GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Austringer on March 06, 2016, 12:13:23 AM
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Like a lot of guys, I’ve been working on QB’s lately and having a ball. Currently I’m modifying a QB 79 to add extra plenum volume by swapping out the 79 tube for a slightly longer 78 tube. Of course, I’ll be running high pressure air at around 1500 psi, so therein lies the issue of safety with the stock QB79 tank block. The screws on the QB block are just too close to the end of the tube. I know there’s only been one reported failure and I know using the screws through the stock can help, but I came up with an idea I think might make the QB block a little bit safer.
On my QB block, I had drilled out the inside a little to gain a cc of volume. The tiny bit of extra volume only added 10 or so fps to my performance which is why I an converting to slightly longer 78 tube. My plan was to drill new support screw holes in the new tube and pin the block for additional safety. Unfortunately, since I had drilled the block, ther was not enough material to allow pinning of the block as Jason has done with his QB78. SO I got to thinking...what if I bored or milled a circular slot a little deeper into the block to allow the new tube more penetration onto the block which would increase the distance from the screw holes to the end of the tube?
I don’t have a mill, but I do have a file, so what I did was file some cutting teeth into the cut end of the scrap piece of tube I cut off my QB78 tube. This provided a custom fit, precision hole saw which used the existing diameter of the tank block as a guide.
(http://i1070.photobucket.com/albums/u490/troynicolls/100_2448.jpg) (http://s1070.photobucket.com/user/troynicolls/media/100_2448.jpg.html)
I don’t have a chuck big enough to turn the hole saw by machine, so I just turned it by hand. It was slow but did a perfect job and allowed for an additional .055” of tube material for added strength far superior to the cheesy thinned holes in the stock QB78 tube. http://i1070.photobucket.com/albums/u490/troynicolls/100_2450.jpg (http://i1070.photobucket.com/albums/u490/troynicolls/100_2450.jpg)
You an see the additional length in the last photo of the 78 tube compered to the 79 tube. Of course I had to drill new holes and will upgrade the screws to 12.9 grade. I do not plan to pin or add additional screws and I can use my homemade hole saw to modify any QB blocks I receive in the future. I do recommend anyone wishing to try this, take the time to file very sharp teeth into the tube as mine did not drill/mill very fast. I have a blister to prove it.
Troy
http://i1070.photobucket.com/albums/u490/troynicolls/100_2453.jpg (http://i1070.photobucket.com/albums/u490/troynicolls/100_2453.jpg)
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Excellent idea, and results.... Looks like you now have at least 1 hole diameter from the end of the tube.... That, plus high tensile screws, should give you considerable extra safety margin.... especially if you are still running the 1.8K burst disc in the regulator (which I would do)....
Bob
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Hi Troy,
I did about the same thing on a QB79 tank block. I chucked mine in a 4 jaw lathe chuck and carefully centered up the tube insert. I then removed about .15 inches from the tube entry portion extending the insert by that much. Worked great for me, and made for a cheap and safe modification for the block. I used it to add a bottle to a Disco tube.
Roger
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The limiting factor, assuming that the holes are far enough from the end, and the screws are strong enough in shear.... will become the ability of the tube wall to withstand the load from the two small screw shanks without yielding.... The Disco valve, for example, sets the screws down against flats on the valve, with the heads of the screws bearing in larger holes in the tube, to spread out that load.... The combination of only two screws, bearing on the tube wall at their shank diameter, and the thin tube wall.... will likely make that the first place to give.... Hence my recommendation to stay with the 1.8K burst disc....
Bob
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Roger
If I had a lathe, I would have done the same thing...if I had a lathe. Some day!
Bob. 1.8 k burst disk still in place! Only plan adjust the regulator to 1500 as I don’t want to chance blowing a disk on an all day, out of town ground squirrel hunt.
And frankly, if you look at how little tube material lies between the screw holes and the end of a tube on a stock QB79 tube, it is a wonder more failures don’t happen. Especially considering half that tube material is thinned to provide a flat for the screw heads.
Troy
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Do think the weak spot might be the “web” of the air tube. Once you use strong screws, seems that it’s more likely to elongate the screw holes and possible have that thin web of tube steel fail.
And as popular as QB79’s are for conversion to HPA are on this forum, I’ve got some reservations about the QB79’s tubes.
Issue QB79 tube:
Has been used mostly with 850psi output, but did see some use with a 1250psi output tank. Old rifle, been in use for years, and still in use with an 850psi output tank as my 12foot pound rat-rifle (also the one that manages 205 shots at that power level).
Factory cuts away about ¼ the thickness of the air tube to make those little screw flats, so the tube is thinner in that spot. Screw holes in the tube are a little larger than the minimum that would allow the screw to pass though (no air, no o-rings on the gas block, and you can feel the little back and forth play). Also seems the gas block isn’t really firmly seated dead flush with the tube when it’s installed.
(http://i157.photobucket.com/albums/t50/ribbonstone/QB%20mods/4e1cacf7-a04c-4d3f-977d-4aa54242dfd0.jpg) (http://s157.photobucket.com/user/ribbonstone/media/QB%20mods/4e1cacf7-a04c-4d3f-977d-4aa54242dfd0.jpg.html)
There is evidence of some elongation of the screw holes, with a little “lip” being formed at the forward part of the hole.
Thinking that the factory thinning the air tube right where the screws put stress on it was a bad idea.
Shortened QB78 tube:
Been in use longer than the QB79 above, also mostly used with 850psi tanks, but did see a bit of use with a 1250 tank.
(http://i157.photobucket.com/albums/t50/ribbonstone/QB%20mods/DSCF0346_zpsd8aac5a8.jpg) (http://s157.photobucket.com/user/ribbonstone/media/QB%20mods/DSCF0346_zpsd8aac5a8.jpg.html)
For this one, made sure the tank block was flush to the tube, drilled the smallest holes in the tube that would let the screws pass, and the walls of the tube weren’t reduced by adding flats. No evidence of elongation.
Screws can fail, esp. “mystery screws”. Too hard/brittle, could snap. Too soft would deform, which might be the safest type of failure. Best quality screws are neither of those, but that still leaves the air tube’s little web of steel to take the load.
Sorry guys, even though lots of high pressure HPA QB79’s are built and the owners seem happy, I’m going to have to pass on building a high pressure HPA on a QB 79 tube.
As of today, the above QB79 will stay gassed down and out of use. Do have a QB78 tube around the shop someplace, which will eventually be cut to QB79 length. May be over cautious, but then again I'm still here with both eyes and all my fingers.
