20.0mm H7 Tolerance with GD&T Circularity 3

[engineeringspread]

Hello All,

New to the forum and would just like some other opinion on this subject. I have specified a hole tolerance to accept a bearing, which is a 20.0 H7 +0 / +0.021 the centre position is defined by GD&T position and datum references, the hole depth is around 20mm. The selected manufacturer is asking for a circularity tolerance which confuses me somewhat as in my opinion the H7 tolerance accommodates this as it is a relatively tight tolerance and doesn't really allow for any significant circularity runout.....if I were to add circularity a tolerance of 0.01 would be required to have any effect due to the H7 tolerance.

Interested to know others opinions on this situation.

Thanks - James

 

[greenimi]

What standard are you using/ specified on your drawing?

 

[engineeringspread]

ISO 2768

 

[pmarc]

engineeringspread,

If ISO 2768, then has a general tolerance class for geometric tolerances been specified in conjuction with the "ISO 2768" specification?

For example: "ISO 2768-mK" or "ISO 2768-K",
where K represents one of the available classes of general geometric tolerances (the other two are H and L).

 

[engineeringspread]

Yes this is correct so unless otherwise specified the general tolerance applies....in this case mK

My real main point was.....unless I specify a very tight 0.01 or less circularity tolerance it would have no effect on the 0.021 margin on diameter.

 

[pmarc]

In the first place, I am trying to determine whether the manufacturer is correct by asking you to add an explicit circularity tolerance to the drawing, and to me (based on your additional input) it looks like the answer is no, there is no need to add extra circularity tolerance (unless you think it is needed for other functional reasons).

I am saying this because para. 5.1.2 in ISO 2768-2 states that: "The general tolerance on circularity is equal to the numerical value of the diameter tolerance, but in no case shall it be greater than the respective tolerance value for circular radial run-out given in table 4."

So in your case - because you don't have a circular radial runout tolerance specified for the considered feature (there is a position tolerance used instead) but you do have the diameter tolerance value specified (0.021) - the general circularity tolerance is already enforced by the drawing and is equal to 0.021.

Tightening the circularity tolerance to any value less than 0.021 is OK, but if the design requirements for the feature don't really force you to do that, then putting it on the drawing is just the addition of extra costs.

 

[engineeringspread]

Thank you for the reply, this was also my understanding and the reason I dimensioned this way and didn't add the circularity tolerance, I was just trying to get some other opinions.

Although I was keen to know how others deal with a 'press fit' situation under ISO 286 values. As these values are very precise it would seem unnecessary to add a circularity GD&T tolerance.

 

[Kedu]

https://www.eng-tips.com/viewthread.cfm?qid=452085

This discussion could be helpfull too....there are some pictures attached.

 

[engineeringspread]

My understanding that on a hole basis if a 0.021 tolerance is stated this is the maximum value of the runout.....so the GD&T circularity would theoretically apply to a dimension of 20.01 with a +/-0.01 circularity tolerance to meet the ISO 286 fit diameter specified of 20 -0 +0.021.

 

[Kedu]

states that: "The general tolerance on circularity is equal to the numerical value of the diameter tolerance, but in no case shall it be greater than the respective tolerance value for circular radial run-out given in table 4."

Would be a difference between calling out ISO 2768-mK-E and the original indicated standard ( ISO2768-mK)?
I personally don't see any.

If there is, which one?

Edit: I submit it too early, without asking my applicable question.

 

[fsincox]

I thought that the size tolerance was "independent" from form under ISO, specifically there has been no statement that the "E" was invoked?
The bearing tolerances like those mentioned here, that I see, are generally from catalogs in ISO countries
What grade of bearing are you looking at?
Frank

 

[greenimi]

So in your case - because you don't have a circular radial runout tolerance specified for the considered feature (there is a position tolerance used instead) but you do have the diameter tolerance value specified (0.021) - the general circularity tolerance is already enforced by the drawing and is equal to 0.021.

Frank,
That would be one of my questions too.

I thought that the size tolerance was "independent" from form under ISO, specifically there has been no statement that the "E" was invoked?

"a hole tolerance to accept a bearing, which is a 20.0 H7 +0 / +0.021"
If no Envelope requirement, why the "default" circularity cannot be 0.021 and an extra approx. 30%= 0.0273...

This value is anyway smaller that the "default" circular runout per tolerance class "K" table 4 which is 0.2.

Pmarc,
What I am doing wrong in my thought process? I have a "shortcut", but not sure where?

 

[engineeringspread]

Sorry Guys,

My question really is that surely the 0.021 tolerance constrains the boundary conditions of circularity, as worse case scenario would be a circularity of 0.02 to maintain this diameter size.

This was my point why would you need to specify both. I am sceptical any tighter tolerance than 0.02 is actually achievable without blowing costs out of the window.

This is a polymer bushing, which is very sensitive to tolerance as this contracts slightly when pressed into position, which determines the play on a rotating shaft.

 

[3DDave]

Under Y14.5 the MMC size constrains the form of the feature. Not under ISO, as far as I know.

Common bearing tolerances are for pressing a steel race into a steel housing.

Since you left until the very end to mention that a material with both a very low modulus of elasticity and a very high coefficient of expansion is involved and that cost is the largest concern then I'd say there isn't a D&T problem to be solved yet.

 

[engineeringspread]

The manufacturer specifies the tolerance for the bearing, its not something I've made up. Shaft and hole fits are specified under ISO 286.

 

[3DDave]

The manufacturer did not supply a stress analysis to back it up, did they?

 

[3DDave]

And if your company had done an analysis at the beginning this part would not have been chosen, because it costs to much to use.

 

[engineeringspread]

The question was really trying to establish whether a circularity tolerance is required in conjunction with a +0.021 fit. Personally I do not believe it is. I was interested in others opinions that all.

 

[3DDave]

You need to buy a copy of the standard, but from the online preview which I just googled for you:

 

[engineeringspread]

Thank you for the information 3DDave, that is actually very helpful, as this gives some standard reference to the issue I have encountered.