Tuning FX Impact Mk2/M3 - slower first shot - no reg creep - quasi-scientific analysis

[mubhaur]

Further we see that in mk2 and m3 FX puts one O ring of 70 shore over the 90 shore PU O ring.

This may be to reduce wedging. 

Will the wedging be eliminated if we put One 70 shore O ring and then 90 shore PU O ring and then again 70 shore O ring.

Just sandwiching the 90 shore PU O ring with two 70 shore O rings.

Just thinking loudly. 

But I still feel that the arrangement of mk1 should resolve the issue. 

Bhaur 

 

[Airgunmann]

Mk1 arrangement would definitely help there maybe some sticktion still but I feel it would definitely be better than what is right now couldn't the existing part be removed and have a oring groove cut into it ? 

 

[Airgunmann]

Groove would be put in the valve housing not the rod correct? 

 

[mubhaur]

Yes I think converting the system into mk1 system will be helpful. Looks like the machining will be required to open the front end of plenum, insert a bush with a groove and then making threads to put the end retaining screw.

Here is the pic of mk1 system.

 

[Seeker]

Further we see that in mk2 and m3 FX puts one O ring of 70 shore over the 90 shore PU O ring.

This may be to reduce wedging. 

Will the wedging be eliminated if we put One 70 shore O ring and then 90 shore PU O ring and then again 70 shore O ring.

Just sandwiching the 90 shore PU O ring with two 70 shore O rings.

Just thinking loudly. 

But I still feel that the arrangement of mk1 should resolve the issue. 

Bhaur

There is a sealing 2.84x2.62 oring and there is a retention oring 5.5x1.5. The sole purpose of the latter one is to just keep the first one in place when there is no pressure in the plenum. Once the plenum is pressurized that oring does not matter at all.

As of the approach used in Mk1. I think it may be better. Firstly because it's brass which is used for bushing. It means that the titanium rod will have better movement charactertistics if it rubs against the brass rather than anodized alu. I can see there are very subtle wear marks on the plug itself. If you make a hole in the brass as tight on the rod as possible then it will minimize the wedging for sure. However, it must be big enough to allow for free movement of the rod.

To sum up, I think we are dealing with two issues:

1. Increase in the static friction over valve inactivity time. I included a figure from Parker Oring book presenting that PUR suffers from that much more than NBR.

2. Wedging caused by oring extrusion over valve inactivity time. Many thanks to Bob Sterne. He identified that behavior being critical with balanced valves.

These two contribute to the stiction problem. I am not sure which one is more critical. If we eliminate the second one we may realize that the stiction is no longer an issue.

I have one more observation. When I replaced my old PUR90 oring which was slightly deformed with a visible lip caused by its extrusion with FKM80(Viton) my v0 increased by 10m/s with the exact same settings.

Lastly, plenum pressure contributes a lot to the issue itself. The higher it is the more evident the issue is. Some folks with 80 bars in the plenum may not experience that issue at all.

 

[Airgunmann]

They went away from this design because the oring didn't last as long ? I suppose as the oring wears it gets pushed farther into the wedge helping it seal or something ??

 

[Seeker]

The conical cavity should, in theory, compensate the oring wear. However, it also contributes to wedging.

My feeling is that they changed the design because the plug is not concentric any more. I mean the rod is not in the same axis as plenum's one. Because of that the machining is more tricky and making it in the same way as in Mk1 may not be feasible or at least practically justifiable.

 

[Bigragu]

Ok, question- which o ring actually does the sealing of the high pressure air? The PUR that is at the very front of the end cap “beak” that's inserted first or the buna one behind it? If the PUR o ring is said to be the seal, then back to my comment earlier, that everytime a leak is discovered with air coming from behind the power wheel knob, I always find upon disassembly that the buna o ring is destroyed or missing with just a few fragments left that tells me the majority of that o ring got shot out.

Personally I think the PUR o ring is the “backer” o ring meant to sit in front of the beak and act as the “guide bushing “ for the valve rod due to its material design of being able to withstand the stresses of a rod sliding in and out for hundreds of shots(abrasive resistance, what polyurethane is best known for), and the buna o ring resting against it is the main seal against the HPA.

upon firing a shot, the plenum quickly fills with air and outward side pressure of the air pushes that buna o ring against the PUR, creating the seal.

Someone tell me if I’m off base in my thoughts here, and someone tell me that they are able to still hold air without that buna o ring in place, and just using the single PUR o ring in the beak. 


Great topic here, lol. The college of pcp knowledge. Thank you to all that have contributed

 

[Airgunmann]

Further we see that in mk2 and m3 FX puts one O ring of 70 shore over the 90 shore PU O ring.

