What if some water left or entered the vacuum barrel

[tromic]

In this case, when the amount of the water is small, the shoot of the speragun might appear as everything would be OK. But the water left would be the actual damper for the piston, not the regular damper. However some water would pass by the piston behind it accumulating and mixing with the oil.
If the amount of the water would be higher the shoot of the speargun would be noticeable worse, but maybe no water would pass by piston behind of it because the speed of the piston and the shaft would be much lower.

pw - the average pressure of water after shoot with some water entered the vacuum barrel

 

[popgun pete]

If the hydraulic lock as the piston runs into water pushed to the front of the barrel causes it to stop then the spear will be jerked out of the piston and the water will exit behind the spear tail allowing the piston to reach the anvil of the damper or shock absorber. When the excess water volume is very small it will stretch the nozzle type vacuum seal before the shoulder of the spear stop hits it thereby providing an easier passage of the spear tail through the rubber nozzle. Too much excess water and it would appear that some can push rearward past the piston "O" ring as the gun's oil gets contaminated with water. However at the same time hydrostatic pressure behind the spear tail tang will blow the spear free of the piston, so the period of high hydrostatic pressure in the muzzle will be very brief. The lower the grip of the piston on the spear tail from conic friction (which is what Mares call it) then the easier it is for the spear tail to be blown free as for an instant hydrostatic pressure will be greater behind the spear than it is in front of it outside the gun. For example the Russian "Taimen" has no grip on the spear tail as it relies on the vacuum in the inner barrel to hold the spear in the gun, the spear tail tang being a cylinder that fits in a matching hole in the piston nose with zero taper.

 

[popgun pete]

Note that even "Taimen" guns have been found to have water get into the oil inside the gun after presumably a total fail of the muzzle’s nozzle type vacuum seal, hence the duration of a high hydrostatic pressure spike inside the inner barrel is very important as that will determine if water can pass rearwards into the gun's interior spaces. The disadvantage of using the inner barrel vacuum as the means of stopping the spear falling out of a cocked to shoot gun is that there is an upper limit on the weight of the shaft, however the deeper you dive then the increased ambient pressure outside the gun will support a greater shaft weight with the gun pointed downwards.

 

[Zahar]

In Italian guns, there is practically no guide for the piston in the shock-absorbing bushing + the rubber of the shock absorber can be compressed unevenly upon impact! This warps the piston at the moment of impact on the shock absorber and leads to water getting behind the piston and flaring the front of the barrel, which also increases the possibility of water getting behind the piston! Also, water can get behind the piston with 1 Oring as a result of piston hydroplaning at high speeds! The presence of 2 and 3 O-rings on the piston reduces the probability of water getting behind the piston to 0!

 

[popgun pete]

In Italian guns, there is practically no guide for the piston in the shock-absorbing bushing + the rubber of the shock absorber can be compressed unevenly upon impact! This warps the piston at the moment of impact on the shock absorber and leads to water getting behind the piston and flaring the front of the barrel, which also increases the possibility of water getting behind the piston! Also, water can get behind the piston with 1 Oring as a result of piston hydroplaning at high speeds! The presence of 2 and 3 O-rings on the piston reduces the probability of water getting behind the piston to 0!

Are you sure about that? In the metal piston and shock absorber models the piston nose fitted into a matching cup shape on the shock absorber anvil. When the guns changed to plastic pistons and anvils the rear of the shock absorber has a cup shape moulded into it to direct the piston, although these were slighter flatter and not so curved in side profile as you can see on this one from a Sten. Some guns like the last ones from Omer had a flat anvil, but these ones were the rubbish from Taiwan where somehow they got confused about what they were doing, including placing bumps on the piston nose that struck the anvil face!

 

[tromic]

The value of deceleration of the piston in the example at the upper image is a= - 91408 m/s2.
The braking time is t = 0.328 ms. If the speed of the shaft were 30 m/s than the shaft would travel about 10 mm in that time.
The braking distance for the piston is 5 mm. Because the length of the shock absorber body is about 18 mm, the shaft tang would be still inside the shock absorber, immediately after braking. The OD of the tang is 8 mm for 7 mm shaft, and the ID of shock absorber is 8.5 mm, so that is the only space for the water to go forward.

