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Thread: Driven-Pinion Spur Gears?

  1. #1
    Associate Engineer
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    Jul 2020
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    Driven-Pinion Spur Gears?

    Hi All,

    I'm researching spur and cycloidal gear design for small mechanisms. The sources and standards (mostly horological) advise on using cycloidal gears for wheel-driving-pinion applications only, or where "the pinion is sometimes the driving member".

    They recommend spur gears for pinion-driving-wheel application but I've found no concrete answer as to whether spur gears are also suitable for wheel-driving-pinion applications. This may be a fairly simple question, but I have yet to find it in writing.

    For my own piece of mind, would somebody be able to weigh in on this? Are there any particular factors that would make spur gears unsuitable for wheel-driving-pinion?

    Many thanks in advance!

  2. #2
    Principle Engineer
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    May 2015
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    The term spur gear usually refers to a straight (parallel to the axis of rotation) cut gear regardless of tooth geometry. This differentiates them from helical gears which have teeth cut at an angle to the axis (other than zero).

    Cycloidal gears are used in clocks and perhaps because of some connection to his fascination with pocket watches Ford used cycloidal gear teeth in the Model T transmission. Find a discussion of clock gears here: https://www.csparks.com/watchmaking/...ears/index.jxl

    The involute gear form has come to be the standard for most applications. You mention pinions and gears with small numbers of involute teeth often require the tooth flank to be undercut for clearance to the conjugate gear. The number depends upon the pressure angle but when your tooth count gets to 16 or less and undercut is likely. This reduction in tooth thickness at the root reduces the tooth strength. It is not impossible to drive a conjugate gear with an undercut pinion tooth, it is done frequently. You can make the gear wider to increase the strength.

    I cannot think of a good reason that you would ever use a different type of gear geometry on mating gears. So pick one that fits in your package and approximate the tooth loading. Almost all gear tooth stress calculations are based on the "Lewis Equation". This equation gets tweaked based on empirical data by different groups. Your mechanism may have more wear issues than strength concerns. Good luck.

  3. #3
    Associate Engineer
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    Jul 2020
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    Quote Originally Posted by Hudson View Post
    It is not impossible to drive a conjugate gear with an undercut pinion tooth, it is done frequently. You can make the gear wider to increase the strength.
    Hi Hudson, thank you for the response. This part here is the critical bit. I guess prototyping will have to be the determining factor, but as long as there isn't a definitive 'no' against it, I'll press onward.

  4. #4
    Principle Engineer
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    May 2015
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    If you are concerned about undercuts and using involute profiles, look into long and short addendums.

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