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GD&T - Difference between positional tolerance, concentricity and runout

  1. Jan 27, 2013 #1
    Hi,

    I have a great confusion existing between posititional tolerance, concentricity and run out. When I read the definitions in ASME Y14.5 M, I feel them mathematical. Can somebody pl. explain me their significance in function? Say, I have a primary axis "Datum A" and there is another cylinder in series to that. What will happen to this cylinder, when I give a positional tolerance or concentricity or run out? When to use what among these?

    http://imgur.com/uTNJJWw

    Thanks.
     
    Last edited: Jan 27, 2013
  2. jcsd
  3. Jan 31, 2013 #2
    Runout sets a limit on how out-of-round the shaft at each place along the shaft can be relative to the datum. Even if the shaft is perfectly round, if its axis is offset from the datum axis it will have runout. It does not control the size of the shaft. It does not control taper or other shapes - just how much variation there is in the radius to the datum at each place. Total runout does control taper as it controls the variation in radius to the datum for the entire surface

    Concentricity sets a limit on how non-symmetrical the shaft is relative to the datum axis. If the shaft is oval it can still be concentric. It controls mass balance about the datum axis by enforcing diametral symmetry. It does not control the size of the shaft, or the taper of the shaft. It compares the radius on one side of the shaft to the radius on the opposite side of the shaft at the same axial point along the datum axis.

    Position sets a volume the shaft surface must stay in or the volume the axis of the shaft must stay in. The volume the shaft surface must stay in is based on the largest allowable diameter of the shaft plus the position tolerance. The volume the axis must stay in is the position tolerance plus any MMC tolerance allowance. The surface method is the recommended one. Either method should give very similar results for a real part. Mathematically, they are identical.
     
  4. Jan 31, 2013 #3
    Hi Dave,

    Now I have found one clear difference between concentricity and other two tolerances. Conentricity will say Ok even the shaft is elliptical / non circular but if its axis lies within the tolerance band. This won't happen with other two (run out and position).
    Now, if an RFS is assigned to position tolerance, is there a difference existing between this and run out? How is a position and run out measured practically.

    Thank you very much for your answer. Like I needed, you have given the explanation in a non-mathematical way which leads to more curiosity. :)
     
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