Type of O-rings to be used on 1st Stage

[HenrikBP]

I'll go ahead an order a batch (100) of the PU 2-010, duro 90 and repack in quantities of 5 or 10 for whoever wants them.

Please put me down for a pack of those, if they're not all spoken for.

Thanks
Henrik

 

[redacted]

I would caution against puttng too much faith in the very abbreviated tables found in the O-ring catalogs. It's fun to play engineer, but there are factors, and combinations of factors, that effect performance but that they cannot cover.

For example, one parameter that it hard to quantify (I'm not even sure what the proper name for it is!) and often gets overlooked or lumped into the general category of "mechanical properties" which is how readily and smoothly an 0-ring can overcome stiction and move. This, in key dynamic O-rings like the piston O-rings on a piston 1st will be one of the key variables in how the stage will perform. That's why my general rule is not to try to outguess the factory engineers unless there is a very good reason to (O-rings failing or bursting into flames, for example), but rather use what they do whenever possible.

Also don't assume that just because Parker says a Parker Superthane O-ring has a particular attribute that Joe's Inet Bargain O-rings superthane ones will also have the same attribute to the same degree. Quality can very incredibly these days, with O-rings being made all over the world.

I agree that the answer to which o-ring is not so simple you can just take 5 minutes to select it off a chart. But I don't have absolute faith in the mfgrs engineers either. I know Scubapro seems to be constantly revising its o-ring strategies and has introduced a problem or two along the way. My schematics show Scubapro using at least 3 different o-rings (136, 137, 394) in that HP piston application. I suspect the 137 shown 12/1997 schematics was a PU o-ring. And I suspect the 394 which is used in their Nitrox 1sts is Viton (designated VT in one schematic).

When I had a problem with inflated IP at high tank pressures in a couple of my Mk10s, I did a piston and o-ring shuffle and found the problem was minimized with a piston with a smooth finish on the shaft and using a PU o-ring. Surprisingly, none of my Mk5/7s are showing the same problem.

I suspect, at this point, that Scubapro is not really paying much attention to the Mk5/7/10s that are still in service and they are basing their HP piston o-ring selection on the Mk25. My newer Mk10 schematics show the 136 (same as Mk25) o-ring being used in place of the 137. And using an O-ring material other than PU, as long as performance is satisfactory, does lower production and service costs.

So far, I have been quite satisfied with my unknown mfgr o-rings as they appear to have helped cure the problem I was experiencing and are holding up nicely even in HP tank (3442) usage. Actually, I'd be happy to pay the $1.50 or more each for a Parker o-ring in smaller quantities, but I have not been able to find a source yet. And Joe's seems to be doing the job.

 

[pescador775]

I doubt that manufacturers know much more about the subject than the present chat group. Otherwise, they would not be constantly casting about for alternate solutions. Of course, some of this is influenced by cost and the legal dept. I mean, if Viton could, under some unusual circumstance, produce toxic fumes then that is out. Same goes for plastic compounds used in valve seats, etc.

Viton seems to be performing better than the mechanical tables would imply and I was pondering how to account for that. I suggest that the tables are correct but do not deal with some variables. The most obvious is lubrication and the other is hardness. I suspect that the increasing use of synthetic greases like Christolube can partially account for the better than expected tolerance to abrasion. Highly polished moving parts and harder compounds could accord the rest.

 

[Couv]

Hello All,

Let me start off with a disclaimer: I am not an engineer. However, I have a background in aviation and aerospace maintenance. Therefore, Pesky, LuisH or another onboard engineer may be better able to shed light on this subject.

One of the problems associated with cylinder/piston and seal interfaces is inadequate lubrication. Oddly enough, too high a gloss on a piston or cylinder's surface is the culprit as such a finish (superfinish) can not retain lubricant very well. This is why in machining a cylinder a crosshatch pattern is intentionally honed in as part of the manufacturing or rebuilding process.

