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Tower Clock Gearing and Motor Speed

  1. Oct 17, 2014 #1
    Hello all, first post on here.

    My fathers friend recently died of cancer and he was the person we used to turn to for this question, I've been racking my brain and searching the net for info, found this link (http://www.schsm.org/html/gear_ratio_calculations.html) and have got the ratio down to 6 and 17/18ths.Not much use.

    I have an "escape wheel" on a turret clock. The "escape wheel" has 9 teeth on it and it meshes with a wheel with 145 teeth. The 145 needs to turn at 1/2 rev per min exactly.

    What I am looking for is a motor speed and gear ratio to drive the "escape wheel" to give the desired output, preferably not stepped if at all possible.

    There are several motor speeds that are stock from the supplier, 5, 6, 8, 10, 12 and 15 rev per minute, but I have no idea how to factor these in to the ratio.
  2. jcsd
  3. Oct 17, 2014 #2
    Sorry, the 145 should be 125! My brain hurts.
  4. Oct 17, 2014 #3


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    I assume you want to drive the 9 tooth gear from the motor gear, in which case the 9 tooth becomes an idler and it's value doesn't' matter, but I'll leave it in for completeness.

    Input speed * gear ratio = output speed

    And applied to your situation:

    n * (x/9)* (9/125)=1/2rpm

    where n the input speed (either 5, 6, 8, 10, 12 and 15rpm) and x is the number of teeth on the motor shaft gear.
    x needs to be an integer, you can't have 1/2 a tooth. There won't necessarily be a solution to this problem.

    You can get wolfram to solve if you cant be bother doing the math.

    If you try for n = 15rpm you get x = 25/6


    25/6 ≈ 4.17, 4 teeth is not practical but for arguments sake lets check the error if you used one:

    15 * (4/9)* (9/125)= 0.48rpm

    Try the other input speeds and you'll get a better tooth count and may get closer to 0.5 rpm output.

    Another option is to control the motor speed.
  5. Oct 18, 2014 #4
    Thanks for the reply billy j but I recounted the teeth with my dad and the big wheel has 120! I had used dividers to do this for the first time and had back counted the last tooth. Definitely 120.

    So I had a think, it's 120/9 in 1/2 rpm? So to bring it into revs per min that would be 60/9?

    So if I put a 5 rpm motor on it that would be 60/5 =12

    That's a ratio 12/9 which I know I cannot get a 9 tooth pulley from the supplier

    So double them up to 24 on motor and 18 on escape wheel?

    Edit: used the link you kindly pointed me in the direction and got this out changing the value to 120

    Last edited: Oct 18, 2014
  6. Oct 18, 2014 #5


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    I don't quite follow you.
    If you use a a 5 rpm motor then you need a 12 tooth gear on the motor, as calculated like this:

    n*(x/9)*(9/120)=1/2rpm, n=5rpm
    gives x = 12 teeth

    http://www.wolframalpha.com/input/?i=n*(x/9)*(9/120)=1/2, n=5

    If you can't get a 12 tooth, try with other drive motors to find a tooth count you can find.

    Remember, the speed of many motors is a function of load, presumably your load is unknown (gearing losses, bearing friction etc are difficult to calculate accurately) So you may need to speed control your motor anyway. The manufacturers motor rpm values may be no load speeds or peak efficiency speeds and it's very unlikely your device will operate exactly at that point. Keep in mind I know nothing about clock making! I'm sure there are very simple solutions to your problem. Maybe posting in a clock makers forum would help?
  7. Oct 21, 2014 #6
    Thanks billy for your responses, you are correct, I would need a 12 on the motor and a 9 on the shaft. However, I cannot source a 9, the smallest HPC gears have "off the shelf" is 10. I am doubling up the the 12 to a 24 and doubling up the shaft one to 18 keeping the same ratio.

    Thanks for your help on this matter, much appreciated. The little formula that you provided will more than likely prove to be valuable in the future.

