Need help sourcing a custom-made Buna O-ring.

[MechEngineer2012]

I am looking to have a custom-made O-ring produced with tight tolerances. The required cross-section diameter is 0.139" ± 0.004". can you see the attached drawing and advise on the most cost-effective manufacturing method for this O-ring.

 

[3DDave]

You are looking for a custom formed gasket? Also, there is no way those tolerances can happen with an elastomer. At best an all-around profile tolerance in the restrained condition.

In a similar application I used a vertical slot and rectangular gasket material such that more than half the width of the gasket was in the groove. In hindsight I would have put a matching groove on the other part. This allows controlled compression of the gasket and retains the gasket during assembly. With the circular profile, especially if it isn't custom molded, the gasket will continuously fight to leave the groove during assembly as even the slightest twist will be enough to not lie flat.

With rectangular material it's just a matter of cutting to length and gluing the ends. For a better seal use a scarf joint.

Other than that there are a number of custom gasket molding companies who can make the part you have above and you can apply those tolerances to the mold.

 

[Brian Malone]

You are possibly over specifying on your design. An o-ring or any flexible seal is by definition: flexible - the actual final shape is going to vary unless measured in a constrained condition. Specifying +.001/-.000 on size dims is unrealistic for most fabrication processes for a flexible/elastomeric seal. Your specification as shown is more for the fabrication mold than the final part. If you are planning on using a face seal groove then you don't necessarily need a preformed seal, measure the interior path length of the seal groove and choose an o-ring with an equivalent inside circumference +/- . Your corner radii are gentle enough to lay the o-ring in the groove and it will follow. Sorry if I am describing something you already know but your question kind of leaves that undefined. Any major seal manufacturer can work with you on your design. Look up Apple Rubber, Parker, Pacific Seal, etc.

 

[3DDave]

Having used a regular circular ring in a rectangular groove I can tell from experience that it absolutely will not stay. It will resist the bend in the corners with enough force to climb the groove wall and escape unless the groove is undercut to capture it, making a good estimate of compression more difficult. It ended with having to glue the ring in place, defeating the value of the extra volume in the groove. It wasn't too big a deal as it was to discourage water entry, but it was a hassle.

 

[Compositepro]

Custom O-rings are commonly made from rolls of O-ring stock. The groove should be slightly narrower than the O-ring cross-section to keep the O-ring in the groove. The stock should be cut slightly longer than needed so that the splice is under compression. Then you do not even need to glue the splice to get a perfect seal. O-ring seals are self-energizing and quite robust, which is why they are so commonly used.

 

[Brian Malone]

I commonly use o-rings in regular (non-undercut) grooves without issues of ride-out/lift-up as long as the corner radii are not extremely tight.

 

[Brian Malone]

3DDave and  Compositepro, your guidance on o-ring cord stock is correct since the OP's seal is not close to a standard AS568 size. An option would be to redesign the seal parameters to be nearer to a -268 or -269 to allow use of an off-the-shelf part.

 

[3DDave]

I commonly use o-rings in regular (non-undercut) grooves without issues of ride-out/lift-up as long as the corner radii are not extremely tight.

View attachment 14765

That looks like glue is being used at several places to retain that seal and note that the seal is out of the groove on the right side.

 

[TugboatEng]

The groove needs to have clearance to the o-ring or the o-ring will rupture when compressed.

A dovetailed o-ring groove will allow the use of a round o-ring without glue.

O-Ring Groove Design Dovetail

Dovetail grooves are used to hold the O-ring in-place on a face seal groove during during assembly and maintenance of equipment.

www.sealanddesign.com

The challenge is not stretching the o-ring as it's pushed through the dovetail opening.

Association for Rubber Products Manufacturers has tolerance classes. Class E1 is high precision and gives a diameter of 0.139 ±0.007".

A Buna o-ring is not going to maintain its original cross section for very long.

 

[Brian Malone]

No glue used - the schmutz is just debris on a part returned from service. The part of the seal on the right is just I was lazy and did not push that area of the seal down and took a bad photo. I place o‐rings in uncircular grooves without issue. No ruptures or squirms. Certainly, this is possible because of the use case of the equipment I design. 3DDave and Tugboat, the design details you point out are valid for the many uses and environments. The OP has not specified any operational parameters, so all options are open.

 

[3DDave]

Show me the movie.

 

[3DDave]

It does raise the question about using a soft wire core in extruded o-ring stock to enable the pre-forming of the stock for complicated routes.

 

[Jboggs]

To the OP: Just taking a wild guess here - are you an Aerospace Engineer? Only an Aerospace engineer would put that kind of tolerances and G,D,&T on a drawing of a rubber o-ring!

1. Get a copy of the O-Ring bible: The Parker O-Ring Handbook. Has everything you could ever possibly want to know about O-rings. Everything!
2. Redraw your o-ring as a circular component with the ID and OD you want to match the total perimeter length.
3. Look in the above handbook. I'll bet lunch you will find one that will work for you.
4. If you don't find an exact match, you will find one that is close enough that you can redesign your sealing geometry to accept it.

I don't think you really need a "custom-made" O-ring.

 

[dgeesaman]

I commonly use o-rings in regular (non-undercut) grooves without issues of ride-out/lift-up as long as the corner radii are not extremely tight.

View attachment 14765

Wow my experience with that kind of thing is the complete opposite.

I can think of an automotive gasket that is NLA and its shape is 80% circular 20% triangular with huge radii. The OEM gasket, when you could buy it, was an o-ring that was somehow formed a bit to hold that shape. Now we just fight with standard o-rings and it's really difficult to have it stay put even long enough to squeeze the parts together. That photo feels implausible to me, especiall with the groove depth barely reaching the centerline of the o-ring body.

What can help, and I don't think has been mentioned, is full dovetail or half-dovetail grooves. Those will lock in an o-ring and leave enough protrusion to form an effective seal. The downside is the cost and complexity of getting the dovetail surfaces sufficiently smooth and free of edges.

 

[Brian Malone]

 dgeesaman - "That photo feels implausible to me, especiall with the groove depth barely reaching the centerline of the o-ring body"

I am on the fence over whether I should post a more complete photo of the example I offered to counter your skeptical remark but the full geometry is part of a proprietary design, so no. The small section of the seal geometry I show works for a device that must maintain a seal with exposure to biological fluids at 300 psi and compatible with autoclave sterilization, and compatible with exposure to laboratory disinfectant solutions. Does this work for 3000 psi hydraulic systems? No. As I stated, the the OP has not said what is their sealing conditions and environment only they need an o-ring with 'tight tolerances'.

 

[btrueblood]

I agree with Brian, that a well-made (generous radii) groove will accept a common round molded o-ring with minimal fuss. A bit of super-lube or silicone grease (or plain old lithium grease for bog standard Buna rubber) will help hold it in place if there is a lot of movement or even inversion of the mating parts. Lube also helps the installed oring to shift under pressure and squeeze to the corner and form a seal. Also - don't put an o-ring in compression, that is what causes it to squirm and try to pop out of the groove. Dimension the oring to be a snug to slightly stretched fit on the inside edge if you need to.