Oring DURO and material? specifically in regulators (another mostly Taipan post)

[Franklink]

Bought a used Veteran recently that was having a bit wider fps spreads than I'm used to seeing in the platform. Decided that the first step would be to check out the regulator. The OEM orings that Taipan uses in their regulators are colorless and kinda opaque. Verified in two different guns cuz I cracked a different one open to check. The orings appear like they were maybe white at one point b/c they have a mottled inconsistent appearance around the circumference, whiteish in places and almost transparent in a different section of the oring. The orings in the reg with consistency issues also had some places harder and softer than others, so it was difficult to get a guess for how hard they were when new. 

A few weeks prior to receiving the aforementioned used Vet I had made a big oring order so I was set to reseal, etc. The order had been placed after only a cursory glance online at the virtues of various durometers and materials. I went with 70D Buna-N, partially from the half-baked research and partially because all the sizes I needed were available in that material and hardness. 

I replaced the orings in the troublesome Vet regulator and went ahead and did another long-owned Vet while I was that deep into it (after seeing the discoloration in that gun too). They both resealed just fine and are now shooting consistent shot strings. Neither have had any leaks in the couple weeks that it has been. 

Yesterday a friend asked me what I know about durometer and material for regulators. His RAW was acting up and he diagnosed it as a regulator issue. I admitted to him that I know very little in this regard. He did some research and found a service guide for the old Theobens to use 70D nitrile in those regulators. 

Common sense would say that an oring on a probe should probably be the harder 90d for abrasion resistance, etc. Same (perhaps) faulty logic makes me think that a 70D is more likely to seal in a potentially less than perfect oring groove? But, again, I have no idea about best duro for regulators or, more generally, the best oring material for all the seals in our guns. 

So, what's the consensus on DURO and oring material, in general, and also specifically in regulators?

 

[Camaro454]

For pressures under 3000 psi 70 durometer high nitrile Buna n is just fine. For higher pressures 90 durometer would be preferred. Viton material can also be used in either instance for the static application like a regulator or plug/gage. You cannot be positive with any color grade as there is no certified standard in use. For dynamic oring application (seal for sliding surfaces) like a probe, they have been using orings where as a square quad seal would have been better, but, since technically an oring groove is not the same as a square ring groove, use a Vinton 90 durometer or research other materials...but cost may be prohibitive. I have seen fuel line cut to replace probe orings...like a square seal and very inexpensive, it will work and can be changed cheaply. My 2 cents. I worked as a hydraulic engineer with some exposure to pneumatics.

 

[nervoustrig]

The choice of durometer carries really just one consideration: operating pressure. It has to be hard enough to resist extruding through whatever gap is present between the mating parts. For parts with typical tolerances, 70 durometer suffices for 3000psi systems and you need to step up to 90 durometer for 4500psi systems. However if tolerances are loose (i.e. there is a large extrusion gap) and the O-ring keeps blowing out, go ahead and step up to 90. Why not just go with 90 all the time? Well for the most part you can but if there are any sharp edges, it is more likely to get damaged on installation.

Regarding your point about a softer material conforming and sealing better to an irregular surface, that is true. However even at low-ish pressures equivalent to CO2 (~800psi), the odds of a 70 durometer sealing and a 90 durometer _not_ sealing are approaching zero. The cheapest of the cheap...the sub-$100 Crosmans and QB78s and such are not so roughly machined that a 90 durometer will struggle to seal.

In terms of material choice, there is no compelling reason to use anything but Buna-N for static seals in PCPs. And for dynamic seals--those subject to abrasion--Buna-N is still often the best choice.

In places where abrasion is especially problematic, like a breech or bolt O-ring that is constantly getting scrubbed and is subject to environmental contaminants like wind-blown dust or sand, polyurethane may be a better choice. However as wear resistant as it is, it isn't a magic wand. If your bolt O-ring is eaten up after one tin of pellets, polyurethane isn't going to significantly improve things. That's a sign the parts are too rough and need to be smoothed, or a there is a burr or other sharp edge that needs to be removed, or a misalignment that is putting undue stress on the O-ring. Elsewhere, polyurethane tends to be a false economy because its shelf life is the poorest of all the common O-ring elastomers. Another place it is useful is the end cap of a CO2 gun because most materials will swell in the presence of CO2 whereas polyurethane does not.

For regulators specifically, go with Buna-N. You will find it in the regulators of some of the finest airguns known to man.

 

[Franklink]

@camaro454 and @nervoustrig, I very much appreciate the information. 

 

[Motorhead]

Given the spec of SAE or Metric often try too make the smaller HP end of spool o-ring a 90 duro buna/n and larger LP side a 70 duro buna/n.

Some instances use Poly "cast" 70's in regs such as RAW and a few others.