Dovetail versus half dovetail gland
Dovetail versus half dovetail gland
(OP)
Hi guys,
I have a large mold that I'm designing. An o-ring gland will be cut into an end plate in a static face seal orientation. Pressure is low (vacuum to 45 psi), but I am heating the mold to 350F. Prior molds have had the o-ring fall out during mold manipulation. I'd like to try a dove-tail or half dove-tail orientation, but every source (including the Parker handbook) says "Only use if absolutely necessary." Is this primarily due to cost? Is there a performance advantage to full versus half?
I'm sure I'm missing some critical detail, but what are your first thoughts?
Thanks,
Scott
I have a large mold that I'm designing. An o-ring gland will be cut into an end plate in a static face seal orientation. Pressure is low (vacuum to 45 psi), but I am heating the mold to 350F. Prior molds have had the o-ring fall out during mold manipulation. I'd like to try a dove-tail or half dove-tail orientation, but every source (including the Parker handbook) says "Only use if absolutely necessary." Is this primarily due to cost? Is there a performance advantage to full versus half?
I'm sure I'm missing some critical detail, but what are your first thoughts?
Thanks,
Scott





RE: Dovetail versus half dovetail gland
Machining an o-ring dovetail face groove is an expensive operation, primarily due to the very small diameter dovetail milling cutter that must be used for the undercut.
Your operating temps are high, but have your considered bonding the seal in place?
Good luck.
Terry
RE: Dovetail versus half dovetail gland
Dow-Corning 111 valve lubricant is good for this if your process is tolerant of silicone.
RE: Dovetail versus half dovetail gland
At your temperature you are somewhat limited to materials you may want to try a type of X-ring with a tight groove
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We use a lot of 3M 77 adhesive spray for holding O-rings and gaskets in place. && does not lubricate, For real tough conditions we use DuPont Krytox Grease to hold and lubricate the O-rings.
RE: Dovetail versus half dovetail gland
But when the dovetail is cutting below a flat surface (like your gasket sealing surface) then the tool has to cut "down" into the surface before it can begin cutting sideways to make the groove. That entry point is difficult to seal. A rectangular O-ring groove has the same size cut at the top as well as the bottom.
RE: Dovetail versus half dovetail gland
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Or some other shape?
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In either case, would an o-ring with a slightly undersized length hug the groove better?
RE: Dovetail versus half dovetail gland
This is a very low production, very high value part. We can't tolerate any risk of part failure, so the o-ring is replaced each casting. We've used 77 spray adhesive, but it was a mess and only work partially. Silicone of any type is not tolerated with this operation.
We have a smaller scale mold that we are going to run trials on. I'm leaning towards trying a 2% undersized x-ring prior to jumping into the dovetails.
Our machinist is recommending a 1/2 cord rather than the 3/16 we are currently using. The face seal is 12' diameter and the longitudinal seal is ~20' long. Thoughts here??
RE: Dovetail versus half dovetail gland
Production hates any kind of goop (which isn't allowed in your application in any case) and loves dovetail grooves.
A rectangular groove is machined first, then a plunge cut is made with the dovetail tool & the dovetail feature is chased in the existing rectangular groove. The plunge cut leaves a circular opening at one spot equal to the maximum diameter of the tool. We haven't had any leak issues there; the seal is still primarily made on the faces.
As with any o-ring seal, especially one used in production, be sure the surface finish requirements are met & then protect those surfaces. If possible, make an adapter tool that will permit the seal to be tested when an expensive production run isn't on the line. Make it part of your preventive maintenance progream.
Rob Campbell
Rob Campbell, PE
RE: Dovetail versus half dovetail gland
The last time I worked with the applications people they were extremely accommodating.
http://www.helicoflex.com/index.jsp
Sorry about the 77 spray. As I post we also use Krytox Grease which is essentially inert, along with Hylomar as a tacking agent for gaskets and O-rings. hylomar is a very thin coating.
RE: Dovetail versus half dovetail gland
RE: Dovetail versus half dovetail gland
I agree with Rob - dovetails work well for us. The biggest downside for us is the cost of the groove, so we've started making our grooves a kind of "hybrid". MOST of the groove is standard rectangular cross-section, with short segments (especially at corners) of a dove-tail cross-section to keep the o-ring retained.
Steve R.
RE: Dovetail versus half dovetail gland
RE: Dovetail versus half dovetail gland