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Testing the Salvimar Pneumatics

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.
tromic said:
I am glad to hear that. Italians are also happy with it. Maybe it is really not a problem except in a few cases?
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Well, I don't know. It's still pretty new on the market. I believe that the number of shots also could be a relevant factor.
The shown problems/tear could be a result of water in the barrel i.e. initially wear/a non-sealing cuff.
Jégwan
 

It could be the reason for damage of the sealing cuff if only once the gun had been loaded out of water or discharged the water before inserting the shaft, if cuff is not greasy. Damage would onset during shooting in deep water because of ambient pressure acting on sealing cuff. This is quite opposite to Tomba, Evo-Air or Tovarich. Accommodation water, as Pete explained on his image (http://forums.deeperblue.com/threads/testing-the-salvimar-pneumatics.100351/page-3#post-934392), prevents damage to the sealing cuff, if sealing is anyway good.
 
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Yes. Next thing is a hight speed water column passing through the reduce square section of the cuff...

By the way; just been one month in Croatia. Not many catches - but what a lovely country <3

Jégwan
 
Looking at the first photos supplied by Gazz the sealing lip looks OK on his torn vacuum cuff, hence there it appears the nozzle body developed those tears or cracks first, but unless he can tell us for sure there is only so much you can tell from a photo. The evidence is confused in the second example by the gun being used after the seal failing and then more damage to the vacuum cuff developing subsequent to that failure, such as those grooves discussed in my previous post.

Tipping water out of the muzzle before inserting the spear tail would be a factor, but we have not heard from the users on this aspect. Regardless of this the causal problem may be that the nozzle section of the vacuum cuff has too much room to stretch forwards when pulled and that cracks it. Another possibility is a bad batch of rubber compound has been used in the cuffs, it can happen as I used to work in the rubber industry.

Although unlikely, the front section of the Salvimar muzzle needs to be screwed up tight, it may be possible to have a seal and it still slightly opened up. That gap in the "accommodation space" diagram is there because that image has the front section not screwed right back on the rear body, the outer rim of the front section has yet to hit the step in the rear body.

I have now adjusted the image to fully tighten the front section as can be seen here. The circumferential grooving observed before should not occur as the edge of the contacting "ring" is right back against the rim of the vacuum cuff flange.
 
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My thoughts are that these vacuum muzzle systems should be dedicated to one shaft size only rather than a multi-size unit. Therefore you would have separate vacuum cuffs for each shaft diameter and a separate muzzle nose for that shaft size, the rear body of the muzzle can remain the same. Or the muzzle body front section has yet another element that changes the exit hole diameter which may require an additional gasket to seal its connection to the front body. Inventory of parts then becomes an issue as the market for each shaft size may not be the same, hence there could be a reluctance to manufacture the less popular size if that was the case.
 

Pete, this is the part where you and me have quite opposite view! When I recall my previous attempt to make Tomba7 using O-ring(s) fixed in muzzle the main problem was damaging of O-ring because it was pushed against muzzle body while the shaft tail step was passing by it. Well lubricated, O-ring lasted for a few shoots. But every new shot made some damages until the vacuum being lost.
It would be good to know, what occurs first, the rupture of the body or damage to the lips of the cuff?
Another thing regarding accommodation water. This is also more complex than might look like. I know that because there are some similar fenomens like I had while had designing hydro damper. It might be important if you shoot downward or upward. If shooting downward water will be where it should be, but if shooting upward water will be between piston and shock absorber.
 
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Well this is essentially a rubber tube being stretched, not an "O" ring. Actually if the front end of the muzzle is screwed right on then the space the front of the vacuum cuff can move forwards is less than I thought before, but the vacuum cuff may seal OK by just sitting on that thin circular rim if it is a tight fit on the rim (until you shoot the gun and it pulls off!), in which case the front section may not have been fully tightened up. Only our users can advise one way or the other, but not much more info and a few questions raised earlier remain unanswered, however they may not have the time available to respond yet.
 
If I can conclude from previous reports, shaft must be loaded in water and water must not be tipping out before loading the shaft - if the sealing cuff is not greasy. Sealing cuff would last longer if it is greasy.
Italians mainly empty the water before inserting the shaft and have the sealing cuff always greasy!
Some of them reported that the seal have been lasting for more than 9 months, with tang!
 
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A comparison between the inside/dimensions of the Salvimar, Pelengas and the Devoto vacuum system would be interesting.

Have any of you found a cross section picture or drawing of these or know how the vacuum cuffs in these perform over time?

Jégwan
 
I just got my Salvimar 13 mm kit for 7 mm shaft, with DHL from Scubastore, 75.9 Euro! Nice kit, but without any manual how to use it. Pete, clearance from the tip of the sealing cuff to the front inner surface of the muzzle is about 1,5 mm! Sealing cuff is well centered and firm in place. Sealing lips ID is 5.8 mm.
 