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Valid and logical concerns, indeed. But pinning the block is so easy, provided you haven't hogged it out, that I should think makes it a minor consideration to anyone thinking of doing a 79 conversion.
Plus as Bob has pointed out, CO2 will reach 1800-1900psi in a gun left in the sun and no anecdotal evidence has ever been presented of that happening.
By the way, the factory M4 screws I've seen, I would not describe as brittle. That's usually a property of hardened, heat-treated...or heat-mistreated...materials. They are more like a mild steel. Very soft but not very tough. No telling what they've used over the years but apparently tough enough to not have QBs blowing their tanks off.
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Ribbonstone
The exact issues you describe with the thinned tube material and minimal area between screw hole and tube end are why I did what I did. I now have %50 more tube material between hole and tube end AND a full thickness tube in the area of the holes. With stronger screws, I feel I’m way better off than before.
Troy
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Would there be any potential problems just adding 2 additional screws to the block? I am thinking eliminate the O ring closest to the valve and drill and tap for additional screws. Could you even just eliminate the front screw in this case, similar to the JDS air blocks?
Thanks - Brian
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Brian, yes what you describe will work very well.
When you say the o-ring nearest the valve, I assume you mean the valve on the tank and not the one in the air tube.
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Lots of PCP's only have one o-ring sealing the front end cap...the dual O-rings of the QB block won't make it seal better than one good 0-ring.
But if you do drill/tap between the two O-rings, then add the front one anyway to help keep the block centered in the tube...won't do "jack" for sealing.
Shortened QB tube (without the added flats of the 79 tube), block all the way tight against the tube,and minimal sized tube-holes seems to work without deformation, so I'd not really worry about it with a shortened 78 tube. With a 79 tube, and those flats, would keep an eye on it.
Got too many irons in the fire right now, that QB79 is just going to have to stay out-of-service until I got some already underway projects completed.
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Brian, yes what you describe will work very well.
When you say the o-ring nearest the valve, I assume you mean the valve on the tank and not the one in the air tube.
Yep I meant the one closest to the guns valve. I thought about just drilling in this fashion for a crosman conversion I want to do and not even bother drilling for the factory screws on the block. On a QB79 I would use the stock screw as well.
Brian
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The other thought is that if you do relocate the mounting screws, aren't limited to tiny screws.
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I put another 8x32 allen screw in my block and changed the factory screw to hardened steel....but ... I'm going to drill a through hole and add a pin just like Jason did .... I don't feel warm and fuzzy inside with 1350psi outgoing being held by very little metal. The cap on the '78 is another thing and darn secure, but I added and extra roll pin for security on that too. I'm playing around with to much pressure to have a mistake !
wll
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The obvious choice is to add a couple screws between the o-rings, but since I had previously drilled out the tank block to get more volume in the QB79 mode, drilling any holes between the o-rings was not possible as the wall thickness was only about .160”. I didn’t have an extra tank block, so what I did worked out well for me.
Troy
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The obvious choice is to add a couple screws between the o-rings, but since I had previously drilled out the tank block to get more volume in the QB79 mode, drilling any holes between the o-rings was not possible as the wall thickness was only about .160”. I didn’t have an extra tank block, so what I did worked out well for me.
Troy
I sure wish someone would make a tank block at a reasonable price that had the another .100+ of space and 3 holes for larger screws ... That would be a done deal
wll
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Ribbonstone I agree with your assessment of the thinning of the tube where the tank block holes are in the QB79.... The single failure I saw documented had the two stock screws installed, and the action was out of the stock, so the block received no additional support from that.... At 2600 psi, the factory screws sheared, but the thin web between the holes and the end of the tube was also distorted, ie about to fail.... Simply fitting higher grade screws would, as you suggest, move the likely failure mode to tearing out the end of the tube.... I would be a lot happier if the factory had not thinned the tube, and always recommend that anyone using the QB79 block on HPA firstly make SURE that there is a 1.8K burst disc installed to prevent the pressures from exceeding what can happen with CO2 on a hot day.... and secondly, never remove the stock with the tube under pressure....
I agree that a full thickness QB78 tube is stronger.... but the hole location in a stock tank block is still too close to the end of the tube.... Modifying the block to move the holes further from the end.... or additional screws or pinning is a great improvement.... I will put the photo of the one I did here, again, to remind people of how I did it....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20PCP/QB3.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20PCP/QB3.jpg.html)
The cross pin, installed between the two O-rings, is a 3/16" steel pin with a small flat filed in the center, against which the top 8-32 screw tightens to prevent it from falling out.... The head of the 8-32 screw fits snugly in a hole in the top of the tube.... There are actually five shear points and load points in the tube.... You can't be too careful....
Bob
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Was kind of new at this back when I put those together...wasn't a lot of information back then. Were I to do them all over again, would likely NOT use a Qb79 tube's screw location. Considering I run them slower (for more shots) with 850psi air, not too worried about the cut QB78.
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Was kind of new at this back when I put those together...wasn't a lot of information back then. Were I to do them all over again, would likely NOT use a Qb79 tube's screw location. Considering I run them slower (for more shots) with 850psi air, not too worried about the cut QB78.
I just pulled the trigger on an AR2079 yesterday and plan on using a 850 PSI bottle I have on hand for this one. Will most likely not play around with hogging out the tank block and restrict this to Bobs stage 1 mods initially. (And I am fully aware that I said "initially"!!) Can you give me some idea of what kind of FPS you are getting on your setup and shot count? Paper puncher here so max power is not a necessity.
Brian
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With this set-up I was getting about 820 fps with JSB 18.1’s and 900fps with CPHP’s 14.3’s for a shot count of about 55 shots. However, tuned to a lower velocity I’m sure it would be considerably higher shot count. Drilling the tank only increased my max fps by about 10 fps. I really like Bob’s method of pinning. I’d do something like that regardless what pressure you run.
Troy
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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
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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
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Bob,
I'm very much looking forward to your test !
wll
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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 (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 :)
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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
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Bob
+1 on the interest of the test.
Mike
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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.
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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
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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
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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
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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 (http://www.doinker.com)
wll
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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
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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
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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
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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.......
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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
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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
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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
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What is the thread size of the tank, does anyone know ?
wll
I have notes that say 5/8-18 unf.
