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Motor to Push Air into a Piston

  1. Nov 29, 2009 #1
    Alright. I need a motor that is capable of pumping air into a piston filled with compressed air. The motor needs to pump the air into the piston at a constant speed, as the piston is being used to raise up a load. The problem is that the weight of the load on the piston will be between 10-200 lbs. Since the weight of the load will change the pressure of the air in the piston, I'm concerned that it will also affect the speed at which the motor is able to pump in the air. I've already calculated that the motor needs to be at least 1/2 horsepower. Is there a way I can make sure the speed of the air pumped into the piston is constant?
  2. jcsd
  3. Nov 30, 2009 #2


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    Staff: Mentor

    The way that is worded is a little confusing. Have you already constructed the pump and are looking for a motor to power it or are you looking for a pump?

    In any case, since air is compressible, it isn't possible to construct a positive displacement air pump (a pump where the volume moved is always constant). The closest thing you can hope for is a variable speed pump with some sort of feedback control.
  4. Nov 30, 2009 #3
    what pressure do you estimate that it would need? What travel thru the bore? How is the piston sealed? Does the piston need to go at a specific rate? How many times does the unit need to go up and down? Does the downward travel also need a specific rate of travel? As previously stated, a constant output pump will not work, it would need a constant pressure, and flow rate, and both would vary with the mass applied on top. My first impression would be to use a motor/pump, pressure holding tank, and a control mechanism to supply a set pressure, and displace the volumn as the piston raises.

  5. Nov 30, 2009 #4


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    Gold Member

    I agree that some sort of feedback loop is essential, and an accumulator would be of great help. One thing eludes me. Do you mean that the volume of delivered air must be constant, or the speed of the piston? Wouldn't those diverge during travel due to the compression factor?
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