Tooth counts in high-end clocks

Discussion in 'Clock Construction' started by doc_fields, Feb 16, 2012.

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  1. doc_fields

    doc_fields Registered User

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    I've heard it said that the larger, higher tooth count clock movements are better timekeepers than say, an American 8 day clock movement. Why is that?

    Secondly, I've also heard that lantern pinions are better than machined shaft pinions on a movement. For instance, many accurate chronometers use them rather than the machined shaft pinions. Why is that?

    Thanks in advance for any contributions you can make towards my enlightenment....................doc
     
  2. Uhralt

    Uhralt Registered User
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    I guess I can answer the first question. The higher tooth count by itself doesn't make the clock run more accurately but it does make the clock show the time more accurately. The reason is that with the high tooths count there is less "slack" or "play" in the movement, especially in the motion works. In a typical American 8 day movement you can move the minute hand easily a couple of minutes back or forward without feeling resistance. The clock will appear to run behind when the minute hand is moving upwards and appear to run ahead of time when the minute hand is moving down (appearing fast from the full hour to 30 minutes past the hour and slow from 30 minutes past the hour to the next full hour. This problem is avoided with a high tooth count.

    Uhralt
     
  3. jhe.1973

    jhe.1973 Registered User
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    Hi doc,

    The attempt with high end clock movements is to maintain as consistent a transfer of power from the driving weight to the escapement to keep the impulse constant.

    The higher tooth counts are the attempt to eliminate or reduce as much as possible, the additional friction caused by the teeth of the pinion and wheel contacting each other before they are in a straight line drawn through the centers of gear set. The term given to this line is, "line of centers".

    If the teeth contact before this line, the wheel 'digs into' the pinion (engaging friction) as it engages until the teeth reach this line and then the action is sliding apart (disengaging friction). This engaging friction takes more power to overcome than the disengaging friction. The results of this 'digging' action can be seen in the pockets worn into smaller, softer pinions of some old clocks.

    I'm only using the terms sliding and digging to make this easier to picture. In reality, horological tooth shapes are designed to give more of a rolling action than sliding. But that's another topic.

    With each step in the train away from the escape wheel this 'engaging friction' is more critical because each step has the teeth in contact for a longer time than the prior step.

    To use my clock as an example, the 120 tooth great wheel takes 8 hours to make one turn. Each tooth is in contact for 4 minutes. If the start of the tooth contact had a higher friction than the last part of the contact, that would mean 2 minutes with higher friction and less power into the train. I.E a 2 minute reduction in power to the escapement with the meshing of each tooth.

    This magic numbers to avoid the engaging friction is 120 teeth wheels to 12 tooth pinions and above.

    As far as the lantern pinions I have heard it said that they can give a better rolling action than sliding action. As I understand it, it is more due to the shape of the pins than it is to weather or not they actually rotate.

    I don't have personal experience to go by. I made cut pinions because I feel that the small pins of a lantern pinion can flex more than the teeth of a solid cut pinion.

    But that's strictly an opinion.

    Hope this helps.
     
  4. jhe.1973

    jhe.1973 Registered User
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    #4 jhe.1973, Feb 17, 2012
    Last edited: Feb 17, 2012
    Hi doc,

    Last night or more accurately this morning at 3:45 I woke up in my recliner. Before I toddled off to bed, I checked these forums and found your question. Being tired, I didn’t go into the rolling vs. sliding action I mentioned. But when I awoke for the day, I realized that I should explain this detail because it helps understand the use of lantern pinion in higher-grade movements.

    To try to explain this w/o math and diagrams:

    In a perfect world gears wouldn’t be gears, they’d be perfectly round disks perfectly concentric w/perfectly round arbors rotating in perfectly round holes w/no clearance. The O.D. of each disk would be it’s pitch diameter and each disk would contact the other perfectly at this diameter.

    But in the real word we need to keep the disks from slipping so teeth are added above the pitch diameter of each gear. The overlap between the two gears is the tooth mesh or depth.

    If the contact is dead on the line of centers that I mentioned in the last post, it will be right at the pitch diameter. But any contact before or after the line of centers will be above or below if the teeth were straight radial lines. So, the tooth of the wheel is curved to bring the contact back as close as possible to the pitch diameter of the pinion. This radial flank of a cut pinion stays flat and is the actual working surface. The rounded top is only clearance for the next & last tooth.

    The curve of the wheel tooth is an attempt to keep the leverage as constant as possible because, after all, the teeth are just the working part of a lever that pivots from the center of the arbor. High tooth counts allow for less angular rotation before the next tooth comes into play, making the drift from the pitch diameter easier to control.

    With an accurately designed lantern pinion and its corresponding wheel shape, this contact at the pitch diameter, as I understand it, can be held a bit closer through more of the angular rotation of the two gears.

    But this means that the pin has to be accurately sized, accurately located on the pitch diameter and accurately held in place parallel to the arbor. The tooth of the wheel also has to be accurately shaped to provide the pitch diameter contact that a designer strives for.

    In lesser grade clocks lantern pinions are used for a lower cost alternative to cut pinions. In these clocks the pins fit loosely in the holes to allow them to turn. This is done to allow them to be run with a lower tooth count, not for quality timekeeping.

    Because we see accelerated wear (flat spots) to the pins that have been soldered in previous ‘repairs’ lantern pinions have been given a bad rap. I am only guessing here, but I suspect that in a chronometer the pins fit tightly in their mounts to take full advantage of the shape of the round pin. If they would turn, it would add play to a system that would only contribute to a power and accuracy loss.
     

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