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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....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/co2.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/co2.jpg.html)
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....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20PCP/QB3.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20PCP/QB3.jpg.html) (http://i378.photobucket.com/albums/oo221/rsterne/22%20PCP/QB1.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20PCP/QB1.jpg.html) (http://i378.photobucket.com/albums/oo221/rsterne/22%20PCP/QB2.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20PCP/QB2.jpg.html)
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
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I must have a newer style tank block as mine has 3 orings on it. I am afraid pinning may be a bit trickier with this block so am leaning towards additional screws instead. Any suggestions as to where to put the extra support? Any issues if my additional screw holes are drilled into the vacant oring groove?
Brian
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I think the latest block actually has 4 O-ring grooves.... Why, I have NO idea, one is all you need.... and that should be the innermost one (closest to the trigger).... Obviously it would be better if you can drill between the O-ring grooves, rather than losing the support of the metal for the screw, or shear pin, near the tube.... If you can't find enough room for a 4mm or 8-32 screw, then I would use a 3/16" diameter shear pin, which would be almost impossible to bend.... Locate the hole so that the inside edge of the pin is supported by the full diameter of the tank block stem, so that it cannot bend when the tank block is forced forwards by the air pressure.... I don't know how strong one is, but I would think even a 3/16" roll pin (spring pin) driven into place would be better than doing nothing.... Use Jason's trick of 200-300 psi of air in the tube holding the tank block forward so that when you drill the hole straight through the tube and tank block, the pin, when installed, will be sharing the load with the existing screws.... You will probably destroy the O-ring when you pull the block out after drilling, from the burrs on the hole in the tube, and don't forget to deburr all the holes before reassembly.... No guarantee this method will work, but it's easier to align than trying to drill the tube and block separately.... Worth a try?....
Bob
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You are correct it has 4 orings. It has about 1.5mm between the front 2 and the same between the rear 2 and a fat 2mm seperating the 2 pairs. Sure seems like over kill but it never leaked when I had it on a bulk fill co2 crosman. Fortunatley the Ninja bottle I have on hand is only set to 850psi so should be good to go with the soon to be arriving AR2079.. I double checked the burst disk and it has a 5k on the bottle side of the regulator and a 1.8k on the downstream side.
Brian
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Guess we are in agreement…sort of.
\Pretty sure it’s not going to stop folks from running +1250psi tanks on their standard gas block fixated QB79’s.
It should make all of us think about better block fixation a little harder, but we’re battling the mind set of “it hasn’t launched the tank/block like a rocket yet, so it must be OK.”
Degassed, removed the gas block screws, and took a look at the 3 cut QB78 tubes. Screws aren’t bent and the holes in the tube aren’t elongated. Only running 850psi, but I had to check them all anyway.
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Thanks Guys for driving the safety issue home,
I was originally looking over this thread with a little bewilderment ::) as it seemed to be a bit overkill.
However last night, I remembered when changing a trigger spring in my (unpressurized) QB78/HPA a few weeks ago, I had noticed a bit more forward movement of the tank block, stock screws and air cylinder/tank block screws than I originally remembered seeing? My regulator is set at about 1400/1500 psi, so was a little alarmed :o. Further inspection had shown the air cylinder tank block holes had elongated a whisker (see photo below taken -> tonight). I also remembered, that when first made/assembled, the holes in question needed to be widened one drill size over 4mm to screw into the block correctly :P.
(http://i113.photobucket.com/albums/n227/kirk_schwarz/IMG_1590_zpsmy4p4tiu.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/IMG_1590_zpsmy4p4tiu.jpg.html)
To remove this shadow of doubt (I have a 10 year old son who sometimes shoots this gun), I will be either adding a pin as discussed or buy another QB78 tube. If I go for the former (per Jason), I will drill the pin hole through the entire air cylinder/tank block with about 400 psi in the 13 CI tank. After this process, the block will look identicle to my current picture, but be held simultaneously in 4 spots instead of 2, 6 if you count the stock screws.
Thanks all, much cheaper than hospital bills and just a little elbow grease ;)
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So glad I asked this question. Any future tank blocks I use will look like the one Bob pictured.
So Bob...as I’m still digesting everything you wrote, I have a clarification regarding this:
"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”
So since my modified tank block allows my 12.9 class m4 screws to be installed .250” on center from the edge of the full thickness tube, then my MSWP is 1474, right??
If true (by your non engineer best guess:)) then my 1475 psi set point is 1 psi over the safe limit. That being the case, I feel WAY better off than I did with the 79 tube and cheesy factory screws. Cant wait till my next build!!
Thank YOU Bob!
Troy
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Sound right....
Bob
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So as I am new to this also, I’m looking at the picture SpiralGrove posted and see his tank block has been pushed slightly out of the air tube. This is consistent with my limited experience upon both initial fillup of my QB79 airtube and to a lesser degree my modified 78 tube. On both occasions, the gap between the end of the tube and the shoulder on the tank block increased as pressure was applied. My thought was this is somewhat normal as the load seats on whatever bearing surfaces are present. In this case, it is screw threads against the hole in the air tube. The thinned tube of course is much thinner so has less bearing surface so it moved more. Screw threads are naturally going to cut into the tube. I’m guessing movement might also depend on the accuracy of alignment of the screw holes in the block vs the tank. One hole may bear more load until the block moves a bit and the load is equally shared. I’m thinking this is normal to an extent. Too bad we can’t find screws with a .062” unthreaded shoulder?
Having said that, I flat out do not like the QB79 tube holes OR the cheesy screws. I’m sure I could safely live the rest of my life on that marginal MRWP many of us 1500psi’rs are working with, but I’ll be adding extra safety precautions to my future QB’s depending on what new ideas are out there. For now, I’ll be monitoring my inletted block and adding at least one screw where possible.
Troy
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I agree, this is for many a case of blissful ignorance, it certainly was for me until I did the math.... I'm actually a bit shocked that the gun wasn't given a larger safety margin when you consider the pressures that CO2 can reach on a hot day.... You are absolutely correct that the load will imbed the screw threads into the thin portion of the tube until the loads on both screws are evenly distributed.... which causes a gap between the block and the end of the tube.... If you look at a Disco valve, same thing, the screw heads are always right back against the rear of the holes....
Bob
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Bob
I did some measuring on my tank block since in your response to me you stated in may be a bit thin between the o ring grooves with the 17/64Ths heads of the 8-32 screws in done with the heads in shear, but if done like yours in the picture you have on page 2 of the thread where they are located towards the front o ring groove there is room to set the heads in shear with the tube instead of just the threads.
