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Homework Help: Pulleys connecting a motor and shaft - Determining loads in shafts

  1. Jun 3, 2010 #1
    1. The problem statement, all variables and given/known data
    A 5hp, 1755rpm, WEG Motor (part #00518OP3E184T) needs to turn another shaft that is 12" away at 1000rpm.

    2. Relevant equations
    T=F*r -> F=T/r
    Drive Ratio = Driver Pitch Diameter/Driven Pitch Diameter
    Sum Forces
    Sum Moments

    3. The attempt at a solution
    I plan on using a pulley (aka sheave) system with v-belts. I want to use a 8" sheave (7.95" pitch diameter) on the motor and a 14" sheave (13.95" pitch diameter) on the shaft that needs to turn 1000rpm. This will give me a Drive Ratio of .5714.

    1755rpm*.5714=1000rpm --- Which is what I want.

    My problem is, I don't know how to figure out the loads in the shaft. Since I know the torque in the motor (Torque = 5hp*5252/1755rpm=14.96ft*lb=179.52in*lb), I thought I might be able to figure out a force at a point on the motor sheave diameter by knowing that T=F*r so that F=T/r. However, when I do the calculations, I get that the force on a point on the outside of the sheave is F=179.52in*lb/(8in/2)=44.88lb. This sounds like too small of a force.

    I also think that that might not even be what I need. I think I need to figure out the tension in the v-belt? Basically I need to figure out how many v-belts I need (do I need just 1 belt or 2 or 3, etc.?), and the loads on the shaft that I need to spin at 1000rpm. If I can get the resulting loads on the shaft, I can figure out the type of bearings I need to support the shaft and such.

    I don't know where to go with this and could greatly use some help. Any help at all would be helpful. Thanks so much in advance.
  2. jcsd
  3. Jun 5, 2010 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Hi MrBlue, welcome to the board. I'm not sure I understand the question. What class is this for? Is this a design problem that you're supposed to determine a design solution to? If so, to what degree do you want to extend this design solution? As you say, there are a number of issues to consider, such as drive ratio (you have that), shaft sizes for stress and fatigue analysis, bearing analysis, V-belt or other belt selection, part selection, mounting bases, belt tensioning device, etc... If you can clarify what you're expected to do I can probably help you out.
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