QB79 tank block improvement

[Buldawg76]

Definitely like the ideas presented her for securing the tank block in the tube and very good idea of cutting relief in block to allow tube to recess into the block for more tube material between screws and tube end.

Mike

[rsterne]

In reference to Ribbonstone's reply # 5, concerning a slight bulge in the end of the QB79 tube where the screw holes are very close to the end, I will be doing a simple stress analysis on the screws and holes and will present the results in this thread.... When I got my first QB79 many years ago, long before I even knew how to calculate stresses and safety margins, I questioned how close those holes were to the end of the tube, and virtually everyone told me that with the thousands of QBs around, hundreds of which had undoubtedly been left lying in the hot sun or inside a car on a hot day, not one had ever let go, so as long as the gun was protected by a 1.8K burst disc, to keep the pressure from exceeding what can happen with CO2 under those conditions (1900 psi @ 120*F), not to worry about it....

I took those opinions at face value, although with my own QB79s I don't take the stock off with pressure in the gun, because I feel better with those extra two screws in the tank block.... Any time I use a QB tank block in another tube, I leave the tube full thickness, and I add at least a 3/16" steel shear pin between the O-rings.... I have decided it is time to have a look at the way the tank block is secured, and I will publish my findings in this thread.... Please remember, I am NOT an engineer, all I can do is give my opinon as to what I would do, and you are free to make your own conclusions, and are responsible for your own results and safety....

Bob

[wll2506]

Bob,

I'm very much looking forward to your test !

wll

[nervoustrigger]

To anyone interested, there is a convenient calculator for determining bolt (pin) shear strength on Engineer's Edge
http://www.engineersedge.com/material_science/bolt_single_shear_calcs.htm
 
Keep in mind a pin that goes all the way through gives you two shear planes.  Meaning that as long as both sides are carrying the load when you pressurize the system, it's as good as having two pins.  Conversely, if they don't both carry the load when pressurized, then it's basically like having only one pin because the one carrying the load can fail first and then the second one is left to carry all the load.
 
For this reason, when I'm ready to drill for a pin, I like to install the tank block and secure it with the two M4 screws and then lightly pressurize the tube to 300 or 400psi.  That pushes the block into a position where the two M4s are carrying the load, so when I drill for the pin it means both the pin and the screws will all be responsible for carrying the load simultaneously.
 
I think it also has the added benefit of keeping the tank block axial to the tube so as to reduce the chance of the bottle touching the barrel.
 
Please do be careful if you follow my method.  You're not drilling a pressurized vessel...unless you mess up and drill in the wrong place or unless the O-ring you think is sealing, isn't sealing.  It's those unforseen circumstances that jump out and bite hard.  At the same time, it's deliberately not a lot of pressure so the biggest risk is probably that you'd get metal shavings hurled at you so make sure those safety glasses are on!
 
Bob, please feel free to make comments and admonitions of safety

[rsterne]

To clarify, this is NOT a test, just some calculations, based in part on a best guess of the materials used.... First, a quick guide to safety margins on pressure vessels.... This information is the best I have been able to find (and it's not easy to come by).... The DOT basically use rules that were designed for boilers over a century ago, that have been updated as materials science is better understood.... They use the Ultimate Tensile Strength (UTS) of the material, and a century ago the standard was a 5:1 safety margin.... That was reduced to 4:1, and more recently (2001?) to 3.5:1.... Even more recently (2006?), a more rigourous set of calculations were established, such as is used in computer FEA programs (Finite Element Analysis) that if applied, allow that safety margin to be reduced to 2.4:1, which brings it in line with European standards.... By "bring in line", I read that to mean "economically competitive", which IMO means they are trading off safety for dollars.... However, since the formulas are much more complex (and presumably accurate), I understand they actually yield similar results for the Maximum Safe Working Pressure (MSWP).... I do not have access to an FEA program, so when designing from scratch, I use 3.5:1 using the UTS for a design point....

There is another standard that should not be exceeded, and that is a 1.5:1 safety margin based on the Yield strength of the material (the point at which it distorts permanently without breaking).... I believe this is what we are seeing between the holes and the end of the tube in the QB79 that Ribbonstone noted in Reply #5.... Most of the time, the 3.5:1 calculation using UTS will result in a lower MSWP than the Yield calculation.... When working with steel, metal fatigue is an issue if the working stress is greater than about half of the Yield strength.... I therefore use 2:1 to yield as a minimum for my personal projects.... About the only thing that applies to, in any of the designs I have done, is the bearing load of the screws against the holes in the tube.... This is precisely the situation I think we are seeing in the case of the holes in the QB79 tube....