 

[Zahar]

Are you sure about that? In the metal piston and shock absorber models the piston nose fitted into a matching cup shape on the shock absorber anvil. When the guns changed to plastic pistons and anvils the rear of the shock absorber has a cup shape moulded into it to direct the piston, although these were slighter flatter and not so curved in side profile as you can see on this one from a Sten. Some guns like the last ones from Omer had a flat anvil, but these ones were the rubbish from Taiwan where somehow they got confused about what they were doing, including placing bumps on the piston nose that struck the anvil face!
View attachment 58234

Sure! For reliable centering, at least half the piston diameter is required for the depth in the piston damper! 1.5 mm, taking into account the idiotic pimples on the piston, is nothing at all!

 

[Zahar]

The value of deceleration of the piston in the example at the upper image is a= - 91408 m/s2.
The braking time is t = 0.328 ms. If the speed of the shaft were 30 m/s than the shaft would travel about 10 mm in that time.
The braking distance for the piston is 5 mm. Because the length of the shock absorber body is about 18 mm, the shaft tang would be still inside the shock absorber, immediately after braking. The OD of the tang is 8 mm for 7 mm shaft, and the ID of shock absorber is 8.5 mm, so that is the only space for the water to go forward.

The disengagement time must be calculated from the amount of work A performed when the harpoon shank enters the piston hole! In tapered shanks, this work A is minimal, since the minimum distance traveled under the action of friction is about 1mm! So we are not talking about any 10 mm! 10 mm goes through a cylindrical shank in Zelinsky guns!

 

[tromic]

The disengagement time must be calculated from the amount of work A performed when the harpoon shank enters the piston hole! In tapered shanks, this work A is minimal, since the minimum distance traveled under the action of friction is about 1mm! So we are not talking about any 10 mm! 10 mm goes through a cylindrical shank in Zelinsky guns!

If we know that the indicial speed of the piston during braking is vo = 30 m/s, and the deceleration is a = 91408 m/s2 than to obtain the final speed of the piston = 0 m/s, we have
0 = vo - a * t,
0 = 30 m/s - 91408 m/s2 * t.
t = (30 m/s) / (91408 m/s2) = 0.000328 s or 3.28 ms.
Because speed of the shaft is 30 m/s, during time t = 3.28 ms it would travel the distance d:
d = v * t,
d = (30 m/s) * (0.000328 s) = 0.00984 m.
So distance d = 9,84 mm or near 10 mm, as I wrote.

 

[tromic]

Sure! For reliable centering, at least half the piston diameter is required for the depth in the piston damper! 1.5 mm, taking into account the idiotic pimples on the piston, is nothing at all!

Zahar, if you are talking about the shaft end and the piston like at image a) you are maybe right, but in case of shaft end having cone, image b) as Italian shafts have, and corresponding piston, you are wrong. In this case centering is very good with most of the shock absorbers.

 

[Zahar]

If we know that the indicial speed of the piston during braking is vo = 30 m/s, and the deceleration is a = 91408 m/s2 than to obtain the final speed of the piston = 0 m/s, we have
0 = vo - a * t,
0 = 30 m/s - 91408 m/s2 * t.
t = (30 m/s) / (91408 m/s2) = 0.000328 s or 3.28 ms.
Because speed of the shaft is 30 m/s, during time t = 3.28 ms it would travel the distance d:
d = v * t,
d = (30 m/s) * (0.000328 s) = 0.00984 m.
So distance d = 9,84 mm or near 10 mm, as I wrote.

Calculations are wrong! The piston will travel as long as the shock-absorbing piston sleeve will allow it! You are interested in the problem of water getting behind the piston into the barrel! I gave reasons! The taper shank of the harpoon in Italian guns has a minimum distance for the work of landing in the piston hole and the same distance when undocking from the piston! This is the whole feature of the cone landing!

