Lowering Barrel Friction (Project Polishing Inner Barrel)

[tromic]

Well spotted, Diving Gecko!

 

[Diving Gecko]

Thanks Tomi!
What is interesting about your numbers is that they show that a reduction in friction doesn't pay much off in terms of overall efficiency.

For example, I have heard that stainless steel barrels could have 12-15 times less friction than the stock aluminum barrels but if a stock barrel still only accounts for about 1% of reduction in efficiency/energy loss due to friction, it is not a whole lot of gain you get with a steel barrel.
Also, it will be a struggle keeping the gun from gaining weight with a steel barrel.

So, from a cost-benefit or workload-benefit point of view, there are other modifications that may make more sense.

 

[spar]

OK, even better.
Can you tell me a bit more - did you make it yourself...? Did you use the ends of the old alu barrel in the handle and nose cone area or is the whole barrel made from stainless steel? How did you solve the issue about the barrel needing to be 18mm at those areas?

Sorry for all the questions, but it is a modification I am interested in having myself.

i made yourself all new barel of inox

 

[MarsArtis]

Nice collection of spear guns and - Tomba!
I suppose that hard loading of LG has the same cause as the elder Tomba700 had!?

It's hard to say.
But actually I think is mostly due to high pressure and yes to axial loading action too.

But we are rough men.



Inviato dal mio SM-T705 utilizzando Tapatalk

 

[MarsArtis]

Aha, very good to know. Anyways, it was a fun project and my mental spearo is happy about knowing he gained about 0.5% of efficiency, haha

From practical experience, the dynamic friction at low speeds is certainly at the least 5 times less than the static one. How do I know? Because of the way my test was set up;-).
Eg. with the Mares piston and unpolished barrel it took 2178 g to get the piston moving but as soon as it did, it would continue to move on its own - even without the force of my hand on top of the scale. The weight of the scale and "scale holder" was 415g, so that is about 5 times less than what it took to get it moving.

I think it is very plausible that dynamic friction could be only 1/10 of the static.

I think you're mixing up dynamic and static friction.
Your tests were all about static friction, mostly because no external force was involved. Except gravity and friction itself.

Tromic pointed out clearly the relationship with a 30 ATM pressure and the resulting 0.5% efficency gain.

A side tought:
major brands like Mares and Cressi still continue to use classic skirt orings for their pistons.
Obviously this is related to their long testings that resulted in best performance/efficency+security ratio configuration.



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[Diving Gecko]

I think you're mixing up dynamic and static friction.
Your tests were all about static friction, mostly because no external force was involved. Except gravity and friction itself.

Tromic pointed out clearly the relationship with a 30 ATM pressure and the resulting 0.5% efficency gain.

A side tought:
major brands like Mares and Cressi still continue to use classic skirt orings for their pistons.
Obviously this is related to their long testings that resulted in best performance/efficency+security ratio configuration.



Inviato dal mio SM-T705 utilizzando Tapatalk

I could have, but I don't think I did...?

You are right in that I tested for static friction and most of the talk has been about that but the post you are referring to talks about my small, unscientific observation of what happened once the piston started moving.

I point out that once the piston actually moves, it does so with a weight on it that is 1/5th of what it took to get it moving (since my hand is no longer pushing down on the scale). Granted, we are talking low speeds, but nonetheless it is moving and as such no longer static(?).

And yes, it was indeed very illuminating to see Tomi's math on how little friction impacts efficiency overall.

 

[Diving Gecko]

A side tought:
major brands like Mares and Cressi still continue to use classic skirt orings for their pistons.
Obviously this is related to their long testings that resulted in best performance/efficency+security ratio configuration.



Inviato dal mio SM-T705 utilizzando Tapatalk

You could be right.
But I wonder if it doesn't also have a lot to do with economics. Surely, one could design a slightly more efficient piston 30-40 years since they designed the ones they use now. But it would cost money in R&D and that could very much be the reason they are sticking with a known design. Nothing wrong with that, but sticking with it is not proof that it is the best.

As much as I would like to think the major manufacturers test and test and then test some more before they release their products to us, recent years have shown that may not to be the case.
Mares Cyrano Evo HF (or whatever it is called) has some pretty bad issues with line releases breaking or getting stuck. Sporasub has quietly upgraded pistons and the tail end on their One Air but its line release is still too short. Salvimar PredaTHOR seems to be doing well so far except for reports of sliders breaking. And the Vuoto seals themselves don't really seem to last long if using slider setups. (I got less than 50 shots on one of those seals.)

