GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: lloyd-ss on January 26, 2012, 08:35:42 PM
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The Benjamin Discovery, a true classic. Like many of you, I love this airgun.
Tom Gaylord offered the idea of an “Everyman’s PCP” to Crosman, and the rest is history. Crosman continues to do a phenomenal job with it.
At a fill pressure of 2,000, it’s easy to hand pump.
But that's not much pressure, why not put some more air in it and get more shots and power?
So, what are the dangers of over-pressurizing a Disco?
Here are 6 things I can see happening, depending on how careless someone is.
1) O-rings - leak, or blow out.
2) The knock-open valve - it leaks.
3) Air tube - distort, or burst open.
4) Threaded end plug at the front - shear the threads and blow out the front.
5) Pressure gauge threads - shear the threads and blow out.
6) Valve at the rear end - shear off the 3 screws and launch the valve backwards.
Has anyone ever seen a failure other than an o-ring failure or a leaking valve? If you are lucky, that is all you will see.
So if you decide to over-pressurize, what are the odds of a catastrophic failure, which numbers 3, 4, 5, and 6 are?
What do the calculations show?
Air tube
The yield strength of the steel in the tube is maybe 60ksi.
At 2000 psi fill, the Hoop stress in the tube is 13.1ksi.
Safety factor is over 4 to 1. Very good.
Threaded end plug
At 2000 psi fill, a force of 884 lbs is trying to push the plug out the end of the tube.
The aluminum threads on the plug have a shear strength of about 5,200 pounds.
Safety factor is over 5 to 1. Very good.
Pressure gauge threads
At 2000 psi fill, a force of 252 lbs is trying to push the gauge out of the threaded adapter.
The aluminum threads in the adapter have a shear strength of 1,770 pounds.
Safety factor is over 7 to 1. Very good.
Force trying to blow the valve body back into the hammer.
Those three 8-32 screws.
At 2000 psi fill, a force of 884 lbs is trying to push the valve body back into the hammer.
Please give that some very serious thought.
The three 8-32 screws holding the valve in place have a total shear area of .042 sqin x 48ksi shear strength (an estimate) of the steel used in the screws, gives a total shear strength of 2,520 pounds. (see note at the end of the post)
Safety factor on the 3 screws is 2.28 to 1. OK, but no more.
Safety factor (S.F.) at fill pressure and at unsafe overfills.
Safety factors of less than 2 to 1 are definitely a bad idea, IMO.
Fill pressure Air tube End plug Pressure Gauge Valve body screws (8-32)
2000 4.5 to 1 5.8 to 1 7.0 to 1 2.28 to 1 This is the rated fill, stick with it!
2500 3.6 to 1 4.7 to 1 5.6 to 1 1.82 to 1 Danger
3000 3.0 to 1 3.9 to 1 4.6 to 1 1.52 to 1 Danger
3500 2.6 to 1 3.3 to 1 4.0 to 1 1.30 to 1 Danger
If you go to the larger 10-32 screws in the valve body.
Fill pressure Valve body screws
Using 10-32 screws
2000 3.26 to 1
2500 2.61 to 1
3000 2.17 to 1 Danger
3500 1.86 to 1 Danger
Note-
These calculations are for information only, come with no guarantees, and are based on available information. Safety factors of less than 2 to 1 area a bad practice and should not be used, IMO. In particular, the shear strength of small screws is difficult to pin down and is only given by manufacturers as a factor of the tensile strength, with that factor and advertised strengths varying. Crosman does not publish the shear strength of the materials in question, but one can assume that sound engineering practice was used int he design of the Discovery. The fill pressure is rated at 2000psi. Heed that.
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That should be a sticky for sure, very very good info to have.. I'm very happy with the power of a disco as is out of the box so I won't be upping the pressure.. just using a power adjuster to tweak the stock hammer spring and power..
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I dont know how much clearer it could be. High pressures are not a joke, and can become deadly serious real quick.
Richard
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Excellent data Lloyd. Safety is so very important when making modifications and pushing the limits.
Thank you very much for crunching the numbers. Opened my eyes for sure.