I'll let you digest that information while I work on the next post, which is my analysis of the QB tank block installation.... Please understand that any results I post are my own, based as I said on a guess of materials, and are not a direct comment of the safety and suitability of the QB7Xs for the purpose they are intended.... use on CO2.... Jason's comments on the importance of all fasteners sharing the load equally are 100% correct, and are assumed to occur....

Bob

[Buldawg76]

Bob
+1 on the interest of the test.

Mike

[Ribbonstone]

Can't help but think that the larger holes in the tube make for point-contact with the block screws...better fitting screws/pins would spread that load.

Pressure is pressure, co2 or air, so if it is safe with normal co2 pressure than it's safe with mild pressure HPA.  Do think a lot of shooters have left their guns out in the sun and seen really high co2 pressure without failure (although I hope the tank's burst disk would have let go someplace near 1800psi).....but I wonder if they pull the screws and see if there is the same type of deformation at the screws as shown.

Regulators do fail on occasion, sometimes the "slow" way were they "creep" (just slowly leak air into the rifle).   Can fill end up with 1800psi inside the rifle even with a "creeping" 850psi output tank.  Not the same effect as a failed regulator on a 3K PCP...if it fails, are just back to the normal pressure limit of a PCP tube.  When it fails on a converted QB, may give stresses you never planned on.

Which is why (in the eyes of many posters) I over do the safety aspects.

[rsterne]

OK, so let's look at the tank block on a QB79.... It is machined from aluminum, and inserted into a steel tube that is nominally 22mm OD x 19mm ID (ie 1.5mm = 0.059" wall).... It is secured into the tube with two 4mm x 0.7mm screws, through holes in the tube just slightly larger than the maximum diameter of the screw, which is nominally 0.157".... The root diameter of the screw thread (the minimum diameter of the shear plane) is nominally 0.122".... I am assuming that the screws are made from mild steel the equivalent of a Grade 2 bolt, with a UTS of 74Ksi, which using the standard 60% value of that for the shear strength gives a Shear strength of 44.4Ksi.... This results in a load to failure of (0.122^2 x PI/4 x 44,400) = 519 lbs. per screw, or a total force of 1038 lbs.... The ID of the tube is 19mm (0.748"), so the area is 0.439 in^2, so that load would occur at 1038 / 0.439 = 2364 psi.... That is consistent with the one observed failure I have heard about, where the screws sheared at 2600 psi.... If we strictly apply the recommended 3.5:1 safety margin, then we get a MSWP of just 2364 / 3.5 = 675 psi.... At 850 psi, which is CO2 pressure at 70*F, the end load on the tank block is 373 lbs., which gives a calculated safety margin of 2.78:1.... subject to the actual material being used for the screws, of course.... It must be noted that the QB79 tank block is also secured by two additional 4mm x 0.7mm screws through the wood stock, which may certainly increase that safety margin to some degree, depending on how good a job the wood does of supporting the load....

The two screws pass through holes in the steel tube that are fairly close to the end of the tube.... On the sample I have here, the center of the holes is 4.4mm from the end of the tubes so that is the length of the shear plane, multiplied by two, because there are two such planes, one either side of the screw.... On a QB79, part of this distance is the full thickness of the tube wall (1.5mm = 0.059") and part of it is thinner than that because of flats machined to allow the head of the screw to seat on a flat surface.... About 0.015" has been removed in the middle of the flat, less than that at the shear plane of course.... I estimate that 0.050" of the length of the shear plane is 0.059" thick, and the remaining 0.120" is 0.051" thick.... That results in an area in shear of 2 x ( (0.050 x 0.059) + (0.120 x 0.050) ) = 0.0179 in^2 per screw.... If we assume that the tube is 1018 DOM steel, which has a UTS of 64Ksi, that means that in shear it is 38.4Ksi, so the force required to tear the screws through the tube wall would be 0.0179 x 38,400 = 687 lbs. per screw, for a total of 1375 lbs.... This is 32% greater than the shear strength of the screws, with a MSWP of 1375 / 0.439 / 3.5 = 895 psi.... It should be noted that in the failure mentioned above, the holes in the tube were elongated but had not yet failed (consistent with the prediction).... Installing stronger screws would quickly make this the weakest point in the system, but it sure can't hurt....

