o-ring groove as datum feature 1

[bxbzq]

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?

 

[EMorel]

Think logically about how a datum feature is supposed to work. You are supposed to simulate a datum when checking a part, so if I am envisioning what you are saying correctly, that may be difficult if you make the o-ring groove the 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.

 

[CheckerHater]

Could you provide a sketch or at least better description of what you are trying to achieve?

What is that other feature you are trying to control wrt diameter of the groove?

 

[pmarc]

Based on description provided and imagining what may be really important for such application I would say that:
- 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.

 

[MikeHalloran]

Agreed that the shaft OD is generally a better datum than a groove root, for most any purpose.

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

 

[EMorel]

Thinking about it more, I agree with Mike. If you are designing your o-ring groove to something like Parker's design guide, you're better off specifying the depth of the groove anyway, and therefore, given the tolerance of your depth, you won't need a concentricity tolerance from the shaft.

 

[CheckerHater]

EMorel, Mike,

Out of curiosity: how many depth measurement around the circle will you consider sufficient?

 

[EMorel]

Measurements? Like when the part is being inspected?

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.

 

[pmarc]

What if shaft diameter close to the groove has maximum roundness error allowable within stated limits of size, which are, or at least may be, way wider than size tolerance for groove root diameter? In that case having only groove depth dimension specified could mean that perfectly centered circular/cylindrical groove root would be rejected because depth dimension would not be met.

 

[CheckerHater]

The person who will inspect the part will quickly realize that the rod of depth micrometer won't fit into 2.2 mm ring groove
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.

 

[MikeHalloran]

In the case of the parts I was forced to buy off on, 6" long x 1" OD cylinders made from PVC with grooves at one end, the groove was circular, the groove root was in tolerance, and the OD was in tolerance. ... but the part was warped like a banana. The grooves had been machined with the part sticking _way_ out of the chuck, so they were eccentric to the OD by maybe .025" at the grooves. You could see the eccentricity with the naked eye, from a good distance. ... and of course the o-rings didn't seal, being pinched on one side and uncompressed on the other. We were relying on the concentricity default specified in our standard manufacturing instructions, which of course the vendor claimed he didn't get with the order. ...

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

 

[bxbzq]

Sealing is the most important requirement. There is clearance between shaft OD and housing ID. I don't want the contact force (in case) between the shaft surface and the housing inner wall to push the o-ring groove offset from housing ID axis. So I think I would control Coaxiality of the groove and shaft OD, and Straightness of shaft.
By the way, I like the approach suggested by Mike.