Piston seal Material Help?

[Zed Sniper]

I want to turn some piston seals and was shopping for material and came up with 90A PUR and 95A PUR. Which would be a better choice and any other suggestion would really help? Also any tips on turning this stuff would be greatly appreciated! Thanks

[QVTom]

PUR?  Polyurethane?  Any how, I have had great success with virgin HTPE and PEEK.  I have made a couple from polyurethane 90A with limited success. the 95A may be a little easier to turn.  I'd suggest extremely sharp, positive/positive tooling with a nose radius no more that 1/64. I cut dry at about 200sfm.

Good luck and let us know how they come out.

Tom

[Zed Sniper]

PUR?  Polyurethane?  Any how, I have had great success with virgin HTPE and PEEK.  I have made a couple from polyurethane 90A with limited success. the 95A may be a little easier to turn.  I'd suggest extremely sharp, positive/positive tooling with a nose radius no more that 1/64. I cut dry at about 200sfm.

Good luck and let us know how they come out.

Tom

Is it HDPE or HTPE, you list "HTPE" and I draw a blank on Google but see HDPE everywhere? Thanks

[QVTom]

My bad, meant PTFE.

[QVTom]

A couple of samples.

[sperho]

I'm no expert when it comes to having personal experience with making piston seals, but I do know a few things about polymers.   According to what I have read, I believe most seals are made from higher density nylon or urethane.  I'm surprised PEEK worked very well for QVTom - it's very hard and I wouldn't think would conform well to imperfections.  PEEK also has a higher modulus compared to nylon and WAY higher than PTFE - also important for conforming to shapes.  PTFE, I would think, would start off ok, but would wear and deform relatively quickly compared to a good nylon.  It's very soft and can tear up easily on imperfections.  For good longevity with PTFE in moving seal applications, very smooth bearing surfaces are critical. 

[QVTom]

I'm no expert when it comes to having personal experience with making piston seals, but I do know a few things about polymers.   According to what I have read, I believe most seals are made from higher density nylon or urethane.  I'm surprised PEEK worked very well for QVTom - it's very hard and I wouldn't think would conform well to imperfections.  PEEK also has a higher modulus compared to nylon and WAY higher than PTFE - also important for conforming to shapes.  PTFE, I would think, would start off ok, but would wear and deform relatively quickly compared to a good nylon.  It's very soft and can tear up easily on imperfections.  For good longevity with PTFE in moving seal applications, very smooth bearing surfaces are critical. 

PTFE seems very fragile but I have had great results with it.  I have a PTFE seal in a Gamo that has been going strong for over 3000 cycles. I do hone my cylinders with 600 stones and only use a smidgen of moly on the piston skirt.

I've been testing the 2.79" seal in the photo, also PTFE for a while now.  This one runs in an AL tube coated with Tufram,  two Delrin AF skirt rings and no lube at all.  It's a test rig so I don't have a lot of data or cycles yet but seems to be holding up.

The Peek is about 10% stiffer (not very scientific) than the commercial seals (urethane I believe) I've examined.  If found it to be very sensitive to correct sizing but it performed excellent once I had the correct size.  I chose not to use it for the 2.79" seal because it is just too expensive in the larger diameters.

Nylon is interesting, good mechanical properties but maybe not so good in heat resistance department.  Maybe worth looking into though.

Tom

[sperho]

PTFE seems very fragile but I have had great results with it.  I have a PTFE seal in a Gamo that has been going strong for over 3000 cycles. I do hone my cylinders with 600 stones and only use a smidgen of moly on the piston skirt.

I've been testing the 2.79" seal in the photo, also PTFE for a while now.  This one runs in an AL tube coated with Tufram,  two Delrin AF skirt rings and no lube at all.  It's a test rig so I don't have a lot of data or cycles yet but seems to be holding up.

The Peek is about 10% stiffer (not very scientific) than the commercial seals (urethane I believe) I've examined.  If found it to be very sensitive to correct sizing but it performed excellent once I had the correct size.  I chose not to use it for the 2.79" seal because it is just too expensive in the larger diameters.