Why is this related to o-ring selection? Well, think about this. If you a have a problem with a first stage piston moaning, groaning, chattering etc. you may be correct that the o-ring is not properly lubricated. You take out the piston and discrepant o-ring (stem or piston head) put in a new, well lubed o-ring and viola! You fixed it….maybe, maybe not. A few week, months, dives later….same problem so you put in a more expensive, harder, better, super o-ring and viola! You fixed it for sure this time, right? Maybe, maybe not. The next time your o-ring needs attention, you put in an even more expensive, harder o-ring with $20 worth of Crisco and just to make sure-a new piston. By God, you fixed that rascal this time…. Probably, but perhaps one small step the first time you removed that piston could have saved you a lot of time and effort. Now I'm not going to advise you to go out and buy a machine to finish your piston stem and cylinder bore with an intricate cross-hatch pattern, but you should inspect the sealing surfaces for any anomalies such as pits, scratches, corrosion etc. Complete the job by breaking the glaze on any surface that sees lubrication and stop scaring away the fish with that piston chatter. 400 grit emery cloth should do the trick…don't overdo it just break the glaze then thoroughly clean away any particles and lube the piston stem and o-ring like you did the first three times.

couv

 

[pescador775]

Yes, back in the 1950's, attempts to rechrome Mercedes cylinders resulted in failure of the cylinder due to oil wiping. Nobody knew that Daimler originally used a special, porous chrome to line the cylinders. I don't know how that applies to a sticky substance like PFPE grease but I have given some idle thought to the subject after noticing shininess of the SP piston stem.
Pesky

 

[halocline]

I don't think I'd use 400 emory cloth. I'd start with 1500 micromesh which has a very high quality grit and is designed to leave a more uniform scratch pattern. The 1500 is the coarsest micromesh grit that I know of. I have used this on 2 different MK20 piston edges to stop IP creep, and it worked great both times. I followed with finer grits of micromesh.

Couv, I imagine you're familiar with micromesh because I heard it was designed to polish aircraft windows.

 

[Couv]

Hello Matt,

Yes, I am familiar with micro mesh kits. I have used them to polish Lexan canopies etc. However, the point here is NOT to polish, but to break the polished surface. If you could do it perfectly you would have what is known as a crosshatched pattern of tiny "scratches" if you will. The pattern is designed to hold a very thin amount of lubricant in the valleys of the crosshatched pattern as opposed to a very smooth surface. Think about windshield wipers on a brand new windshield. The wiper blades remove the water very efficiently. After a decade or so of driving and sandblasting your windshield, even the best blades will still leave tiny droplets of water on the windscreen. While smooth and polished works well for a windscreen, it does not work well for metal surfaces that require lubrication.

400 grit may be a bit coarse, (will have to get back to you on that) but at any rate the process should only require one level of adrading.

Finishing a knife edge on a piston is a different story, start coarse and finish fine.

Couv

 

[halocline]

Sure, I understand that the point here is not to polish. But the micromesh has a very uniform grit size and shape and leaves a nice uniform scratch pattern. I think the 1500 grit would be perfect for it; in fact I'm going to try it with a spare MK10 piston I have. I'd be afraid that the 400 emory cloth would leave some big scratches, maybe with sharp edges. You want to just break any glaze, right?

I think it would be best to do this with the bullet installed to protect the knife edge.

 

[redacted]

I think it would be best to do this with the bullet installed to protect the knife edge.

That's why god made Duct tape.

 

[Couv]

Sure, I understand that the point here is not to polish. But the micromesh has a very uniform grit size and shape and leaves a nice uniform scratch pattern. I think the 1500 grit would be perfect for it; in fact I'm going to try it with a spare MK10 piston I have. I'd be afraid that the 400 emory cloth would leave some big scratches, maybe with sharp edges. You want to just break any glaze, right?

I think it would be best to do this with the bullet installed to protect the knife edge.

Right, just break the glaze. Remember, you do not have to work the entire piston stem, just the section that will come into contact with the stem o-ring. It is a good idea to have the bullet installed ANY time you have the piston removed from the housing. As Awap said, duct tape or one of my favorites is a plastic straw if no bullet is available. This is good excuse to go to Whataburger and get a malt...yes, malt not a shake, tell the counter person you want extra malt added...get a few extra plastic straws and pretend you are getting away with something. Whilst you drink your malt and while you have the piston out, don't neglect the main body surface that is in contact with the piston head o-ring, but take extra care as this is brass not stainless.

Make double sure everything is clean before putting it back together...you do not want any grit or metal shavings cutting your expensive PU o-rings.

c