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Choosing spear for dry barrel gun

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.

After a "low" power shot the gun is easier to reload because the only air that expanded in the gun was that in the inner barrel and the rear "pre-chamber" which are always connected. The front tank, which is on the other side of the partitioning bulkhead that forms the "pre-chamber", will still be at cocked gun pressure. You leave the gun on the "low" power setting so that when muzzle loading for the next shot you only need to compress the air in the sections that the air actually expanded in for the previous shot. That way you don't lose the effort expended in raising the front tank to cocked gun pressure when first loading the gun, the energy stored in there will stay there until you flip the power regulator to "high" power and take a shot. Moving the power regulator from "low" to "high" after a low power shot and before muzzle loading for the next shot causes the pressure to equalize throughout the gun, so that stored loading effort is then wasted. You only do that at the end of the dive as pneumatic guns are best stored without any pressure differentials existing inside the gun, or you take your last shot of the dive at "high" power.

If you want to use the "easy load" feature, which is one of the main reasons for the manufacturer incorporating the partitioned reservoir system in the guns, then you always muzzle load the gun on the "low" power setting. You can then choose to use all of that loading effort at "high" power, which lets all the air in the gun expand, or use only some part of it by selecting "low" power instead. This is all explained in the "Pneumatic Speargun Compression Ratio" thread which is somewhere on this forum.

http://forums.deeperblue.com/pneumatic-spearguns/86054-pneumatic-speargun-compression-ratio.html
 
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Tomi, this procedure makes a lot of sense. The O-ring is actually a bit too big for the 6.75mm spear, so when not compressed by the outer part of the muzzle, I think it might have enough clearance to let the water out when inserting the spear. Maybe this is one of the reasons they designed it like this(?).
 

If O-ring is not compressed, water will get out, during loading, with any shaft from 6.5 to 7 mm. That is not a problem. The only problem is would water (left in muzzle from any reason, poor sealing...) get out fast enough during shooting! If not, regular shock absorber will be almost useless, with any usual vacuum barrel kit for free shaft. The result will be premature breakage of piston. The safest way of loading the gun is to empty water from muzzle and insert the shaft above water (of course sealing must be good). This is not very convenient and that is why I am looking for some other solutions, like usage of hydro damper with free shaft.
 
Hey Tomi,
I promised to get back to you with some info on the Evo-Air.
So, I finally got around to measuring the insides as much as I could on the spare one that I have - for a Cressi (sent to me by accident).
I have a digital caliper and the measurements are all done with that. But I can't reach deep enough into the muzzle to measure directly, so I had to probe with pencils etc. until I found the perfect fit and then measure those. Though it doesn't sound like it, I feel the measurements are still quite precise.
So, for the o-ring. It is 13 X 3 (OD X W) which makes it 7mm ID.
That's hardly a surprise as they originally advocated the use of 7mm shafts as far as I recall.
The OD of the o-ring seat is app. 12.2mm (I'd say +/- 0.1mm) and when the o-ring sits in its deepest seat its ID is 6.2.
I'd say that explains why it will work with spears from 6.5-7.0mm but also, it sounds like a lot of compression and thus extra friction if used with a 7.0mm spear. I use a 6.75mm, so that might be better for this o-ring.
I was wrong in saying that the outer muzzle puts any compression on the ring when screwed in all the way. Actually, it leaves a gap of 3.2mm and the o-ring width is 3.0mm. When unscrewed by one turn which the manual advices to do before firing, then the outer part of the muzzle screws out about 4.4mm (in addition to the 3.2mm). "Up there", lifted off the bottom of the seat, the o-ring is no longer compressed at all. That was an easy test - I let one o-ring sit deep in its seat where it can not just be shaken out but has to be plied out and then I put another o-ring (the kit comes with two extra rings in the same size) on top of it. That one easily falls out by itself.
So, it seems like it is actually pretty much a Tomba but with the extra screw-in tip of the muzzle. But I suspect your o-rings and the groove design are chosen to work with less compression, right?
I think I might look for an o-ring which would be a better mix for my 6.75mm spear, though especially guys with 7mm spears should be looking.

Best,
David
 
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I was wrong, actually it seems like the compression is not bad at all, maybe even on the light side. Even with a 7mm shaft, compression is just around 13% and for 6.75mm it's close to 9% - That is if I used the calculator correctly...;-)

I used this online o-ring calculator here:
ERIKS O-Ring Calculator

And the results for both 7mm and 6.75mm spears are attached:

Tomi, can I ask how much compression you are aiming at when you design the TombaFs?
 

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Excellent job David!
I make such a design for TombaF that Calculated Values at Excentrical Position of Rod would be at least 3 %. If this value is 0 or negative flooding might happen. Medium compression is about 13 - 14 %. :friday

But if you are sure your measurement were accurate (OD of well for 7 x 3 mm O-ring) than there is no moving O-ring. The friction is not high enough with 13 % compression nether for 7 mm shaft nor for thinner shafts!
 
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I redid the measurements just now. I can lower the D of the o-ring seat a tiny bit. It's likely 12-12.1mm but that doesn't change compression much.

Also, I can't definitively say whether the seat is tapered (like your 4% angle) but for now, I would say that it is not. It looks more like it has the same width (12.1mm) in a groove that is about 3.2mm wide. But then the bore flares out to at least 13mm for the next 4.4mm or so. The transition between these two diameters is at an angle. This angled step looks to be app. 1-2mm wide.

Quick sketch below.
 

