Tomba - All in One barrel sealing

Why not to use both shock dampers together?

This might be the easiest to use. Just load and shoot, all under water, without empting barrel before inserting the shaft. Damper might be just regular, with rubber tubing, which can operate flooded, or combo damper (additional hydro damper). Mode of operation depends on design dimensions.

I will make this for my Cyrano 850 that has normal, rubber, damper, so I can load it without emptying water before inserting the shaft. I suppose it might work! Very similar to this is version for hydro damper. Difference is possibility of taking additional water for hydro damper.

A step to realisation!

You could run the shock absorber in the inner barrel bore and create a larger "hydropump" damper by in a sense turning the usual shape around in the gun. Water pumps out through a ring of small holes in the face of the muzzle. The piston (not shown) pushes the damper body forwards until it strikes the front face of the muzzle, which screws out to access the seals.

Thanks Pete! Despite of numerous different arrangements I analysed, this is a new one! I will think about it! :friday

Hydro damper would work fine. Only problem would be if loading under water (b). Would be better to insert shaft after emptying water from muzzle (a).

One more possibility, TombaF650W. The spring has force of about 1.5 kgf pushing O-ring on taper of 15 deg. O-ring is 6 x 2.5 mm for 6.5 mm shaft. This is not for hydro-damper, but for normal, rubber tubing, damper.

trromic said:

Thanks Pete! Despite of numerous different arrangements I analysed, this is a new one! I will think about it! :friday

Hydro damper would work fine. Only problem would be if loading under water (b). Would be better to insert shaft after emptying water from muzzle (a).

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Layout is what I thought was inside "Taimen" muzzle when I first saw external photos of that gun and did not know very much about it. Only thing that puzzled me was the ring of holes appeared to be too large, but now I know the "Taimen" does not have a hydro-braking muzzle. Instead it uses the vacuum cuff as an absorber. The ring of holes is to lighten the muzzle which is also the nose cone, hence the large diameter compared to the air tank.

Photo is of my "Black Sea" muzzle, but I have added extra holes to illustrate general idea of hydro-brake muzzle. Diameter of hydro-damper is large, but has relatively short stroke. When the hydro-damper is pushed by the piston with the gun in a discharged state the water flow ports in the front face are closed off by the front face of the damper. Developing vacuum in inner barrel during muzzle loading causes hydro-damper body to move rearwards in muzzle. As the hydro-damper moves rearwards water is sucked in to fill the hydro-damper cavity through the front water flow ports. The gun does not need emptying as water cannot get in behind the damper as the piston seals off the access there via the locating hole in the face of the piston because the front face of the piston and the rear face of damper inside the inner barrel butt up tightly against each other. Only requirement would be to load gun when near the surface, but still submerged.

If a cylinder of water is trapped by the spear tail entering the muzzle seal then it will end up down the rear end of the inner barrel when the gun is held with the muzzle pointing upwards to load it. During the shot that trapped water will form into an annular cylinder (refer to second diagram) that will hydro-lock on the rear of the hydro-damper and push it forwards causing it to absorb energy from the shot by pushing water from the front cavity through the ring of water ports in the front face of the muzzle. A question would be will the spear jerk free of the piston and the trapped annular cylinder of water around the spear follow it out of the barrel or will some of it go into the hydro-damper rear cavity as it opens up? Each time the gun is loaded any water in the hydro-damper rear cavity will find its way into the inner barrel as the hydro-damper moves rearwards to push it out (there is no seal at the rear of the hydro-damper), being drawn in by the developing inner barrel vacuum. It will only be a problem if the water gradually accumulates instead of escaping. The hydro-damper's rear cavity will have a bubble of air inside it initially, before any loading, so for any water to stay inside there that air has to come out, that air being at surface pressure. An alternative would be to taper the spear tail so that the muzzle seal only takes up from the moment when the spear tail first lodges in the face of the piston, then the same volume accommodation for water would be available both at the start and the end of the shot thus avoiding any water accumulation problem.

popgun pete said:

If a cylinder of water is trapped by the spear tail entering the muzzle seal then it will end up down the rear end of the inner barrel when the gun is held with the muzzle pointing upwards to load it. During the shot that trapped water will form into an annular cylinder (refer to second diagram) that will hydro-lock on the rear of the hydro-damper and push it forwards causing it to absorb energy from the shot by pushing water from the front cavity through the ring of water ports in the front face of the muzzle. A question would be will the spear jerk free of the piston and the trapped annular cylinder of water around the spear follow it out of the barrel or will some of it go into the hydro-damper rear cavity as it opens up? Each time the gun is loaded any water in the hydro-damper rear cavity will find its way into the inner barrel as the hydro-damper moves rearwards to push it out (there is no seal at the rear of the hydro-damper), being drawn in by the developing inner barrel vacuum. It will only be a problem if the water gradually accumulates instead of escaping. The hydro-damper's rear cavity will have a bubble of air inside it initially, before any loading, so for any water to stay inside there that air has to come out, that air being at surface pressure. An alternative would be to taper the spear tail so that the muzzle seal only takes up from the moment when the spear tail first lodges in the face of the piston, then the same volume accommodation for water would be available both at the start and the end of the shot thus avoiding any water accumulation problem.

