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Centrifugal Force: Balancing a swing bucket rotor

  1. Aug 26, 2015 #1
    A lab mate and I got into a discussion on how to properly balance a swing bucket rotor. This is different than a normal centrifuge in that there are buckets that swing out once spinning begins. In a standard centrifuge balancing is simple - it's just the same weight directly opposite your sample.

    I've attached a picture of "A" and "B" - which is the correct way to balance the centrifuge?

    I argue that there is a vector Force (both a horizontal and vertical). One balance accounts for both, while the other only accounts for the horizontal. Arguably, the horizontal is much larger than the vertical, and it might not make a difference to not balance the vertical force..unless you have a huge amount of weight or extremely fast spin (and extremely fast spins do not use swinging buckets).


    Attached Files:

  2. jcsd
  3. Aug 26, 2015 #2

    Andrew Mason

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    Welcome to PF DJVan!

    I am a little confused by your picture. But I think I understand your question. You want to balance a tethered rotating bucket so there is no net horizontal force on the post. You cannot eliminate the downward force on the post. That will increase if you add another bucket. It is only the net horizontal force that you can eliminate.

    The inward force that the post must exert on the rotating bucket is proportional to the square of the rotational speed and proportional to radius (the distance from the bucket to the post). That radius depends on the speed. So in order to perfectly balance the rotating bucket so that there is no net horizontal force on the post you should attach another bucket of the same mass using an identical tether attached to the same vertical position on the post but on the opposite side and fixed to the post so that both buckets rotate at the same speed.

    The system will naturally rotate about its centre of mass with the centre of mass remaining an inertial point (no net force acting on it). So long as you keep the horizontal position of the centre of mass at the post, there will be no net horizontal force.

    Last edited: Aug 26, 2015
  4. Aug 26, 2015 #3
    Hi Andrew,

    Yes, essentially that is what we're trying to do. Except, it's a little more complex because the bucket does not have just one space where mass is held during the spin. These buckets have several slots, and hold multiple tubes. When swinging, there are tubes that sit vertically above and below the "post". Thus, when swinging, I expect there to be both a vertical and horizontal force (the vertical force being a result of the centrifugal force).

    I've attached a picture of an actual swing-bucket rotor:

    So, you can imagine that, when spinning these buckets swing out, so the blue caps of the tubes face inward toward the rotor. If you only wanted to load two tubes (in the 5-slot buckets), how would you balance all of the forces? Also, assume you don't load the tubes in the center slot (too easy!)

    I've attached another drawing of the buckets while spinning. Basically I just want to know if there is a vertical force component.

    Attached Files:

  5. Aug 26, 2015 #4

    Andy Resnick

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    One of our centrifuges is like this. Balancing the rotor is the usual procedure- the tubes are identical sizes and equal weights, arranged symmetrically. to use only 2 50 ml conical tubes, put one in each bucket, opposite from each other (in this case, each would take the center position. If you have 3 tubes, make a 4th 'dummy' tube with water and arrange 2 tubes on each side.

    To be sure, we aren't spinning faster than 1500 rpm, so we can tolerate small loading imbalances.
  6. Aug 26, 2015 #5
    Dr. Resnick,

    We do know the basics of balancing the centrifuge, and this question is more of a technical one, as loading in either orientation as I described above results in no disturbance while spinning. The vertical force is so small compared to the horizontal, that it's likely negligible. That is, at standard spin speeds and tube weights. However, now we have egos at stake and must have the technical answer.
    Last edited by a moderator: Aug 26, 2015
  7. Aug 26, 2015 #6


    Staff: Mentor

    The correct loading would be option C, with the tubes in the center. A would be better than B, but neither A nor B are "correct".
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