Ratchet wheel dimensions

Discussion in 'Clock Construction' started by Raynerd, Dec 3, 2018.

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  1. Raynerd

    Raynerd Registered User

    Apr 11, 2004
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    When building a clock from scratch with the obviously luxury of being able to correctly position the click to ratchet, could anyone shed any light on ratchet wheel dimensions and the significance of specifically the OD of the ratchet wheel and angle of the ratchet. I’m just curious as there are very few details of ratchet wheels in my growing collection of reference books! Is this because it simply doesn’t particularly matter?

    My presumption, and probably nievely is that:

    The angle is fairly irrelevant providing the ratchet angle fits the click and is steep and deep enough to engage?

    The outside dimension is fairly irrelevant but obviously impacts the number of revolutions needed to wind the clock. I.e the bigger the wheel the fewer revs needed for a full wind?

    Or is there more to it?

    Cheers
    Chris
     
  2. David S

    David S Registered User
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    I am no clock designer, but here are my thoughts on what you said.

    Firstly diameter won't affect the number of turns, since one turn is ..well, one full turn. The diameter of the arbour and length of spring determine number of turns for full wind.

    Depending on how strong your spring will be, I wouldn't want the tip of the click that does the engagement to be too close to the center of the ratchet wheel since it will put a lot of force on the click rivet....if I am thinking this through correctly.

    Let's hear from the real clock makers.

    And Chris looking forward to your next video.

    David
     
  3. Hessel Oosten

    Hessel Oosten Registered User

    Apr 26, 2017
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    Attached Files:

  4. Raynerd

    Raynerd Registered User

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    Hi David, yes great point and a stupid statement I made. The clock I’m currently building is weight driven and not spring. I was therefore linking the size of the ratchet wheel with the barrel size and therefore I presume I am correct in thinking there would be more winding length fed on to the barrel with every revolution on a larger barrel. However, totally take your point and accept that my statement was not correct.

    Regarding your other statement, are you saying that the larger the wheel, the further the click is away from the wheel centre and therefore less pressure is loaded onto it?
     
  5. Raynerd

    Raynerd Registered User

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    Truth is, I also don’t really understand and can’t picture the necessity of the undercut as well! Hessel, you plans call for 5deg undercut as do my plans on the clock I’m building.

    Don’t get me wrong - I’m just going to cut the ratchet as per the plans I have but I’m just trying to understand a bit more about it!
     
  6. John MacArthur

    John MacArthur Registered User
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    #6 John MacArthur, Dec 3, 2018
    Last edited: Dec 3, 2018
    Hi Raynerd -- I posted some info on this in thread: Problem is with the click

    It includes diagrams and reference to Goodrich. I have always referred to this principle as "draw"; in other words the pressure of the wheel "draws" the point of the click into it. It is entirely dependent upon the location of the pivot of the click, the point of the click, and the angle of that line to the line from the point of the click to the center of the wheel. It doesn't have anything to do with the undercut, which is just a safety mechanism once the click is in place. I use very little undercut.
    Hope it helps;
    Johnny
     
  7. Hessel Oosten

    Hessel Oosten Registered User

    Apr 26, 2017
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    In fact same as John says above: The click has a better bite.
    See pic.

    Hessel

    Better bite.png
     
  8. John MacArthur

    John MacArthur Registered User
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    My other pic is a little misleading - here's another
    Johnny click geom.jpg
     
  9. Jim DuBois

    Jim DuBois Registered User
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    I generally prefer a bit of rake on the locking face of the teeth as well as a matching angle on the click face. Just enough, like 5 or 10 degrees so that the force of the weight deflects the click inward and locked rather than neutral or out. A bit of insurance I guess but I like it. I have seen such approaches in other clocks. With 90 degrees on both parts it leaves us entirely dependent on the spring always working properly does is not ?
     
  10. glenhead

    glenhead Registered User
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    Expanding on an earlier point: The larger the ratchet wheel, the lower the pressure that is put on the click assembly and the ratchet teeth. You're giving the hold-it-in-place mechanism a longer moment arm, so the force necessary to keep it in position is lowered. The teeth on a ratchet wheel of diameter 1 will be under twice the load of the teeth on a wheel of diameter 4. (The moment arm is the radius of the wheel.) It won't make any difference to the number of turns or the force needed to wind the clock, it'll just result in a lighter load on the teeth and click.

    Glen
     
  11. Phil Burman

    Phil Burman Registered User

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    I think that if the diameter ratio is 4 to 1 then the radius ratio is also 4 to 1. So the applied force ratio is also 4 to 1.

    Phil
     
  12. glenhead

    glenhead Registered User
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    Well, bleep. You're right, Phil. It made sense when I typed it - how about that for an excuse! (That's what happens when I overthink things.)

    Glen
     
  13. Raynerd

    Raynerd Registered User

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    Sorry, I’ve been working late the last few nights!

    I understand the concept of the rake angle now.

    Glenhead, phil - could you elaborate a little more on what you are talking about in terms of this 1 to 4 ratio. I apologise if it is obvious but I don’t understand and would like to. Do you mind just giving me a little more info about what you both discussing?

    As always, great info taken from this forum!

    Cheers
    Chris
     
  14. glenhead

    glenhead Registered User
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    <Said with a laugh> Yeah, the only obvious thing is that I goofed!

    The length of the lever created is from the center of the arbor to the tooth. In other words, it's the radius of the wheel. If you double the diameter you double the radius, too. I overthought it and came up with more than that.

    A longer lever makes it easier to hold a load. If you have a ratchet wheel that's the same size as your barrel, the teeth are being subjected to the same torque as the barrel. A larger ratchet means the teeth are holding a smaller load; for example, if the ratchet is twice the diameter of the barrel, the teeth only get half the load. A smaller ratchet means a heavier load on the teeth.

    Is that a bit clearer, or did I muddy things up worse? :)

    Glen
     

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