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Mechanical efficiency with regards to torque and rotational speed

  1. Sep 17, 2014 #1
    In what way does mechanical efficiency in gears affect output torque and angular velocity? Mechanical efficiency affects power which is the product of torque and angular velocity. Let's say we have a gearbox with a mechanical efficiency of 0.95, hence the output power is 95% of the input power, but how does this affect speed and torque? Is only the torque reduced by 5% or is it a combination of torque and speed reduction? Does this vary from system to system, for example, a worm drive differs from a bevel gear drive?

    I wish to design a gearbox whose input and output run at the same speed, however in between is a series of gears. Let's assume these gears have a resultant system efficiency of 0.9. My input and output has a theoretical gear ratio of 1:1 because I intend them to move at the same speed. As my system efficiency is 0.9, does this mean my output speed is no longer the same as my input speed so my gear ratio isn't 1:1 as designed? I am assuming this isn't the case as gears are timing devices, and what efficiency should affect is only torque.
  2. jcsd
  3. Sep 17, 2014 #2


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    The mechanical efficiency of a gearbox does not affect the output speed of the GB: the gear ratio determines the output speed relative to the input speed. If your gears are made out of silly-putty, there may be additional variation in speed beyond the nominal gear ratio, but there should not be any such effects if metal gears are used. The input torque will be reduced by the mechanical efficiency of the GB, and the output torque must be calculated based on the gear ratio and this reduced input torque.

    Designing a GB with a 1:1 ratio is a bit of a puzzler: you add complexity to the drive train while reducing the amount of torque transmitted from the motor to whatever is being driven.
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