If the tank block was installed in a shortened QB78 tube, where the wall was full thickness, the load at failure is increased to 2 x (0.170 x 0.059) x 2 x 38,400 = 1541 lbs.... and the MSWP to 1541 / 0.439 / 3.5 = 1003 psi.... If the holes were further from the end of the tube, which could be achieved by cutting a channel in the tank block, or by machining some material from the back of the block, to allow the tube to slide on further, that could be increased again.... The standard minimum dimension for the distance of a hole from the end of the material is 1.5 diameters (to the C/L), which would be over 6mm, let's call it 0.250".... Using that dimension, and the full wall thickness of the QB tube, we have 2 x (0.250 x 0.059) x 2 x 38,400 = 2266 lbs. force.... That works out to 2266 / 0.439 / 3.5 = 1474 psi MSWP.... If this modification is done, then it makes sense to use stronger screws as well.... If Grade 12.9 Metric screws are used (UTS 177Ksi), their shear strength would increase to 106Ksi, and the force to shear them to 1238 lbs. each or a total of 2477 lbs.... That would give a MSWP, at 3.5:1, of 2477 / 0.439 / 3.5 = 1612 psi.... If the holes were at least 5mm (0.197") from the end of the tube (to the edge of the hole), a MSWP of over 1600 psi could be achieved.... That would be the maximum that could be achieved using two Grade 12.9, 4mm x 0.7mm screws.... To get more strength than that, additional fasteners would be required....

There is one more factor that we have not examined, and that is the bearing load of the screws on the tubing material.... The area under load is the width of the screw times the thickness of the tube, but since we are dealing with a compression load, the full Yield strength may be used.... For a full wall thickness QB78 tube, the area under load is (0.059 x 0.157) = 0.00926 in^2 each, and the Yield strength of 1018 DOM tubing is 54Ksi, so the yield point is 500 lbs. per screw, or 1000 lbs. total.... That would occur at 1000 / 0.439 = 2278 psi, and we are allowed a safety margin of 2:1 in this case, which means a MSWP of 1139 psi.... For the QB79, the area is reduced because of the flats milled for the screw heads.... My best estimate for the yield point on a QB79 tube is 1900 psi, however, the compression load on the thin strip of metal between the hole in the tube and the end of the tube, likely reduces that because some of the load becomes tension in the thin metal strip.... Ribbonstone's tube, which only saw 1250 psi, clearly shows some distortion at that point, which is concerning.... It is possible that this distortion (stretching) has settled in, and will not increase further unless more pressure is applied.... It may be due to the point loading of the screw threads causing the metal to "flow" (yield), pushing out into a slight bulge.... The fact remains that to maintain a 2:1 safety margin in a QB79, the MSWP would be about 950 psi....

To summarize, for a QB79, if we use the suggested 3.5:1 safety margin, the shear strength of the screws limits the MSWP to ~ 675 psi.... the tearout (shear) strength of the tube would limit it to ~ 895 psi, and at 2:1, the bearing (yield) load to ~ 950 psi.... These are all subject to the materials I chose, but my choices can't be too far out of line, because the calculated failure points are just shy of the one documented failure of the screws (and nearly the holes) at 2600 psi.... Based on that single failure, and a 3.5:1 safety margin, the MSWP of the QB79 is 743 psi.... Obviously, since we know that the QB79 stands up just fine to the loads of CO2, we can, and IMO are, dealing with a safety margin of less than that.... However, doing the math has been quite an eye-opener for me.... Looking at the numbers, my guess is, and it is purely a guess, that when the gun was designed they used about 3:1 at 850 psi, and didn't worry about the potential for the pressure to be twice that, because in reality that would not happen very often, only on a very hot day with the gun full of CO2.... I'll let you digest this for a bit before I present my recommendations, based on the above....

Bob

[rsterne]

The burst disc on CO2 tanks, which are rated at 1800 psi, are a 3K burst disc, because that is the (5/3) hydrotest pressure for a CO2 tank.... I wonder if anyone has ever noticed a bulge in the ends of the tube on a QB79, adjacent to the holes, on CO2.... The thing that is different, of course, with running HPA at, say, 1500 psi compared to CO2, is that with HPA you are running that load every time you fill the tank, instead of only when the temperature is very high....