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It would be great if Crosman would actually tell us what the tensile strength of the screws are.... In the absense of that, I would have to agree with Lloyd's calculations.... I do have an article written when the Disco first came out, but the source of the numbers is not stated.... One would assume that it was Crosman, but who knows....
http://www.americanairgunhunter.com/benji_discovery.html (http://www.americanairgunhunter.com/benji_discovery.html)
The pertinent part of that article is:
Though the recommended fill pressure is 2000 psi, the tube and valve assemblies have been hydrostatic tested to 11000 psi (the tube alone to 15000 psi).
I wouldn't trust that without confirmation from Crosman.... and I'm betting that because of liability issues they won't tell us.... It's a shame Crosman used a pan head screw instead of one of the standard socket head ones to locate the valve, as they are available in high-tensile versions, up to 200,000 psi.... that would triple the safety margin.... Unfortunately, the head diameter on #8 SHCSs is smaller, at only 0.28", and it won't fit the holes in the tube properly....
I remember that this discussion took place over on the Yellow forum, and the concensus reached there was that the screws locating the valve were good for a 6,300 psi fill (ie they had over a 3:1 safety margin at 2,000 psi).... How that conslusion was reached, I don't know.... In the absense of anything conclusive, I would recommend people follow Lloyd's advise and stick with the 2,000 psi fill.... It's the safe thing to do....
Bob
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Lets say the numbers are right(and they look right to me) and the tube won't fatigue...the end cap won'.t shoot off...and the issue screws won't fail. They probably won't.
The following does NOT apply to the guys already posting on this thread..they already know all this stuff. This is for the new guy, first PCP shooter.
But would your average guy with an as-issued Discovery see a giant advantage to higher fill pressure? Will the issue striker spring would open the valve under 2500psi...3000? Or would you be so far over the spring's ability that velocity would be seriously decreased by a 25%-33% increase in starting pressure?
So...change the spring...put in a "power adjuster"...fiddle with the transfer port...bolt probe...etc. Get it to run right at its new pressure.
And you get it to run like every other 3K PCP out there.
Why?
To get any real advantage, couldn.t
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Without "fiddling" with the hammer spring.... either adding an RVA or a heavier spring.... There is no point in filling above 2000 psi as the velocity will drop off.... That is actually the only warning Crosman have in their instruction manual about filling to more than 2000 psi....
DO NOT fill the airgun to more than 2000 psi. See “Overfill” information below.
Over Fill
• This airgun is designed for optimal performance at a pressure no greater than 2,000
psi. Over filling does not improve performance. It will actually lessen performance and
may cause the valve to lock. If you discover that the airgun has been over filled, try to
dry fire the airgun (cock and shoot with no pellet loaded) in a SAFE DIRECTION until
pressure on your airgun drops to 2,000 psi.
The above is a direct quote from the Benjamin Discovery Owner's Manual....
Bob
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A lot of good comments. As everyone has said, fooling with high pressure is no joke, so stick with what the manufacturer recommends.
Bob- To get that 6,300psi failure point with those 3 screws, I calculate that the tensile strength on them would have to be 110,000 psi, derating them to 66ksi (60% of tensile). They "might" be that when new, but take them in and out a few times, and they won't be the same.
The tube itself might be stronger than I calculated, but I chose a typical figure for steel tubing. Anyway, the tube is not the weak link.
Ribbonstone- You are absolutely right about over-pressurizing a stock Discovery. A little gain, then negative gain, and then valve lock. I wonder at what pressure the valve will lock with a standard spring? I bet it's a lot less than 3,000.
Lloyd
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Really don't know where it would valve lock...so far, have avoided that in a builds using Crosman valves/strikers, but not pressurizing Crosman tubes past their stated limit.
Stock 2260 spring and striker pretty well peaks at 1300-1400psi. Same spring with a "power adjuster" cranked to where the sear will just engage peaks at 1500-1600psi.Past those points, vel decreases, but I never pushed it until it wouldn't shoot at all (came close...estimated wrong and had pellets dribbling out the barrel at BB gun speeds).
Disco striker and issue spring peaks about 1700-1800psi. Power adjuster and cranked to it just engages the sear and it peaks at 2100-2200psi.
Enough variation in production springs that the above can be off by 200PSI either way and I'd not be surprised.
To get one running well at a peak of 2600-2700psi, would probably divorce the bolt from striker cocking, use an extended end cap to gain some spring room, and either slot the rear tube for a separate cocking knob or try a variation of the old Crosman 180's cocking end cap.