Check to see if screwing the front section of the muzzle onto the rear section appears to be tight before the front section is fully back on its screw thread. It seems to me that the cap type vacuum cuff will seal OK without the front section screwed on properly, provided the vacuum cuff is a close fit on that thin cylindrical rim it sits on. I queried the closeness of the fit there before, but had no answer. Problems will occur if the front section is not fully screwed on as that increases the gap to the face of the cuff, but it may still provide a vacuum seal. Otherwise I don't see how the damage we have been looking at has been generated.
 
This is what I think is the problem:

If not lubricated, due to friction and ambient water pressure (2.4 cm2 of the cuff outer surface is exposed to the pressure) the tip of the cuff will be squeezed between the muzzle and the tang of the shaft. The tip of the cuff is inside similar to O-ring. That is the same problem what I had with Tomba7 as I told before. The top side of Salvimar kit would be better with 8 mm shaft than with 7 mm as on image. For 7 mm shaft boring in front of the cuff should be maybe 8.6 mm, not 9.6. I would also make more clearance, more than 1,8 mm, maybe 3 mm. Similar problem I had once before with rubber tubing on Sten 11 shock absorber. Because being greasy and free to move in-out, it was pulled down from the shock absorber body and became pinched in front of the its body. After that Marko made a delrin ring behind of it to prevent moving of shock absorber.

Before front section of muzzle become tight on lower part it makes contact with the sealing cuff rim. About 1/8 turn should be made more to be tight. The central part of the lower muzzle part, where comes the cuff supports the cuff 4.8 mm deep while the cuff rim is 3.9 mm thick. It means there is no support to upper part of the sealing cuff, only 0,9 mm from total 7 mm. Also the space for accommodation water is very small. You can estimate that from the image.

There is still another possibly cause of damage that might be responsible for rupture in cuffs body:



The rubber body in a designated place, bends and stretches over a sharp rim. Maybe the rim height should be shorter for 1 mm, or just rounded little more.
 
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The height of that inner rim is much higher (B) than I thought that it would be (A), in which case the vacuum cuff collapsing from ambient pressure could fatigue the rubber on any sharp transition in bending by the edge or corner positioned under there. However there were no internal marks apparent in Gazz's cuff and we have not seen inside the other one that was similarly damaged. The extended inner rim may serve to hold the nozzle section open and resist it collapsing at the rear, but one large crack or tear is very close to the flange base in Gazz's photo.

The circumferential grooves or gouges worn in the rubber cuff on the outside, near the base of the nozzle section, still need to be explained, but maybe they are irrelevant if they only occur by shooting with an already damaged and leaking vacuum cuff.
 
I also think that this second damage scenario is more likely than the first one I explained. The reason for that is because the initial friction, shaft to sealing cuff, is highest only in first moment on shot. As the speed increases the friction decreases so the tip of the cuff would probably not be in contact with the muzzle when the tang is passing by it. That would be a good news because it would be easy to make correction, to solve the problem.
 
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In the absence of a large numbers of failures being reported this may all be a non-issue and related to operator error or just fair "wear and tear" after a large number of shots through the gun. What I have learned is that the Salvimar vacuum cuff seals in the muzzle body as a cap, I did not know about the thin rim or fence it presses onto before, whereas the "Taimen" and the "Pelengas" cuffs seal inside the muzzle body as washers. If the latter guns do not have tightened muzzle body sections then the sealing will not be achieved, but maybe it will be with the cap type cuff and that allows the gun to be fired without the cuff completely immobilized. Maybe you could check if the gun still holds a vacuum with it loaded (use very low pressure in the gun) and the forward section of the muzzle then unscrewed and removed.
 

I do not need to check it. It will hold the vacuum. OD of the tubing (rim) where the cuff comes over is 10,35 mm, while the cuff ID is 9,2 mm so it must be stretched to be in place.
This cuff could be used with 6.5 mm "free shaft" too, because the ID of front, sealing, end of the cuffis 5.8 mm. But that would require better centering of the shaft - different front part of the muzzle, possible with delrin guide.
I have read some comments on Italian forum after I stated that the sealing must be regularly lubricated. They were talking about what would be good for lubrication like silicone grease, lithium grease, vaseline, lard...
 
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Whatever grease they use it should not be too sticky or affect the rubber as some silicone greases can do. The same grease as is used in the gun should be OK, just wipe it on and then rub most of it off as only the smallest amount will suffice. You can also do that with the spears, rubbing most of it off to leave a gleam or shine on the spear shaft surface. Too much lubricant and sand will stick providing an abrasive paste to damage the cuff and the spear with repeated shots. Personally I would use oil as the lubricant film on the vacuum cuff which will be very thin and less likely to hold sand particles. That is what I use on the "Taimen" before each dive, just a couple of oil drops.
 
Reactions: Jegwan
Adjusting and using your muzzle diagram this may be how those circumferential grooves are formed in the vacuum cuff.
 
What you drew, Pete, is also a possible scenario for damage of the cuff.
 
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