Bob

[Buldawg76]

Bob
If the tank block was installed in a QB 78 tube with the two factory 4mm x 0.7 screws in the factory location and then two 8-32 grade 8 screws added between the 2 o rings at 180 degrees apart with the tube drilled to 3/16" to allow the 8-32 screws heads  to be flush against a spot face on the side of the block surface like the Disco/ Prod use. Would that have a similar safety margin to the 3/16" pin with the third screw securing it in place like you have shown here or does the pinning and third screw add a significant more safety margin than just the four screws with the two being flush to the block like the disco/ prods design.

Mike

[wll2506]

I am happy with my '79 getting 810 fps with 18,1 gr Exact Heavies, but .. I will be putting a pin  in between the "O" rings

If I would have known about the '79 vs '78 early on, the two '79's I bought for hunting I would not have bought, I would have bought '78's and modified them.

The two AR2079A's I have are fine in that I keep the out going to ~125o0, they are in .177 and used for target shooting, and have hardened screws and an extra one for safety .... they only see a bench !

I would very much like to see an improved block for the '79 ! If my company should buy a milling machine I will have some of my guys draw up plans and make a block that will hold and is doable.... but that is a bit down the line ! I very much no body has done it that has the ability too ! and the one that has is a pretty much copy of the OEM unit... mine would look a lot cleaner for sure.

If you want go to my company web site and see what we make   www.doinker.com

wll

[rsterne]

If you add two more screws between the O-rings, you may be cramped for room to drill the tube for the screw heads.... On an 8-32 screw, the heads are 17/64".... However, you will have more than doubled the strength by using two additional high-tensile screws, so you really don't need to worry about setting the heads down through the tube.... I still would stay with a 1.8K burst disc.... I got tied up tonight, so finishing up my comments will have to wait until tomorrow.... One thing I will mention now is that the JDS block, if used with high-tensile screws, will be much better than the stock QB79 tank block....

Bob

[Buldawg76]

Bob
Thank you very much as that's what I was wanting to know is if putting the heads in shear instead of just the screw threads themselves was really necessary as I will not be going over 1500 psi on the regulator. I would use standard thread 8-32 screws since I have several from my Prod builds on hand and they are easy to get in grade 8 at the local hardware store as well here in the states.

I will be keeping the 1.8k burst disc as all I am looking for out of my 79 is 890 to 900 fps with 177 JSB 10.34s for a nice light FT gun since my arthritis makes it hard for me to shoot a 10 pound gun in off hand and kneeling without swaying all over the place. It is truly a hit or miss type affair for me without a bipod to rest the gun on if its over 6 to 7 pounds.

I already have the QB 79 block so cannot justify the cost of the JDS block at this time anyway.

Thanks Mike   

[Austringer]

Well Bob, I never dreamt presenting a solution to my own little problem would prompt you to perform a tier 1 analysis! But, I think I speak for a lot of guys here who are very thankful. I’m really looking forward to your final comments and perhaps your insight combined with this discussion wll inspire someone to produce a somewhat more safe and affordable tank block for the QB78/9. Not that the JD block isn’t affordable, it’s just that at over half the price of the entire gun, economy seems to have been lost. And I’m guessing most of us here are really into makin somethin outa nothin. Frugality, a disease derived from poverty, but hard to shake when poverty is no longer an issue. I think at that point it’s just called tinkering...

Troy

[mackeral5]

Unfortunately I do not think anyone can make a "better" qb79 tank block at an attractive price point and it be worth their effort.  Too many machine operations, then some type of coating.  Even if they ran upwards of 1000 units, it would likely take years to sell them.  Not a very attractive nor successful business model.......

[wll2506]

Bob, This info is of great interest to me as I live in the High Desert and the summer months are very, very hot. I'm getting another '79 tube modified for extra strength and my '78 will get another screw for security also.

Do you remember whose '79 gave way and how it happened ?

wll2506

[wll2506]

Unfortunately I do not think anyone can make a "better" qb79 tank block at an attractive price point and it be worth their effort.  Too many machine operations, then some type of coating.  Even if they ran upwards of 1000 units, it would likely take years to sell them.  Not a very attractive nor successful business model.......

It would be a small specialized run for sure, really a one run kind of thing. It would have to be someone that has the machine and is not using it and willing to do a small run. It may be able to be done in a different fashion than what is out there now  You may be able to make it on a lathe as a two piece unit fastened together. It would be very strong and could be much cleaner looking and a smaller profile. I will talk to my machinist today.