I can offer another theory - since a lot of these products, if not all, are produced in China, there is a real risk that the production runs of these parts are not made to the standards specified in the contracts. I live in China and a lot of my friends work in production and I hear a lot of these examples.
This is why the biggest brand names producing here have very strict quality control and try to control the supply chain more rigorously than they might have had to do 20 years ago in, say, Italy, Germany or the US.

The handful of issues with the newest designs could be a combination of not enough testing or not enough quality control (which again costs money) but of course, these guns are still fantastic guns when they work, which luckily they do most of the time. No doubt about it.

 

[popgun pete]

Stainless steel barrels can be polished to a mirror finish inside the bore as that is the final contact surface, but alloy barrels have an oxide surface that has some microtexture over the parent metal surface which itself may be smooth underneath. If you over polish the alloy tubing then you may just be making the ID larger, especially if you remove the original anodized layer. Anodized aluminium can be hard and scratch resistant, unlike the parent metal itself which is easily scratched.

 

[Diving Gecko]

Stainless steel barrels can be polished to a mirror finish inside the bore as that is the final contact surface, but alloy barrels have an oxide surface that has some microtexture over the parent metal surface which itself may be smooth underneath. If you over polish the alloy tubing then you may just be making the ID larger, especially if you remove the original anodized layer. Anodized aluminium can be hard and scratch resistant, unlike the parent metal itself which is easily scratched.

I agree on all counts.
But the real question now, I guess is how much more overall efficiency can be had from changing to a highly polished SS barrel.

I have seen estimates saying that an SS barrel has about 15 times less friction than an alu one. But if barrel friction, as have been mentioned, above only accounts for about 1% loss of power in the gun, then even lowering that loss by a factor of 15 wont give you any real world advantage. Or am I misunderstanding something?

I am asking because I know of a few "high end" tinkerers who in their quest to build the most powerful airguns go for SS barrels.

 

[popgun pete]

The "Taimen" has a mirror finish stainless steel inner barrel, so I guess it must have an advantage, but there the motive was to extract performance from a small capacity gun where every efficiency was going to be needed. On a "Sten" you can have sufficient power without going to such attention to detail, plus there are costs to be considered and how much the market is willing to pay for a gun compared to the price of other offerings. Aluminium tubing is cheaper to buy and easy to machine. Stainless steel inner barrels are used more for their scratch resistance in turbid waters with suspended grit, such as in rivers where bottom material is being lifted by currents into the water column. That is why you see Russian spearguns with these types of barrels and before any move to vacuum barrels where you would expect the grit to be kept out to some extent.

 

[Diving Gecko]

One thing that was touched upon and which slipped a bit again was that the measurements should be taken under pressure. While the dynamic friction is substantially lower than the static, in the above example it amounted to so little that it seemed to have no tangible influence on the efficiency of the gun and thus the verdict would be that the modification is not worth the hassle.

But under pressure, the o-rings will be jammed against the wall and thus very likely increase friction a whole lot. So, if the friction reaches a level where it does indeed have an impact on the efficiency, then a 30% decrease in friction would be substantial.

Anyways, that is the what the people in favor of the stainless steel barrels argue and what I personally forgot to take into account.

I might actually order one for a future project. If I do, I will of course as always, keep you updated;-)

 

[popgun pete]

To maintain buoyancy stainless steel inner barrels are usually machined to be very thin at other than the ends where there are usually screw threads to be cut, so that adds to the fabrication cost. If the gun does not need to float and is required to stay put if and when dropped in say a river then the barrel weight is not a consideration. The Russian Pirometer (Seabear) is a heavy gun thanks to its stainless steel inner barrel, but was never intended to float, so the OD of the stainless steel tubing was left as is except for polishing where it extends forwards from the grip, the gun being of the classic rear tank, mid-handle layout. In that case the barrel polishing is just for the sake of improved external appearance and resistance to rusting (no tiny grooves for saltwater droplets to sit in).

 

[Diving Gecko]

The stainless steel barrel I have in mind is from 12x14mm ss tube. It's made by a very good Ukranian spearo, engineer and machinist living in Cyprus. The tube itself is not that much heavier than 13x18mm alu, but he says, the tube is a bit frail if you use it as is for the whole length of the gun. So, he adds a bit of 14x18 alu to the ends. That also helps it work fit in easily in standard guns. Plus beefs it up, especially at the muzzle end.
I can share his contact info for anyone interested. He also makes titanium vacuum muzzles and sliders and have a pretty neat tail end, too. Supposedly, he was actually the one who came up with the system that Salvimar now is selling a ton of... Down to the exact seals, he supposedly, has been doing this muzzle for around a decade.