Concentricity vs Runout 3

[Randy1111]

Our shop produces shafts. The shafts have 3 stepped diameters at each end. The designers have been using concetricity callout on the shaft diameters in relation to one of the bearing journals.

From any research I do it says to not use concentricity if at all possible, since most shops are unable to accuratly measure it.

What is a good method for using geometric tolerancing on a shaft with multiple turndowns to ensure it can be accuratly reproduced if contracted out? Should they just be using runout?

An example of a typical shaft may be a 5" diameter for the majority of the center portion, turned down at each end to a 4" diameter, then a 3" bearing journal, then a 2.5" drive journal. One 3" bearing journal would be the datum. With all other journals wanting to be inline to it.

Sorry if this is a very basic question.

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Randy

 

[TheTick]

Every time I have seen concentricity used, the designer really meant runout, and the inspectors actually measured runout (with dial indicator).

You may have to force your designers to dig deep to understand the true meanings of concentricity vs. runout. Once they do, it should be no problem.

 

[KENAT]

I started writing a long reply and then realized I didn’t know well GDT enough to go into detail. I think the Tick is right tho’.

On all the parts I’ve designed lately where ‘concentricity’ is an issue I’ve used a combination of run out and/or positional tolerance. The senior design checker here helped me with it, and he does know GDT well.

The positional may not be relevant to your application but I think it’s probably worth looking at as an option as well as run out.

Ken

 

[ctopher]

Since it is for a bearing and it's a spinning part, I would use Total Runout.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-05)

 

[Randy1111]

From what i've researched it appears total runout and maybe position control combined with total runout would be most appropriate.

A spinning shaft mounted in bearings is a fairly common part spanning many industries. I was hoping there was a common prefered method for dimensioning it.



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Randy

 

[ewh]

Am in agreement with the above posters. Concentricity on a shaft simply isn't worth the expense of proper inspection when runout returns the same result.

 

[aardvarkdw]

From pg.203 of "Design Dimensioning and Tolerancing" by Bruce A. Wilson (ASME Senior);

"Concentricity is a control of one axis to another. It is always applied on an RFS basis. The use of any other modifier is incorrect.
A very small number of tolerancing applications call for a concentricity tolerance. The tolerance should only be used when the location of one axis to another needs to be accurately controlled. Concentricity should only be applied when it is certain the axis relationships are the only means of producing a functionally acceptable part. verifying concentricity tolerances is very difficult when using manual inspection equiptment.
Most coaxial requirements can be met with either a position tolerance at MMC or a runout tolerance at RFS. These tolerance types can both be easily checked on the basis of surface conditions. They are preferable to concentricity requirements because of the ease in verifying position and runout tolerances. Concentricity should only be used when absolutely nessesary.
There are many applications where concentricity is incorrectly applied, and runout is the control the should have been applied."

 

[Randy1111]

Great, so i can toss out the concentricity requirement, since we cant accuratly measure it anyways.

So I'm left with runout, total runout, or position, or a combination of these which is what i was hoping was the correct method.

Is the proper method to use either total runout, or position at mmc, or both?

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Randy

 

[caseynick]

I would suggest using the 2 bearing journals at the ends two create the datum axis, then use total runout. It is a composite control. It controls orientation to the datum axis, form of the feature being checked, and it's location related to the datum axis. Total runout also controls taper of the feature being checked since it checks the entire surface as opposed to circular runout which checks each circular line element independently. You would also, of course, need size tolerances on the diameters since runout is a surface measurement.

 

[CheckerRon]

caseynick just nailed it. As one who used to do a lot of long lathe turned shafts and lead screws, using the bearing jounrals to establish a datum axis that is controlled by a TOTAL RUNOUT callout checks taper, bowing, and will be ideal for minimal eccentricity and balancing if neeeded. In this case, position doesn't do much for you, especially if dynamic balancing is needed. Any good GD&T guy like me and others will tell you, FORGET CONCENTRICITY, and SYMMETRY too for that matter. The european method of measuring these, adopted by ASME in the 1982 and 1994 eds. of Y14.5 made these callouts useless for most applications as aardvarkdw correctly noted above.

 

[MechNorth]

Aardvarkdw, Caseynick are right, and CheckerRon added the final detail for your consideration...the dynamic balancing of the rotating shaft which is best controlled by total runout. None of the other controls will achieve that, though positional tolerance and cylindricity may come close.
There has been talk about removing symmetry and concentricity from the next revision of the Y14.5 standard because it doesn't provide any functionality that can't be achieved better by other controls separately or together, and because there really is no practical way to verify it.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

 

[ctopher]

I think Total Runout was suggested earlier in the thread, also.
TheTick was correct about designers and inspectors.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[Randy1111]

So 'total runout' alone, although with size tolerances on the journals also is the prefered method. The following is why I still have some confusion as to the best method.

One site I looked at used both runout and total runout to define one circular feature with respect to another ...

http://www.tec-ease.com/tips/march-00.htm

The same site shows circular runout alone being used to define circularity and concentricity....

http://www.tec-ease.com/tips/june-97.htm

From looking at those examples, I should be using both circular and total runout as in the first example above. Does that sound like the proper method to you folks?

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Randy

 

[MechNorth]

Hi Randy,
Looking at the first reference (March '00), notice that the two controls have different tolerance values; specifically, the Total Runout has a larger tolerance zone than the Runout. So, the overall variance is larger than the variance at the each individual cross-section (which is where Runout is applied).
The second reference only shows the topographic view (an edge), so no depth is idicated; as a result, Total Runout cannot be indicated until an actual surface is shown.

As to which to use, if the length of the cylindrical surface makes it difficult to maintain a constant diameter, then use both controls. If the cylindrical surface is short, then Total Runout alone should be adequate.
Hope that helps.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

 

[aardvarkdw]

Randy,

In the first link, if you just used total runout, you could have a variance at any one POINT of up to .15 as long as the overall variation for the entire surface did not exceed .15. On the other hand just using the runout tolerence, you can only have a variation of .05. The problem with that is that it only controls the size at any given point not the shape of the whole surface. By using both, more importantly the total runout first, You can loosly control the overall shape of the surface and but prevent drastic variance at individual points.