What is the thread size of the tank, does anyone know ?

wll

[rsterne]

The QB79 that failed was around 2008, and in Ontario, Canada.... The regulator did NOT have a downstream burst disc, hence why when it leaked the full tank pressure went into the gun and caused the failure at 2600 psi during filling.... The tank block, regulator, tank assembly went spinning across the floor, but fortunately nobody was hurt.... just a few underwear changes, I'm sure!....

Bob

[cclingma]

What is the thread size of the tank, does anyone know ?

wll

I have notes that say 5/8-18 unf.

[rsterne]

I apologize for taking so long to get back to finishing the analysis of the QB79 tank block.... To summarize....

As installed in a QB79, with stock screws, if you expect a 3.5:1 safety margin, you won't even get it with CO2 at 70*F.... However, it does appear that it is about 3:1, which personally would not concern me.... I am less than happy with the way it is constructed for use on a 120*F day, which with a full CO2 tank could push the pressure to 1900 psi.... However, we have never heard of one letting go on CO2.... and in fact the only failure I am aware of, let go at 2600 psi, and the action was out of the stock, so if there is any additional support from the stock screws that would not have been there....

If you are converting one to HPA, things change a bit.... If you are using a Paintball tank at 850 psi, you should be in better shape than on CO2, because the pressure won't increase to any significant amount on a hot day, and if the regulator fails, the 1.8K burst disc will blow at (usually before) 1800 psi.... Basically, if the gun was OK on CO2, you shouldn't need to be the least bit concerned, IMO....

If, however, you use more pressure, like I did in my QB79 Ninja $200 PCP thread, where I used the Ninja SHP regulator which comes set for 1100 psi (and mine was 1200), your normal operating pressure will be the same as if you were using CO2 every day at 85-90*F.... My personal gun is set up like this, and the lowest safety margin is down to about 2:1.... Now that I have done the math, I'm not too happy with that.... but compared to CO2 on a hot day I'm still way below those loads.... Here is the chart for CO2 pressures for reference....



Since you should always have an HPA setup on a QB fitted with the 1.8K burst disc, and since if you push over 1500 psi you will inevitably have it fail, you have to stay below 1400-1500 psi.... You are still only at the pressure you can hit on a 100*F day with a full CO2 tank, but you now have that load 24/7.... This might be better than having the pressure going up and down as you use up CO2.... or it might not.... Ribbonstone's gun showed some bulging between the holes and the end of the tube at 1250 psi.... We would be wise to react to that by being more cautious when running elevated pressures in the QB79.... I would, at this point, recommend pinning the tank block with either two additional screws, between the O-rings, or a shear pin right through at the same location.... Using 4mm or 8-32 screws, or a 5/32" shear pin, would double the strength in shear (more if high-tensile), and the bearing load.... and greatly increase the resistance to tearout because you would have a much greater distance between the new holes and the old ones.... As a reminder, here is a tank block I modified with such a setup....

   

This one is complete overkill, using a 3/16" shear pin, with a small flat in the middle, against which the additional 8-32 screw tightens to hold the pin from falling out.... The head of the 8-32 screw is turned down to 1/4", and fits in a 1/4" hole in the top of the tube and into a recess in the block, putting the head in shear.... as I said, complete overkill.... When you do this type of modification, you can only (and in fact should only) use the single, inboard O-ring groove....

For those of you using a QB79 style tank block in a shortened QB78 or Crosman 22XX/Disco tube.... you have other options.... You can use the JDS tank block, with high-tensile screws.... As I mentioned above, you should be able to achieve a 3.5:1 safety margin in shear and tearout at 1600 psi, but with just those two screws, to maintain the bare minimum 1.5:1 on yield for the load on the tube pockets, your limit is 1500 psi.... That happens to work with a 1.8K burst disc.... The same thing would apply if you modified a QB tank block so that the screw holes were at least 5mm from the end of the tube, and the tube wall left full thickness, and used high-tensile screws.... Better yet, of course, would be to pin the block as I did above.... Under no circumstance would I use pressures that would require replacing the 1.8K burst disk with a higher one.... unless you do a complete safety analysis of the gun, looking at other things such as valve mounting, tube strength, etc.etc....

Please remember, I am NOT an Engineer.... I offer the above only as suggestions that I feel OK with for my personal use.... You are completely and totally, 100% responsible for anything you do when modifying a CO2 gun to HPA....

Bob