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Why does high contact ratio in gear result in inefficiency?

  1. Aug 2, 2013 #1
    As titled.


    This was taught to me in machine design, but I don't see how that makes sense

    I would think that doesn't matter how many teeths are in contact, since the force between gears are distributed on each pair of teeth in contact, more engaging teeth just mean the friction is distributed to more teeth pairs(as the normal force is distributed to more teeth pairs).


    thanks
     
  2. jcsd
  3. Aug 4, 2013 #2

    Baluncore

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    There is no real need to have more than one tooth in contact except during the transfer from each tooth to the next.

    To have more teeth in contact requires smaller teeth on the same diameter gear wheel, or the same sized teeth on a bigger wheel.

    Teeth should be designed to carry the entire load. Smaller teeth are a riskier proposition since wear and misalignment can unload one tooth and transfer all the load to another.

    So multiple teeth in contact requires bigger gears which are heavier, they need to be more accurate and so cost more.
     
  4. Aug 4, 2013 #3


    so are smaller teeths more vulnerable to friction and wear? => less efficiency?

    or it's because they are heavier hence need more rotational kinetic energy when transmitting power?
     
  5. Aug 4, 2013 #4

    Baluncore

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    Unfortunately, the precise context and the meaning of the word “efficiency” have not been recorded.

    The answer to your question is certainly not simple. It is multi-factorial and so requires many interrelated effects to be considered. The line between theory and practice is determined by how accurately the gears can be manufactured and supported.

    Are you talking spur, helical, internal or external gears ?

    Indeed, a high contact ratio can lead to greater efficiency, if you are prepared to pay for the increased mass and machining accuracy, as is evident from this extract.
    The efficiency of power transmission is one thing. The efficiency of the machine as a whole is another. These days, minimum material and shipping weight is the primary requirement. This applies to the gear and it's mountings.

    Because the teeth will not share the work fairly under all conditions, every tooth must now be capable of carrying the full load. Therefore the tooth size cannot be reduced. In order to have multiple teeth in contact requires a longer contact zone and so the gear radius must be linearly greater. Mass and cost will rise as to the square of the radius.

    So to have a greater contact ratio you must have more accurate tooth profiles, with bigger gears in a more rigid mounting.

    But what do you gain from such a high contact ratio? You may change your friction loss from 1.5% to 1.4% but the total weight of your machine may rise by 10%. At the machine level that may be very inefficient both in terms of energy and economy.
     
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