I have not fully decided whether I am going the two additional screw route or the hardened pin and third retaining screw like your design as of yet in my QB. I did notice the two front screws that come as standard Philips head screws with star washers in the photos you show have been replaced by you with better grade allen screws, so are they still 4mmx .07 pitch screws or have you switched to all 8-32 screws for both front screws and the third pin retaining screw as well.
Would a 3/16" drill bit shank cut to the proper length be considered the same as a hardened pin in respect to strength in shear in the tube or does it need to be an actual drill rod or a hardened pin.
Mike
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If you drill the tube for the heads of the screws, you may not be able to leave enough room between that hole and the existing one in the tube.... You should have at least one full hole diameter to the edge of the hole....
A 3/16" drill would be plenty strong enough because of the diameter.... Mine is a piece of O1 Drill Rod, but not hardened....
I used 4mm x 0.7mm SHCSs to replace the stock pan-head screws.... If you can get Grade 12.9, that is the Metric equivalent of Grade 8 screws in 8-32....
Bob
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Bob
I have a 78 tube so it has no holes at all as of yet but understand what you mean now with the holes for the 8-32 screws in shear will not leave the full screw hole distance to the factory 4mm screws so some more inspection and measuring is in order to insure I have at least 5mm of metal between the front two screw holes and the rear holes with heads in shear and if not then pinning with a 5/32 or 3/16" drill shank cut to proper length with a top retaining screw will be the route I go.
Now I just need to decide if it want to leave the 78 tube its full length and cut it right behind the threads for the cap or cut it back 2 to 3 inches to get the bottle more rear ward to help balance the gun better. Can you tell me the difference in length between a stock 78 tube and a 79 tube as all I have is a 78 to compare to and I know from what you have said the added length of the 78 tube will add needed volume for the power I am after from the gun. So I guess I am asking is how short can I cut the 78 tube and still have sufficient volume to achieve my sub 20 fpe goal with 10.34 pellets.
Mike
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Just to add another anecdotal data point, 2 years ago when I set out to build my first QB78, I made a post asking about the safety margin of the tank block at 1500psi. It was to be a rifle for my uncle and based on the feedback, I decided to pin the block. I was so pleased with the outcome that I quickly turned around and built one for myself but this time I omitted the pin in order to monitor it for movement. Bear in mind this is a model 78 tube cut to accept the tank block, meaning it has the full tube thickness. Well, 2 years later it has not moved from the point it was when it was new. This is a picture taken yesterday:
(https://www.gatewaytoairguns.org/GTA/MGalleryItem.php?id=5347)
I had the rifle apart after a few months of use and there was no observable deformation of the holes in the tube under a 5x loupe. The machine screws have a very short (~1mm) unthreaded section just under the head so there wasn't even any marring from the threads. I had taken care to locate the holes and drill so as to maintain as much material as possible so I imagine that has helped.
That said, I still recommend that folks use a pin or additional screws because of assembly and materials variables, and because of possible abnormal use/abuse such as dropping the rifle or slipping on leaves in the woods and falling on it.
(https://www.gatewaytoairguns.org/GTA/MGalleryItem.php?id=5345)
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Jason
Were the 4m screws with the 1mm unthreaded section difficult to find? I have not yet procured my class12.9 4m screws, so whether they are easy to find...or is a particular source, I'd sure like to know because I see this factor as being pretty important to maintaining minimal deformation of the screw holes and movement of the tank block, regardless whether or not the block is pinned. Really glad you posted this!
Troy
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Hi Troy,
To be honest I didn't deliberately select screws with that feature but the ones I'm using are McMaster #95263A247 (http://www.mcmaster.com/#95263A247)
They are considered full thread but in my experience most screws will have a tiny bit of unthreaded section just under the head. Here's a closeup of the screw in question:
(https://www.gatewaytoairguns.org/GTA/MGalleryItem.php?id=5344)
If memory serves, I think I needed to shorten them 1mm or so. Not because they butted against each other (0.863" - 10mm - 10mm = 0.075" of clearance) but because I think the factory taps the holes from each end so there's a bit near the very middle where it's not tapped fully. So if you need to order, you may want to save yourself the trouble and instead get 8mm length McMaster #95263A234 (http://www.mcmaster.com/#95263A234).
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Mike, I did a thread on the importance of tube volume on the QB78-79 question a long time ago.... http://www.gatewaytoairguns.org/GTA/index.php?topic=21681 (http://www.gatewaytoairguns.org/GTA/index.php?topic=21681)
I ended up shortening the 78 tube about 3" (basically half way between the two stock tubes, to move the tank back, and that was an excellent solution.... The only time you really need a full length 78 tube is if you are trying to build a .25 cal (or a really hot .22).... The volumes between the tank block and valve are about 10cc in a 79 and 50 cc in a 78.... using my guide of 1 cc per FPE, if the tube is half way between, at 30 cc, that is adequate for a 30 FPE gun.... Not many QBs shoot hotter than that and stay under 1500 psi....
BTW, I believe 8mm is the correct length screws....
Bob
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Yeah Jason,
I'm sure at least 1/2 of the tank block movement was due to my poor hole drilling of the QB78 tube :P
I will take off the tank block again and take a photo of the actual tube.
Ohh...... the holes were drilled larger than I thought:
(http://i113.photobucket.com/albums/n227/kirk_schwarz/IMG_1592_zpsowmvagzk.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/IMG_1592_zpsowmvagzk.jpg.html)
Kirk
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Sorting through 3 old QB78 tubes, looking for a clean one to replace the QB79 shown in post #5.
Took some measurements, and have to say, not all QB tubes are all that uniform. To be fair, two of the tubes are from “back in the day”.
So I’m going to post an average. You could end up at a thick part or a thin part of the tube, as they aren’t all that dead-even.
OD: .862” (=/- .03”)
ID: .748” (+/- .03”)
There for the difference is .115” (or .057” per side).
The diameter over the two flats on the QB79 tube pictured in post #5 is .836”. So at those machined flats make the tube wall thickness .044”.
Considering the flats serve no use, are just there more-or-less as decoration (maybe to give a little less of the “bolts in Frankenstein’s neck” look), it’s intentionally weakening the system for the sake of style.
Picked the cleanest of the two, am going to simply make a QB79 tube from a QB78 tube. NO extra length, just without that flat and with tube screw holes that have less "slack" to the screw. BUT, I do only run at 850psi. If you run at higher pressures, would relocate the screws farther back.
