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 Post subject: Re: Build from scratch?
PostPosted: Sat Jan 04, 2014 2:59 pm 
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rrdstarr wrote:
Just saying Bob, that I would feel better with a steel air tube then an aluminum one.


Depends what type of aluminium you are talking about though, there are endless varieties of aluminium. Air Arms is using aerospace grade aluminium in the new(ish) S510 Twin Cylinder guns, not sure exactly what it is they are using, but the TC SL Carbine has a weight of only 6.3lbs, pretty impressive for a gun that is 37.38" long and has two air tubes... :drinkers:

http://www.air-arms.co.uk/products/our- ... tra-detail

http://www.airgunsource.com/store/index ... 30ADO&c=77

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 Post subject: Re: Build from scratch?
PostPosted: Sat Jan 04, 2014 3:14 pm 
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Posts: 231
Location: Edmonton
EverHopeful wrote:
Thanks JSP.

Let's see if I'm following the conversation. I have a hopefully modest project in mind - a stretched 1377 with the valve separated into two halves to form an air reservoir between them and an increased pump stroke to make pumping more efficient. I purchased .75" OD .065 wall steel tube which the vendor says comforms to ASTM A513 which I found online at http://www.industrialtube.com/client/images/A513.pdf

TABLE S5.1 Hardness Limits and Tensile Properties for Round Tubing gives the yield strength for the various grades. The tube vendor doesn't specify exactly which grade it is, but gives a range: 1020 - 1026. Looking in the table and picking the lowest value to be conservative gives 60 ksi for 1020 DOM. Engineers Edge has a handy calculator for getting pressures from material strength and tube dimensions at
http://www.engineersedge.com/calculator ... t_calc.htm

So feeding in the tube dimensions, 60k yield strength and Bob's recommended 3x safety factor gives a working pressure of 4200 psi against a calculated deformation pressure of 12600 psi (and checking with the ultimate strength value from the materials table suggests the burst pressure would be 14600 psi), all assuming that any fittings could be made stronger than the pipe itself.

Did I get that right?

Now, of course the next point of concern is that I have no idea what the typical operating pressure of a 1377 is, but it seems unlikely that the simple, low tolerance pump mechanism can get anywhere near 4000 psi, so it looks like everything is good on paper. Whether I can make it without proper tooling is another question entirely.

Jim


I have devised a wonderful reply which this browser is too small to contain (I had a bunch of stuff typed out, but my browser crashed)

It looks good. Be aware that a513 dom can come in stress relieved which takes the yield strength down to 55ksi for 1020 and a worst case of 1008 at 45ksi. 45ksi would give you a MSWP of 2600 psi with a 3x safety factor. To calculate the max pressure of your pump you can take the area of the piston (0.307 square inches for a 5/8" piston) the leverage factor (as a wild guess I'm going to say 3, you can calculate it by dividing the length of the long arm of the lever by the length of the short arm) and the force pressing on the long arm of the lever (again a guess says 50lb).

So, 50 lb force multiplied by 3 gives us 150 lb force on the piston. Divide that by .307 and you get 488 psi at the pump. That number seems quite low to me so one or both of my guesses are probably way off, but you can use the math to get better numbers on your own. Even with a leverage factor of 6 which would mean a 3 inch pump stroke and a 18" long arm on the lever you're under 1000 psi. I'd guess you'll be safe.

You should go into the site search and look for the uber pumper by rsterne (bob). I seem to recall he did a version with a reservoir. He's sort of the local mad scientist and he's been very generous with showing his work including calculations. Just be careful, it's a pretty deep rabbit hole.