This may be to reduce wedging. 

Will the wedging be eliminated if we put One 70 shore O ring and then 90 shore PU O ring and then again 70 shore O ring.

Just sandwiching the 90 shore PU O ring with two 70 shore O rings.

Just thinking loudly. 

But I still feel that the arrangement of mk1 should resolve the issue. 

Bhaur

There is a sealing 2.84x2.62 oring and there is a retention oring 5.5x1.5. The sole purpose of the latter one is to just keep the first one in place when there is no pressure in the plenum. Once the plenum is pressurized that oring does not matter at all.

As of the approach used in Mk1. I think it may be better. Firstly because it's brass which is used for bushing. It means that the titanium rod will have better movement charactertistics if it rubs against the brass rather than anodized alu. I can see there are very subtle wear marks on the plug itself. If you make a hole in the brass as tight on the rod as possible then it will minimize the wedging for sure. However, it must be big enough to allow for free movement of the rod.

To sum up, I think we are dealing with two issues:

1. Increase in the static friction over valve inactivity time. I included a figure from Parker Oring book presenting that PUR suffers from that much more than NBR.

2. Wedging caused by oring extrusion over valve inactivity time. Many thanks to Bob Sterne. He identified that behavior being critical with balanced valves.

These two contribute to the stiction problem. I am not sure which one is more critical. If we eliminate the second one we may realize that the stiction is no longer an issue.

I have one more observation. When I replaced my old PUR90 oring which was slightly deformed with a visible lip caused by its extrusion with FKM80(Viton) my v0 increased by 10m/s with the exact same settings.

Lastly, plenum pressure contributes a lot to the issue itself. The higher it is the more evident the issue is. Some folks with 80 bars in the plenum may not experience that issue at all.

So viton helped and a increase in fps/mps was that just for the first shot or across the board at 3 mm rod I suppose is to darn thin to include a oring groove into the rod to bad a piece of peek wouldn't work to do the sealing 

 

[nervoustrig]

If you make a hole in the brass as tight on the rod as possible then it will minimize the wedging for sure.

Agreed, same principle as a backer ring...normally thought of in terms of preventing an O-ring from suffering extrusion failure by minimizing the gap, which simultaneously serves to minimize the wedging action.

However, it must be big enough to allow for free movement of the rod.

Knowing the size of the extrusion gap at present will inform you as to how much potential there is to reducing the wedging effect. Can you get a measurement of both the rod OD and the hole ID to within a thousandth or half thousandth of an inch (0.01 to 0.02mm)? Calipers will suffice for the rod OD. For the hole ID, preferably use gauge pins or you might get away with a finely graduated drill set.

If the difference in these measurements is more than 0.05mm (0.002in), I would drill out the hole to accept a brass bushing for a nice slip fit to the rod. This is a job that can be accomplished with modest tools (drill press and/or hand drill...doesn't require a lathe). All you would need is a piece of 3mm ID brass tubing and cut off a ring from it with a Dremel emery wheel. Drill out the aluminum part to accept it. Clean the surfaces with a solvent and apply threadlocker or bearing compound and set the brass bushing. Now you have a minimal extrusion gap plus a more favorable material through which the rod can move.

Before I had a lathe, I did this very thing to a number of valves very successfully.

 

[Seeker]

@Bigragu, please note the pressure works in all directions. I am sure 5.5x1.5NBR70 oring is a retention ring when the plenum is not pressurized. Does it also help when the plenum is pressurized? I am not 100% sure. It might help when the plenum is emptied once the shot falls to help keeping 2.84x2.62PUR90 in place. But again, the sole purpose of 5.5x1.5 oring is retention. The whole sealing job is on 2.84x2.62 oring.

I can confirm that Viton 2.84x2.62FKM80 did not help with stiction. After sitting for 12 hours the first shot was slower by 25FPS.

@nervoustrig, I am not sure I understand your idea. Whould you be that kind and make some drawing?

As of the measurements, I just have a caliper. The hole is 3.04mm and the rod is 2.96mm. The measurements might be a little off because of the improper tool which was used.

 

[nervoustrig]

@nervoustrig, I am not sure I understand your idea. Whould you be that kind and make some drawing?

Cut a segment from a piece of brass tubing:

Drill / enlarge the hole in the end of the hammer rod guide to accept the brass tubing:

Apply a thin film of film of red threadlocker (or bearing compound) to both parts and insert the bushing and leave it overnight to cure. Of course clean them first with a solvent to remove any possible oil from their surfaces to ensure a good bond.

If you think you may have drilled a little off axis from the original hole, reassemble everything so you can check that the rod isn't binding. If it slides smoothly, leave everything assembled while the compound cures. Just make sure you have cleaned up any squeezeout or you might come back to find the rod adhered to the inside of your new brass bushing.