Tom

That's about what I would expect:  excellent preparation of the metal bearing surfaces for PTFE helping to make it last and PEEK being very sensitive to correct sizing.  In other words, if you do your part with not so forgiving materials, you can have winners on your hands...

[QVTom]

I'm no expert when it comes to having personal experience with making piston seals, but I do know a few things about polymers.   

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

[sperho]

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...

[Rescue912]

I've had good luck with 95D. The 90 seemed a little soft, slowed piston travel a bit, wore quicker and cut FPS.

[nced]

I want to turn some piston seals and was shopping for material and came up with 90A PUR and 95A PUR. Which would be a better choice and any other suggestion would really help? Also any tips on turning this stuff would be greatly appreciated! Thanks

Years ago I was into cutting my own polyurethane piston seals from different durometer rods (70-90) and was less than satisfied with the longevity. I had also bought some aftermarket piston seals and they suffered the same fate in my .177 R9. Here are a couple pics of factory R9 piston seals showing "transfer port erosion" of factory R9 seals..........



Back to polyurethane piston seals:
The issue for me was that the polyurethane would erode at the transfer port "high temp/high air velocity area". Initially the seals would work well, however my velocity would start dropping after about six months and a tear down would reveal that the urethane would be "hard and cratered" at the transfer port, yet the edges of the seal would be fine. The service temp of 90 durometer rod is less than 200F. I even did a "torch test" and found that the polyurethane would melt like wax and form a sticky "glue". LOL....one of the ways I would machine the polyurethane was to melt the surface of the polyurethane rod with a torch, then stick it to my steel holder for mounting in the lathe.

 I have no experience with PTFE seals except to note that the stuff will "cold form" and on some of the British sites I've read that they may fit a PTFE "puck" to the piston and then "fire form" the seal to the receiver ID by shooting the gun. Seems that this is satisfactory for 12fpe max guns but I don't know how this would work for higher power springers we have in the states.

Anywhoo........I ended up nixing parachute and flat faced seals all together (a couple decades ago) and replaced them with home turned oring sealed piston caps made of 6061 T6 aluminum. To my delight I found that not only did the piston caps solve the transfer port erosion issue by putting aluminum behind the transfer port, but the minimal amount of "rubber against the road" (I use 1/16" cross section orings) also increase velocity about 50fps over a factory seal for my .177 R9 & HW77. I take advantage of this velocity gain by using lighter wire aftermarket springs with little preload to maintain my R9 CPL velocity at 860-900fps depending on the tune. The oring seal also made the gun less temperature sensitive with a 30 degree temperature shift during a spring day since there was less "rubber" to change durometer and less lube to change viscosity. Another feature of orings is the fact that it only takes a couple shots to stabilize the poi instead of the couple dozen needed when I was running a factory R9 seal. Also, at a cost of a nickel each from McMaster Carr there is no problem replacing the oring yearly when I do my "winter dustin'-&-cleanin'". Anywhoo......I've found that the aftermarket VAC seals from Vortek are superior to a factory R9 piston seal and have used them, however I always return to the oring sealed piston cap due to the good poi stibility I get.

Some pics of various oring............




I realize that my reply doesn't directly address you question about "best seal material" but it does explain some issues I've had with seals cut from polyurethane decades ago (perhaps there are better compounds today) and how I solved my piston sealing problems.

[ezman604]

95 A urethane...
http://www.ebay.com/itm/1-1-2-DIA-x-11-URETHANE-POLYURETHANE-95-A-RED-/370488886022?pt=LH_DefaultDomain_0&hash=item5642dbc306

[OleTomCat]

nced, how do you secure the O-Ring cap to the piston?  Are there set screws under the little plastic buttons?

I like this idea and may venture into it a little later when I get my little lathe for the house......

[nced]

nced, how do you secure the O-Ring cap to the piston?  Are there set screws under the little plastic buttons?

I like this idea and may venture into it a little later when I get my little lathe for the house......

