This Is What Can Happen.....

[cilami]

Just a quick question, as I have two of these Hipacs.  I am using the stainless steel valves sold by Powermax, these are retained by a single 8/32 stainless screw.  Is this sufficient if filling to 2200psi?

This is what the designer of the HiPAC said about the screw used in the Powermax SS HiFlo valve, quoted from the HiPAC forum:

"The screw is a custom 303SS with a 1/4" set. It looks like any orher screw buy I actually designed it for the HiFlo and with a lower head to clear the frame. My test consisted of me putting a valve and HiPAC in, losking the whole thing in a vise, and wrenching the HiPAC into the tube until it bottomed out. While all under 2500psi. It dammaged the &^^& out of the tube but the screw stayed perfectly straight. SS valve and SS screw, no give to give...so to speak."

and this what Gippeto posted in the thread about pinning...

"For Hipac use with the product being used as the manufacturer INTENDED, the valve retention screw needs to be changed to an 8-32 low head socket head cap screw...that's it.

You can find the math and reasoning here;

http://www.network54.com/Forum/275684/thread/1328765973/Examining+the+hipac....the "

IMO, is up to you to reinforce the valve or let it as it is, but always remembering that too much safe is a good thing most of the time.   

[Voltar1]

The Hipac failure was due to the incredibly thin weak construction where the Hipac parts fit together.
I have two of them here and the wall is very thin where the failed unit broke, came apart.
One is 0.025" wall and the second is 0.033" wall. IMO if you have a Hipac, degas it and stop using it until such time as it is checked over and re-engineered  for a certain fill pressure.
Anything over 1000psi may be a ticking time bomb.
Be safe,
Walter

[Voltar1]

okey not smart to work on the gun with with pressure in the tube .i always empty the tube before working on it

To work on the hammer or trigger? Really? I doubt it

[rsterne]

Al, you are right, calculating a safety margin without knowing the material is of little use, other than to show that unless the material is pretty good it doesn't approach the 3:1 value that we like to work to with HPA (or CO2, for that matter).... The HiPac was never intended for CO2, its whole purpose is to allow HPA to be used in a stock 22XX....

Walter, a thickness of only 0.025" at the bottom of the O-ring groove is particularly disturbing.... but using the visual proportions in the photo of the one that broke it doesn't surprise me.... Having a variation from 0.048" down to 0.025" in just  examples meaured is even more distressing, IMO....

I'm certainly not saying that falling on a gun is using it properly, but accidents do happen, and obviously if the tube is less than half it's thickness at one point, and that point is right where two tubes are screwed together, if you should fall on it, the strength there could be quite important....

I am wondering about just how the forces act on the material at the bottom of the O-ring groove.... we can assume that the HPA leaks along the threads, so that the O-ring is under pressure from all sides, being forced into the outer corner between the shoulder of the rear portion and the back edge of the front portion.... That would transfer the pressure load, and therefore the hoop strength calculations, to the outer tube which is threaded, would it not?.... This is not to say that you don't need a minimum wall thickness at the bottom of the O-ring groove, to stand the end forces pulling the two tubes apart.... but maybe we should not be over-reacting to pressure calculations based on that area?.... Al?.... Walter?.... Your thoughts?....

Bob

[Voltar1]

Bob, I am thinking that weak area is under two stresses that compromise the lack of engineering.
In tension it seals at 0.770" applying a load of 1400 Pounds and a stress of 27,155 psi
Pressure hoop stress is 40,800 psi.
Both of these values are higher than any other pcp setup I am aware of. Hydraulic tubing stress is figured at 15,000 psi as I recall.
I inclined to add these stresses as it makes the failure more plausible
Cheers.
Walter

[rsterne]

Walter, we are both thinking along the same lines.... sort of.... I agree that the sealing line is larger (you measured it at 0.770"?) so that is the area needed to calculate the end force, not the .616" I used to get 894 lbs.... Using the 1400 lb. figure at 3000 psi makes a lot more sense.... However, if you look at the comment I just added above, there may not be any hoop stress on the inner tube, just on the outer one.... Still, that leaves 1400 lbs. of end force to be carried by just a 0.025" wall in the thinnest one you measured.... That works out to 0.05 sq.in., which makes the stress (1400/0.05) = 28,000 psi.... With 4130 CrMoly (yield 63K) that would only give a safety margin of 2.25:1 at 3000 psi, without any added stress from a side force....

Has anyone looked at the hoop strength of the threaded portion of the outer tube?....

Bob

[Gippeto]

Bob, I would figure the back side of the o-ring groove to be under hoop stress and would calculate it as being so.

Going to proof both of mine and use or not as the results indicate. Will be keeping that information to myself to avoid further confusing any issues.