As of the measurements, I just have a caliper. The hole is 3.04mm and the rod is 2.96mm. The measurements might be a little off because of the improper tool which was used.

Okay so we have reason to believe the gap is at least 0.08mm (0.003") because calipers aren't very good at measuring small inside diameters. That means the actual difference between the parts is likely greater, and more reason to believe this mod will be beneficial to preventing the O-ring from wedging so strongly.

 

[Seeker]

It does not sound safe. The pressure will work on the face of the tubing and it can push it out. I would rather insert a conical brass (or PTFE) piece from the other end and top it with an oring. There is not that match of space because there is a hole between two external orings so that plenum can be pressurized.

 

[nervoustrig]

It does not sound safe.

Yes, good to give it the proper consideration. If the adhesive bond were less than 4mm long, I wouldn't use this approach. However you might be surprised when you run the numbers. For example consider a brass bushing of 3mm ID and 4mm OD (0.5mm wall thickness). At an elevated pressure of 210 bar (~3000psi), the force acting on it is just 12kg (25lbs). A bushing as described here isn't even thinking about budging at such a modest force. I have several valves with bushings fitted in this manner that have been cycled thousands of times and have not once had one come out.

 

[Airgunmann]

Could you bond it to the rod if it wasn't that thick of a bushing just would have to be longer piece of brass just spit balling I don't know the internals of it that well 

 

[Vetmx]

I hope FX is paying attention to this because I think I have the cure for what ills us and it comes from the hydraulic world. They need to return to the M1 plug. Then instead of cutting a groove for an internal oring, they need to cut the internal groove to accept a tiny poly pack. They would have to source a micro poly pack or poly seal but they have the resources to do it. When using a poly in a hydraulic gland, you also have what we refer to as a rider. So another groove would be cut to accept an oring with good wear characteristics but we are not relying on it to seal, it’s more of a guide. With a poly, the flared end faces your high pressure side. The high pressure acts on that lip to seal things up. This is what a poly looks like.

 

[Vetmx]

These poly packs are used in the gland of a hydraulic cylinder because of the constant in and out use. I believe in the PCP world it was initially thought that if we can use internal orings to seal a pellet probe in a barrel, we can use that same concept on a valve rod. Not the same thing though. Your pellet probe seal is not under constant pressure. Yes, it does see the same amount of cycles as the valve rod but it’s not living in the same environment. With the M2 and M3 they tried to create a situation that wasn’t so harsh on the orings. Maybe the use of two different orings in there was their attempt at giving the sealing oring a rider. I hope I didn’t lose anyone with what I’m trying to explain.

 

[Seeker]

I have another potential solution which is somewhat similar to yours. However, it is much easier to implement.

We know that oring wedging caused by its extrusion seems to be the root cause of the first shot being slower after extended period of a valve inactivity under high plenum pressures. So ultimatetly we want to avoid this situation:



My idea comes from the pneumatics world as well. There as so called back-up orings preventing from oring extrusion: https://www.parker.com/Literature/O-Ring%20Division%20Literature/ORD%205700.pdf -> Back-up Rings – Section VI

I propose this approach:



The gap in the plug itself should be big enough to allow the rod to move freely. Its diameter can be about 0.05-0.08mm bigger than the rod itself. Next, a PTFE washer should be placed. It should be a tight fit over the rod and it should also be tight in relation to the external cylinder. The role of that PTFE washer is to be a backup oring. Next, we place some Dur90 oring - it can be the one suffering from stiction issues the least. We know that wedging will be eliminated with this approach. The rod will be guided by the PTFE washer, no longer rubbing the plug itself.

Sounds like a win win approach? Any weak points?

 

[Vetmx]

It’s not really a backup oring you’re describing. Your concept is basically a guide with tighter tolerance that’s more forgiving than FX making the aluminum plenum plug with tighter tolerance and ultimately wearing out quickly. For your idea, the washer could also possibly be made out of peek or Delrin. Not sure what the friction wear characteristics of those two are though.

 

[Airgunmann]

Peek would probably be better because it is harder a derlin does seem to have a sticktion problem on some valves also for back up orings a notch would also have to be made correct the back up orings go in to the notch also right ? So either way a notch or two would have to be made if this part unscrews from the plenum it would be best if we could get FX to make the part to fix the problem they have everything to do so maybe contact FX or something I don't know it would be nice since the guns are supposed to be under a warranty what's the warranty 3 years ? I would actually be willing to pay for the piece if they wanted to call it a upgrade or update I suppose it would be 2 different pieces one for mk2 and one m3 I don't know