J


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 Post subject: Re: Build from scratch?
PostPosted: Sat Jan 04, 2014 8:47 pm 
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With stock springs a 13XX valve starts retaining air at about 1200-1500 psi, but you can double that by changing springs.... My .25 cal Millenium Pumper runs at 1800 psi using a Disco tube.... I had a 140 based .22 cal Uber-Pumper with twice the swept volume of a 13XX and at 20 pumps it just started retaining air with a valve about twice normal volume and a hammer spring heavier than stock, but not quite as heavy as a Disco spring.... It was shooting 900 fps (14.3 gr.) which is a bit stronger than a Disco, so I would put the pressure at about 2400-2500 psi, jugding by that and how hard it was to pump.... I would never attempt that again, the gun was MUCH more pleasant to use at 10 pumps and 743 fps....

Bob

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 Post subject: Re: Build from scratch?
PostPosted: Sat Jan 04, 2014 11:07 pm 
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Thanks for the replies - much appreciated!

JSP, Bob's pumpers were indeed the inspiration that got me started on this. I'd love to try and reproduce those designs more closely, but I think they are well beyond my skill level. Starting with something that re-uses 1377 parts is a first baby-step on what I hope will be a long learning curve :) Your comments about calculating the pressure produce by the pump had me thinking for quite a while. I think the key thing is that the hinged lever has a non-linear factor. Towards the end of the travel the pump head moves a very small distance as the end of the pump lever closes. Looks like the leverage factor is limited only by the manufacturing tolerances and the elasticity of the parts.

Bob, I'm heading in the other direction, clipping the stock spring to keep it under 500 fps. The current build with a FTP retains air at four pumps and maxes out at six. The tune would be nicer if I didn't have to worry about the 500 fps restriction, but it is what it is. On the plus side, at least the cocking force on the bolt is nice and light :) Somewhat amusingly, even with just the normal valve volume I noticed it gives two shots from nine pumps. What I'm hoping for with the next build is something that can get perhaps 10 to 20 shots per fill in the 450 to 490 fps range (with 177), without having to pump it forever (with that small diameter piston) between strings. It's that trade off between reservoir size and piston stroke that has me scratching my head at the moment! I made an estimate of the original valve volume, and it looks like you get about 4.5 times that volume per inch of tube given to the reservoir. Based on the very limited information I have (those 2 shots at 9 pumps), that suggests I need 1 to 2 inches of reservoir to get my 10 to 20 shots, and that it might take 50 to 100 pumps to fill it, if the pump was standard. I think I can double the stroke, so that brings it down to 25 to 50, and maybe a longer stroke is also more efficient (the lost volume at the end of the stroke being a smaller percentage of the total), so perhaps not too bad. It sure will be interesting to see what happens. I suspect this is a next winter project for me, so I'm going to have a long time to chip away at the options.

Jim


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 Post subject: Re: Build from scratch?
PostPosted: Sat Jan 04, 2014 11:52 pm 
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The pump on a 2100/2200 is 50% more swept volume than a 13XX, and the 140/1400 pump is double.... A stock 13XX valve is about 1.5 cc, and the 3/4" OD x 0.065" tube is 5 cc per inch, so you're pretty close.... Trying to get 10-20 shots without repumping, however, will be darn tough.... Let's say you are shooting an 8 gr. pellet at 475 fps, that is 4 FPE.... If you can get an efficiency similar to most PCPs (which is what you are building, with an onboard pump), then at 1 FPE/CI, 10 shots will use 40 CI of air at 1 atmosphere.... If you run an average pressure of 1450 psi (100 bar), you will use 0.4 CI of that 100 bar air to produce the 40 FPE.... and that works out to about 6.6 cc.... Your reservoir will have to be at least double that size (13 cc) so that the pressure drop is only 1/2 of the fill pressure, otherwise you will get too much velocity change over the string.... Pumping to 2000 psi and ending at 1000 works with the math in my example....

Now look at the pump.... The swept volume on a 13XX is 18cc.... You need to fill a 13cc valve to 2000 psi (138 bar), so from empty that is 138 x 13 = 1800 cc.... That is 100 pumps at 100% pumping efficiency, and you can probably count on only about 80%, so more like 120 pumps from empty.... To refill from 1000 psi to 2000 would likely be about 60 pumps.... If you double the swept volume of the pump to 36cc, cut that in half, about 30ish pumps to refill after 10 shots.... If you want 20 shots, double all the above numbers....