 

[Zahar]

Zahar, if you are talking about the shaft end and the piston like at image a) you are maybe right, but in case of shaft end having cone, image b) as Italian shafts have, and corresponding piston, you are wrong. In this case centering is very good with most of the shock absorbers.

View attachment 58236

Tomislav! I repaired more than 10 Italian guns in 3 months! Almost all aluminum barrels were ground and turned into new piston shock absorbers! All barrels were flared with a piston near the beginning of the barrel and had scratches! There is no reliable centering there! We must accept the Truth as it is! Most damage to the barrel is caused at the beginning of loading the gun due to the large leverage and backlash of the harpoon relative to the axis of the barrel! I always write what I know for sure from my repairs!

 

[tromic]

Here is an interesting diagram regarding O-rings and pressure. Most elastomers for O-rings are made for pressure up to 103 bar (1500 psi)
In example I gave at image above the average over pressure is more than 220 bar.

 

[Zahar]

Almost all rubber O-rings after 50 atm begin to behave like plastic with a decrease in elasticity and a decrease in the coefficient of friction! Silicone O-rings work better!

 

[tromic]

Calculations are wrong! The piston will travel as long as the shock-absorbing piston sleeve will allow it! You are interested in the problem of water getting behind the piston into the barrel! I gave reasons! The taper shank of the harpoon in Italian guns has a minimum distance for the work of landing in the piston hole and the same distance when undocking from the piston! This is the whole feature of the cone landing!

Yes Zahar, the piston will travel as you said, until landed on the face of shock absorber, it is obvious. I was calculating something other. I think you did not catch it because I omitted some additional data. Energy of the piston would be consumed over the distance of 5 mm. That is because the amount of water in front of the piston having the speed 207.27 m/s at the exit from shock absorber would have the same kinetic energy as the piston, 4.8 J. The mass of the water in front of the piston on distance 5 mm is 0.2238 gr.
(0.0002238 kg) * (207.27 m/s)^2/2 = 4.8 J

 

[popgun pete]

The piston is aligned in the inner barrel during the shot by the spear which bridges the distance to the muzzle and which is centred in the muzzle by the stop ring sitting in a recess in the outer muzzle nose. Scratches in the inner barrel at the muzzle end are due to foreign materials in turbid water being dragged into the gun during loading on wet barrel guns via the various muzzle openings which was the majority of guns through the major development years. When Mares made the first Sten, and I have two of those, the metal piston was much shorter and carried a single short back to back cone type seal. This piston gave a very fast shot, but under loading effort could rock in the barrel as evidenced by a burst of air escaping, however this only happened rarely. During the shot the piston hit directly in the centre of the metal shock absorber anvil, so the piston did not wobble around during the shot. Mares moved to a three seal longer metal piston, then they replaced the front cone seal but retained the rear one. The piston then carried a front annular plastic cylinder that acted as a piston nose guide. When Mares switched to plastic pistons this front guide just became the long front piston body. The piston nose is long because it has to bridge the length of muzzle that opens out in diameter where the muzzle relief posts are located. All these designs hit dead centre on the shock absorber anvil as the piston nose locates in the cupped recess in the anvil as evidenced by impact marks.

Sten and Cyrano muzzles have a small short bore in the piston nose cap that is just slightly larger than the stop diameter on the shaft, which is why unless you drill it out a Cyrano original model will not accept an 8 mm shaft as the 9 mm stop diameter on the spear will not go through that hole. Why I mention this is any shaft wobble caused by the piston rocking could cause the stop diameter to hit that bore on the way out, but I have never seen any sign of that happening.

Mares later put a ring of tiny bumps on their plastic piston nose, but these sit inside the shock absorber bore as you can see them when looking inside the muzzle of an unloaded gun. Omer put much bigger bumps on the piston nose which sit on the anvil as when looking through the muzzle ports you can see a gap between the piston nose and the anvil with the gun pressurised which is completely stupid. It virtually makes the piston a meat tenderizing mallet hammering away at the anvil surface which is why in later offerings they gave you a spare anvil with each new gun! These bumps should be removed, however a check needs to made that as the piston now advances in the gun in the resting condition that the piston seals are still in the inner barrel.

