Piston seal Material Help?

[QVTom]

Ed,  Have you ever seen any signs of rolling or abrasion issues with the nitrile?  Dynamic o-ring applications seem to be limited to relatively low sfpm applications when looking through manufactures data.  Maybe an irrational fear on my part, I know scotchmo uses them successfully as well.

Tom

[nced]

Love it Ed, thanks for the detailed information......

Now all I have to do is buy a lathe...

LOL.....if you're a machinist then you dispense with another requirement I had to learn (and still learning) with my "seat of the pants" machining skills.............the requirement to convert perfectly good stock into scrap before you get the first good part! 

[OleTomCat]

Love it Ed, thanks for the detailed information......

Now all I have to do is buy a lathe...

LOL.....if you're a machinist then you dispense with another requirement I had to learn (and still learning) with my "seat of the pants" machining skills.............the requirement to convert perfectly good stock into scrap before you get the first good part!

I am not a machinist but I do know how to run manual lathes and mills....

I too have turned plenty of good stock into scrap....

[nced]

Ed,  Have you ever seen any signs of rolling or abrasion issues with the nitrile?  Dynamic o-ring applications seem to be limited to relatively low sfpm applications when looking through manufactures data.  Maybe an irrational fear on my part, I know scotchmo uses them successfully as well.

Tom

I've had no issues with Viton and have had the orings last longer than the mainspring, however the Nitrile rings are a "new study" for my application.......time will tell. I used to use only molly paste for my oring piston seal lubing however I've switched to a non-dieseling lube that's recommended for orings and the stuff is slick. Perhaps this lube (Krytox GPL205) will mitigate any rolling or abrasion issues IF they are present. I do have to say that I size the width of my oring grooves narrower than suggested in "the charts" to reduce the chance of the oring rolling. Still, at "only" 3000psi pressure for less than 1/4" of the piston stroke I really don't consider the springer receiver as being a very harsh environment. It's not like an air or hydraulic cylinder that gets hit with an instant 3000 psi and maintains this pressure while sliding a long distance. Actually, the relatively low duty cycle of a springer compression area is the reason I believe the oring lasts at all considering the less than desirable receiver geometry and surface finish.     

[CharlieDaTuna]

Great write-up Ed and very thorough.

CDT

[benj397]

You said you were going to do a "grease strip" on one of your rifles.  Do you use krytox on all parts including the trigger area?  This area is obviously all metal and it appears the GPL205 is mainly for o-ring type material, although I did read that it has a low surface tension and wets metal parts easily.

[nced]

what seems to be the general lifespan of the orings?? it seems you change yours out pretty much regularly but what about the people that cannot pull their guns apart every year to change orings out???

Properly fitted and lubed 75 durometer 020 size orings in a good receiver tube usually lasts longer for me than a good aftermarket spring. My brother doesn't break down his .177 R9 unless something breaks (like the spring) and he used the same oring for 4 seasons till the aftermarket spring broke which was about 40,000 shots. Generally, I replace the oring whenever the gun is broken down for WHATEVER reason since they're easy and cheap to replace in a R9.

[nced]

You said you were going to do a "grease strip" on one of your rifles.  Do you use krytox on all parts including the trigger area?  This area is obviously all metal and it appears the GPL205 is mainly for o-ring type material, although I did read that it has a low surface tension and wets metal parts easily.

Actually, Krytox is a "space age" lubricant for many applications that's also compatible with "rubber" orings.
http://www2.dupont.com/Lubricants/en_US/applications/Applications.html
I've been using the Krytox GPL 205 grease successfully in my R9, however I'm wondering if perhaps a 100 series Krytox OIL would be a better lube for my piston seal. Don't know, haven't tried the oil yet...........but I am wondering.

[TimmyMac1]

I'm going to put you on the spot

If you had your choice of any material, cast or extruded machinable, thermoset or injection moldable; cost is not an issue.  What are your thoughts starting with the most suitable?  And why?