My Millenium Pumper is a .25 cal and gives 3 shots at 40 FPE from a 28.5cc valve running at 1800 psi.... It takes 40 pumps of a 53 cc pump to get back to 1800 after those 3 shots....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 05, 2014 7:39 am 
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Thanks Bob, that helps bring a healthy dose of reality to the project. I'll aim for the larger end of my reservoir volume estimate and see what happens. If I could get 10 shots with 30 pumps between strings that would still be a very useful result. With the current single shot version I normally use 4 pumps/shot, and being able to shoots strings of 10 would be a relative luxury! With a longer pump stroke I'm hoping I can get the number of pumps down a bit.

What are the practicalities and costs of adding a small pressure gauge to the build? I assume it has to go through an unpressurised part of the tube wall and screw into the valve body? Are the gauges readily available on their own?


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 05, 2014 12:49 pm 
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The gauge in my Millenium Pumper is threaded into the front portion of the valve body, into a 3/16" thick wall to provide enough strength.... It passes through a 1/2" hole in the tube.... http://www.gatewaytoairguns.org/GTA/ind ... ic=55930.0 .... Some sort of pressure indicator is a necessity on a retained air pumper, IMO, as otherwise you will have no idea where you are in the shot string.... Ninja have a really tiny 6K gauge, (less than 3/4" OD) but it's only 500 psi markings.... The Disco gauge is the most readliy available, but is really only colour coded with 1K divisions, you have to extrapolate between them.... The good thing is that it covers the range you want.... There are lots of Chinese gauges on eBay, but I've had spotty success with them, falling apart from the vibration of the air when shooting, and the smaller the valve volume, the greater the pressure swings and the worse luck with them surviving....

Steve in NC uses a spring loaded pin that pops out at a certain pressure to indicate when the gun is full, search "Air Conserving Pumper Pressure Indicator" on the Yellow or Green Forums....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Build from scratch?
PostPosted: Mon Jan 06, 2014 12:53 am 
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Location: Stavely, Alberta
rrdstarr wrote:
rsterne wrote:
Quote:
4140N is rated to 180,000PSI

All that means is that the tensile strength is 180,000 psi, not that it will stand that much pressure.... It means is you had a 1" square piece and tried stretching it, it would fail at 180,000 lbs. of load.... The MSWP depends on the thickness/diameter ratio just as much as the tensile strength.... Larger diameters or thinner walls reduce the safe working pressure....

Bob



Just saying Bob, that I would feel better with a steel air tube then an aluminum one.


My Ripley uses an aluminium tube, Rick. It's the only PCP I've ever had where the manufacturer states it's Safe Fill Pressure at 300 bar :D

DaveD :D

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 Post subject: Re: Build from scratch?
PostPosted: Mon Jan 06, 2014 1:12 am 
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Just saying Bob, that I would feel better with a steel air tube then an aluminum one.[/quote]

My Ripley uses an aluminium tube, Rick. It's the only PCP I've ever had where the manufacturer states it's Safe Fill Pressure at 300 bar :D

DaveD :D[/quote]

Yeah, I know DaveD! My Pardini cylinders are aluminum and good to 250bar.

But steam pipes are steel, high pressure hydraulics are steel, etc..