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Guys, thanks for the work you have done on this. I need to take my 79 apart to put a spring guide on the rear face of the block, and when I put it back together I intend to follow Jason's advice of pressurizing the tube a bit before drilling for a crosspin. Bob, your idea of a setscrew for the crosspin is ingenious! I'll probably copy that.
BTW, I really don't see how the stock screws can add any strength against the pressure. Seems to me the most they could do is direct the movement downwards in the event the block blows out of the tube.
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WECSOG
If you drill the back of the tank block for a spring guide like Jason did (and as I did to gain plenum volume), you will likely not have enough material left to safely drill for a cross pin. Measure twice, drill once just to be sure.
Drilling the back of the block is why I had to inlet my tube into the block in order to beef up my screw hole strength as outlined at the very beginning of this thread.
Troy
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Have no illusions that the stock screws would really do anything than slow an ejecting block/tank down a little (by whatever the force would be to split the wood at the forend).
But I still inlet long tubed HPA 78's into their stock, use the stock screws, and even glass bed the block...want to isolate the tank block from stress from tank-bumps.
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+1 ^ Ribbonstone,
Plus the QB78 just looks better with the block in the stock ;).
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WECSOG
If you drill the back of the tank block for a spring guide like Jason did (and as I did to gain plenum volume), you will likely not have enough material left to safely drill for a cross pin. Measure twice, drill once just to be sure.
Drilling the back of the block is why I had to inlet my tube into the block in order to beef up my screw hole strength as outlined at the very beginning of this thread.
Troy
Good point, Troy. I might have to just use setscrews.
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Mike, I did a thread on the importance of tube volume on the QB78-79 question a long time ago.... http://www.gatewaytoairguns.org/GTA/index.php?topic=21681 (http://www.gatewaytoairguns.org/GTA/index.php?topic=21681)
I ended up shortening the 78 tube about 3" (basically half way between the two stock tubes, to move the tank back, and that was an excellent solution.... The only time you really need a full length 78 tube is if you are trying to build a .25 cal (or a really hot .22).... The volumes between the tank block and valve are about 10cc in a 79 and 50 cc in a 78.... using my guide of 1 cc per FPE, if the tube is half way between, at 30 cc, that is adequate for a 30 FPE gun.... Not many QBs shoot hotter than that and stay under 1500 psi....
BTW, I believe 8mm is the correct length screws....
Bob
Thanks Bob
That's exactly what I want to know and is the route I will likely go as I am only trying to build a 19.5 max fpe gun for Ft use, but will keep my tune in the mid range with hammer spring setting so I can turn it up some to maybe 22 or 23 fpe if I decide to hunt or shoot at longer ranges than 55 yards we max out in FT.
If this one turn out as I hope/know it will I will likely buy another to go for much more power in 22 or 25 for solely hunting and long range spinner shooting at our FT sight in range with spinners out to 100 yards.
Thanks Mike
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This thread has been a fountain of very important info for the safety of all of us. Bob, you did a great job of explaining your conclusions and Jason and the rest of you helped me on deciding about what I can do to add insurance that I don't have a HPA bottle flying around the room after the holding screws let loose.
wll
EDIT:
Using my blocks I made for my QB78, does this pin at 2700lbs shear OK ? http://www.mcmaster.com/#98404a010/=11i9cw1 (http://www.mcmaster.com/#98404a010/=11i9cw1)
Just drill and put in or take out, no threading needed or extra screw to hold in place ?
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I know I'm a little late to this thread, but I have a question for Bob. First of all, I thought the calculations were great and well done, and as a metallurgical engineer, I agree with the material choices and strengths chosen. I often look over the shoulder of mechanical or structural engineers while these types of calculations get done, and what Bob did seems entirely consistent with what I have seen.
The question I have is this: Have you given thought to grade 8.8 fasteners rather than 12.9? My concern is for potential stress corrosion cracking of the harder fasteners. I know these guns are not operated in a chloride-rich environment, but if they EVER come in contact with such a thing, I would expect the potential for failure of these fasteners. Guns stored near the beach would be one example of this environment. I think the 8.8 fasteners, if strong enough, would be a better choice.
Mike
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Will, that pin, at a 3.5:1 safety margin, could support up to 771 lbs. of load per side, so you double that in this application.... Just that pin, by itself, should have sufficient shear strength.... and the load on the pockets in the tube should be reasonable as well.... The tube wall at each end of the pin would support 598 lbs., for a total of 1196 lbs.... giving a yield point of 1196 / 0.439 = 2724 psi.... Using a 2:1 safety margin for the bearing load, that works out to 1362 psi.... so it would still be a good idea to keep the stock screws in place if you go over that.... Keeping the pin in place would be the only issue.... I would remove the ring, install the pin, and then safety wire it around the tube, through the hole where the ring was.... You might be able to put the ring around the tube before installing the tank block, and then work it back through the hole after installing the pin to retain it....
Grade 8.8 Metric screws are 116Ksi UTS, or 70Ksi Shear strength.... They would have a load to shear of 818 lbs. each, or 1635 lbs. total, which works out to 1635 / 0.439 / 3.5 = 1064 psi for a 3.5:1 safety margin.... Using those would be a good choice if you are using the stock holes, because instead of being the weakest link, the screws would then be the strongest.... They would also be a good choice if you were drilling and tapping additional holes between the O-rings but still using screws in the stock holes as well (ie 4 screws).... If you were using them in a block like the JDS, with just two screws, I would use the higher UTS Grade 12.9 screws, but in fact the JDS block is tapped 8-32, so you would be using Grade 8 equivalent anyway.... Note that Metric grades and American grades are NOT the same.... If you wanted to use the Grade 8.8 screws at pressures approaching the limits of a 1.8K burst disc, then I would use three of them instead of two....
It would be a good idea to check into environmental considerations, as Mike says, if that might be an issue....
Bob
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Will, that pin, at a 3.5:1 safety margin, could support up to 771 lbs. of load per side, so you double that in this application.... Just that pin, by itself, should have sufficient shear strength.... and the load on the pockets in the tube should be reasonable as well.... The tube wall at each end of the pin would support 598 lbs., for a total of 1196 lbs.... giving a yield point of 1196 / 0.439 = 2724 psi.... Using a 2:1 safety margin for the bearing load, that works out to 1362 psi.... so it would still be a good idea to keep the stock screws in place if you go over that.... Keeping the pin in place would be the only issue.... I would remove the ring, install the pin, and then safety wire it around the tube, through the hole where the ring was.... You might be able to put the ring around the tube before installing the tank block, and then work it back through the hole after installing the pin to retain it....