Tom

Oh man.  I don't know...  I'd probably start looking at composites.  Carbon-filled PTFE has over 2X the Young's modulus and better wear and compressive characteristics than virgin PTFE, yet retains it's conformal abilities and lubricity (which PTFE is very good in that regard), so I'd probably start there.  No, actually, I'd probably dig into the literature and look at everything composite material that has the deformation, compressive strength, modulus, etc. that's between PTFE and PEEK and go for it, but a carbon filled PTFE would be on the list for sure; probably in the 15 to 25% fill range.  Problem is, you've seen good results with both PTFE and PEEK, so....  The shape of the bearing surface of the seal would probably be important as well to balance ability to seal while minimizing wear, but the details of that are above my pay grade.  

Seriously, though, composites will often offer the best properties of both worlds without sacrificing too much - they are just usually more expensive and can sometimes be more difficult to work with.  That said, price and how you get to the end result in the part isn't a criterion, so...

I know the answer!
I don't think a solid Piston seal made of 95A hard material in Urethane is gonna cut it for very long. It doesn't tolerate heat well and A scale 95 is not all that hard or tough in material you are going to turn. Like Ed said Urethane works great but can't take heat and will fail. The harder it is the longer it lasts and that's what has worked in the famous Venom Power Kits. Harder than A scale but not Peek hard. They use 90A urethane o-rings in the last stage of the high pressure hand pumps because it is so tough but heat does kill it eventually I've found that it helps to use a aluminum head on Pump pistons. Teflon(PTFE) is tolerant of the heat but has little memory so you need to mash it like in a P1 or parachute it via Apex seal design to get it to seal good. I love PTFE as an Intake seal. There is nothing better. As a Piston seal it is tricky but it has been done well. I like the Idea of the Carbon filled PTFE. I need to try that. The P1 uses a solid chunk of PTFE floating on piston a bit and it gets constantly mashed to have it be pushed outboard to maintain a seal. An occasional dry fire restores speed lost to piston seal leakage. That may be the Carbon filled Stuff as it is not the same consistency as the Check valves we make from Virgin PTFE
The best spring gun ever was an oring piston. The Venom Mach1 used a Durometer oring made of Carboxylated Nitrile. This material is so slick. It was a classified item for NASA use only till 1987. I was the first to purchase the material after its declassification. The commercial market learned about the material in Aviation week and space technology magazine. My roommate at the time was in A&P school so the AWST mag was our regular reading room stuff.
I've always been a techniques and technology geek so when I read about the material I was on the phone that day to the Parker Seal Rep asking for a Demo. He had to read up on the material because I knew more than the salesman did. We bought a few hundred pieces and used it in Some Magnum RI/HW80 Tunes/Builds and the Venom Mach 1. This stuff makes PTFE seem not very slick. It started moving on cardboard at 1/2 the inclination of virgin PTFE. I was stunned. You rub this stuff and it feels like you have moly on your hand. CARBOXYLATED NITRILE. I do not own any Parker Stock. It is very tough stuff, slick as snot, can tolerate heat and is very abrasion resistant. It worked flawlessly.

TimmyMac1

[nced]

Tim.......a few years ago I tried to buy some XNBR (the Carboxylated stuff) orings and found only one supplier, however I was told over the phone that XNBR orings were "special custom" compounds and the minimum order was about $250. I just did another internet search and it SEEMS that it's still a custom compound.

I've been doing ok with military spec viton orings but I really liked the description of XNBR when I first heard about it. Matter of fact, I think it was one of your forum replies to one of my posts years ago that gave me the "heads up" of the stuff!

Do you know a source for size 019, 020 & 021 XNBR?

