Whether to apply "Concentricity" or "Total Runout" tolerance characteristic.

[Ron Exley]

Which tolerance characteristic, "Concentricity" or "Total Runout" would be best for different machined diameter scenarios? Practically all descriptions and illustrations that I have found display a shaft containing two different diameters with same or different lengths. For the sake of this discussion these would be considered external cylinders.

My scenario involves two bores (internal cylinders), each having different diameters and lengths, that are spaced 5.740" apart. (These bores are in a 400 LB gearbox.) A torroidal bearing will be installed in each bore, a 6.000" diameter roller between them, and a splined shaft installed through all three components.

The diameter of the top bore will be used to establish my datum axis. The lower bore will be tolerance relative to that datum axis. I believe both bores to be easily accessable for CMM tooling and inspection from either end.

So now the questions. What is the availability of CMM tooling to establish the needed datum axis of the first bore and then qc the second bore for "Total Runout"? I envision the tooling with the dial indicator having to be spun. How costly and practical would this be versus requiring "Concentricity"?

Thanks,
Ron

[Belanger]

Three questions:

1-- Do you know which tolerancing standard would be governing the drawing? (If you're in the US, it is most likely ASME Y14.5.)

2-- Are you concerned about the two choices simply based on ease of inspection, or might there be a functional concern that "drives" the decision?

3-- From the functional point of view... is the roundness of the bores (form) just as important as the alignment? This would be the key question to swing the vote.

I can tell you that total runout is a more stringent control. But it is often the easier one to measure, using the dial indicator method. Concentricity is sometimes harder to measure, but it actually is more forgiving (because the bore could be oval-shaped yet still be concentric).
The fact that the part will be measured with a CMM shouldn't matter. I don't think the part literally has to be spun; the computer can do all the math as long as sufficient points are sampled.

[Ron Exley]

Thanks for the reply Belanger.

Ques 1-- I am in Houston and use ASME Y14.5.

Ques 2-- I'm concerned about both to know which may be more applicable in varied applications and functionality. It is my understanding that concentricity would apply to a diamtrical feature that functions as a component mounting location. Where as total runout would be applied to mating surfaces that have movement between them. For example, the surface of a bearing race that the roller bearings and ball bearings move across. The runout of these types of surfaces are more critical where friction affects the component life.

Ques 3-- Functionally the bores will have a slip fit diameter for installing, SKF, "Torroidal" bearings. These bearings are designed to compensate for a certain amount of mis-alignment between the axis of each bore. Potentially the bores could be oval and still retain the axis of each bearing within their operating parameters. With that design intent I'm inclined to think "Concentricity" would be sufficient.

Do your (or anyone else) know if there is CMM equipment to have a dial indicator rotated inside 3.125" and 4.125" bores so that 100% of the bore depth can be checked. Pictures would be helpful. This would be required of the component that we have. It is a 400 LB casting of a gearbox.

Thanks,
Ron

[Belanger]

Sure -- that helps a bit more. As for Q1, I asked because many people latch onto the word concentricity because it sounds like the right word. But if you go by the definition actually given in Y14.5 then it is a bit different than how Webster's Dictionary might define the word! (In fact, many times concentricity ends up getting verified using a dial indicator method that is really checking runout.)
I might also throw the position symbol into the mix; it is a very common way to control features such as you describe.

Yes, I agree that runout might make more sense when something is actually riding the surface in the function of the part. But if you are strictly worried about alignment of the average centers (you said the bores can have some ovality and be OK) then concentricity or position might suffice.

For deciding on a proper tolerance symbol, here are the four things to consider:
size
orientation
location
form

I presume the size will already be called out as a traditional diameter with a ± tolerance. And orientation is usually a by-product of location, so the focus is on location and form. If the location and form of the bore have to be held to the same accuracy, then use a runout symbol. But if the location and form require different accuracies, then use position or concentricity to focus on location, and then form can be called out separately (with circularity) or simply default to the diameter's ± tolerance.

I don't know of any way to rig a dial indicator directly with a CMM. Other folks might chime in?...

[Kelly_Bramble]

What is the availability of CMM tooling to establish the needed datum axis of the first bore and then qc the second bore for "Total Runout"?

I don't know of any way to rig a dial indicator directly with a CMM.

​Modern CMM's are very capable and measuring total or circular runout is easy. The stylus or measuring probe are programmed or manually positioned to measure many surface elements about a diameter cross section along the length of the diameter. The initial orientation of the diameters is irrelevant. Also, all this assumes that the CMM is large enough to handle the part..

To establish an axis from a datum diameter all that is required is access to the diameter to collect enough surface elements. Depending on the CMM, a long stylus should be available. I've seen a Zeiss CMM with a 4 ft. long stylus.

See http://www.pmargage.com/styli_main.shtml

My only comment about total runout vs concentricity is that you probably don't want ASME Y14.5-2009 concentricity.

For a bearing contact or race surface installation there should be an OEM specification that you would design these contact surfaces to. This information should be available on the OEM's website or in a specification document provided with the toroidal bearing. Likely, you will need an interference or locational interference fit - again see want the OEM specifies.

If this is a custom bearing size - then look at the installation specifications for the adjacent bearing size. Usually, the smaller the bearing the smaller the interface tolerance.