O-ring surface finish specification 6

[trevorault]

I am having difficulty specifying a proper surface finish for the gland surfaces of a static vacuum o-ring face seal. The drawing currently specifies a 16 microinches Roughness Average. The vendor has supplied parts with a few large pits of depth greater than 4 times the specified avg of 16 and claims the average roughness of the entire surface is still below 16. These large pits may increase the vacuum leak rate. Is there a more detailed method of specifying surface finish that would minimize these anomalies?

 

[CoryPad]

Yes, you can specify R_z, R_3z, R_max, or many other parameters. You can learn more at:

http://www.predev.com/smg/parameters.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jetmaker]

Cory,

Great site.

jetmaker

 

[boo1]

ya Cory got my vote

 

[trevorault]

Very good, thanks a lot Cory.

 

[israelkk]

trevorault

As informative and interesting the web page brought by Cory unfortunately, as far as I know it is not applicable to O-rings made of rubber. Inspection of O-rings is usually a visual inspection (under magnification) and by comparing the actual surface defects to a group of known defect photos. See MIL-STD-413 VISUAL INSPECTION GUIDE FOR RUBBER ELASTROMERIC O-RINGS wHich is still valid for military and aerospace. You can find a PDF version on

http://assist.daps.dla.mil/quicksearch/

. The photos in the pdf version are not in good shape (very contrast). However if you can get a hold of a printed version you can see clear photos with all types of defects. In addition I will recommend contacting Parker Hanniffin Co.

 

[profengmen]

One more note. There are symbols that denote the machining marks for the surface finish. For example; an ORing may accept a lower than specified surface finish if the lay marks are parallel with the sealed surface, ie A round gland should be turned and not milled.

 

[jetmaker]

israelkk,

Think you misunderstood the post. The issue is about the surface finish of the o-ring seating/sliding surface, not the actual o-ring itself. Thank you for the link tho, as the information was still informative.

Regards,

jetmaker

 

[israelkk]

jetmaker

See your point I was mistaken. MIL-G-5514G GLAND DESIGN; PACKINGS, HYDRAULIC, GENERAL REQUIREMENTS FOR covers the issue of the gland design. Regarding surface roughness MIL-G-5514G refere\s to as defined in ANSI/ASME B46.1-85

 

[profengmen]

Using the Ra, roughness average, specifies (ISO 468) that the average is over five sampeling lengths.

Read the Ra definition in the link Corypad sent you. Pits as much as 4 times the 16Ra finish are Not in spec and the parts are defective. The surface finish is NOT the average of the entire surface, it is measured off the sampling lengths.

From the link on Ra:
Graphically, the average roughness is the area between the roughness profile and its center line divided by the evaluation length (normally five sample lengths with each sample length equal to one cutoff):

 

[Cockroach]

Man, that is a fine finish. You must surface grind that puppy or something.

I use 63 Ra with all oilfield o-rings and never back them up unless over 2500 psi. My peak pressures have been 15,000 psi on some wellhead designs, o-rings with backups work fine on 63 Ra finish surfaces. On ocassion I may call a 32 Ra finish, but this is rare. I've never had a problem with high pressure, compressed hydrocarbon gases. Try to get a hold of Parker-Hannifin Corporation, Elastomer Design Handbook, ORD5700. You can order off the website. This has pretty much everything I ever wanted to know for the past dozen years or so!

In all honesty, I've never specified an o-ring for vacum service, but I would imagine that instead of the internal pressure trying to leak out, now you have external pressure trying to leak in. I know your vacum pressures can't be that high, that would entail heavy pumps, that sort of thing. I think 16 Ra on an o-ring is overkill, I could be wrong for vacum service.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[israelkk]

Cockroach

You are correct, I have developed and designed a solenoid valve for vacuum use for an automatic cow milking system measurment and we used nothing better than Ra 63/32 which is easy to achieve with turning machining.

 

[Tmoose]

Whether grinding or turning it may be important to plunge with no traverse to avoid making a thread. I think the Parker book mentions plunging is a requirement. Sometimes Hi pressure is easier to seal than low pressure. 500 psi stuffs the O-ring hard enough to seal against most any surface. 10 or 20 psi with air or thin liquids needs a continuous surface.

We were asked to reverse engineer and manufacture some
compicated hydraulic cylinders (part of a machine tool clamp mechanism). We shipped several that held hi pressure all afternoon. The customer was (justifiably) wild because after a few hundred cycles the oil was oozing out the spindle nose. This was partly Because the design could not manage its leakage properly, but mostly because a stationary O-ring groove was generated creatively, but poorly, by plunging with a wear-and-breakage-prone cut-off tool. Sometimes with 2 separate plunges, which often made a step, and sometimes with a final clean-up traverse. All The returned spindles were found to have a marginal-to-crappy groove bottom finish. The best theory was it weeped a few drops as the pressure rose slowly. The parts were reclaimed by grinding slightly undersize, and charging the cylinder with thicker oil. If that had not worked I would have smoothed the groove bottom with epoxy.

 

[Cockroach]

Without question TMoose, gland design and machining are critical when it comes to soft seated seals. But basic machining practice(s) can be applied to guard against "making a thread", "plunge with no transverse", etc. In general, the piece is imparted a high surface finish in turning by increasing RPM and slowing down feed. I've seen pieces that show your reflection, thus the seal was truly superior in an o-ring application.

As mentioned earlier, I've used o-rings in excessive pressure applications, 5 ksi, 10 ksi, 15 ksi, without a problem. You need to be diligent about o-ring stretch and squeeze in addition to gland design. This is why the Parker-Hannifin Handbook is so valuable, it discusses in detail the engineering practice while providing suggested gland geometry for a particular selection of o-ring. It is possible to bastardize a gland by interpolating between set values, thus maintaining o-ring stretch and squeeze. You still need to select these proper values based upon static, dynamic, reciprocating, face seal, etc, stretch and squeeze vary tremendously depending on application. Material selection is also another important consideration, fluids interact with the base rubber matrix and introduce secondary effects such as explosive decompression, hydrocarbon mitigration, o-ring degragation, etc. There is no magic material that is a silver bullet against ALL fluid applications.

So while your situation of gland manufacturing is an important one, it is often not the only issue surrounding soft seat sealing. It is generally known that o-rings can seal against liberal tolerances, but stepped gland ID, coarse surface finish and poor design are killers!

At any rate, you've brought up some nice points!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[handleman]

This thread has been inactive for a while, but I'll go ahead and put in my two cents. You mentioned that you are trying to seal for vacuum. If you are talking about low vacuum application, I agree with Mr. Hueston that the finish you're calling for seems a bit extreme. If you are talking about high vacuum, however, there are usually other factors involved. Every seal leaks. The question is how much does it leak, and how much is tolerated by the intended application. A leak size that is undetectable in a hydraulic system can leak helium into a mass spectrometer leak detector enough to ruin the background level.

Josh

 

[mloew]

Here is a link to the Parker O-Ring Handbook Catalog ORD 5700A.

Best regards,

Matthew Ian Loew


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Philrock]

O-rings are pressure-energized sealing devices. To a large extent, pressure tends to seat the seal material into minor depressions in the surfaces of the gland, enhancing the sealing capability. In vacuum service there is little pressure available to do this, so a finer surface finish is required.

While it is often desirable to state requirements in terms of standards, there is nothing that says you can’t use sketches and the king’s English to communicate whatever requirements you want.

 

[BillPSU]

Helium not only leaks but can actually migrate through the rubber. You will never get a complete seal using rubber against helium. I agree with Philrock pressure increases sealing. The more pressure the less problem of leaking.