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

In summary, concentricity and run out are used to control the size, shape, and mass of the shaft while position tolerance only controls the volume.
  • #1
k.udhay
169
13
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.
 
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  • #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.
 
  • #3
three_d_dave said:
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.

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. :)
 

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

1. What is the difference between positional tolerance, concentricity and runout?

Positional tolerance, concentricity, and runout are all geometric dimensioning and tolerancing (GD&T) controls used to specify the allowable variation in the location and orientation of a feature on a part. However, they each have their own specific requirements and applications.

2. How does positional tolerance differ from concentricity?

Positional tolerance is a control that specifies the allowable deviation of a feature's location from its true position, while concentricity controls the allowable deviation of a feature's axis from its true center. Positional tolerance is used for features that have a defined center point, while concentricity is used for features that are circular or cylindrical in shape.

3. What is the purpose of runout tolerance?

Runout tolerance is used to specify the allowable variation in the circular or cylindrical shape of a feature, such as a shaft or hole. It ensures that the feature does not deviate from its true circular or cylindrical form, which is important for proper functioning in assemblies.

4. Can positional tolerance, concentricity, and runout be used together on a single feature?

Yes, these GD&T controls can be used together to fully define the location, orientation, and form of a feature. For example, a hole may have a positional tolerance to control its location, concentricity to control its axis, and runout to control its circularity.

5. What is the difference between circular runout and total runout?

Circular runout controls the deviation of a feature's circular form relative to its true circular form, while total runout controls both circularity and the location of the circular feature relative to its true position. Total runout is more comprehensive and stringent than circular runout, as it includes both form and location requirements.

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