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

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I personally want to explore the exotic orings territory.

We may have some machinists here so they can do their part.
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The design is flawed. Doubt the issue will completely go away.
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I really believe in a PTFE oring. The slickest material on earth. What is more, when I apply TefDri layer on the rod then the stiction issue goes away or it is greatly reduced. However, as nervoustrigger pointed out, there may be an issue with proper sealing. I believe it will be good enough thanks to the conical cavity.
 
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I just checked the stickines of PU and NBR O rings on the titanium rod. Its after 12 hours of keeping all this in Atmospheric pressure. 

The PU took tripple the force to move as compared to NBR. Both are 90 hardness and as per size mentioned on the packing, these are identical. No oil applied on any one.

Only there is one thing that I do not know that sometimes there is a bit of difference in manufacturing tolerances in O rings. If the PU sticks more due to tighter tolerance as compared to NBR, then there may be error in the findings.

If we conclude that PU is more sticky on the titanium rod, can someone guide me the comparison of stickines on PU vs NBR on stainless Steel rod ?

20220125_224840.1643183680.jpg


Bhaur
 
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If we conclude that PU is more sticky on the titanium rod, can someone guide me the comparison of stickines on PU vs NBR on stainless Steel rod ?
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In the Parker reference I linked earlier in the thread, they provide the following table ranking friction factors in order of importance. Notably, they list coefficient of friction at the very bottom and include the footnote "A minor factor and should be ignored in design work other than for ultra high speeds."



However near the top they list surface finish (metal) as being a major factor so that suggests stainless vs. titanium is of little significance but being super smooth and uniform is a big help.

Intuitively that makes sense because the time-dependent nature of the stiction is attributed to the elastomer slowly conforming to the microscopic hills and valleys of the metal part and forming an intimate bond with it that becomes harder to break free.
 
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If we conclude that PU is more sticky on the titanium rod, can someone guide me the comparison of stickines on PU vs NBR on stainless Steel rod ?
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In the Parker reference I linked earlier in the thread, they provide the following table ranking friction factors in order of importance. Notably, they list coefficient of friction at the very bottom and include the footnote "A minor factor and should be ignored in design work other than for ultra high speeds."



However near the top they list surface finish (metal) as being a major factor so that suggests stainless vs. titanium is of little significance but being super smooth and uniform is a big help.

Intuitively that makes sense because the time-dependent nature of the stiction is attributed to the elastomer slowly conforming to the microscopic hills and valleys of the metal part and forming an intimate bond with it that becomes harder to break free.
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Makes sense. 
 
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I do not feel one thing. How the decrease of oring hardness makes the friction lower? The rubbing surface is greater then and it seems that the softer oring flows into the microscopic metal imperfections easier than the harder one.

Lastly, if we want to lower the coefficient of friction then we need to increase oring durometer. I know that the coefficient of friction was qualified as a low priority factor but that sounds kinda contradictory.

Also, mubhaur confirmed my observations that PUR90 generates a lot of stiction. Much more than NBR90. It is aligned with the diagram which I included stating that the coefficient of friction of PUR increases more rapidly comparing to NBR in a function of a valve downtime. It does not seem to confirm the statement that the coefficient of friction is of low importance in relation to stiction.

Am I missing something?


 
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I do not feel one thing. How the decrease of oring hardness makes the friction lower? The rubbing surface is greater then and it seems that the softer oring flows into the microscopic metal imperfections easier than the harder one.

Lastly, if we want to lower the coefficient of friction then we need to increase oring durometer. I know that the coefficient of friction was qualified as a low priority factor but that sounds kinda contradictory.

Also, mubhaur confirmed my observations that PUR90 generates a lot of stiction. Much more than NBR90. It is aligned with the diagram which I included stating that the coefficient of friction of PUR increases more rapidly comparing to NBR in a function of a valve downtime. It does not seem to confirm the statement that the coefficient of friction is of low importance in relation to stiction.

Am I missing something?

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Looks like there is a catch that we are missing.

I understand that there should not be any air pressure behind the O ring as is in mk2 and M3 set up.

In mk1 there is no back pressure causing expansion of O ring so there was no issue. 

In mk1 this O ring used to go bad a bit quickly than in mk2.

I think that the reason is that the continuous back pressure in mk2 causes expansion of O ring and as much as wears is caused with usage, it expands accordingly. 

So there are less chances of leakage in mk2 and M3 set up.

But the side effects we all bear.

Bhaur 


 
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I didn' pay attention to this OP earlier only now got bored to read all of the comments.

I can recall a conversation in email with my local FX distributor AGS back somewhere in Oct/Nov 2021 about this PUR90 oring.

I wanted to buy couple pieces for my own stock, because I was re-doing some power / porting upgrades including the plenum internals, and me personally I like to replace every oring which I remove onse.

Travis told me no available out of complete PP kit, but he sent me some NBR90 orings with a note, that he knows people using these orings in the PP succesfuly.

This much I can repeat the message (buried deep somewhere in my emails), me personally didn't install/replaced this specific PUR90 oring yet.



Edit:

Back to OP, I noticed a speed difference as well especially with my power mods, but doesn't bother me much. I shoot BR 100 and a first couple of shots I dispose anyway to "get on paper", I don't hunt or doing anything a first shot to be soo much valuable.
 
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How the decrease of oring hardness makes the friction lower?
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I agree, the third line in the table says decreasing durometer will decrease friction...that doesn't make sense to me either. The example familiar to most people would be racing slicks made of a soft compound and running low pressure to increase the contact patch and grip. It doesn't follow that a soft O-ring would exhibit less friction than a harder one. At least not for its running friction. 

However for the break-out friction, I could see it going that way. For example both a 70 duro and a 90 duro, given sufficient time, will probably conform similarly well to the microscopic hills and valleys of a machined metal surface. We can imagine the surface finish being more like coarse sandpaper to help visualize it. So when everything is at rest for a while, our O-ring has conformed neatly around each and every granule of grit. Then when the metal is subsequently cycled (e.g. the metal shaft driven axially), it has to displace the O-ring material that is gripping the grit. The softer, more compliant material being easier to displace so the shaft can break free and slide. But then again, it may be that the softer material has an appreciably larger contact patch that offsets the difference.

I have some O-ring sizes in both 70 and 90 durometer so I'll try your experiment this evening and see what happens.
 
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Regarding the break-out friction of a 70 vs 90 durometer O-ring, I tried a quick experiment with two -012 size Buna-N over a 3/8” steel shank. At both 1hr of dwell and 3hrs of dwell, I could detect the 90 durometer requiring slightly more force to initiate movement. However that may simply be because the 70 durometer is more supple so it isn’t hugging the shank as strongly. FWIW the choice of -012 and 3/8” shank only produces about 0.011” of stretch because I wanted to minimize that effect but it may still have a significant influence on the break-out friction.
 
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When shooting slugs and how you have to tune for them my first 2 shots being slow is like having a different hammer spring setting. That puts things way out of whack. Pellets? They don’t really care. It’s not a matter of a slightly less elevation with those first shots, it’s a matter of not knowing where they’re even going. Could be low left, low right, straight right or left. 
 
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