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Pardini K10
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 Post subject: Re: Build from scratch?
PostPosted: Mon Jan 06, 2014 5:51 pm 
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Something to think about. Aluminum starts to fatigue from the first time it's used, steel does not. Aluminum doesn't have a lower elastic limit, so every time an alloy tube is filled it fatigues, and will eventually crack. :shock: As long as steel is not taken near the yield point it has an indefinite fatigue life. :) This is why there are very few aluminum springs. :mrgreen:


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 19, 2014 4:38 pm 
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Posts: 231
Location: England.
rsterne wrote:
(eg. tungum?)....
I do have one question, however, on what planet is it OK to engineer a pressure vessel for only 1.5 times working pressure?.... They are hydrotested (when required) to 5/3 MSWP, and typical safety margins run at least 3:1 to yield and 4:1 to burst ratings on the tubing and similar on the fasteners and end plugs.... I wouldn't even want to be in the same room with a gun with only a 1.5:1 safety margin.... You must be the only gunsmith in the last century to make his own screws....
Bob


Well Bob unlike 95% of manufacturers I have always gone over board, you will find it very hard to find a manufacturer that exceeds operating pressure by 1 1/2 times which require no test. You will also find many that operate up to 200bar fail at 245bar. Or a well respected producer make the cylinder ok but overlook the thread in to the action when the reg fails.

I have seen loads of removable German cylinders in date with serial number, operating pressure 200bar, test pressure 300bar etched, fail at less than 200bar. Point is just because its stamped under German law, they are not all safe.

5/3 MSWP I would like to see that a law worldwide for any modifications which after all is not much above 1.5 as stated! So you don't get wrong end of stick again i'll spell it out operate 200bar, withstand 300bar = 1.5 times comprende.
Hydro testing is not a requirement neither is individual testing of each cylinder under a certain cc or a fixed tube.

Tube rating is meaningless if end plugs or other items are weaker.

We wont go in to the cheap co2 designed stuff (110bar) used at twice operating pressure.

Screws, well some of us do things right and don't need to copy other peoples work. The best guns made all have hand made screws to this day, how would they line up otherwise.

Tungum, I could have told you to do some homework but here goes http://www.tungum.com/


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 19, 2014 4:47 pm 
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rrdstarr wrote:
My Ripley uses an aluminium tube, Rick. It's the only PCP I've ever had where the manufacturer states it's Safe Fill Pressure at 300 bar :D


The main difference to the run of the mill production builds, these are made correct using thick wall tubing with decent threads. They do however weigh more than most.

Quite agree about aluminiums fatiguing, when they go theres no warning.
At least you may get some warning when a steel is about to let go providing the end plugs are adequate.


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 19, 2014 5:59 pm 
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Jon,

I'm unsure if there is an issue with terminology here or not. Different folks may do things differently...and so....

5/3 MSWP is a proof or hydrotest pressure. On this side of the pond, pressure vessels over 2" diameter require a regular hydrotest. I'm aware that most pcp reservoirs are smaller than that....some folks choose to surpass the minimums.

The safety factor (point of yield failure) that Bob and I use in a build is 3 or 4 times (I use 3) the maximum working pressure...ie...if MSWP = 3000psi then Yield failure = 9000psi or 12000psi depending. This safety factor applies to any parts that play a part in pressure containment..ie tube, valve and fill assembly threads, retention screws, etc.

With this in mind, if the safety factor at yield were 1.5 times the MSWP...the tube would never survive a hydrotest. If there are manufacturers producing pressure vessels to this spec I don't know what to say, but "utter stupidity" comes to mind. :roll:

Tungum seems to be the name of the manufacturer, and applied to what further seems to be a proprietary alloy. Speaking for myself, although it may be popular in the UK...I've never heard of it, and thought it was a spelling mistake on your part. Thank you for the link clearing it up.

Regards,

Al


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 19, 2014 6:36 pm 
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Well, Al, that makes two of us that feel the same way.... :roll:

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Build from scratch?
PostPosted: Sun Jan 19, 2014 9:51 pm 
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Location: West Coast
When I built my .510 air shotgun, I used a 3x yield safety factor (9000PSI). I was slightly concerned about weight, but after it was complete it wasn't much heavier than my hunting rifle. And I lug that thing around for up to 12 hours some days. The weight really isn't that much of an issue if you think about it.
I'd rather have a gun that I don't have to worry about exploding if I accidentally drop it from 3 feet.

Mike


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