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I think I may have found a solution for those who wish to run a bit more pressure in a Disco.... McMaster Carr sell a Stainless Steel pan head 8-32 x 3/16" machine screw, part # 91400A189.... It is made from 300 series Stainless to MIL spec 51957 which requires a minimum of 80,000 psi tensile strength.... However, it appears that the typical tensile strength of 300 series Stainless in small fasteners is more like 125,000 psi.... Here is some data I found....
http://www.marfas.com/mechanical.html (http://www.marfas.com/mechanical.html)
There isn't any specific data for screws under 1/4", but the trend is clear, smaller is stronger.... Although these screws meet the MIL spec, they are listed in the McMaster Carr catalogue as "not rated".... Your thoughts, Lloyd?....
Bob
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RE peak velocities vs various hammer springs.... here is what I have found in a Disco with a stock transfer port....
22XX spring (0.030" wire) peaks at about 1300 psi, up to ~1600 near coil bind with RVA
13XX spring (0.032" wire) peaks at about 1500 psi, up to ~1800 near coil bind with RVA
Disco spring (0.035" wire) peaks at about 1700 psi, up to ~2000 near coil bind with RVA
Trakar spring (0.040" wire) peaks at about 1900 psi, up to ~2200 near coil bind with RVA
Those are with 15.9 gr pellets.... The peak velocity will occur at a higher pressure with a lighter pellet and a lower pressure with a heavier pellet.... Like Ribbonstone said, individual springs will vary somewhat, so the above is only a guidline....
Bob
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I think I may have found a solution for those who wish to run a bit more pressure in a Disco.... McMaster Carr sell a Stainless Steel pan head 8-32 x 3/16" machine screw, part # 91400A189.... It is made from 300 series Stainless to MIL spec 51957 which requires a minimum of 80,000 psi tensile strength.... However, it appears that the typical tensile strength of 300 series Stainless in small fasteners is more like 125,000 psi.... Here is some data I found....
http://www.marfas.com/mechanical.html (http://www.marfas.com/mechanical.html)
There isn't any specific data for screws under 1/4", but the trend is clear, smaller is stronger.... Although these screws meet the MIL spec, they are listed in the McMaster Carr catalogue as "not rated".... Your thoughts, Lloyd?....
Bob
Bob,
I have used those MS51957 and MS51958 pan head screws by the bucketful and the 80,000 psi tensile is about right for a non-heat-treated stainless machine screw. Also, for the similar SS socket heads. To get above 100ksi you normally have to go to something that is specifically purchased as high strength or heat-treated. Some of the smaller sizes that are roll formed out of work hardening alloys may achieve the higher strength, too.
Lloyd
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Really don't know where it would valve lock...so far, have avoided that in a builds using Crosman valves/strikers, but not pressurizing Crosman tubes past their stated limit.
Stock 2260 spring and striker pretty well peaks at 1300-1400psi. Same spring with a "power adjuster" cranked to where the sear will just engage peaks at 1500-1600psi.Past those points, vel decreases, but I never pushed it until it wouldn't shoot at all (came close...estimated wrong and had pellets dribbling out the barrel at BB gun speeds).
Disco striker and issue spring peaks about 1700-1800psi. Power adjuster and cranked to it just engages the sear and it peaks at 2100-2200psi.
Enough variation in production springs that the above can be off by 200PSI either way and I'd not be surprised.
To get one running well at a peak of 2600-2700psi, would probably divorce the bolt from striker cocking, use an extended end cap to gain some spring room, and either slot the rear tube for a separate cocking knob or try a variation of the old Crosman 180's cocking end cap.
I agree that it will take some work to get the gun operating above 2400. It really is a balancing act with these PCPs and trying to push them too far off their original design spec usually requires several changes that must work well together.
I am not familiar with the Crosman 180 cocking end cap. Do you have links to any pics of it?
Thanks,
lloyd
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I'd just be happy with screws that would give me a 2:1 safety factor at 2500 psi.... :o
Bob
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Bob,
Ya know, with your one piece valve front end that incorporated the gauge port.... if the hole spacing were precise enough, doesn't the gauge adapter act as another shear point?
I think you just forgot that that was one of the added benefits of that design, he, he. ;)
Lloyd
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It would, except I never fit the hole in the tube precisely enough for that.... However, it wouldn't be difficult to use the front trigger screw for extra strength.... or to simply add one more screw in the extension.... The orientation of the hole would be critical because of "thread clocking".... but it could be done....
