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Homework Help: Torque and rpm calculation

  1. Jun 16, 2013 #1
    Just wondering if someone can help explain how to go about figuring out this paper. Thanks!

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    Last edited: Jun 17, 2013
  2. jcsd
  3. Jun 17, 2013 #2


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    For one thing, the units of torque are ft-lbs not just lbs.

    Another thing, it would be much appreciated if the image were presented in the normal orientation, so helpers don't have to break a neck trying to read it.

    Your diagram is not clear. Are the weights just suspended from the sprockets?

    Some explanation of what the diagram represents and what you are trying to do with the shaft would go a long way to avoid needless confusion. Why does the shaft have to turn 3600 rpm? What's the deal with the sprockets on the right?
  4. Jun 17, 2013 #3
    So what this person wants me to build is a way to run a water pump using counter weights. As the weight falls it turns the pump. The pump has a working torque load of 36 ft lbs @ 3600 rpm. I dont even know if this is possible so any help would be awesome.
  5. Jun 17, 2013 #4


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    Is it feasible? Well that depends...

    Forgive me if i switch to metric/SI but...

    36ftlbs is 49Nm.
    3600 rpm is 377 rads/s

    So the power required is 49 * 377 = 18.5kW

    A human can generate around 200-300W (eg cycling) so you would need about 90 people working flat out raising the counterweights.

    Not allowing for any rest time or losses in the mechanisim.

    EDIT: or perhaps one person working flat out for 90 hours raising counterweights so that the pump can run for one hour. Lots of ways to look at it. I suspect it might be more efficient for people to raise the water directly rather than use counter weights.
    Last edited: Jun 17, 2013
  6. Jun 17, 2013 #5
    OK so there will be a continuos duty air compressor at the site so I was thinking of using long travel air rams to lift the weights and then using limit switches attached to solenoid's for engagement and disengagement. So continuos duty wont be an issue.
  7. Jun 17, 2013 #6


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    The problem is that gravity is quite weak.

    18.5kW = 18500 J/S

    Lets say your rams have a 1m travel. They would need to lift 1800kg, 1m in 1 second, then while that weight is falling back down (also in 1 second) and driving the pump they have 1 second to lift another 1800kg.

    So in short...
    1800kg 1m every second or
    1800kg 2m every 2 seconds or
    3600kg 1m every 2 seconds or
    or any similar combination that meets..

    mass * height * g/time = 18500

    not counting losses in any mechanisim.

    Clearly if you only want the pump to run part time you have longer to store energy in raising counter weights.

    But this is all crazy. Why store energy by raising counter weights? Why not do what most people do and store energy by raising the water itself eg build a water tower. Use the compressor to drive rams (or whatever) to raise the water up the tower. It's pointless adding complexity and inefficiency with a two stage process.
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