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Yes, at the moment I see that as the only solution. Another problem or disadvantage with this upside-down design of damper is that the piston running distance is reduced for 1 - 2 cm. I have still one idea using this "upside-down" design, to use it without water exit holes. I was previously trying to implement that idea with piston but it would be much easier to do it with damper. I will make a sketch on Monday.
Cheers, Tomislav

Here is a new possible design, but... this would not work without water exit holes. Al least a few small holes are necessary because water volume on the right is always higher than on the left.

trromic said:

One more possibility, TombaF650W. The spring has force of about 1.5 kgf pushing O-ring on taper of 15 deg. O-ring is 6 x 2.5 mm for 6.5 mm shaft. This is not for hydro-damper, but for normal, rubber tubing, damper.

Click to expand...

Here is a modification. Maybe this would be more convenient for maintenance and part replacement, and there are less holes in a muzzle. This is supposed to be used with normal damper and to facilitate loading the gun under water not emptying water from the barrel before inserting the shaft. Stainless steel spring is inside delrin housing so I suppose there would not be significant electro corrosion to aluminium adapter.

Here I had shown several variants of possible solutions to "free shaft". The aim was to enable efficient and reliable work of shock absorber. Because it is not reliable enough in any vacuum system for "free shaft" that I know (this is only my opinion). The main cause of this is water that can be found in the shock absorber and that can reduce or prevent its operation. Existing shock absorbers can best work in guns without vacuum system or system as Tomba with slider or LG where water can exit after the slider leaves the muzzle. There are some solutions with hydro damper but they are not very conveniet for maintenance and are not very reliable for longer usage. It is not easy to inspect or change O-rings inside muzzle. Only safe way of using "free shaft" with vacuum barrel kit, is to empty all water from muzzle before inserting the shaft. This is neither very convenient, especialy with long guns. Not doing so, maybe nothig would happen after few shoots. Chance for some damage is higher if aiming down direction, and less if aiming horisontal-up. Aiming horisontal-up water would be between piston and shock absorber. After shooting shock absorber could work, but some water might be enjected behind the piston passing by the skirt seal. Pistons with O-rings are mybe better choice in this case.

The problem is that most pneumatic spearguns are designed for a pressure differential which is always in the same direction and hence the seals only have to operate in one direction. An "O" ring in itself has no directional preference, but the seat that it sits on may not seal equally well with the pressure differential going in either direction or during the transition from one side to the other. Certainly bi-directional sealing can be achieved and is used in many industrial hydraulic and pneumatic devices, but that will not have been necessary on the standard pneumatic speargun piston, particularly the ones that rely on the rear conical rubber or cup type seal and its flaring action under pressure which causes it to seal even better in less than perfect inner barrel bores. Those seals are also used in some hand pumps, one way they seal and push air, the other way they collapse their skirt and allow air to pass by thus causing the pump to breathe ambient air for the next pumping stroke. Water will equally well push past these seals by collapsing the rubber skirt on the seal if the pressure differential is reversed and that is a real possibility in a "sealed" vacuum barrel gun which has unintentionally flooded with water. What works in theory is not always realized in practice and I think this is why pneumatic speargun manufacturers have stayed away from designs where the seals may be overwhelmed and water can then penetrate the pressurized chambers inside the gun. Saltwater is very corrosive if it can get inside a pneumatic gun and it will then destroy smooth surfaces over a period of non-use where it can work on the same internal spot without being disturbed by any movement of the gun. Freshwater poses less of a problem and I think that this explains why hydropneumatic spearguns have persisted in places like Russia and the Ukraine and is also the reason that pneumovacuum spearguns have appeared there. Water on its own can cause corrosion, but salt in the water improves its conductivity and that allows a much faster reaction or galvanic corrosion to occur. Even 316 stainless steel, the most corrosion resistant of stainless steels, can have pits burned in it by saltwater over a period of time, so stainless steel inner barrels are not immune to corrosion.

I suspect that pneumatic speargun manufacturers, being realists, anticipate that their guns may receive little or no maintenance and hence have opted for designs which can tolerate a degree of neglect. If you are prepared to strip a gun down regularly and don't mind pumping it up on a frequent basis then more adventurous designs can be tolerated, but most gun users just want a gun that performs with little attention to maintenance and lasts for a respectable period of time before they need to replace it. Like light bulbs, manufacturers have some interest in spearguns not lasting forever and the introduction of the plastic piston has probably helped in this respect!

I had actually find the solution for safe "free shaft" 4-5 years ago. Sealing plug! My very first Tomba, made without a lathe, just using hand drill and screwdriver. Just have to make it more convenient for usage. I noticed that the "plug" had been expelled after each shoot, so I tied a thin spectra to it not to lose it in water after shoot. Of course, this strange slider would not be used!

trromic said:

I had actually find the solution for safe "free shaft" 4-5 years ago. Sealing plug! My very first Tomba, made without a lathe, just using hand drill and screwdriver. Just have to make it more convenient for usage. I noticed that the "plug" had been expelled after each shoot, so I tied a thin spectra to it not to lose it in water after shoot. Of course, this strange slider would not be used!

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"Plug" od slider like TombaF650W. This might be the most reliable and easy for maintenance Tomba for "free shaft". Easy to inspect and change O-rings. Loading under water. Water in muzzle should not be a problem.