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

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http://www.real-seal.com/blog/how-to-improve-your-product-performance-with-slippery-rubber-seals/

Some compounds are better suited for dynamic applications than others. While nitrile and EPDM are suitable for dynamic applications, they do tend to have above-average break-out or running friction when used without lubrication.

Silicone and fluorosilicone have poor tensile strength, meaning that they rupture very easily. This makes them a poor choice for high-rate dynamic applications. Reserve these materials for low movement dynamic applications with smooth gland surfaces.

Fluorocarbon is significantly costlier than most other compounds, and cannot be used with steam. It does, however, have better temperature and chemical performance, as well as better running and break-out friction than most other elastomers.



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​I think I will try to rub some WS2 powder into the rod to reduce friction even further.

I will also pay more attention to the rod finish. I mentioned it was polished. However, ERIKS book states that 8 to 16 RMS is the best surface finish for orings in dynamic applications. It must keep some oil in place.

Repeatable valve movement is crucial to v0 consistency. Placing an oring there complicates that to great extent. FX decided to use PUR because it has really high abrasion resistance but it lacks other parameters. Viton is fine stiction wise but it is poor when it comes to abrasion resistance. I tend to think NBR with PTFE particles embedded in its structure might be the best.

So guys, that simple design decision to use oring around the rod may result in a PhD dissertation.
 
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Right, that is a valid point.



Minimizing break-out friction is still worth investigating because it allows to open the valve easier. Less hammer strike, less vibrations. But that is a different aspect.

Anyway, I guess there should be some correlation between low initial break-out friction and its tendency to grow over time. For example (arbitrary values), Viton80 initial stiction can be X. It can grow to 2X over two weeks. NBR90 initial stiction can be 3X and it can grow to 6X over two weeks. Viton's 2X after two weeks is lower than NBR's 3X which is its initial stiction. So stiction increased in both situations but Viton's increase may be insignificant in terms of v0. That's my point here.
 
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True, if we were to identify an arrangement with super low break-out friction, one would reasonably expect it to be more consistent. 

For example, let’s say we had a sticky O-ring and a slick O-ring. I’m going to make up numbers for them, somewhat absurd to simplify it. The sticky O-ring needs 5J of energy to unstick it right away and 10J to unstick it after it has been idle for a while. So it soaks up 2x as much of the hammer’s energy after being idle, or 5J difference in absolute terms.

Meanwhile let’s say the slick O-ring needs 0.1J to unstick it right away and 1J to unstick it after it has been idle for a while. As a percentage, it’s very inconsistent…it soaks up 10x as much after it has been idle. But because it was so low to begin with, the difference is only 0.9J…far less than the 5J difference with the sticky O-ring. 

I just want to leave room for the possibility of an O-ring that we wouldn’t necessarily think of as low friction (slick), but maintains a pretty consistent amount of friction. Indeed, it would need a little more hammer spring tension but I think most people would happily trade that for the confidence of knowing the first shot is going to be spot on.
 
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Many of us who having been working with these "Reduced opening effort" valves ... AKA balanced valves have been fighting this sticksion issue all along and just like this conversation what we found then still applies now. Yup an issue that while able to minimize to large extent we can't eliminate it but one way .... OVER STRIKE meaning striking the valve HARDER than required so as the drag of breakaway friction does not really matter. In this case LIFT LIMITING miens become required to control lift and dwell for more broad range tuning flexibility. ** That is unless your running at a very well defined tune where power / speed one achieves is based on PRESSURE and adding more hammer strike does not increase power. Thats going to allow for a heavy hammer strike and lowest 1st shot variance. Not exactly the most efficient tune but one that mitigates concerns of first shot or two lower velocities.



JMO ..

Scott S


 
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Many of us who having been working with these "Reduced opening effort" valves ... AKA balanced valves have been fighting this sticksion issue all along and just like this conversation what we found then still applies now. Yup an issue that while able to minimize to large extent we can't eliminate it but one way .... OVER STRIKE meaning striking the valve HARDER than required so as the drag of breakaway friction does not really matter. In this case LIFT LIMITING miens become required to control lift and dwell for more broad range tuning flexibility. ** That is unless your running at a very well defined tune where power / speed one achieves is based on PRESSURE and adding more hammer strike does not increase power. Thats going to allow for a heavy hammer strike and lowest 1st shot variance. Not exactly the most efficient tune but one that mitigates concerns of first shot or two lower velocities.



JMO ..

Scott S

Click to expand...

