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Moving O-ring Muzzle For Large Slider Shafts (Inspired By The Seatec Evo-Air)

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.
As to whether the "O" ring moves I expect that it does. The annular ring over which the pressure differential acts is very small, basically the gap between the shaft and the muzzle bore. If the shaft had an 8 mm tail stop and a 7 mm shaft body then the muzzle bore may be 9 mm, hence the annular gap is only 1 mm wide. In the Seatec Evo-Air which uses 7 mm shafts with no tail stop the gap will be 0.5 mm if the muzzle bore is 8 mm. Calculate the cross-sectional area and you will see the force holding the "O" ring against a vacuum is very small when you apply the pressure differential of 1 atmosphere. At 10 meters underwater the pressure will be 2 atmospheres as we need to think in terms of absolute pressure.

14.7 psi is equal to 1.0335 kg per cm squared. To convert to mm squared we need to divide by 100, therefore 1 atmosphere is equal to 0.010355 kg per mm squared. Multiply that number by the gap cross-sectional area and you will have the force on the "O" ring holding it back due to the vacuum in the inner barrel.
 
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It took me a while to figure out why you were talking about the gap area and not the whole o-ring area instead. But I think it is because the o-ring in the most rearward position is not sealing on its sides (against shaft and side wall of the bore) but rather against the shaft and the inner edge of the lip?
If so, you are correct that the pull of the vacuum on the o-ring is really small, but the frontal area subjected to the pressure from depth is still large.
Some quick math shows that for a 7mm x 3mm o-ring (ID x CS) in an 11.5mm bore (to give app. 25% compression) we have 0.67kgf per bar of pressure. So, at 20m you have almost 1.5kg pushing the o-ring backwards, not counting any vacuum pull.

Also, I would think that even in shallower dives if the friction from the shaft does manage to push the o-ring forward then as soon as the o-ring no longer seals on the smaller lip it will seal on the sides of the bore and the shaft - which exposes a much larger area of the o-ring to the vacuum. The result is going from very little "pull" to at least 0.67kgf in a short amount of time and that might stop the o-ring from moving further forward. It may, in theory, start going back and forth between the lip and the bigger bore.
(Actually, this last paragraph could be flawed as I don't know how much vacuum the piston ends up pulling on the seal - is it safe to say it is close to 1bar?).

But thanks for bringing this up as it made me reevaluate the best size for a moving o-ring for a shaft with a slider. I was thinking that it would be better to maximize its size to guard against o-ring extraction rearwards through the gap. But since the idea for a gun like this would be to shoot fish that are often at around 20m I now think, it's better to go for the smallest possible o-ring section as the frontal force will be smaller. This should make it easier for the shaft tail to move the seal forward to the bigger bore where it can expand, letting the shaft tail through.
I just did a few more quick calculations and a 3.5mm section wide o-ring @ 20% compression, which I was thinking of using, would have about 1kgf per bar of pressure pushing it rearwards, so 2kgf at 20m, not counting the vacuum pull which could add another kilo. That's a whole lot of force for the shaft tail to bang against!
If I can get away with using a 2mm thick o-ring then that drops to half that at 0.46kgf per bar. That's still potentially close to 1.5kgf holding the o-ring it in its place at 20m when the shaft tail comes knocking...
If I manage to go all the way down to a 1.5mm o-ring, the force on it from depth will be 0.35kgf/bar - a 24% reduction over the 2mm o-rings. But again, it would be around 1kg total at 20m of depth holding that o-ring from moving forward with the shaft tail. I am beginning to see why @tromic couldn't get it to work.
 
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Pete, your assumptions are wrong. The boring for 7 x 3 mm O-ring in Evo-Air might be 12.1 mm to 12.5 mm for 7 mm shaft.
I forgot how I actually obtained the measures, I think somebody who had the kit told me.. He could not measure it precisely.
Maurizio (the Inventor) did not want to tell me the right value..
I also noticed that with the Tomba having 7 x 2.5 mm O-ring, O-ring is in place retained by vacuum even out of water, if the shaft is wiped with oily rag and I shot (on low pressure controlling the shaft by hand).
 
Reactions: Diving Gecko

Hi Tomi,
It's not really important but I think 12.5mm bore is way too big, it would only give around 6% compression on a 7mm shaft. 12mm would give app. 12% compression but I used 11.5mm in my calculations above as that gives around 23%.

I think what @popgun pete is saying is that for the vacuum pull on the o-ring, the whole o-ring surface is not the right one to use as the seal might be on the inside of the much smaller lip or inner bore and not on the outside of the o-ring.
Illustrated in this sketch, Pete is saying the sealing is not happening at the red arrows but at the much smaller area at the cyan-blueish arrows (in front-tied design, this gap is much smaller than in this sketch):


But my point is that even if Pete is right, then as soon as the o-ring moves just a tiny bit forward, the seal will be on the outside of the o-ring - at the red arrows - and the whole area of the o-ring should be taken into account again.
And also, I do believe that any frontal pressure from the depth will be pushing on the full area at the red arrows.
 
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Well I am right, any other parts on the "O" ring lean on the gun’s bodywork in terms of the "O" ring seat and there will be water in any of the blind ended gaps, not a vacuum. The only force differential is across any unsupported gap with nothing behind it.
 