[TimmyMac1]

I will check with my supplier Monday and get back to this thread when I find out. I assume you want 90 Durometers in those sizes? I also assume you are adjusting the interference by stretching different sizes onto the same groove? If I were to get ONE size which size? 020? I think the thing to do is ask for a sample if any of those sizes are stocked in the durometer you want. That way you can put one into service and get a feel for how it works. I'll be expecting a need to reduce spring preload.
Ivan was using the 1/8" wall and your asking for 1/16th. The stuff is so slick it can drag a lot more surface and still be lower drag. I'd be a little concerned the oring would want to roll in the groove at 1/16" wall. Ivan kept the groove width at Nominal and let the interference from width and depth make the sealing surface proud by the interference in a ROUND groove. This gave much surface area in contact with the groove and eliminated the orings tendency to want to roll in the groove. I liken it to a slick tire on a smooth track giving great traction via surface friction
Have you had any problems with that happening? Maybe a 3/32" would be a good compromise. You have more experience with the 1/16" wall than anyone I'd imagine. I think the 1/16" will be a little cheaper tho than the other two. I'll find out from the seal supplier. Generally we are facing a 100 piece minimum and special order status for high durometer special runs. We see savings in the 20% range for 200 QTY and a bit more at 500. They tell us it will take 4-6 weeks because that is the time frame if the molding department is backed up. They have not been backed up for going on two years now. In most orders I have orings in hand within 10 days even on specials cause the business is way down and they are treating old school customers like me quite well lately. With any luck the part will be tooled and on stock rather than a special. I can't imagine there isn't a lot of products that have got used to using this stuff. The application that comes to mind is Rams for car hoods and trunks. Low stiction means they run smooth and last.
Mil Spec Viton is great stuff. It is a good choice in off the shelf stuff. Probably the best option to avoid specials. Fortunately we don't need to tool o-rings unless we have a funky size. We just need to wait in line till the tooling is available to use.
XNBR is also referred to now as CNBR. RT Dygert is a source.  Apparently it is a 1978 patent.

http://www.archive.org/stream/USPTO_Patent_Full_Text_1978_April-June/19780411.txt

http://www.rtdygert.com/technical/design_oring.cfm is a cool standard o-ring groove size chart

TimmyMac1

[nced]

I will check with my supplier Monday and get back to this thread when I find out. I assume you want 90 Durometers in those sizes? I also assume you are adjusting the interference by stretching different sizes onto the same groove? If I were to get ONE size which size? 020? I think the thing to do is ask for a sample if any of those sizes are stocked in the durometer you want. That way you can put one into service and get a feel for how it works. I'll be expecting a need to reduce spring preload.
Ivan was using the 1/8" wall and your asking for 1/16th. The stuff is so slick it can drag a lot more surface and still be lower drag. I'd be a little concerned the oring would want to roll in the groove at 1/16" wall. Ivan kept the groove width at Nominal and let the interference from width and depth make the sealing surface proud by the interference in a ROUND groove. This gave much surface area in contact with the groove and eliminated the orings tendency to want to roll in the groove. I liken it to a slick tire on a smooth track giving great traction via surface friction
Have you had any problems with that happening? Maybe a 3/32" would be a good compromise. You have more experience with the 1/16" wall than anyone I'd imagine. I think the 1/16" will be a little cheaper tho than the other two. I'll find out from the seal supplier. Generally we are facing a 100 piece minimum and special order status for high durometer special runs. We see savings in the 20% range for 200 QTY and a bit more at 500. They tell us it will take 4-6 weeks because that is the time frame if the molding department is backed up. They have not been backed up for going on two years now. In most orders I have orings in hand within 10 days even on specials cause the business is way down and they are treating old school customers like me quite well lately. With any luck the part will be tooled and on stock rather than a special. I can't imagine there isn't a lot of products that have got used to using this stuff. The application that comes to mind is Rams for car hoods and trunks. Low stiction means they run smooth and last.
Mil Spec Viton is great stuff. It is a good choice in off the shelf stuff. Probably the best option to avoid specials. Fortunately we don't need to tool o-rings unless we have a funky size. We just need to wait in line till the tooling is available to use.
XNBR is also referred to now as CNBR. RT Dygert is a source.  Apparently it is a 1978 patent.

http://www.archive.org/stream/USPTO_Patent_Full_Text_1978_April-June/19780411.txt

http://www.rtdygert.com/technical/design_oring.cfm is a cool standard o-ring groove size chart

TimmyMac1

Thanks Tim for the time you took giving me such a detailed reply......It's appreciated!