The piston caps are secured with (4) nylon screws through the cap skirt that engage the piston seal retaining groove. Here is a pic of my early piston cap experiments showing the tapped holes in one experimental cap and another cap installed on a R9 piston........

Since the TX200 piston has a piston seal retaining dovetail I first sharpen the point of the nylon screw to a conical point, then the screws wedge the cap in place when tightened. When I do my caps I first secure the piston cap "puck" to the piston, then machine the cap to size while the piston is chucked in the lathe.

I found that the R9 receiver tube geometry isn't normally very consistent so it did no good for me to try measuring the receiver ID, then cut the piston cap so size. What I do is to counterbore the piston cap for a tight slip fit to the piston "button", then drill and tap the cap flange, then put a bit of "super glue" on the face of the piston, then press the cap blank into place over the "button", then tightly screw the nylon screws till they bottom out on the bottom of the piston seal retaining groove, then center the piston in the lathe and machine the blank for about .002 clearance to the tightest restriction in the receiver, then I machine the oring groove for a proper fit to the receiver ID. Since the pistons aren't modified in any way the conversion is reversible by drilling out the nylon screws, then prying off the cap with a pair of vise grips or in a vise.

There are a few tricks to oring sealing and one of them is paying attention to the geometry of the receiver tube. If the end plug end of the receiver is considerably smaller than the transfer port end it can be impossible to get the proper oring squeeze at the "compression area" of the receiver if the oring cap is cut to clear the constricted end plug end. This has been an issue with a couple R9s I've converted so I made up a home made hone to enlarge the end plug area constriction  like this..............

Using oiled silicon carbide abrasive it's a pretty tedious process but it has worked well for a couple R9 "problem child" mods. One thing about oring sealing the R9 is that all the receiver dowel holes, knockouts, and slots need to have all sharp edges removed and the oring sealed piston needs to be carefully installed to prevent chipping the oring. In contrast, oring sealing the HW77 and TX200 are a "piece of cake" because the sliding compression tubes that the piston goes into are nicely machined (especially the TX200) without a bunch of slots and knockouts to be concerned about. Here's a pic of a TX200 oring sealed piston and compression tube........

 

[MichaelM]

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???

[Zed Sniper]

WOW Nced! I really like the o-ring conversion, most excellent. Thanks guys I hope you can keep them coming, I am learning a lot!

[Zed Sniper]

Outstanding work!
How do you lube them O-rings and have you tried different hardness's of o rings or thicker?
Also is there a slap noise when the aluminum reaches the chamber face?
What material did you use for your piston buttons?
Thanks

[nced]

Outstanding work!
How do you lube them O-rings and have you tried different hardness's of o rings or thicker?
Also is there a slap noise when the aluminum reaches the chamber face?
What material did you use for your piston buttons?
Thanks

Here is my a reply to a few questions I received..........

Hi, I have a few questions about the seal conversion you just posted hence I am turning parts tomorrow.
 Greetings from North Carolina!

"1. Is there piston slap at the end of the stroke?"
a. As long as the shot cycle is "balanced" there is no issue with "slap" since the piston is decelerated on the compressed air column just before the pellet "pops" through the leade. In the beginning I shot my .177 R9 a few thousand shots at the 15fpe power level and upon disassembly I found NO impact damage on the 6061 T6 aircraft aluminum cap. Here is a pic of a recent piston cap that was removed when testing a Vortek VAC seal..........

This piston cap has THOUSANDS of 13fpe shots on it and the only thing noticeable is a slight "combustion colorization" from the molly paste lube I used at the time. After the VAC seal testing I reverted back to the oring seal because I found that the poi was more consistent if there was an atmospheric temperature shift of 20F or more during the shooting session. This happened quite often during early spring and late fall field target matches. I do have to mention though that the oring sealed shot cycle is a BIT snappier than the shot cycle of a parachute sealed piston since the oring gives little "rubbing friction".     
 
