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Gearing Question

  1. Sep 5, 2010 #1
    I have a question regarding gearing and torque transfer. If I had a receiving stator that requires 3600rpm/26 lb ft torque, and needed to get a drive motor to turn it at those parameters using an 18:1 ratio, what would the drive motor need to be capable of, performance-wise? This will be a real world application, under continuous-use, so longevity will be a large factor.

    I have formulated 1HP @ 200RPM (1HP X 5252/200=26.26 lb ft), but this is at the drive motor. At 18:1 gearing, the drive motor will be capable of initial torque value required, but as it goes through the gearing, will the torque value increase at 18:1, meaning i can now drop to a different drive motor of approximately 1.45 lb ft/torque (26.26/18=1.4588). I know that the RPM transfers at 18:1, but does the torque do the same?

    I am trying to determine the drive motor's performance values to apply it into this application. As long as the final drive at the stator is 3600RPM/ 26 lb ft, and gear ratio is a maximum of 18:1 (preferably lower), any advice on what 12V-BLDC motor would fit will be greatly appreciated.
  2. jcsd
  3. Sep 5, 2010 #2
    In theory torque and speed are (inversely) proportional through a gear-train. Taking your 18::1 as output::input, you need to run 18x _slower_ at the motor to get 3600rpm at the drive shaft -- so 200rpm -- and your torque at the drive shaft would be 18x _higher_ than the motor -- so a motor of about 1.5 ft-lb could do. However there are losses in the gear-train due to friction and inertia. You might be able to measure these, or just do the usual seat/pants-engineering and double everything... You also might want to consider acceleration, which means having to figure out the inertia of the whole system and compare to the amount of power you need to get it going.

    I think most BLDC motors like to run at higher than 200 rpm, but you would need to look at torque/rpm curves to see if you can find something that works. Or change your gearing to match the motor you find. Or...use a regular garden variety 1Hp AC motor?
  4. Sep 5, 2010 #3
    Thanks for the input, as that was what I needed to know. I am alternately thinking of using a lower HP gear drive motor to alternate as the BLDC motor, so cost is a factor in the whole thing :)
  5. Sep 6, 2010 #4
    Ooooopsy..... I got the torque backwards... sorry. I usually gear things down, so I wasn't thinking clearly. The _product_ of torque and rpm is constant. So if you are gearing _up_ from 200rpm you need to scale the torque _down_ by the same ratio. Which means you need an even bigger motor. duh.

    Brain death is a horrible thing to waste.
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