A few comments..........
I assume you want 90 Durometers in those sizes?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When running Viton and Nitrile in a R9 receiver tube I found that 75-80 durometer conformed better to the "less than true" tube ID so I've been using 75 durometer orings.

I also assume you are adjusting the interference by stretching different sizes onto the same groove?
No, I've been using 019s for TX200 & R7 orings, the 020s are for my R9 and HW77 orings, and the 021s are for the occasional R1s I get in for modding.

If I were to get ONE size which size? 020?
Yep.......020 be the one!


I think the thing to do is ask for a sample if any of those sizes are stocked in the durometer you want. That way you can put one into service and get a feel for how it works.
I agree, that seems to be the best option.

I'll be expecting a need to reduce spring preload.
Yep....even with the Viton orings I have to go with minimal preload and I've taken advantage of this "feature" by using 34 coil .120 wire springs and still getting my preferred velocity. Real nice setup!

Ivan was using the 1/8" wall and your asking for 1/16th. The stuff is so slick it can drag a lot more surface and still be lower drag. I'd be a little concerned the oring would want to roll in the groove at 1/16" wall. Ivan kept the groove width at Nominal and let the interference from width and depth make the sealing surface proud by the interference in a ROUND groove. This gave much surface area in contact with the groove and eliminated the orings tendency to want to roll in the groove. I liken it to a slick tire on a smooth track giving great traction via surface friction
Hasn't been an issue with Viton as far as I know. My understanding is that the oring deforms into the "square corner" of the "low pressure side"of the oring groove so it doesn't roll (I don't think). I also keep my oring groove width at .070 which is nominal for a 020 oring, however I don't maintain the recommended compression of 15-25% for a dynamic oring. I'm currently running about 10% compression and it's been working fine. The main reason I even tried oring sealing was an attempt to minimize that amount of "rubber" and lube rubbing the ID of the receiver. At an early spring field target match at DAFTC near Harrisonburg, VA (I think you shot with my brother at one match there) we were zeroing our guns during snow flurries, however the temps rose to the mid 50s during the match and both my brother and I has major poi shifts during the match. At that time our guns were lubed with molly paste and spring tar so I thought that perhaps "grease thicking/thinning" with the temp shift was the issue. My R9 was stripped of all tar and relubed with only minimal molly paste and this did help a bit, but then I thought that perhaps the factory parachute seal was also changing durometer with the temp shift. Long story short.......I cut an aluminum oring sealed piston cap, used an Ace home center water oring, found that this arrangement really helped reduce my temp induced poi shifts! 

Maybe a 3/32" would be a good compromise.
Years ago I tried 3/32" orings in an oring sealed piston cap and when the dust settled the result was too "factory like". My thinking was that the extra oring thickness would make the seal more tolerant of receiver tube variations. 3/32" did WORK, but I preferred the results I got with the thinner ring. Another issue with an oring sealed piston cap on a R9 or HW77 piston is that the cap has to be thicker so the oring gland doesn't "hit" the c-bore for the piston seal retaining button. This is especially an issue with 1/8" orings.

I can't imagine there isn't a lot of products that have got used to using this stuff.
When searching out this stuff I noticed that it's also used for orings in motocycle chains, but not the sizes I need! LOL

Tim....lately I've been running my .177 R9 completely lubed with Dupont Krytox GPL205 grease and the combination of Viton and Krytox works great since the Krytox stuff doesn't diesel!

Anywhoo........your interest is GREATLY appreciated and if you could score a few 75 durometer 020 XNBR orings I'd appreciate it! There is no way that I can justify a 100 count lot of expensive (when I checked a while back the minimum order was $250) orings for experimentation.

Thanks for the Dygert link.......I've bookmarked the page!

[willbird]

Really "deep" subject :-).