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Very good David!
I would say it is basicaly same as TombaF for 7 mm shaft. Neither Tomba has 4 deg angle. It had 4 deg angle long time ago. I use grease to have low friction between the O-ring and the shaft. I would not recomand to use Evo-Air neither with 6.75 mm shaft nor with 6.5 mm. I would use it same as TombaF700. I would also put grease on O-ring because it is not movable anyway. You can screw the tip in and out one turn to facilitate water exit if loading in water, or you can modify the shaft tail as I suggested for TombaF, to let exces water out during inserting the shaft.

Calculated Values at Excentrical Position of Rod:
O-Ring Compression (%) = - 2.09% for 6.75 mm shaft.

Calculated Values at Excentrical Position of Rod:
O-Ring Compression (%) = - 10.7% for 6.5 mm shaft.

When O-ring Compression is like in examples flooding will happen!
Flooding would, maybe, not happen if you use something similar to my Easy Loader or Giga-Press! You must not bend the shaft during loading.
 
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Yes, I do think your modification idea of the shaft tail was an easy and clever one. I think, I'll do that.
But what is Excentrical Pos of Rod? What does it mean?

I did put grease on the o-ring, but I think it would be a good idea to regrease it pretty often. I think it gets "blown out" and rubbed away after a while.

I am beginning to wonder what the benefits to the Evo-Air is then. If I leave the outer muzzle turned loose by one revolution, then I guess it behaves like a Tomba... Remember, Seatec recommends to screw in the muzzle before loading, I am speculating that they do that to make sure the o-ring is pushed back in its seat and wont furl or jam when inserting the spear. But a 13X3 ring is a pretty sturdy one in itself. I don't think the risk of it jamming is high. Or maybe Seatec feels that if the o-ring isn't pushed back in its seat by the outer muzzle, it is not sure that the shaft and beginning vacuum would push/pull it back there? But then it does on your Tombas...

D.
 
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It would be better to screw in and out because you already have that posibility. You might make a modification to the shaft if you will have the screw tight.

Excentrical Pos of Rod: Position of rod during loading the shaft if bending it, in case of pneumatic spergun, or any other cause of excentrical position of the rod.

Before each using the gun put some grease on O-ring and wipe the shaft with oily cloth. But if Evo-Air would work as it is supposed you must not use grease! You want to have friction to move the O-ring in front position.

If you will have the screw loose before loading you would not have sealing! You must have it tight! You must have that minimal compression of 13% all the time.
Compression could be lower (8 - 10%) but than you should use loading device not to bend the shaft.
 

Got it. And also understand about the excentrical issue now. But how does the TombaF make sure that the o-ring is moved back into the tighter seat when loading then? You have more compression adding more friction or what?

D.
 
Diving Gecko said:
Got it. And also understand about the excentrical issue now. But how does the TombaF make sure that the o-ring is moved back into the tighter seat when loading then? You have more compression adding more friction or what?

D.
Click to expand...

Very first Tomba for fee shaft TombaF7 had movable O-ring and 4 deg cone. TombaF700 (TombaF650...) have no movable O-ring. It has fixed position O-ring and same compression all the time, the lowest possible, but to prevent flooding. I think similar to Tomba is Tovarich but it have (or it had) O-ring near to the tip of the muzzle. You have to use some dentist tool to change it. Tomba has O-ring deep in a muzzle adapter. Look at the picture: https://www.dropbox.com/s/x8x9r64o0kuxhxh/Tomba.pdf
You change or inspect the O-ring unscrewing the delrin part.
 
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Ah, so now you don't have the 4 deg. angle or another step with a bigger diameter? The o-ring has the same compression all the time and stays in a groove with the same diameter when loading and when shooting? It looks like that in the last TombaF drawing on the PDF.
Do you mind me asking why you changed the design?
 
My guess is that with another step on an angled groove, you couldn't be sure that the o-ring wouldn't actually be pushed into the tighter end of the groove by the spear and vacuum alone? If that is the case, then the Evo-Air starts making sense - it manually manipulates the o-ring into the tighter seat to make sure it goes in there.
 

That was just an early idea. It worked but the design was unnecessarily more complicated than it shoud be. And the benefit in energy was less than 0.7 %. There was no sense to go with it. It is better to keep things as simple as possible, and to be more reliable. Same benefit would be with Evo-Air (over TombaF700 or Tovarich) if it had worked as it was supposed.
 
Ah, get it. Yes, machining it without the step or angle is surely easier. But the Evo-Air does work as intended - You have probably seen the video where they let a spear slide out by itself in the direction of shooting whereas it is held firmly in the other direction. I tried that myself and it does intend work like that. The o-ring expells from its seat quite easily and there is very little friction when it is the bigger seat.

D.
 

David, have you fogot about vacuum and water pressure on O-ring?
In real use in water it will not work as in air, without vacuum and pressure of water! It will be in place all the time except in the last moment when water left in a muzzle will shoot it out.

About what I said less than 0.7 % benefit.

Here is a simplified estimation:

Let say you have a shaft under constant force of 30 kgf on distance 1 m, after shooting. The work of that force, lets say to be

W1 = 300 N * 1 m = 300 J

Lets say O-ring friction to be 0,2 kgf. That will be

W2 = 2 N * 1 m = 2 J

So due to O-ring friction we lose 2/300 * 100 = 0,66 % of energy.

In real friction of O-ring will be even less than 0,2 kgf. It drops down with speed of the shaft.
Did you get it?
 
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Yup, the point is that the friction is relatively small and negligible because the energy of the spear is quite high, right?
 
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