Bob
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Bob,
If the 3 holes are tapped out to 10-32, and these McMaster 10-32 low profile socket head capscrews in black ox alloy steel 92220A171 at 145ksi tensile are used (they will have to be shortened from .250 down to .187), that will give an ultimate shear failure of over 11,000 psi (tank pressure), which will give a nice safety safety factor, and the screws might no longer be the weak link (the threads in the valve body might fail first).
10-32 screw, effective cross section = 0.02 sqin x 145,000 tensile = 2900 pounds x .6 (reduction for shear stress) = 1,740 pounds x 3 screws = 5,220 pounds of shear strength.
Now, back figuring the valve: .75 dia cross section = .442 sqin x 11,810 psi tank pressure= 5,200 pounds force.
So, if I figured right the failure point of the three 10-32 high strength screws is approx 11,810 psi, which gives a very generous 4.7 to 1 safety factor at 2,500 psi.
A little alteration to the tube and valve body are required, but not bad considering the gains.
Lloyd
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Is there anybody out there who is capable of performing such a stress test??? If someone is qualified to test the Disco up to failure we all could donate some funds to cover the cost!
Manfred
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I have some high tensile low profile 10-32 screws, I use them for my Hayabusa.... I'm going to try and figure an easy way to enlarge the valve holes to 10-32 today.... maybe a drilling and tapping jig?....
Bob
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all this talk about a screws tensile strength...shear strength is different from tensile.
Like when I was helping my friend and his dad(contractor) build a garage screws aren't allowed by code in the framing because the shear strength of regular 16 penny nail is MUCH higher than any screw and cheaper.
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Nam-
I thought the same thing. But then I had to remember that when a shear is not given, that the tensile is needed to calculate a shear for a fastener. Building a dwelling down here to withstand a hurricane wind load opens ones eyes to fastener strength values.
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The shear strength is typically calculated using 60% of the tensile strength.... That is an industy standard.... If a screw has a tensile strength of 100,000 psi, you use 60,000 psi to calculate the shear strength....
Bob
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anzboi and ratracer,
All of my calculations used the accepted standard method for bolts of using a value for shear strength that is 60% of the rated tensile strength.
Lloyd
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If say the "recommended pressure" in a tire is XX psi, - does anyone really think "higher" would be better??? Let's see how much it will take before it blows. ... Yes, no doubt there is a "safety factor" built in, ... there has to be, so folks don't injure/kill themselves - and then sue Crosman for their own actions. - If someone wants a 3000psi gun - fine, go buy one.
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Bob,
If the 3 holes are tapped out to 10-32, and these McMaster 10-32 low profile socket head capscrews in black ox alloy steel 92220A171 at 145ksi tensile are used (they will have to be shortened from .250 down to .187), that will give an ultimate shear failure of over 11,000 psi (tank pressure), which will give a nice safety safety factor, and the screws might no longer be the weak link (the threads in the valve body might fail first).
10-32 screw, effective cross section = 0.02 sqin x 145,000 tensile = 2900 pounds x .6 (reduction for shear stress) = 1,740 pounds x 3 screws = 5,220 pounds of shear strength.
Now, back figuring the valve: .75 dia cross section = .442 sqin x 11,810 psi tank pressure= 5,200 pounds force.
So, if I figured right the failure point of the three 10-32 high strength screws is approx 11,810 psi, which gives a very generous 4.7 to 1 safety factor at 2,500 psi.
A little alteration to the tube and valve body are required, but not bad considering the gains.
Lloyd
Lloyd, I think your calculations are real close. Industry standard for pressure vessels is a least 4x operating pressure for burst.
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If say the "recommended pressure" in a tire is XX psi, - does anyone really think "higher" would be better??? Let's see how much it will take before it blows. ... Yes, no doubt there is a "safety factor" built in, ... there has to be, so folks don't injure/kill themselves - and then sue Crosman for their own actions. - If someone wants a 3000psi gun - fine, go buy one.
I'd much rather build one. ;D
Lloyd
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Well, after last nights discussion, you just know I had to make a jig for drilling a Disco valve for 10-32 screws, right?.... Here are the photos....