But reading over it the question that comes to mind is how much effect the spacing of the ring back from the piston face has. That distance to some degree shields the o-ring itself from heat ??

Bill

[nced]

Really "deep" subject :-).

But reading over it the question that comes to mind is how much effect the spacing of the ring back from the piston face has. That distance to some degree shields the o-ring itself from heat ??

Bill

That's correct.......however back when I was cutting flat faced HW piston seals from urethane I noticed that the center of piston seal would be toasted to "hard cinder" after about six months of rather constant use in my R9, however the sealing edges were still pliable. At that time I bought a flat faced aftermarket seal and had the exact same performance as my home cut seals. Due to this I believe that the temperatures along the sealing edges aren't a severe as they are at the transfer port so my main intent was to get the "rubber" away from the heat face of the seal.

Anywhoo....obviously the further away the oring is from the face of the piston cap the more the "swept volume" is reduced, however, for practical purposes I find this "swept volume loss" to not matter with my components. LOL.....I already have to "work" to get my R9 velocity DOWN to my desired velocity.

[willbird]

Many years ago I was introduced to Lottie black max, which is a toughened cyanoacrylate glue. I have used it for a lot of things including gluing split sleeves to rifle bolts, then machining them for a closer fit.

Anyway it is a toughened super glue..........if you have a chance to get some give it a shot where you now use  "super glue".

We used it to glue hdpe sleeves inside train wheels that go under an 80 ton crane modified to run on steel railroad rails.....I figured if it worked for that maybe I could find other uses :-). I have glued percussion caps onto the noses of 22 lr bullets and they stayed on :-)

Bill

[TimmyMac1]

Ed,
Finally checked with my current O-ring supplier and much to my surprise I knew a lot more than he did. He told me they don't make o-rings in that material so I had to tell him I had already bought several hundred from my previous Oring supplier. It may be that nobody does them anymore but I have to admit I didn't work that hard looking since I have plenty to do.

TimmyMac1

[nced]

Ed,
Finally checked with my current O-ring supplier and much to my surprise I knew a lot more than he did. He told me they don't make o-rings in that material so I had to tell him I had already bought several hundred from my previous Oring supplier. It may be that nobody does them anymore but I have to admit I didn't work that hard looking since I have plenty to do.

TimmyMac1

No problem Tim and thanks for taking time from your schedule to even check it out! When I spent quite a bit of time looking for XNBR orings I came up with the same result, however I did find one supplier that would do a "special-special" order if the minimum was for a bit over $200. Too rich for my tinkering so I settled on "plain ole military spec Viton", however I'm currently taking some of your advice by using 90duro oring instead of the 75duro molly paste lubed stuff I've used for years, however it is lubed with Ultimox 226 instead of the molly paste I used for years .

I took first place in hft piston class at the recent Maryland State Championship match with the Ultimox 226 90 duro mil spec Viton oring seal. 1st place wasn't a big deal with a field of only 2 HFT PISTON shooters, but I do take some satisfaction in that  I was able to shoot a higher score than 7 other shooters  in hunter PCP and the WFTF classes, and 4 of those shooters were using PCPs. 

The 90 duro stuff lubed with Ultimox (Krytox) is working well and 90 duro is working better than when I tested it several years ago! I have increased the oring squeeze a few thou over what I used "years ago". I've learned that the dimensional tolerances of even military spec orings can be a little loose and since I don't use the recommended 15%-25% oring compression for a dynamic seal I recently got "bit". A while back I did an oring seal for a fellow with a TX200 which it passed my "pull a vacuum for 30 seconds test" and I sent it along with a different oring from the SAME bag. Later I got an email stating that the assembly wasn't sealing properly so I had the piston, compression tube and piston cap returned for inspection. I tested the returned parts and indeed the thing wouldn't pass the vacuum test with the mil spec oring I supplied, however the seal worked perfectly with another oring from the SAME bag. LOL.....I simply removed the piston cap and cut a new cap with a couple thou more "squeeze" and the offending oring even worked well. Oh well.....lessons learned! 

