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

[Vetmx]

It was easier to make an oring groove in gen1 because the plenum was symmetric there. Here the oring axis is off the plenum axis. It makes it more tricky to make such a groove. I guess that's why they decided to use the cone.

However, they should be aware of the consequences I guess.

Anyway, I will try PTFE oring to check if it solves the stiction problem and if it seals properly.

I am totally shocked that noone really noticed this issue with Impact in the past.

My M2 has the same issue. Zero creep, first shot 911, then 925 then where it should be from 932-935fps. Not sure if my M3 does but I will check is soon now that I think I have the gun where I want it. To be honest, I never mentioned it on this forum because when you deal with all the other Achilles heals of an Impact, I finally bowed down to being a guy that has to dry fire exactly 3 shots if the gun sits. I never tolerated this with any other guns but dotting all the Impacts I’s and crossing all its T’s to keep it shooting slugs well has wore me out. If you figure out a way to resolve it, I will follow. And to think I used to make fun of just fire a couple shots into the dirt guys. Now if I plan to not wound something, I’ve joined the club. Hate it.

 

[Seeker]

Guys, I just did a very simple test. I placed a PUR90 oring on a valve rod.



At this point it was moving freely. Even after a minute of inactivity it was easy to move it again. Next, I left it for a couple of hours. It was SO MUCH harder to move it initially. After a couple of movements it was back to its smooth movements. It was all done under atmospheric pressure. 

The static friction can be determined using the following formula:

f = u * F,

where:

- u is the coefficient of friction between the titanium rod and PUR90 oring,

- F is the applied force.

We know how u behaves in terms of a PUR oring in a function of valve downtime:



You must know the mentioned oring is under the plenum pressure which can be between 80 and 160 bars. Based on that one can compute the applied force F.

It is intuitive the behavior which I experienced in the atmospheric pressure is proportionally amplified by the increased pressure. The higher the pressure, the more evident the issue it.

May I ask how such an obvious thing could slip through unnoticed until now?

 

[Corvid_hunter]

It was easier to make an oring groove in gen1 because the plenum was symmetric there. Here the oring axis is off the plenum axis. It makes it more tricky to make such a groove. I guess that's why they decided to use the cone.

However, they should be aware of the consequences I guess.

Anyway, I will try PTFE oring to check if it solves the stiction problem and if it seals properly.

I am totally shocked that noone really noticed this issue with Impact in the past.

My M2 has the same issue. Zero creep, first shot 911, then 925 then where it should be from 932-935fps. Not sure if my M3 does but I will check is soon now that I think I have the gun where I want it. To be honest, I never mentioned it on this forum because when you deal with all the other Achilles heals of an Impact, I finally bowed down to being a guy that has to dry fire exactly 3 shots if the gun sits. I never tolerated this with any other guns but dotting all the Impacts I’s and crossing all its T’s to keep it shooting slugs well has wore me out. If you figure out a way to resolve it, I will follow. And to think I used to make fun of just fire a couple shots into the dirt guys. Now if I plan to not wound something, I’ve joined the club. Hate it.

Do you notice a vertical or horizontal POI shift for the first 3 shots?

 

[Seeker]

Yes, I noticed the first shot is off after a week of inactivity. It can be more than 10m/s slower depending on the inactivity period. It sounds like a reg creep but the reg pressure is exactly the same as I have a Sekhment digital manometer installed. How crazy is that?

 

[mubhaur]

Guys, I just did a very simple test. I placed a PUR90 oring on a valve rod.



At this point it was moving freely. Even after a minute of inactivity it was easy to move it again. Next, I left it for a couple of hours. It was SO MUCH harder to move it initially. After a couple of movements it was back to its smooth movements. It was all done under atmospheric pressure. 

The static friction can be determined using the following formula:

f = u * F,

where:

- u is the coefficient of friction between the titanium rod and PUR90 oring,

- F is the applied force.

We know how u behaves in terms of a PUR oring in a function of valve downtime:



You must know the mentioned oring is under the plenum pressure which can be between 80 and 160 bars. Based on that one can compute the applied force F.

It is intuitive the behavior which I experienced in the atmospheric pressure is proportionally amplified by the increased pressure. The higher the pressure, the more evident the issue it.

May I ask how such an obvious thing could slip through unnoticed until now?

Looks like if instead of PU we use NBR, it will work better.

Can you do the same experiment with titanium rod and 90 shore NBR O ring ?

Further I still feel that this O ring arrangement that was in 1st generation of Impact was better in this respect. 

Bhaur

 

[corny]

What is the diameter of the valve rod?

 

[Vetmx]

It was easier to make an oring groove in gen1 because the plenum was symmetric there. Here the oring axis is off the plenum axis. It makes it more tricky to make such a groove. I guess that's why they decided to use the cone.

However, they should be aware of the consequences I guess.

Anyway, I will try PTFE oring to check if it solves the stiction problem and if it seals properly.

I am totally shocked that noone really noticed this issue with Impact in the past.

