【转帖】concentricity vs runou

yang686526

concentricity vs runout
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
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everyfff"> 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.
i started writing a long reply and then realized i didn鈥檛 know well gdt enough  to go into detail.  i think the tick is right tho鈥?
on all the parts i鈥檝e designed lately where 鈥榗oncentricity鈥?is an issue i鈥檝e 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鈥檚 probably worth looking at as an option as well as run out.
ken
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
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
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.
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."
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
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.
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.
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
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