Grade 8.8 Metric screws are 116Ksi UTS, or 70Ksi Shear strength.... They would have a load to shear of 818 lbs. each, or 1635 lbs. total, which works out to 1635 / 0.439 / 3.5 = 1064 psi for a 3.5:1 safety margin.... Using those would be a good choice if you are using the stock holes, because instead of being the weakest link, the screws would then be the strongest.... They would also be a good choice if you were drilling and tapping additional holes between the O-rings but still using screws in the stock holes as well (ie 4 screws).... If you were using them in a block like the JDS, with just two screws, I would use the higher UTS Grade 12.9 screws, but in fact the JDS block is tapped 8-32, so you would be using Grade 8 equivalent anyway.... Note that Metric grades and American grades are NOT the same.... If you wanted to use the Grade 8.8 screws at pressures approaching the limits of a 1.8K burst disc, then I would use three of them instead of two....
It would be a good idea to check into environmental considerations, as Mike says, if that might be an issue....
Bob
Bob, thank you very much. Yes I was going to remove the ring. I was planning on using that a bit above center line to bypass air vent hole, and do away with any stock modification.
The length I chose may be a bit long , that I will have to figure out !
wll
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Just keeps getting better all the time here with more info than can be absorbed in one sitting and continually adding to my favorites to keep the latest info at hand without logging in to access it.
Mike
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Something for you guys to consider if you are making a tube to hold a QB79 or similar tank block.... Instead of cutting the tube to length first, and then trying to lay out and drill the screw holes the correct distance from the end.... simply reverse the process.... Figure out where you want the tank block, lay out the screw holes, and just drill straight through, horizontally.... THEN cut the tube off with at least 1/8" clearance to the holes, so that when you insert the tank block you can't quite get the screws in.... The use a belt sander, lathe, grinder, or even a hand file to carefully and slowly shorten the tube, keeping the end square, until you can just get the screws to thread into the block.... This will leave the maximum amount of metal past the screw holes possible.... Don't make it so tight that you damage the screws, but if done right, you will have virtually no gap between the end of the tube and the tank block, and maximum strength....
My Dad always used to say, do the most difficult part of the job first, and then the rest is easy....
Bob
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Bob
That's the same way I used to fix the cars I worked on was do the hardest jobs first then the rest was gravy and dessert.
Mike
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I wonder if we can find someone here willing to do the mods for those without the needed skills or tools to make the HPA conversion. And maybe we can set down what we believe would be the BEST method for a machinist to do so. Just and idea.
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Go back and look at the tank block I did on page 2, reply # 39, right at the bottom of the page....
The 3/16" pin, if made of CRS, has a shear strength of 2019 lbs. for the total of both sides.... Just by itself, it would have a safety margin of 2.7:1 at 1800 psi.... This shifts the concern to the bearing load of the pin in the wall of the tube.... Just the pin would support 1192 lbs, scraping the bottom of the limit at only 1.5:1.... However, if you add the 8-32 bolt, which screws against a flat on the pin to retain it.... and set the head of the bolt down through a hole in the tube into the tank block.... a grade 8 screw, by itself, will stand 1068 lbs. in shear.... Add that to the 2019 of the pin, and you have 3087 lbs.... This would give a safety factor of 3.5:1 in shear of 3087 / 0.439 /3.5 = 2009 psi.... The head of the screw, supported by the tube wall would withstand 847 lbs. to yield, assuming the tube is 1018 DOM tubing.... Add to that the 1192 lbs. support from the pin, and you have 2039 lbs.... This would give a safety margin for the compression load in the tube wall of 2039 / 791 = 2.6:1 at 1800 psi.... That is well past the fatigue limit of the steel, I would have no problem doing that.... Note, this is without the original screws even being in place.... but no reason not to still use them as well....
I don't know if the setup I used in that tank block will fit into the new QB79 tank block with the 4 O-ring grooves.... but IMO the installation itself is plenty strong enough providing the gun is equipped with a 1.8K burst disc.... but then again, I'm not an Engineer....
Bob
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thanks Bob for all you info in this thread ,earlier in the thread someone asked about custom made gas blocks
a chap in the uk does make them from time to time
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Ped
Got a link to the UK chap.
Mike
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he sells them on the crosman groups on facebook Mike
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Ped
Don't do social media of any kind so that will not work for me. Is he on the GTA and if so got a user name.
Mike
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not afaik Mike he makes all sorts of custom parts inc ar style trigger frames breech's barrel bands but does them in small runs
i'll talk to him later today and ask if I can pass on his email address to you then you can talk to him direct
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Ped
That will work or give him my email and he can email me direct if he prefers that way.
buldawg76 at cableone dot net
Mike
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Got my 1/4 x 28 hardened allen holding screw in place, hole drilled and tapped when together ... should be enough if something gives way.
http://s20.postimg.org/9a542habh/Large_Holding_Screw.jpg (http://s20.postimg.org/9a542habh/Large_Holding_Screw.jpg)
wll
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Is that tapped into both pieces while assembled?.... If so, looks like a good solution....
Bob
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that's how I do mine
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Is that tapped into both pieces while assembled?.... If so, looks like a good solution....
Bob
Yes, side screws were in place with a small amount of air pressure and then drilled and tapped. I have looked and thought of everything, but this was simple enough for me to do, and screw size was way bigger than what most use so I figured staying at 1350psi outgoing and using a burst disk I was in a good safety margin using this big allen screw maybe not up to using two 10x32 screws or a dowel pin, but still a margin of safety well beyond factory.
Thank you Bob for all your input !
wll
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Will
I would say that is plenty of surface area to prevent any unintentional parts flying out of the tube under the 1350 psi you are running and is an nice clean method of doing it since it is hidden under the barrel when assembled.
My 78 is apart right now getting the mods performed to mount the tank block and other ones talked about on the two threads for modding the QBs. So far have my 177 probe drilled out to .125" in the nose bore and .140" for the breech port interface and barrel port at .140" as well. Getting ready to do the valve throat and angle the port wall out of the valve and then work on the poppet and valve nose and select the poppet spring I think will be the best tension.
Then it will be on to the cock on close mod and RVA mod.
I have one question about the RVA mod since the retaining screw that holds the trigger, tube end cap and breech end cap runs up thru the middle of the tube end cap and threads into the breech cap. To install the screw for the RVA does the hole get drilled off center of the end cap so it pass by the rear retaining screw to push on the rear spring guide or does the tube end cap get tapped for a bigger thread and two screws installed from the trigger side and top of breech side with hole on the top of the breech being drilled to allow for the top retaining screw to thread into the tube end cap like crosman gun are designed.
