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Estimating Drag on Spinning Disk with "Spokes"

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  • Estimating Drag on Spinning Disk with "Spokes"

    I need to find a motor that can spin a disc at 12000 RPM. It also needs to be very compact and have hall effect sensors built in. I am looking at brushless pancake motors but I want to estimate the amount of power that is required to keep it spinning at that speed.

    See link below for drawing of the disk.

    http://www.thorlabs.us/thorcat/2500/...AutoCADPDF.pdf

    Thanks for any input!

  • #2
    Drag before or after it go boom?

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    • #3
      As a first step I would ignore the cut-outs and consider a smooth rotating disc. This NASA paper should be useful: http://ntrs.nasa.gov/archive/nasa/ca...0050241738.pdf
      Allan Ostling

      Phoenix, Arizona

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      • #4
        The smooth rotating disc isn't too difficult, but I am assuming the cutouts will significantly affect the drag. Maybe not...

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        • #5
          I am certainly no engineer but I assume that at some point this has been modeled or tested on the computer and does actually work.
          The disc I mean, to be spinning at said speeds. Say that because to me the disc itself is just fraught with potential problems...a hundredth of an inch thick with, more or less, pie shaped slots, just saying if it is not perfectly flat and balanced one little out of plane movement and I would expect the whole thing to be a twisted mess quick. Unless, I suppose it is spinning within a tight fitting "slot" in a housing.

          I am not so sure once at speed it will take a whole lot to keep it spinning there, at that speed with the cutouts I suspect very little difference than a smooth/solid disc.
          What does the supplier of the drawing use? Or, perhaps more importantly, at what speed does it spin at in their application, if different than yours?

          There are of course numbers attached but IIRC at a certain speed, based on size (diameter) and number of spokes, the disc is "seen" as a solid rather than spoked. Very limited experience and much slower same principle spoked carbon fibre bicycle wheels. Most were 3 or 4 spoke, sort of settled on 3 but there you are dealing with a spoke that has an aerofoil shape. The ones that were 4 pairs of 2 flat spokes about an inch wide were not all that popular, don't know if it was function or some other inherent quality.

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          • #6
            Maxon and faulhauber both make a line of brushless motors that would be perfect. I assume you want to keep the diameter smaller than the inside of the shutter wheel?

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            • #7
              Originally posted by Fasttrack View Post
              ... I am assuming the cutouts will significantly affect the drag. Maybe not...
              I'm not sure, either.

              Is the disk spinning in a containment housing? A narrow gap top and bottom could yield Couette flow. Bessel functions, anyone?
              Allan Ostling

              Phoenix, Arizona

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              • #8
                Originally posted by aostling View Post
                I'm not sure, either.

                Is the disk spinning in a containment housing? A narrow gap top and bottom could yield Couette flow. Bessel functions, anyone?
                Couette flow was my first thought because it will be spinning in close proximity to a flat plate. (The mechanical design is still in its very first stages so I don't have all the details). However, I am imagining some nasty results caused by an intermittent Couette flow as a spoke passes and turbulence as a spoke passes.

                The manufacturer spins theirs at about 6k rpm.

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                • #9
                  Originally posted by macona View Post
                  Maxon and faulhauber both make a line of brushless motors that would be perfect. I assume you want to keep the diameter smaller than the inside of the shutter wheel?
                  Yes - I have been looking at Maxon. I have done quite a bit of business with them in the past but they tend to be pretty spendy. Great quality but I am trying to keep the motor cost to around $100. Actually, I am considering trying something like this: http://www.graysonhobby.com/catalog/...or-p-1260.html because the disc doesn't have to spin for more than 1 minute out of 10. Figured that would give it plenty of opportunity to cool. I just need some hall-effect sensors to get RPM feedback to my control circuit.

                  By the way, the total length of the motor shouldn't exceed about 1.2" - preferably much shorter. Not sure what size those model motors are... figured they were cheap enough it wouldn't hurt just to try one.

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                  • #10
                    Maybe one of the motors from server grade hard drives. They run up to 15krpm.

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                    • #11
                      I'm guessing from your name and the link you're a Thorlabs engineer? Ditto

                      I'd be tempted to stick with the Maxon personally, from a quality point of view mainly. Can you get the price down by ordering in volume?

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                      • #12
                        Originally posted by Rivergypsy View Post
                        I'm guessing from your name and the link you're a Thorlabs engineer? Ditto

                        I'd be tempted to stick with the Maxon personally, from a quality point of view mainly. Can you get the price down by ordering in volume?
                        I have no professional association with Thorlabs. Do you work for Thorlabs or were you just jesting? I guess you could call me a "Thorlabs engineer" in the sense that most of my designs end up revolving around components from Thorlabs but officially I am a physicist working for an R&D firm.

                        Welcome to the forum, by the way!

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