Tolerancing Double-Tapered Seal Ring

[Gilmiril]

I need help! I'm making a drawing for a centrally ribbed, double-tapered symmetrical seal ring, and I need advice for specifying geometric controls. The basic geometry is shown in the attached file, but basically its a hollow ring that's tapered on the OD on both ends and that has a rib on the OD centered along the length.

Functionally, the important characteristics are that the tapers be coaxial to each other, that the faces of the rib be parallel, and that the tapers be perpendicular to the rib. My problem stems from this being symmetrical, so there's no preference for one side versus the other being a primary datum, and I have to admit lack of experience dealing with compound datums and such.

Thanks in advance...

 

[MikeHalloran]

It's helpful to think of a datum as a surface of a fixture, not a surface of the part.

In this case, you could use either face of the rib or either end of the primary cylinder as a datum, depending on how the part is retained and used.

I'm guessing it's made of an elastomer, so you'll probably want another datum comprising a cylinder through the center, with a note about measurements with x dia cylinder through, and resting on datum y.

You'll also need to specify the allowable mismatch at the mold's parting line, and a limit on flash trim.

It may be helpful to scrounge an inspection drawing for an o-ring, a quad ring, or a Greene Tweed T-Seal, for reference.



Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

thread1103-380433 may be of interest as it touches on the 'restrained condition'/'free state variation' aspect which can be significant for flexible parts.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

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

 

[Gilmiril]

I should have mentioned that this is a metal seal ring similar to the Grayloc seal ring used in ubiquitous clamped hub piping connections, but we're using this as a bonnet-to-body seal in a valve. So when installed, the central rib is clamped between mating faces on the body and bonnet, and the two tapered sealing surfaces are forced into mating conical seats in the body and bonnet. Surface finish is of course important to ensure a pressure-tight seal, but these types of rings have been used in 20,000+ psi service.