QB79 tank block improvement

[Ribbonstone]

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.

[SpiralGroove]

+1 ^ Ribbonstone,
Plus the QB78 just looks better with the block in the stock .

[WECSOG]

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.

[Buldawg76]

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

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   

[wll2506]

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

Just drill and put in or take out, no threading needed or extra screw to hold in place ?

[Mikes Garage]

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

[rsterne]

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

[wll2506]

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

[Buldawg76]

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

[rsterne]

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

[Buldawg76]

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

[oldpro]

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.

[rsterne]

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

[ped]

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

[Buldawg76]

Ped
Got a link to the UK chap.

Mike

[ped]

he sells them on the crosman groups on facebook Mike

[Buldawg76]

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

[ped]

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

[Buldawg76]

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

[wll2506]

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


wll