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I know that a gear reduction would decrease the RPM if the motor ran at constant speed, but in reality its speed is a function of the load.

I first tried turning a wheel with a direct drive system, but the motor started getting warm (burning off the lubricant inside) and was spinning much slower than without a load because it didn't have the torque required. I know that a gear reduction would cause less stress (torque) on the motor, which in turn, would allow it to [possibly] go faster. I am essentially looking for the highest speed I can get, so how would I determine the proper gear reduction ratio to use?

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  • $\begingroup$ You would need to inspect the RPM to efficiency graph $\endgroup$ – ratchet freak Jul 15 '15 at 15:24
  • $\begingroup$ @ratchetfreak is the graph specific to the motor? If it is, I do not have it. This (comingsoon.radioshack.com/…) is the motor I used. Is there a way I can make the graph given the few specs listed? $\endgroup$ – electricviolin Jul 15 '15 at 15:59
  • $\begingroup$ why do you want to run the motor at its max speed? If you do that, you'll have to maximize the gearbox ratio (to get minimum torque at the motor shaft), to levels that would practically not work. $\endgroup$ – Gürkan Çetin Jul 16 '15 at 9:03
  • $\begingroup$ The motor can direct drive, but it gets very hot and the wheel spins slowly, so I think it would work with a 1:2 or 2:3 ratio. $\endgroup$ – electricviolin Jul 17 '15 at 18:44
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What you really need to do is measure the power required to turn the wheel at various RPMs. Then, match the power output of your motor to this data. So, for example, if your motor generates 1 HP, see what RPM value corresponds to 1 HP of required power (maybe leave yourself some margin and use 0.9 HP instead), then set your gear ratio accordingly to get these RPMs.

The hard part is, of course, measuring the power at different RPMs. You may have to do this by trial and error. You already know that direct drive (1:1 ratio) is too fast, so maybe try something like 3:2.

Measuring the rotational speed of the wheel is easy with a non-contact optical tachometer: enter image description here
I bought one a few years ago for less than $50. The RPMs that the motor is rated to is usually stamped on the motor somewhere. Once you measure the maximum RPMs that you can turn the wheel at (based on the power of your motor), the drive ratio is just the rated motor RPM divided by the measured wheel RPM.

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  • $\begingroup$ How would I go about doing this? I am not sure how to count the RPMs unless I used an opto interrupter or similar (which would be over complicated). Once I do get the RPMs and HP, what calculation do I do to find the needed ratio? $\endgroup$ – electricviolin Jul 15 '15 at 16:55
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    $\begingroup$ @electricviolin the page you cite gives motor speed at load at various voltages. That can be turned into a rough speed/power /load graph. By measuring current at a given voltage for your load you can see how your load compares to their load. Go from there. Ask more if needed. $\endgroup$ – Russell McMahon Jul 16 '15 at 8:22
  • $\begingroup$ So I should put an ampmeter between the motor and supply and check the current it draws? I am only using 12V for this project... Please explain further $\endgroup$ – electricviolin Jul 17 '15 at 12:47
  • $\begingroup$ @russellmcmahon $\endgroup$ – electricviolin Jul 17 '15 at 13:05
  • $\begingroup$ @carlton would a 1:2 ratio work? I am not sure if that makes the torque half is stressful on the motor, but if it does, it may work best. $\endgroup$ – electricviolin Jul 17 '15 at 18:46

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