DIY Vacuum Cuff (Yes, A Thread Drift...)
I am allowing myself a derail of my own thread as, at this stage, the following does not yet deserve its own thread.
But here's a quick sketch of how a muzzle with a home cast PU cuff could look (as mentioned, I could 3D print molds for cuffs):


The inspiration comes from the STC cuffs and oil seals and the muzzle design could be simplified a bit more. Perhaps with the retainer lip being only on the inside of the seal:


So far, the seal itself is optimized for 8-9mm shafts. And if I went forward with this, I would work on trying to make the "moving" part of the seal as small as possible to reduce how much rubber can be compressed onto the shaft.
The design in this sketch weighs about 44g and is 58mm long. That's a few grams lighter than the Vuoto and a full cm shorter.
 

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Muzzle Wrench (UBL Clone)
Here's how the muzzle wrench would look. It would be much easier for me to machine this and the blind hole in the muzzle than having to mill flats (I don't have a milling machine):

 


Absolutely The Right Solution!

A silicone O-ring can be used as a spring!
 
Zahar said:
View attachment 55051

Absolutely The Right Solution!
Click to expand...

I am not so sure... We have way too much pressure keeping the o-ring from moving forward already. I am not sure we need any more. But I will keep it in mind if I go forward with this type of design.

Since you are here, Zahar - have you seen any examples in Russia or Ukranie of spearguns using a simple o-ring inside the muzzle when using a shaft with a tail end 1mm larger than the shaft (same as what I am trying to make here)?
 

The same strong pressure prevents the harpoon shaft from coming out! There is no point in tightly pressing the vacuum cuff!
 
Zahar said:
The same strong pressure prevents the harpoon shaft from coming out! There is no point in tightly pressing the vacuum cuff!
Click to expand...

Lost in translation... I will try again.
Do you know anyone who have made a design like this work? One moving o-ring and a shaft with slider?
 
Diving Gecko said:
Lost in translation... I will try again.
Do you know anyone who have made a design like this work? One moving o-ring and a shaft with slider?
Click to expand...


Bushing 3 presses sealing O-ring 15! RPS 3

What is the use of a movable O-ring if you clamp it? Nothing! Get the design of the RPS 3! The mobility of the O-ring is important when firing for the free passage of the thickening of the shank of the harpoon! If you use your scheme correctly with the transformation of the O-ring into a valve element, then this will be the Original Scheme! And very useful!
 
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I will look into the RPS-3 but I am not clamping anything. The clamping is only during loading to ensure the o-ring is moved into the smaller bore to be compressed. For the shooting, the front of the muzzle is extended and there is a bigger space the o-ring can move into. Look at the drawings again, please.
 

The argument can't really be settled without real world tests. Tomi has said his o-rings don't move at low speed shots and I have a vague memory of a guy proving that it doesn't move. The bottom line is that we don't know the force of the friction at all and Pete assumes it to be larger than the force of the vacuum and pressure on the o-ring. Maybe one day, I can mount my Evo-Air and shoot some tests with it closed and opened. Personally, I don't expect any big difference. In any case, I have no idea when I would be able to do this.
 

You make a 0-ring clamp by hand when loading with the risk of sand and dirt getting into the threaded connection and spending time unscrewing - twisting! All this can be done by the valve in the same O-ring without unnecessary manipulations with the hands! Put the valve on my advice and call your name - it will be the original valve! Because He will work in two directions! When you enter a harpoon in a muzzle, it releases excess water, when you enter a harpoon with a piston in the barrel, it closes the entrance to the barrel due to the force of a spring or springy silicone O-ring + vacuum force! When fired, the thickened harpoon shank will simply move the O-ring spring and come out freely without destroying the O-ring!
 

Sand is not an issue. This muzzle is for bluewater hunting. Unscrewing the front part takes 2-3 secs - and the front could even be made to slide if time is an issue. Also, we are talking depths of 20m or more, so a lot of pressure on the o-ring already. The spring idea may be easier to use, but the downside is that it adds pressure to the front of the o-ring. But thanks, I will take it all into consideration.
 
Reactions: Zahar

Why do tests where everything has long been verified and theoretically confirmed? I do not argue with you! Just suggested unifying your decision! My experience in gun-making also includes the experience of studying the resistance of O-rings during movement and under prolonged exposure to pressure without movement! Hand-held O-ring does not have the freedom to change geometry for elastic deformation!
 

As to what has been proven to work - it sometimes depends on the hunting style. We have had this discussion before when Vlanik insisted I should adopt his design - but different things work in different circumstances and e.g. a Vlanikgun is not a good choice for a long bluewater gun shooting heavy shafts at higher pressures.
If you are guaranteeing that a moving o-ring will indeed move, then please tell me if you can guarantee that the o-ring will still easily move forward when there is 2.5-3.5 bar acting on it?
I am not arguing with you, I just want to check that you have taken into account the differences in shooting the gun at 20-25m vs. in a 2m deep river. The difference, as I have explained earlier can be a few kilos of force pushing on the seal depending on the size of the o-ring.




Sent from my iPhone using Tapatalk
 
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I sincerely wish you success! Thank you for listening to my suggestions! I consider it an honor to use your solution with my suggestions in your personal design!
Depth 30 m! Pressure - 3 atm. Harpoon diameter 8 mm. O-ring thickness 1.8 mm. The cross-sectional area of the O-ring is 0.55389 cm2. The clamping force is 1.6616 kg + vacuum will be approximately 1.8 - 2 kg! This is a minuscule, which will not significantly affect the operation of the valve with a correctly selected working stroke of the O-ring!
 
Reactions: Diving Gecko
You could always check automotive engine parts lists for engine valve stems that have 8.5 to 9 mm shanks. The oil seals have the job of stopping oil being sucked into the combustion chamber during the piston's downstroke on the fuel/air intake with the valves open and preventing the engine burning oil which would eventually carbon up the valve heads and seats. Truck engines as well as cars, or even boat engines, they all have oil seals on their valve stems.
 
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