Anywhoo.......

[nced]

Many years ago I was introduced to Lottie black max, which is a toughened cyanoacrylate glue. I have used it for a lot of things including gluing split sleeves to rifle bolts, then machining them for a closer fit.

Anyway it is a toughened super glue..........if you have a chance to get some give it a shot where you now use  "super glue".

We used it to glue hdpe sleeves inside train wheels that go under an 80 ton crane modified to run on steel railroad rails.....I figured if it worked for that maybe I could find other uses :-). I have glued percussion caps onto the noses of 22 lr bullets and they stayed on :-)

Bill

I didn't use "home center SuperGlue" but Permabond 105 which costs over $19 per ounce. Anywhoo....I used to "stake" the aluminum piston cap blank to the piston for machining by drilling a shallow recess in the bottom of the HW piston seal retaining groove. The purpose of this recess was so one of the nylon retaining screws would "key into" the recess when tightened and this "key" would minimize the possibility of the aluminum blank spinning on the piston dut to pressure from the lathe tool. A while back I nixed the "recess: in favor of a couple drops of super glue on the piston face to retain the aluminum blank while machining.

[willbird]

Have you ever tried two O-rings, each in it's own groove ?? Maybe the second one would not work properly at all without getting hit with full pressure ??

I wonder if you could make a simple test rig for the O-rings ?? Simpler assembly than a gun ??

Bill

[nced]

Have you ever tried two O-rings, each in it's own groove ?? Maybe the second one would not work properly at all without getting hit with full pressure ??

I wonder if you could make a simple test rig for the O-rings ?? Simpler assembly than a gun ??

Bill

I've never seen the NEED for double orings in the piston cap for my HW springers, however I do know that some of the Theoben "rammers" used double orings.
The mechanical issue for my HW guns is that making the piston cap thick enough to accommodate two orings reduces the "swept volume" which in of itself probably isn't a big issue with an oring seal. There is a fellow on a UK forum that DELIBERATELY installs thick top hats with orings to better balance out the shot cycle for use at 12fpe. A while back I installed a thick top hat just for grinns to see how it worked in my .177 R9 and it worked just fine. The issue I had is that I didn't like the "double tap" cocking stroke feel.

For me a more practical issue with a double oring is that now the amount of "rubber against the receiver" is doubled which I believe makes the poi more prone to shifting when shooting in a cold "snow flurry" spring morning which warms into the mid 50s by the afternoon. My THEORY is that the lubes change viscosity and "rubber" changes durometer with 30F temperature shifts, and this in effect changes the sliding friction of the piston when shooting. My goal for trying an oring seal well over a decade ago was to stabilize "piston seal sliding friction"......hense a single oring.......and I do believe that the oring seal does help minimize temperature induced poi shifts with a springer. The fact that the velocity of my .177 R9 increased 50+ fps simply by changing from a factory seal to an oring seal does indeed indicate that there is less "piston sliding friction"! LOL.....I even took advantage of this velocity increase by substituting a lighter wire, easier cocking, lightly preloaded spring while still maintaining a good velocity.  I used to always tune my oring sealed .177 R9s to shoot CPLs at 900-910fps, however for the past couple seasons I've been tuning to shoot CPLs at 850-860fps which is about as low as I can go without changing my spring for an even lighter spring.

As a side note....years ago I tuned one of my oring sealed R9s to shoot CPLs at 960fps and did some 30 yard grouping. Then I retuned the same gun to shoot CPLs at 910fps and shot some 30 yard groups. The result was that EVERY 910fps group was tighter than the 960fps group which indicated to me that the R9 weight springer became too hold sensitive for my shooting skills with the stiffer tune. Funny thing is that I've reduced my CPL velocity to around 850-860 fps and really like that power level!  Funny thing is the fact that when shooting at 50 yards using current 13fpe (CPL @ 860fps) tune, the CPL drops only about 3/8" than a 14fpe (CPL @ 900fps) tune and only about 5/8" more than a 15fpe (CPL @ 925).