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• Re: Do you know what a moment of inertia is? - ER 16:30:15 01/10/07 (4)
  In Reply to: Do you know what a moment of inertia is? posted by Mark Kelly on January 10, 2007 at 14:07:15:

  
  Hi Mark, I've been boning up...

  So for unknown or "scalar" moment of inertia I would use I=sum of MiRi^2 using the cartridge weight and center of counter weight with the center point of the bearing tube as the axis? Where does arm weight factor since it's mass will not have a center other than perpendicular to the axis?

  This stuff is interesting, I have a mountain bike with 29" wheels, as opposed to the standard 26". That whole moment of inertia thing works well off road.

  The more I learn the more I don't know, never truer.

  Thanks,
  Erik
  
  

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  • Basically - Mark Kelly 02:53:41 01/11/07 (3)
    In Reply to: Re: Do you know what a moment of inertia is? posted by ER on January 10, 2007 at 16:30:15:

    For a first cut the small or at least short items can be considered as point masses and the classic M.R^2 formula applied. Long thin parts such as the arm tube need a further elaboration: use the point pass formula taking the geometric centre as the centre of mass but then add the moment of inertia about the centre of mass. For a long thin object the moment of inertia around the centre of mass is M.L^2/12 where L is the total length.

    The most difficult part to determine will be the "rider" which interfaces with the bearing tube. Fortunately it will also have a small impact on the total effective mass because it is, perforce, so close to the pivot. If you post a drawing of your proposed rider I can work it out for you and run you through the calculation.
    

    Mark Kelly
    
    

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    • Re: Basically - ER 11:20:46 01/11/07 (2)
      In Reply to: Basically posted by Mark Kelly on January 11, 2007 at 02:53:41:

      Thanks Mark,
      The "rider" will be round carbon fiber tubing also, it's distance from the pivot axis will be ~15.375 mm (= r at tubing wall center). The carier or rider tube can be any length, though 4-8 inches would probably work best, with a mass of 5.4 grams per inch.

      Tone arm material will be round carbon, 1.4125g per inch again can be any length to get things right.
      Headshell, cartrige, and mount, 21.6g (cartridge alone 5.5g)
      counter weight and mount, 82.1g adjustable over 3/4 of an inch or so.

      Please forgive my significant digits,

      Erik
      
      

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      • Re: Basically - Mark Kelly 04:55:14 01/12/07 (1)
        In Reply to: Re: Basically posted by ER on January 11, 2007 at 11:20:46:

        The carrier will have an Icm of 3.3 x 10^-8 kgm^2 per inch of length as is but this is useless because it will obviously have some fixings for the arm tube and the counterweight carrier. For a thin walled tubethe error between using the method of subtraction of concentric cylinders and the wall centre is very small.

        The rest of the items you can figure out using the formulae you already have.
        

        Mark Kelly
        
        

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        • Thank you... - ER 16:28:16 01/12/07 (0)
          In Reply to: Re: Basically posted by Mark Kelly on January 12, 2007 at 04:55:14:

          Thanks a lot Mark!

          There will be very few fixings other than some epoxy, the counter weight will be very close to the carrier. I included the mount and hardware weights in my measurements.

          It's amazing how fast this thread got burried, busy board!
          I'll be sure and post the results when I get it all together.

          Thanks again!

          Erik
          
          

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• Re: Do you know what a moment of inertia is? - ER 15:52:51 01/10/07 (0)
  In Reply to: Do you know what a moment of inertia is? posted by Mark Kelly on January 10, 2007 at 14:07:15:

  Thanks mark,

  I'm a little fuzzy on moment of inertia, I'll have to study that one a bit, is that like having a heavy rim on your platter or along those lines? How would estimate that?

  I'm sure I could measure it once the thing was built but, that would be putting the cart before the horse, I'm still in design phase though I will soon know what all the components weigh, along with a weight per inch on the tone arm material to optimize mass vs. length.

  Another quesetion, is it worth doing away with a removable headshell, it would be convineient but what about sound? It would almost be as easy to build a seperate arm and carrier optimized for each cartridge other than changing out the head shell and adjusting down force.

  With what I want to do the "carrier" tube will be ~1 1/4" carbon fiber tubing, it's round so will the pivot point be a virtual pivot at the center point of the stationary air bearing tube?

  If I vary the length of the tube to try and compensate for mass to reach a set resonance I guess I'll have to try and sneak up on it til lenght and mass are where I need them.

  Thanks,
  Erik
  
  

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