    I'll maybe take some photos when it's done so you can get an idea what I was on about, I should have maybe been clearer in my original post :-)
  8. Oct 28, 2014 #7


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    File one of the teeth off? :s
  9. Oct 28, 2014 #8


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    I'm glad to see that you "liked" that, because I didn't want to offend you. It was just an expression of sharing your frustration.
    I did just think of something, though. Do you know anyone with a 3D printer? Either one of those or a local machine shop should be able to fire up a custom gear with no trouble.
  10. Oct 28, 2014 #9


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    The first post says you already have a 9 tooth gear?
    My suggestions were based on the escape wheel (9 tooth) being an idler gear so it's tooth count has no effect on the outcome anyway.

    Maybe a diagram of what you want to do would help us help you.
  11. Oct 28, 2014 #10
    I'll get some photos up lads sometime soon, I've been changing clocks the last 6 days as its gone into GMT time here in the UK.

    My mate is a maths teacher at a posh school, I've checked over the figures with him and they are correct. 5rpm motor with a 24 tooth pulley on its shaft driving the escape wheel with an 18 tooth pulley.
  12. Oct 28, 2014 #11


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    Meaning no insult, it's rather important around here to use proper terminology to avoid confusion. If you were using pulleys, you wouldn't have this problem because there is no restriction on ratios. Gears, on the other hand, have to incorporate a whole number of teeth while maintaining the proper pitch.
  13. Oct 28, 2014 #12
    I'll take some photos mate, I haven't explained well what's what here. I'm using pulleys, one on a motor and one on the initial shaft (escape wheel) along with a small timing belt to get a constant speed out and drive the hands of a large tower clock. I have done a few using this technique, I just had a problem with the maths :-/
  14. Oct 28, 2014 #13


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    Sorry; I misinterpreted a few things along the way. At your first post, I thought that you were dealing with a huge clock (not quite Big Ben, but in that family). Soon after, though, it began to seem that you meant something domestic (a grandfather clock at most). I've never heard the term "turret clock", so that was irrelevant to me.
    I must say that I've never seen pulleys used in clocks in any models later than the old Persian and Chinese "water clocks". That might have remained available (without widespread distribution of the knowledge), but gears pretty much took over once they became economically viable. Now you have doubly confused me by mentioning that you're using a Gilmer drive. That particular system has the same restrictions as a gear train, due to the pitch of the belt teeth, but you should be able to compensate for that by tweaking the actual pulleys.
  15. Oct 29, 2014 #14
    Something like this:

    Attached Files:

  16. Oct 29, 2014 #15


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    Alright, well... I know to start with, even being an amateur, that there are an awful more gears in a clock than what you've shown. Unless you're using multiple terms for the same thing, you haven't even shown all of the things that you've referenced in your previous posts. Estimating my scale from what I believe to be a lino knife on the shelf behind, your motor pack would appear to be about the size of my cordless phone or TV remote control. If that honking huge brass gear next to the Gilmer is just the input, that bloody clock must be about the size of a cargo container. So... your photo shows the motor pack, one huge mother of a brass gear, and two Gilmer wheels with their belt. What I do not see are the pulleys that you mentioned. Those are where you can fine-tune your ratio.
  17. Oct 29, 2014 #16


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    The way to avoid the speed dependence on the load is to use a synchronous motor. This will be locked to the frequency of the power grid, which is probably about as good as you can do with a motor drive.
  18. Oct 29, 2014 #17
    Especially a picture of the escape wheel, that is mentioned. Is doesn't seem to be an actual part of an escapement.
  19. Oct 29, 2014 #18


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    Yeah, I was sort of trying to bypass that whole issue because I just figured that he didn't understand what an escapement is and I wanted to ease him into it. That was probably a mistake on my part. Thanks for putting it front and centre, because that's where it will do the most good.
  20. Oct 30, 2014 #19
    Correct, that is a Crouzet synchronous motor in the pic, with 2 20 tooth pulleys which we fitted and tested but the clock was running slow. Now with the new pulleys the clock will keep perfect time.

    Lesson learnt, I always double check when counting teeth, looks like I have to triple check now.
  21. Oct 30, 2014 #20
    That is the escape wheel (I've taken the fly off) in the photo that the pulley is fitted to with the next wheel in the sequence of the train. It goes through another wheel then through the winding barrel and finally out to the centre wheel which drives the 4 dials.
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