2. After doing this mod , next time would you just JB Welded it to the piston and make it permanent?
a. No I wouldn't trust a hard "epoxy mix", plus I like the idea of being able to remove the cap if desired for some reason. Drilling and tapping a few #6-32 holes around the cap skirt has worked well for me. I've always used (4) opposing nylon screws to secure the cap because I've relied on "screw pressure" to hold the cap in place on the piston while machining using VERY sharp lathe tools with considerable rake. After having more then one cap turn on the piston with a overzealous lathe tool advance, I started drilling ONE shallow hole in the piston seal retaining groove so a nylon screw would "key to the piston" preventing cap rotation. Later I reverted to simply putting some cyanoacrylate (Super glue) on the face of the piston prior to pressing the cap in place. This secured the piston cap when machining, plus it didn't require that I drill the piston, the only requirement was that the piston button was bedded in the glue. While I still use (4) retaining screws out of habit, I really believe that two opposing screws are all that's necessary to keep the piston cap squared on the piston and from "popping off".
 
3. Have you tried different hardness's of o-rings?
a. Yep.....I've used everything from 65 durometer to 90 durometer and all worked fine, but I normally use 75-80 durometer orings. This chart shows that a 70 durometer oring is good for 3000psi with a .020 clearance which is (5) times the clearance I'use for fitting my orings.........
http://www.aceseal.com/gland-clearance.html
I've also used military spec Viton orings for years with good results. Recently I'm testing plain ole cheap Nitrile (Buna N) orings that cost about 5 cents each when bought in bags of 100 (LOL....a several lifetime supply). While they don't have the temperature resistance of Viton, they also don't take as much of a "set" as the Viton. I'm still testing out the Nitrile rings in my .177 R9 however I don't know if they'll "burn" with compression temperature. Years ago I also tried expensive polyurethane orings (about $3 each) and found that while they were really tough and wear resistant, for my application they didn't seem to work as well as the Viton oring costing about 10 cents.
 
4. Have different thickness's of o-rings?
a. Due to the configuration of the HW piston seal retaining flange I've been restricted to the smaller orings unless I made the piston caps thicker. I've been using .070 nom orings (about 1/16" cross section) for a couple decades and they have performed well when correctly fitted. Here is the chart I use for oring fitting..........
http://www.allorings.com/gland_dynamic_reciprocating.htm
However I do revise the gland size for my seals by using a narrower oring groove width and a 10% oring compression instead of the recommended 15-20% compression. LOL.....if I used 20% compression it would be impossible to push the oring sealed piston past all those receiver cutouts and holes without shredding the orig.  Years ago I did experiment with a 3/32" nom oring in a thicker piston cap thinking that the larger cross section would better compensate for the less than perfect geometry of the R9 receiver tube. While the larger oring does tolerate more receiver bore variation, I also found that the extra "rubber against the road" gave shot performance that was too "factory like". One thing about oring sealing is the fact that oring size and oring crush all affect the output. Last year I also learned that orings do vary a bit more in cross section than I thought (even military spec Viton) so I'm using a couple thou more "crush" when cutting the groove than in years past. 
 
5. Lastly what did you lube the o-ring with on assembly?
For well over a decade I've used the same lube that I would use for "regular" piston seals....molly paste. This past year however I nixed the black staining, dieseling prone, not very compatable with "rubber" molly paste in favor of the expensive Dupont Krytox GPL205 grease. This stuff is non-staining white colored, VERY slippery, will NOT diesel at all, is recommended as an oring lube and assembly lube, and I found that it holds up better than molly paste. Initially I bought a 1/2oz tube of the GPL205 for $17, tried it and I liked it so much I stripped my R9 of all "tar and paste", relubed EVERYTHING with the Krytox, and my R9 has never worked so slick. I recently took delivery of a 2oz tube of GPL205 from an Ebay retailer for $38 and free shipping. LOL.....now I have enough of the stuff for YEARS of maintenance work on my HW77 and R9. Matter of fact, I currently have my HW77 broken down for a "grease strip" using brake cleaner and a relube with the Krytox grease.

Hope this helps some!

Regards,
Ed

[OleTomCat]

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

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