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Upside down motor mount and Thrust bearing

  1. Dec 12, 2011 #1
    I wanted to use a disco ball motor to rotate an object weighing approximately 125lbs. The motor was rated for 25 lbs. Motors rated for higher weights had much higher costs, beyond what would be practical.
    It was suggested that using an arrangment like the one described in the attached pdf would solve the problem by using a thrust bearing to take the weight. So I built the item described in the drawing and am now running it in tests. However, it seems to be laboring and oil from the thrust bearing is very black. Can anyone tell me if this design does what it is supposed to do, i.e. take the weight off the motor shaft? Thanks in advance for any help.
    Ralph
     

    Attached Files:

  2. jcsd
  3. Dec 12, 2011 #2
    What is the tolerance of the bushing diameter with the plastic diameter - are they rubbing together?

    And secondly, and probably more important, the thrust bearing should have no or minimal preload except for the weight of your 125 pd object. When assembling your contraption make sure you are NOT pressing down with the motor shaft onto the thrust bearing when you tighten the aluminium spacers. You should be able to turn the motor freely, as if it had not been in the assembly. You could try adjusting your set screw after assembly.

    And thirdly, is your thrust bearing rated for a 125 pd load.
     
  4. Dec 12, 2011 #3
    >>What is the tolerance of the bushing diameter with the plastic diameter - are they rubbing together?

    No they are not rubbing together, maybe about a 32nd of leeway all around.


    >>And secondly, and probably more important, the thrust bearing should have no or minimal preload except for the weight of your 125 pd object. When assembling your contraption make sure you are NOT pressing down with the motor shaft onto the thrust bearing when you tighten the aluminium spacers. You should be able to turn the motor freely, as if it had not been in the assembly. You could try adjusting your set screw after assembly.

    This one I am not sure about. I don't think the bolts are pressing the pices together. The motor doesn't allow the shaft to move unless it is on. I assumed there should be contact between the bushing flange and the thrust bearing top washer.

    (I must admit I don't understand how when the bushing is attached to the shaft and the J hook, it (the shaft) isn't bearing the load, after all the set pin firmly attaches the bushing to the shaft and the J hook is screwed into the other end of the bushing and then has my rotating weight attached.)


    >>And thirdly, is your thrust bearing rated for a 125 pd load.

    Yes it is.
     
  5. Dec 12, 2011 #4

    AlephZero

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    Ideally, you want a sliding joint in the shaft between the motor and the thrust bearing. Your rotor currently has two thrust bearings (one in the motor to take its 25lb design load) so there is no end float in the shaft. The frame doesn't have close tolerance dimensions (and it doesn't need them for its main purpose of carrying the load to the ceiling) and it may distort slightly when you bolt it onto the ceiling.

    If this runs at low speed, you could probably get away with the roll pin being a clearance fit in a slot in the motor shaft, so none of the 125lb load is transmitted into the shaft. Or use a proper flexible coupling, though you might have to make the frame a bit deeper to make room for that.
     
  6. Dec 12, 2011 #5
    The plastic mounting plates allow too much deflection for the current design. As AlephZero said, a sliding and/or flexible joint between the motor shaft and the load shaft is needed.

    As for the bearing load rating, is it rated for 125 lbs maximum, or 125 lbs working load? Also, how is the load distributed? (Is it evenly balanced. Is it symmetrical?)
     
  7. Dec 13, 2011 #6

    AlephZero

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    In other words, there are two different load paths from the 125lb to the ceiling. One is through the lower thrust bearing and then cantilevered out by the lower plate into the bolts and spacers. The other is up the motor shaft and through the motor thrust bearing.

    The way the load will split between the two paths depends on the relative stiffness. The stiffest path will take the most load.

    Geometrical tolerances will also create preloads that add to the uncertainty about the bearing loads.

    For both issues, removing the load path up the motor shaft is the "clean" way to fix the design.

    Another way to delete the load path through the motor would be NOT to fix the motor to the top of the frame, and let the motor shaft support its weight, so the whole 125lb plus the weight of the motor goes through the thrust bearing. You would need pins or dowels through the fixing holes stop the motor casing from rotating. But that seems a "Rube Goldberg" design compared with a flexible joint in the shaft.
     
    Last edited: Dec 13, 2011
  8. Dec 16, 2011 #7
    I am unfamiliar with the 'flexible joint' of which you speak. As you can see I have a roll pin jammed into the bushing's set hole which also corresponds to a hole in the shaft. I think this is what is taking a good portion of the load and transmitting it through the motor, which is NOT what I want. So what is a flexible joint in this regard? Would a roll pin that is much smaller than the set hole help accomplish this?
     
  9. Dec 16, 2011 #8

    AlephZero

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    You want the pin to be a tight fit in the bushing, otherwise it may work loose and fall out. Make a rectangular slot in the shaft, so the pin is a loose fit and the shaft can move up and down relative to the pin. See the attachment for a side view of the shaft and pin.
     

    Attached Files:

  10. Dec 16, 2011 #9
    Thanks! The drawing helped a lot. I'm going to do that right now. I'll let you know what happens.
     
  11. Dec 16, 2011 #10
    OK, now I understand how a thrust bearing works! I knew the original design had a problem and you all were very helpful in making the point that it was too tightly bound to the motor's shaft. AlephZero's picture clarified for me how I could disengage the shaft from the weight load while still using its torque. Once I took it apart, machined the shaft and made the pin hole into a slot, I could see just how it would work when re-assembled. I did re-assemble and load it up and it went into testing at 12 midnight EST. The last motor was making noises of complaint less than 6 hours later, but appeared labored almost from the beginning. This time there is no sound at all and I am hopeful that when I awaken tomorrow it will still be soundless. I'll let you all know. Thanks again to one and all.
    Ralph Z
     
  12. Dec 23, 2011 #11
    Final Report

    The suggestions worked. I have run the motor since last week around this time, and not had a problem, or even bad sounds. Thanks one and all.
     
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