My concern with drilling the RVA screw hole off center of the retaining screw hole to pass by that screw in the end cap is binding of the rear spring guide in the tube since it would be pushing off center of the rear of the guide or does the guide fit snug enough and have enough surface contact on the tube walls that it is a non issue and work just fine without binding of the rear guide.
Any thoughts.
Mike
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Re: RVA - I had no problems with the RVA screw being off-center... No binding.
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Will
I would say that is plenty of surface area to prevent any unintentional parts flying out of the tube under the 1350 psi you are running and is an nice clean method of doing it since it is hidden under the barrel when assembled.
My 78 is apart right now getting the mods performed to mount the tank block and other ones talked about on the two threads for modding the QBs. So far have my 177 probe drilled out to .125" in the nose bore and .140" for the breech port interface and barrel port at .140" as well. Getting ready to do the valve throat and angle the port wall out of the valve and then work on the poppet and valve nose and select the poppet spring I think will be the best tension.
Then it will be on to the cock on close mod and RVA mod.
I have one question about the RVA mod since the retaining screw that holds the trigger, tube end cap and breech end cap runs up thru the middle of the tube end cap and threads into the breech cap. To install the screw for the RVA does the hole get drilled off center of the end cap so it pass by the rear retaining screw to push on the rear spring guide or does the tube end cap get tapped for a bigger thread and two screws installed from the trigger side and top of breech side with hole on the top of the breech being drilled to allow for the top retaining screw to thread into the tube end cap like crosman gun are designed.
My concern with drilling the RVA screw hole off center of the retaining screw hole to pass by that screw in the end cap is binding of the rear spring guide in the tube since it would be pushing off center of the rear of the guide or does the guide fit snug enough and have enough surface contact on the tube walls that it is a non issue and work just fine without binding of the rear guide.
Any thoughts.
Mike
Mike I wish I could help on the RVA but my QB's are all cock on close ... I just like that/ cocking method, even though the other is far easier to adjust.
I set my QB 78 in .22 cal up with a 22ci tank, a 1350 outgoing and a 1/4 inch shim under the striker spring to open the poppet with this much pressure. I'm in the 850's using 18.1 heavies with the '78 and 808fps using heavies with the '79 and the same outgoing ... the '79 is using a 17ci tank . My QB78 in 177 has a outgoing of about 1200+, uses a 22ci tank and is shooting Kodiaks around 880ish last I checked
wll
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I did make a hammer spring adjuster for cock on close qb's ,you had to remove the rear tube cap though
basically a new cocking block with the guide threaded so it can be adjusted can be
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Yup. The rva screw hole is drilled off center. The spring guide fits pretty snug in the tube so binding is not normally a problem. I worried about the same thing. I've done two with no issues. It's a great mod. Improves tuneability and versitity if the qb. Troy
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To all who responded on the RVA and cock on open versus cock on close thanks for the input and I will be going the off center RVA route as I want to have external adjustments capable of being changed easily without disassenbly at all. I want the ability to turn it up or down within certain limits with just the screw and prefer cock on open as well.
Mike
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Hey Mike,
The RVA with cock on open are very easy modifications to do, even though cutting your breach the first time will make you think twice ;D. Again, the hardest part for me is RVA drilling. For a $500 drill press, mine is really a POS :o --> too much spindle run-out :-[
(http://i113.photobucket.com/albums/n227/kirk_schwarz/QB79%20Tune%20010_zpsamqddkts.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/QB79%20Tune%20010_zpsamqddkts.jpg.html)
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Hey Mike,
The RVA with cock on open are very easy modifications to do, even though cutting your breach the first time will make you think twice ;D. Again, the hardest part for me is RVA drilling. For a $500 drill press, mine is really a POS :o --> too much spindle run-out :-[
(http://i113.photobucket.com/albums/n227/kirk_schwarz/QB79%20Tune%20010_zpsamqddkts.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/QB79%20Tune%20010_zpsamqddkts.jpg.html)
Kirk
Yea I am going very slow and thinking out every step closely so I measure 4 times and cut or drill once. LOL My 35 year old craftsman 8 inch bench top drill press is actually still quite tight in the spindle bearing despite have one spindle bent from milling my striker block on the 60C off by .050" and the end mill hung in the steel and stopped the drill press cold. It bent the spindle and the end mill but 35 bucks later with a new spindle and its back good as new. I have more issues with my Chinese cummins tool sale drill press vise not holding the position its set it at while drilling and the gibs and ways are as tight as possible and still be capable of moving without a cheater bar on the handles.
Just making do with what I have at home until it has to be precise then its over to the buddies for his south bend lathe and huge Yakima mill to get it perfect when needed.
Still working on finding room for that 10x 48 south bend in my garage maybe one day in the future.
Mike
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Close on open is the way to go for ease of changing power setting that is for sure even though I don't do it. Because I have a cock to close my fine tuning is very much a pain so I will live with being close. For me, drilling out the breach side for a bolt slot is not an option
I can tell you guys that having a RVA on my Disco is a real pleasure and dialing in the power setting I wanted with the pellet I wanted to use was no problem.... now if someone can figure out a way on a QB to have the bolt stay back when cocked (without a breach modification) so it does not get in the way of pellet feeding ... then I would re-consider ;- )
BTW: On a cock on close is the only thing you do is remove that one bolt (#800) ?
wll
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Hey Wll,
I believe it's part 1000, the pin that fits in part 800 (spring guide).
Not that I'm any expert (as I still have a hard time drilling a straight hole), but I did my first rear cocking mod. with a hand drill and cleaned up with a diamond bit, Dremmel tool. I usually really clean up the whole cocking slot too, so the bolt action is very smooth.
A couple of years ago, I noticed Bob always (automatically) included the RVA on any gun he was working on. I thought that was odd and was a lot of extra work :o But after the mod. on my first QB, I had a "Oh my God" moment, and now try to incorporate it whenever possible.
Sometimes I need to be hit over the head a few time before it sinks in - Just ask Bob ;D.
(http://i113.photobucket.com/albums/n227/kirk_schwarz/More%20QB%20003_zpsgxmaftsu.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/More%20QB%20003_zpsgxmaftsu.jpg.html)
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Hey Wll,
I believe it's part 1000, the pin that fits in part 800 (spring guide).
