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Gears and pulleys?

  1. Oct 23, 2008 #1
    hiya, i need some help for a project im working on, basicly i have a compressor pump that runs at 500rpm and @ 12 hp.

    now the problem is (this is a theoretical problem) i have to power it using a 3480rpm ac motor which has to be less than 10 hp (cost reasons).

    rough calculations show that a 20cm pulley on the compressor pump and a 3cm pulley on the motor will give me the correct speed reduction, but what do i do about power?. will the speed reduction increase torque giving me the power i need? if this is the case what is the minium sized motor i could use?

    what if i used gears, id be looking at a ratio of 1:0.15?

    hope thats clear, any websites or calculations that can help me with this would be amazing as im really baffled.

    thanks alot.
  2. jcsd
  3. Oct 24, 2008 #2
    there is no way you can increase the power at the driven end. In fact you ll end up with a lil bit less at the driven end.

    P = T*ω, Ideally P remains constant(conservation of energy), Torque varies inversely to any variation in ω.

    velocity ratio of almost 7 for a belt drive is a bit tooo much.(I dunno, I m not a practicing engineer yet, ;) ). Have you looked into worm gears?
  4. Oct 24, 2008 #3
    As ank_gl pointed out there is no way you can get 12 hp out of a 10 hp motor.

    But you can still build something. Just remember that the 12 hp on the compressor spec is the power required to drive it at 500 rpm which is propably the ideal for that piece of equipment. If the compressor is a fixed displacement type, the rpm is approximately proportional to the power you deliver to it. So your 10 hp can drive it at 416 rpm. If you can live with the reduced qty of compressed material, gear the 3480rpm motor down to 416.
  5. Oct 25, 2008 #4


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    Science Advisor

    I hate to say it, but even if you had a 12 HP electric motor, there's no guarantee that it would work. It could be completely lacking in torque where you need it, usually at the low end of the speed range. When selecting an electric motor you need to look at the inertia of the system as well.
  6. Oct 25, 2008 #5


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    Staff: Mentor

    Is this a homework problem? Perhaps the goal here is to provide an rpm at the pump which results in the pump requiring only 10 hp....
  7. Oct 25, 2008 #6
    FredGarvin makes an important point. Don't start building this untill you analyze the torque requirements.
  8. Oct 25, 2008 #7
    Try 9 inch pully on motor and 2 inch pulley on the pump AND HANG ON TO YOUR HAT LOL
  9. Oct 26, 2008 #8
    hi thanks for the reply's, sorry i should of said this is a homework problem (i dint notice the homework sub forum when i posted the prob) so im not actually going to build it, does that help. the pump spec is 500 rpm @ 12hp minimum, and i need to locate a motor that can power it which runs at 3000-4000rpm not costing more than $700.

    any ideas? thanks
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