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TIR and ruout or total runout 2

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ozzkoz

ozzkoz

Mechanical
Aug 13, 2009
51
Hi, I am dimensioning an o-ring grove and parkers catalog says the max eccentricity is .002 and states this is TIR between the groove and adjacent diameter. I think TIR is total indicator reading, but is this total runout (two arrows) or circular runout (one arrow)?

Thanks
 
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Eccentricity is the amount of non-roundness and will result in a TIR value (total indicator reading).

The relationship between a diameter and another diameter sharing the same centre is called "circular runout" and combines roundness and off centre in one value which is also reported in a TIR.

I don't think total runout (2 arrow heads) is applicable here so I won't get into its meaning.

So, what do you want? Roundness of the feature or its circular runout to another diameter sharing the same centre that will become a datum?

Dave D.
 
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I'm with Dave -- total runout is not what's wanted here. Circular runout (single arrow) with a specification of .002 (and the correct datum reference) will probably equate to this TIR callout.

But there's one other catch that Dave is getting at: circular runout error is caused by two main factors: eccentricity (coaxiality) and circularity (roundness). So in reality, you could have a groove that is perfectly centered to the adjacent diameter (so there is no eccentricity), but if this groove is slightly egg-shaped it could be rejected by circular runout.

To truly isolate "eccentricity" is rather difficult (pretty much "concentricity" in GD&T lingo), so I would stick with circular runout.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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Thanks, I think circular runout is what I need. The TIR is used to control the minimum amount of squeeze on the o-ring so I think I would want to control both eccentricity if it is perfectly circular and form if it is more egg shaped.

Thanks all
 
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Dave,
Was that a misquote? roundness and excentricity are not the same, they are separate contributors to runout. This is why I am not a fan of them. It leaves me to ask: "well which is it", it does not answer the question. Circular runout is traditional for this application, done many in the day.
Frank
 
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Frank:

I now agree with JP on this one. Eccentricity is not roundness but non-coaxial.

Circular runout is a combination of roundness and off centre.

Dave D.
 
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The TIR mentioned in the catalog could be circular runout or total runout. For the geometry of an o-ring groove, the difference between the two would typically be very small.

The word "eccentricity" is a bit confusing. On the one hand, "eccentric" generally means "not concentric".

On the other hand, "eccentricity" is also used to describe the proportions of an ellipse, or egg shape. An ellipse with zero eccentricity is perfectly circular.

On the third hand, an "eccentric" can also be "a person with an unusual or odd personality". This term has probably been applied to several of us on this forum.

Perhaps we could describe how centered an egg-shaped feature is with the term "egg-centricity". ;^)

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
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For an o-ring groove on a shaft (as opposed to a face), I'd go with total runout as the interpretation rather than circular. Circular just covers one individual segment at a time, whereas total covers the entire surface as a single entity. The o-ring gets deformed to take up pretty much all of the bottom surface of the groove, not just the initial circumferential ring contact at the tangent of the toroid.

Evan ... most of us are third-hands.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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Interesting, Jim...
Your suggestion will also include a straightness tolerance and a parallelism tolerance of .002. Do we really care if the bottom of the groove is that good as it goes across longitudinally?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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I certainly would use a total runout if the groove width was, say, 4 inches wide, but in most cases, it would be rather small in the .060 - .100 or so range.

I agree with JP on this one - circular runout.

Dave D.
 
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JP, yes, you'd get the roundness, cylindricity, longitudinal straightness and position from the total runout. I suppose that in a sense you'd also get the line-segment longitudinal parallelism to the axis as well; interesting thought. As for how good the bottom of the groove needs to be, that's dependant on a number of factors including load magnitude & type, seal compound & deformability/durability. I had to work with some o-rings and custom seals that were made from some interesting polymers that were highly susceptible to abbrasion wear and seatng form; those required even better control of the surface. But to turn your question around, why go for a lesser, and more difficult to validate, control than one that gives you extras for free. If you buy a base-model car these days, it probably has a lot of "extras" like power locks & windows, CD player, etc ... would you tell them to strip the extras out because it doesn't add value to you personally? By more difficult, I mean that you have to verify each segment independently rather than as an entire surface; decision criteria must then be set for how many locations along the surface, etc. As Dave points out, it's a very short surface typically (1.5-2.5mm), so where is the economy and value of using the lesser control?

Personally ... wait for it, Dave ... I prefer a surface profile that goes from the tangency point on the major diameter to the other tangency point on the major diameter. It controls size, location, runout, form, orientation. I've gone thru the work of checking the difference vs traditional linear tolerances & runout ... less time to inspect the profile & well within the tolerances required. Considering that most such grooves are now done by form tools, it's even more likely that the same tolerance can be held on all aspects.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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I mentioned parallelism because total runout certainly detects taper, so I wondered if that along with straightness is really necessary. As you mentioned, it of course depends on the application.

As for why I voted for a "lesser" tolerance when there's another that gives you more for free, I hold to the philosophy that tolerances should not be constraining something that has no bearing on manufacturability or function (i.e., make the tolerances as large as possible while still making sure the thing works).

Total runout is easier to check, but it's also going to reject a part that otherwise might be passed by circular runout. So when buying a car, I love getting more for free, but when tolerancing a part, I would disagree with you. :)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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Dave,
I apologize, I did not realize that eccentric was a word also used for egg shaped. I sometimes forget that communication is not just about what "you" read but also about what was really meant.
Sorry :)
Frank
 
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MechNorth,
Total runout? Please have a dicussion with the Tec-Ease people (very nice site) about inspecting the perpendicularity of short length-large diameter pilots then, if you are willing to inspect TR on an o-ring groove.
To the OP,
SAE specifications AS4395 & AS5202 based on the earlier MS33649 & MS33656 referenced in the Parker book itself use circular runout to define the faces, Parker's acctual catalog page states it as concentricity.
Frank
 
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Fsincox,
Indeed, perpendicularity of a short-length, large-diameter hole is not usually useful, however sometimes it IS necessary and IS done. I am used to working with tolerances down to 0.005mm on pilot diameters, and on occasion it does matter; usually it doesn't. And yes, I have had o-ring grooves inspected for Total Runout on a shaft when seals were failing. I prefer surface profile for o-ring grooves though.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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Jim,
I am really curious, how do you teach students to calculate the correct VC for checking the bolt holes and to create the gauge, if you do not give the perpendicularity value of the pilot diameter to the face? (say: Fig. D-4, pg 216 ASME Y14.5M-1994). Do you really tell them it is not important?
Frank
 
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Frank,
In that particular case, you're reading too much into the figure. The purpose of that particular figure is to illustrate former datum feature symbol applications, not to give a comprehensive GD&T application for all features.

Section 1.1.4: The figures in this Standard are indended only as illustrations to aid the user in understanding the principles and methods of dimensioning and tolerancing described in the text. The absence of a figure illustrating the desired application is neither reason to assume inapplicability, nor basis for drawing rejection. In some instances, figures show added detail for emphasis. In other instances, figures are incomplete by intent.

The example you cite is clearly missing the relationship between datum feature-B and datum-A. No argument there, but it's not relevant to the point of that illustration. Now, let's say pilot diameter was only .020" high; would perpendicularity wrt datum-A actually be relevant in "most" situations? Likely not, and it would be outside of the ability of most shops to verify it; in that particular situation, the perpendicularity back to datum-A would be omitted from the drawing without jeopardizing the intent or quality.

Jim

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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Come on MechNorth, you don't actually expect people to take "Section 1.1.4:" into account do you, rather than monkey see, monkey do off of an incomplete figure that may be illustrating a different point entirely.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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