Not that I'm any expert (as I still have a hard time drilling a straight hole), but I did my first rear cocking mod. with a hand drill and cleaned up with a diamond bit, Dremmel tool. I usually really clean up the whole cocking slot too, so the bolt action is very smooth.
A couple of years ago, I noticed Bob always (automatically) included the RVA on any gun he was working on. I thought that was odd and was a lot of extra work :o But after the mod. on my first QB, I had a "Oh my God" moment, and now try to incorporate it whenever possible.
Sometimes I need to be hit over the head a few time before it sinks in - Just ask Bob ;D.
Yes you are correct, it is #1000, and ya I can often use a good smack too ;- )
I'm looking at the RVA but I don't want to mess with the breech and the rear screw in the back is just ugly I'll need to find something good looking.
wll
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Use a polished brass screw ::).
Not as durable, but will add some bling :D
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For cock on opening, cutting your receiver isn't necessary. Moving the cocking pin rearward on the bolt is all you have to do. Drilling a new cocking pin location in the bolt isn't too difficult--- if you grind through the hardened surface most any drill bit will do the rest.
For RVA's, I prefer the crosman type. this does require drilling/countersinking a hole in the breech for the upper screw. However if you start with a crosman end cap($6 via Alchemy), only a hand drill and a tap is required for the rva adjustment hole. This drilling operation is very easy as the hole is aleady centered, and you only have to drill about a 3/16 deep hole. The breech and trigger screw holes are already drilled/tapped in the crosman end cap. If you make an SSG like the one in my K.I.S.S. SSG thread, NO TAPPING is required.
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Hey mackeral5,
Where can I find this Alchemy? For only $6 it could be worth it :D.
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Alchemy airgun works ebay store. Looks like they've gone up a bit since my last purchase. Today 2 cost me $20 and change after tax/shipping.
http://m.ebay.com/itm/290810725554?_trksid=p2057872.m2749.l2649&_mwBanner=1 (http://m.ebay.com/itm/290810725554?_trksid=p2057872.m2749.l2649&_mwBanner=1)
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Here is a simple solution for pinning a QB79 tank block.... http://www.mcmaster.com/#90692a725/=11lg1pi (http://www.mcmaster.com/#90692a725/=11lg1pi)
This zinc plated steel spring pin is rated at 4,100 lbs. total force to shear in two places (ie into three parts).... By itself, that would provide a safety margin at 1800 psi of 4100 / (.439 x 1800) = 4100 / 790 = 5.2:1 in shear.... It would also provide a safety margin, by itself, in compression (bearing load in the tube wall) of 1.7:1 at 1800 psi, or 2:1 at 1500 psi.... The location would have to be such that the inner half of the pin (the load side in the tank block) was bearing full width between the O-ring grooves, supported to the full diameter of the tank block stub.... When used in conjunction with the original screws, with the 3/16" hole drilled while under a low pressure load against those to insure some support from the original screws.... I see no reason why this would not be an excellent, and simple solution to retaining the tank block when used with CO2 at temperatures to 120*F, or HPA with a 1.8K burst disc installed.... IMO....
Bob
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Thanks Bob,
We'll have to label this as THE Tank Block Fix ;)
(http://i113.photobucket.com/albums/n227/kirk_schwarz/IMG_1670_zpskxbzticj.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/IMG_1670_zpskxbzticj.jpg.html)
(http://i113.photobucket.com/albums/n227/kirk_schwarz/IMG_1672_zpstzrayocb.jpg) (http://s113.photobucket.com/user/kirk_schwarz/media/IMG_1672_zpstzrayocb.jpg.html)
Looking at the two tank block pictures above, we can see that this needs to be a pretty precise hole.
And this needs to be drilled with the block in the gun under slight pressure. So a good fix, but this needs to be a well planned drill job :o. Not too far right, left or low - maybe right on center or a whisker high. 8)
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Thanks Bob,
We'll have to label this as THE Tank Block Fix ;)
Looking at the two tank block pictures above, we can see that this needs to be a pretty precise hole.
And this needs to be drilled with the block in the gun under slight pressure. So a good fix, but this needs to be a well planned drill job :o. Not too far right, left or low - maybe right on center of a whisker high. 8)
This is quite obvious this is not a job for me... if I had ten of these blocks I might be able to get one right.. I would take Bobs advice. drill the hole in the tube first and then work backwards by taking a little of the front end at a time. The 79 tank is in very much need of re-design !
wll
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That advice (drill first, shorten the tube to fit) was for installing a block in a shortened QB78 tube using the original holes.... The spring pin idea is for making a QB79 safer....
Bob
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Hey mackeral5,
Where can I find this Alchemy? For only $6 it could be worth it :D.
Kirk
The best bet for a rear cap for the crosmans is to call crosman themselves at 1-800-724-7486 and ask for part sales and you can get a disco end cap that already has a threaded hole for the adjuster screw in it with threaded holes to hold the cap in the tube or one for a 2240 that has the threaded holes to hold it in the tube but no hole threaded for the adjuster so you can thread to the screw size of your choice.
The disco end cap is part number 1761-012 and the 2240 cap is 2250-004. If you call then any parts ordered are a flat rate 4 bucks shipping so the end cap with shipping should be less than ten bucks total. The threaded hole in the end cap of the disco cap is not for an RVA it is for a degassing tool which is just a long threaded screw that pushes on the hammer to open the valve in case of overfilling and creating valve lock but can be used to install a screw as a RVA. I don't remember the thread off hand but I believe its 10-40 or a fine thread close to that size but is not an oddball thread so any local hardware will have the right thread screw. Its way cheaper than Alchemy and shipping is less than a week.
Mike
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Took a stab at one using the recessed socket head screw method. I drilled a third hole through tube and block with a few hundred PSI on system as recommended for alignment. Tapped hole 10-32 and drilled tube 5/16" to recess screw head. Hit all three with no need to enlarge holes. This rig gets a Ninja SHP 13ci tank left at the factory 1100psi. I went and got hardened socket head cap screws for the block after taking picture.
Now the questions,
This block is one I ran on a converted QB78 for a while so it had a through hole drilled for a pin. I didn't want to pin this setup and therefore drilled for 10-32 SHCS. Also, this a Crosman tube which I've learned from Mr Rsterne is slightly thicker wall. Look okay?
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That tank block looks good with the hardened screw being installed but as Bob has stated a spring steel poll pin in the hole you already have in the block would make it far above the 3.5 to 1 safety margin.
The thicker tube will increase the strength as well.
Mike