My M2 has the same issue. Zero creep, first shot 911, then 925 then where it should be from 932-935fps. Not sure if my M3 does but I will check is soon now that I think I have the gun where I want it. To be honest, I never mentioned it on this forum because when you deal with all the other Achilles heals of an Impact, I finally bowed down to being a guy that has to dry fire exactly 3 shots if the gun sits. I never tolerated this with any other guns but dotting all the Impacts I’s and crossing all its T’s to keep it shooting slugs well has wore me out. If you figure out a way to resolve it, I will follow. And to think I used to make fun of just fire a couple shots into the dirt guys. Now if I plan to not wound something, I’ve joined the club. Hate it.

Do you notice a vertical or horizontal POI shift for the first 3 shots?

Unfortunately with slugs out of an Impact, if anything is even slightly amidst, it equals a big group. Because the first two shots don’t jive where the gun is tuned for 933-935fps, they can go anywhere. Horizontally or vertically. Pellets might not care, slugs do.

 

[Seeker]

The diameter of the rod is 3mm. The oring dimensions are 2.9x1.78.

I agree that the conical cavity exaggerates the problem as the F component will have a higher value. NBR should work better but PUR was chosen on purpose. It is often used in a hand pump on the last compression level as it is very abrasion resistant. However, it absolutely does not matter if we run into stick&slip issue there.

On the contrary, a valve and a hammer system needs to work in harmony to produce the best result. It means we should remove the time component as a distortion factor and minimize the stick&slip phenomenon.

I am going to test 2.9x1.78 PTFE oring firstly. If it does not work then I will switch to the rocket science 2.9x1.78 FFKM Kalrez 7075 oring.

One of the orings should work much better than PUR, hopefully. Even 2.9x1.78 NBR90 should work better in the stick&slip terms but it might be not abrasion resistant and sturdy enough.

 

[corny]

 3mm bore is probably doable with a single point carbide grooving bar.

but otherwise this drawing could work too. Effectively creating an oring grove. 

 

[nervoustrig]

The diameter of the rod is 3mm. The oring dimensions are 2.9x1.78.

Ah, so it's an -006 size...an AS568 (inch) O-ring. I figured it would be a metric. That's fortuitous because I think you'll have an easier time sourcing a replacement. I haven't had much luck locating metric O-rings in specialty materials or 90 durometer when I've needed them, usually resorting instead to buying a nearest fit AS568 and then modifying (abrading) it to fit.

 

[Seeker]

I do not have that much of a practical experience when it comes to the machining itself. Please note the rod is not in the same axis as the power plenum. That may pose some machining challenges. Design change would surely help a lot but it won't eliminate the problem entirely. I think the oring material should also be reevaluated.

P.S. I tried to lube the oring and the rod with Krytox 205. It did not eliminate the initial stiction effect after a couple of hours of inactivity.

P.S.2. The only lube which seems to work is a Boretech Tef-Dri. It creates a dry layer of PTFE on the rod. However, it will wear out over time. So it's not a permanent solution.

 

[nervoustrig]

I like Corny's idea. Yesterday when I was looking at the $50 Kalrez O-rings, it occurred to me that at some point the experimentation may become costly enough that it will instead make more sense to machine something to address the issue at its root cause.

Is the pocket sized sufficiently to allow this drop-in part to do the job?

 

[Seeker]

I personally want to explore the exotic orings territory.

We may have some machinists here so they can do their part.

 

[mubhaur]

I think I have O rings of PU and NBR.

I shall try both on the valve rod at Atmospheric pressure to see which one sticks more.

Just like Seaker has trued PU.

 

[Seeker]

Just place both of them on the rod and leave them for 24 hours.

 

[mubhaur]

Just place both of them on the rod and leave them for 24 hours.

 

[Seeker]

Nice. You will be shocked how much of stiction they generate under the atmospheric pressure. Just imagine the amplification of that effect when you add 140 bars and the conical cavity to the equation.

 

[corny]

I personally want to explore the exotic orings territory.

We may have some machinists here so they can do their part.

The design is flawed. Doubt the issue will completely go away. 

 

[Corvid_hunter]

Too bad it couldn't be replaced with a bearing.

 

[Corvid_hunter]

It was easier to make an oring groove in gen1 because the plenum was symmetric there. Here the oring axis is off the plenum axis. It makes it more tricky to make such a groove. I guess that's why they decided to use the cone.

However, they should be aware of the consequences I guess.

Anyway, I will try PTFE oring to check if it solves the stiction problem and if it seals properly.

I am totally shocked that noone really noticed this issue with Impact in the past.

My M2 has the same issue. Zero creep, first shot 911, then 925 then where it should be from 932-935fps. Not sure if my M3 does but I will check is soon now that I think I have the gun where I want it. To be honest, I never mentioned it on this forum because when you deal with all the other Achilles heals of an Impact, I finally bowed down to being a guy that has to dry fire exactly 3 shots if the gun sits. I never tolerated this with any other guns but dotting all the Impacts I’s and crossing all its T’s to keep it shooting slugs well has wore me out. If you figure out a way to resolve it, I will follow. And to think I used to make fun of just fire a couple shots into the dirt guys. Now if I plan to not wound something, I’ve joined the club. Hate it.

Do you notice a vertical or horizontal POI shift for the first 3 shots?

Unfortunately with slugs out of an Impact, if anything is even slightly amidst, it equals a big group. Because the first two shots don’t jive where the gun is tuned for 933-935fps, they can go anywhere. Horizontally or vertically. Pellets might not care, slugs do.

Thanks. I'll have to keep an eye on this when I finally get my MKII tuned for slugs.