate a certain number of times for the weight drop for the clock to run one day. That means that a certain length of cable is equal to one days run. Therefore if the drum is larger the required length of cable will not turn the drum the same number of times but fewer. And it will require more cable to fill the drum. Earlier there was a statement made that a larger drum would probably require less weight to power the clock. Yes. Think of it this way that the drum arbor center is a fulcrum point and the surface of the drum determines the length of the lever.. Therefore the greater the length of the lever the less amount of force needed to equal the original force produced.
    I went through this a number of years ago with a repro banjo movement that needed more weight to make it run. Plus it had a very small drum. I increased the drum size and was able to get the movement to rum reasonably reliably on a typical banjo weight.
    Hope this might help.
    tom
     

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