o-ring groove as datum feature
o-ring groove as datum feature
(OP)
I have an o-ring application. A shaft like part slides into a housing, o-ring groove is on the shaft, static seal, 2.2mm width. If I want to use the o-ring groove diameter as a datum feature on the shaft, I think it should be referenced at RFS. Similarly on the housing, if I want to use the inner diameter as a datum feature, it should be referenced as RFS. Am I correct?





RE: o-ring groove as datum feature
Also, you may want to specify what you are trying to constrain with the feature control frame. The straightness of the shaft? The alignment of the axis between the shaft OD and o-ring groove?
I would say the outside diameter of the shaft is the better datum feature, because it is easier to simulate. From there, you can put a concentricity requirement on the diameter of the o-ring groove and deference the OD datum feature so that your axis are constrained. Concentricity is always referenced at RMB/RFS.
You can also apply a straightness to the O.D. of the shaft to ensure if fits in the hole straight. Straightness tolerance does not have a datum reference.
RE: o-ring groove as datum feature
What is that other feature you are trying to control wrt diameter of the groove?
RE: o-ring groove as datum feature
- It is indeed functionally justified (centering) to use RMB concept instead of MMB for o-ring groove, but...
- Like EMorel mentioned, I would rather see shaft as datum feature and control the groove to the axis derived from the shaft;
- I would not go with Concentricity for groove cylindrical surface, but rather consider Position (with additional Circularity callout if needed) or Profile as an alternative or Runout even.
RE: o-ring groove as datum feature
For the specific case of an o-ring groove, and especially for a small section o-ring as here, I would specify the groove depth with a tolerance, because that's just one tolerance affecting how the o-ring seals.
If instead you specify the groove root diameter and a concentricity, that's two tolerances that you have to allot from the total tolerance for radial squeeze on the o-ring, which doesn't provide a big budget.
Mike Halloran
Pembroke Pines, FL, USA
RE: o-ring groove as datum feature
RE: o-ring groove as datum feature
Out of curiosity: how many depth measurement around the circle will you consider sufficient?
RE: o-ring groove as datum feature
General practice, I believe, is as many measurements as it takes to be confident that the part is in print. That could mean 3 or 33. Definitely more than 1, though.
I see where you're going though; even without GD&T, I would rotate the shaft about the axis with a dial on the groove and see what kind of variation is there. That's just me though, not everybody who would inspect the part.
RE: o-ring groove as datum feature
RE: o-ring groove as datum feature
On more serious note - come on!
Parker's and other manuals are based on ISO O-ring standard.
The "concentricity" in any ISO standard is not ASME concentricity.
Anyone who cares about "big budget" wil set-up run-out inspection.
If the part passes, say 0.10 runout, it automatically passes 0.10 concentricity (ASME), 0.10 position MMC, 0.10 position RFS.
RE: o-ring groove as datum feature
Since then, while tolerancing parts, I've tried to maintain a mental image of the part as if it were made of some semi-hard plastic like PVC, and warped in every possible way.
Mike Halloran
Pembroke Pines, FL, USA
RE: o-ring groove as datum feature
By the way, I like the approach suggested by Mike.