Finally the shock absorber anvil locates in the muzzle by the rear diameter being a close fit in the muzzle body. On some guns it is held by a removable nose cap, on others it inserts via the muzzle rear and is such a snug fit that you have to move it along with a prod to remove it from the muzzle. That is what I had to do for the photos shown above. That shock absorber is new as it was removed during a muzzle swap on a Sten Competition Line gun that had been damaged during mishandling on land, only the muzzle body needed replacing.

 

[tromic]

One Spanich guy asked me how is it possible for water to get inside the speargun passing by the piston and not to lose the air?
In the speargun you might have a pressure 24 bar - 30 bar depending on the barrel size 11 mm or 13 mm.
When you shoot, the piston can have a speed of 30 to 40 m/s and kinetic energy 4.8 J to 12 J depending on the size of the speargun and the weight of the piston and the pressure, of course.
If there is some small amount of water in front of the piston that stops the piston 5 mm in front of shock absorber an enormous pressure will
build up: 220 bar - 382 bar (average - maximum might be 2 x higher). So you will have on one side of the piston say 30 bar and on the other 382 bar. O-ring can not withstand that pressure and will extrude allowing some water to pass by it into the speargun.
While water is going inside the air cannot go outside because it is on much lower pressure than the water.

 

[Zahar]

One Spanich guy asked me how is it possible for water to get inside the speargun passing by the piston and not to lose the air?
In the speargun you might have a pressure 24 bar - 30 bar depending on the barrel size 11 mm or 13 mm.
When you shoot, the piston can have a speed of 30 to 40 m/s and kinetic energy 4.8 J to 12 J depending on the size of the speargun and the weight of the piston and the pressure, of course.
If there is some small amount of water in front of the piston that stops the piston 5 mm in front of shock absorber an enormous pressure will
build up: 220 bar - 382 bar (average - maximum might be 2 x higher). So you will have on one side of the piston say 30 bar and on the other 382 bar. O-ring can not withstand that pressure and will extrude allowing some water to pass by it into the speargun.
While water is going inside the air cannot go outside because it is on much lower pressure than the water.

Very doubtful conclusions regarding 220 - 380 atm! There are no conditions for creating such pressure! Otherwise, the 0-ring of the piston would simply be cut by pressure! Technological gaps of the movable piston shock absorber can give excess pressure within the minimum limits without destroying the piston O ring!

 

[tromic]

Very doubtful conclusions regarding 220 - 380 atm! There are no conditions for creating such pressure! Otherwise, the 0-ring of the piston would simply be cut by pressure! Technological gaps of the movable piston shock absorber can give excess pressure within the minimum limits without destroying the piston O ring!

What is doubtful?
Energy of the piston is Ek = 4.8 J
It is consumed over the distance L = 0.005 m
Force is F = Ek / L (N)
F = (4.8 J)/(0.005 m) = 978 N
Piston is acting on a water in front of it minus the cross section of the shaft tang so
Area A for 11 mm piston and shaft tang 8 mm is A = 0.000044767 m2
The pressure on water from the piston is
p = F/A = (978 N)/(0.000044767 m2) = (978 N)/9.81/(0.000044767 m2)/10000 kgf/cm2 = 222.7 kgf/cm2 = 222.7 * 1.0197 bar = 227 bar
The time t in which this pressure is present is less than 2 ms! I doubt that it can destroy the O-ring permanently.

 

[tromic]

I have come to the conclusion that there is a downside to loading a speargun underwater. When the shaft is inserted into the speargun under water, the piston already rests on the shock absorber and all the excess water is pushed out by the shaft during insertion. The type of vacuum seal is irrelevant. On shoot, when the piston hits the shock absorber, the shock absorber and the water act together in parallel as a shock absorber, and the increased pressure of the water that has remained in the gun also occurs. However this increased pressure is lower because a part of the energy is absorbed by the shock absorber. Also this amount of water is now smaller because the braking distance of the shock absorber is usually less than 4 mm. Specifically for the Mares Cyrano shock absorber for 4.8 J, the braking distance without water in the head is 3.1 mm.