Yes, there is a note on that in this thread. One way to reduce the severity of stiction is to tune the gun to be on a plateau. However, it's not what I expect. Finding a proper tune for slugs is not easy. If I add one more factor to the equation then it may be extremely hard to find a proper tune for slugs. Pellets are much more forgiving and accuracy nodes are much wider with them.
 
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Got a peek disc for my RAW and one for my Impact. 

But
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I understand that Scott is very right that we need more strike so that the sicky issue could become immaterial for fps.

Now I am going to increase the hammer hit of my RAW and restrict the valve travel that can be done through my modification. 

I hope it will work.

Bhaur


 
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The first shot low fps problem solved in my RAW HM1000X .22.

After 16 hours I did the chrony. I had set it with 18 grain jsb at 950 fps.

This morning the first shot was 945 fps and the 2nd shot and onwards were 950 + & - 3 fps.

I am using PU 90 shore O ring in my RAW.

So I understand that increasing the hammer strike and restricting the valve travel has did a great job for me.

Previously my same RAW at same set fps of 950 used to give 890 fps in 1st shot and in next 3 shots it gradually used to reach the set fps of 950 with same ammo.

Now increasing the hammer tension resulted into fps of around 980 and with the valve restrictor I brought fps down to 950.

After all night stay now the first shot is 945 fps and the 2nd and further shots are 950 fps.

For me 5 fps less speed in first shot is not an issue. Neither in target shooting nor in hunting.

By the grace of God I achieved more than expected out of my modifications in RAW.

I also tested my Impact M3. I had left it at 950 fps in .25 caliber with 25 grain jsb.

This morning it started from 936 fps in first shot and in next 5 shots it went up to 966 fps.

MY Impact also has PU 90 shore O ring. 

I think that a bit of similar treatment with my Impact m3 is needed.

Bhaur
 
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Hoping Seeker reports back soon about his findings with a NBR oring. I have Viton and NBR 70 and 90’s to put in the gun when we have the data. I am not willing to jack up my hammer spring tension and counter it by shutting down the valve more. That tuning method had my slugs coming off the tune as the bottle pressure dropped. If my guns were pellet guns I would try tuning around the sticky oring that way. Slugs with the FX barrel system are just too sensitive. 
 
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Hi,

NBR90 also works fine. I mean the first shot's v0 was slower by 1.5m/s when comparing to Viton80. However, it's still way better when I had no peek washer. My first shot was slower by over 10m/s then.

mubhaur, why didn't you check NBR90?

I wonder which oring to install finally. NBR90, Viton80 or PUR90. Regardless of the compound I will cover the rod and oring with WS2 powder and Huma silicon grease.
 
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In my estimation a Viton 70D (FKM) is likely the best choice, unless of course you're willing to splurge on a FFKM (e.g. Kalrez 7075). I don't think abrasion loss will be much of a factor with the extraordinarily low duty cycle and short travel, particularly if one takes care to ensure the rod is smooth and uniform. The plastic washer serving the role of a backer ring also minimizes any tendency for rolling and pinching.
 
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Hi,

NBR90 also works fine. I mean the first shot's v0 was slower by 1.5m/s when comparing to Viton80. However, it's still way better when I had no peek washer. My first shot was slower by over 10m/s then.

mubhaur, why didn't you check NBR90?

I wonder which oring to install finally. NBR90, Viton80 or PUR90. Regardless of the compound I will cover the rod and oring with WS2 powder and Huma silicon grease.
Click to expand...

Initially I tried NBR 70 in my RAW but it gave me similar good results. Then I again installed PU 90 due to its longer life as a dynamic O ring and chances of less extrusion even if there is still any little gap that can cause extrusion. 

However I could use NBR 90 that I didn't try at this stage. 

Bhaur
 
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In my RAW after keeping it idle for 32 hours the first shot went around 18 fps low than previously adjusted. But the very next shot was spot on.

Practically not a big issue but I opened the gun, mirror polished the valve stem and installed NBR 90 shore O ring and removed PU 90 shore O ring. 

I understand that NBR sticks less than PU.

Let's see the next results.

Any one else did any testing?

Bhaur
 
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Yes, NBR seems to have lower stiction over time than PUR.

I would say the lowest friction has Viton (FKM), then NBR, then PUR. The softer the oring the lower the friction is.

I installed PUR90 and I will check that. However, I am inclined to put Viton80 there as a long term solution but I am a little worried about its low abrasion resistance.

I also polished the rod. However, mirror finish is not the best. I used 400 grit. Then I rubbed some WS2 powder (MoS2 should also work fine) and I used Huma silicone lube. If you combine it with Viton70/80 then you have the best possible conditions friction wise if PTFE oring is not an option.




 
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