Al

[Voltar1]

Bob, yes I see that now. Comment, they could not get the oring to seal so they epoxies the threads to get a seal. Therefore we are back to the thin wall taking both stresses.
IF the oring made the seal then hoop stress goes to the outer tube as you pointed out above.
Tension would be at the id of 0.630" so reduced to 940 pounds.
Walter

[rsterne]

Al, not sure what you mean by the back side of the O-ring groove under hoop stress.... Isn't the O-ring forced into the upper, outer corner?.... Or is the O-ring squeezed radially both inwards and outwards, leaving the inner, rear corner of the groove under pressure?.... How will you determine if there is any movement in just the back corner of the groove?....

Walter.... Yes, it depends on which version we are talking about, the one with the threads epoxied up, or a stock on, where the forces are applied, and how great those are.... which complicates things further....

bob

[cilami]

From Bob, "Walter.... Yes, it depends on which version we are talking about, the one with the threads epoxied up, or a stock on, where the forces are applied, and how great those are.... which complicates things further...".

Since there are two versions of the stock threads (coarse vs thin), I wonder if that should be taken into account into this analysis.

[Voltar1]

In the OEM Hipac seal the hoop stress will occur at 0.800" id and 0.868" od on the unit I have in hand.
Second one 0.780"/0.869"

Threads thin the outer tube to 0.800" id on both units

[Voltar1]

Outer tube would be in tension at the 0.034" wall.......

[Gippeto]

Running on 4hrs sleep..quite possible I'm missing something here, but I think the coffee is starting to kick in...a little.

You two are thinking the actual point of failure (what I would call the front/pressure side of the o-ring groove) was not under hoop stress but only tension?

Al

[rsterne]

Al, I think in a stock HiPac that would be the case (tension only).... However, as Walter pointed out, if the threads were sealed with epoxy, then the bottom of the O-ring groove would be subjected to hoop stress as well.... In both cases, the thin wall below the O-ring groove would be subjected to tension loads, but that would be different as well.... In a stock HiPac, the tension would be on the order of 1400 lbs. at 3000 psi (depending on the actual seal diameter).... In the one that failed, we would use the ID of the HiPac to calculate the end load, so about 900 lbs.... I think Walter's point about the two stresses in the one that failed (hoop plus tension) may increase the total stress on the bottom of the groove.... one force trying to bend it outwards, the other trying to stretch it lengthwise.... Add in a possible collision with something, or pushing sideways on it while working on the gun, would increase the stress (on one side) of the groove even more....

Walter, using the average diameter of the outer tube (0.834") and a wall of 0.034", that is an area in tension of 0.089 sq.in.... If the load is 1400 lbs. that is 15,730 psi of stress, if the load is 900 lbs., it is only 10,112 psi of stress.... Either one should not be an issue, correct?....

Bob

[Voltar1]

Bob that would be ok. I use 20,000 psi when building with 4130
15.000 if unknown or 1018 hydraulic dom

Hoop stress at 3000psi will be 38800 psi
That is higher than I would use

[rsterne]

Yeah, I only looked at the tension on the threaded part, you are right that the hoop stress on the outer tube above the peaks of the threads is extremely high.... It would have to be extremely good material to have a decent safety margin at 3000 psi.... Have you worked it back the other way, using say 4130, to figure out what that would be?.... What are you using for dimensions to calculate the hoop stress?.... 0.868" OD x 0.034" wall (above the threads)?....

Bob

[Gippeto]

I'm with ya...o-ring groove handling hoop and tension...I agree....sealed with epoxy or not...back side of groove (non pressure side) I would always calc as handling hoop stress.

Off camping with kids/grandkids for the weekend ...will catch up on the "goings on" when I get back.

Al

[rsterne]

Have a good weekend, Al.... I agree to be safe we have to consider the outside bottom corner of the O-ring groove to be subject to both hoop stress and tension, regardless of whether the threads are sealed or not.... Looks like the hoop stress on the outer tube above the threads may be just as important?.... I guess it depends on the additive effect of the hoop stress and tension on the bottom of the O-ring groove.... It might take an FEA program to figure that out.... QVTom, are you listening?....

Does anyone have the specs on the fine and coarse threads used to join the two halves of the HiPac?.... They would be needed for that (which is well above my pay grade - zero)....

Bob

[chekbone]

Nope. But I think a good test for things flying out the back of the gun, wish I had the rocks to try it - would be to lock a pressurized tube in a vise and grind the valve screw down until the valve goes rocket. Not suggesting any one try it, just saying i won't. I measured the slot on my hipacs and both of mine have about .050 thickness. I remember somebody say that it was tested it to 5000 psi.

[rsterne]

You NEVER test anything to destruction using air pressure.... you use a fluid that doesn't compress and store energy.... Hence the term "Hydrostatic" testing.... Alternately, for testing things like the shear strength of screws, you can use direct mechanical force, like a hydraulic ram, as used by Lloyd to test the Disco screws....

Bob