(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/LargerScrews.jpg)
The jig is made from a piece of 1" x 1.5" x 2" aluminum.... A block of steel would be better for production work.... I drilled a 3/4" hole 1.5" deep and then ran a 5/8" mill down along the bottom side and removed 0.015" to form a shallow groove (ie the hole is slightly "pear" shaped).... This forms a "V" block so that when the valve is inserted it only touches at two points.... On the opposite side is a 3/8" SHSS with the end ground flat so that it won't damage the valve when snugged up.... There are two holes drilled in the top beside the setscrew.... The front one is a #21 drill (the correct size for tapping a 10-32 thread) and the rear one is a #11 drill (just clears a 10-32 tap).... There are two brass pins, the smaller one has the end machined to 0.128" which just fits in the 8-32 tapped hole in the Disco valve.... The larger one has the end machined to nicely fit in a #21 drilled hole....
(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/DrillingJig.jpg)
To use the jig, you slide the rear valve half in until one of the screw holes lines up with the front hole and then slide the small pin down until it locates in the 8-32 thread hole.... Then you snug up the 3/8" setscrew to hold the valve in place and pull out the pin.... If you were doing a bunch, you would set the block up in the mill, aligning the hole with the head.... but I just used a hand drill and drilled by eye, using the hole in the block to guide a #21 drill to clear out the threads in the valve.... I used a normal drill, very carefully, to just remove the threads, making sure the point didn't dig in as there isn't much meat between the bottom of the hole and the inside of the valve.... I then changed to a drill ground flat on the end to make the hole full depth with no danger of cutting through.... This operation could also have been done with a 5/32" end mill provided you stopped at the correct depth....
I loosened the setscrew, removed the valve, and the threads were evenly removed, so I turned it around and replaced it in the jig and slid it in until the newly drilled hole lined up with the rear hole in the jig.... I slid in the larger pin to locate the valve and snugged up the setscrew again....Using a 10-32 plug tap, with the hole as a guide, I tapped the hole until the tip of the tap just touched the bottom (about 3 threads).... I then changed to a bottoming tap, and got nearly 6 threads total.... I repeated this for all three holes and then ran a countersink into the top of each hole in the valve just to provide a slight relief because the thread on the screws stops a few thou from the head....
I had to shorten the 1/4" long low-profile 10-32 screws to 3/16", properly finished the ends, checked the thread in a nut (rather than risk galling the aluminum valve), and then installed them to make sure they went all the way down and that the head was seated on the flat on the valve.... Everything worked perfectly, so it was time to install it in my 9mm and give it a try.... The screws I am using are rated at 180,000 psi tensile strength, which means 108,000 psi in shear.... That will give me a 4.4:1 safety margin at 3000 psi, making the screws no longer the weak link in the Disco.... The OD of the head on the screw is a few thou smaller than the holes in the Disco tube, but the head still seats well enough to spread the load and the transfer port still lines up as good as they ever do....
Bob
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Excellent job, Bob. Now you know for sure that the screws aren't the weak link. I might have to do the same on mine!
Lloyd
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Crosman 180:
The 180 (and some othere old-time Crosmans) seperated the bolt from the striker cocking. Were ytwo diffenet actions.
Gun at rest.
(http://i157.photobucket.com/albums/t50/ribbonstone/crosman/DSCF1628.jpg)
Cycle the bolt to load the pelllet.
(http://i157.photobucket.com/albums/t50/ribbonstone/crosman/DSCF1630.jpg)
Cock the stricker. The plunger is reverse spring loaded to pull itse3lf back down flush (where is started):
(http://i157.photobucket.com/albums/t50/ribbonstone/crosman/DSCF1629.jpg)
Point being, that if you seperate the cocking effort from the colt/loding effort, can accept a stronger spring without feeling that you are really yanking on the bolt.
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Ribbonstone,
Thanks for the pictures of the 180. That is like a lot of the big bore airguns that need the heavy hammer springs to open against the high pressure.
Does the knob on the hammer spring handle scrape the skin off the back of your thumb, or is there enough clearance?
Lloyd
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It is spring loaded to self-retract back to the starting (down) position...I jammed it for the photo.
But it does take two motions to load. Cycle the bolt (pinky finger effort) to load the pellet. Cock the striker separately,
If you wanted more striker weight (which could help open a Disco valve at high pressure), could make the cocking knob/end cap captive to the striker...adding its weight to the striker'.s weight, and getting the valve to run at high pressure.
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I think this information should be posted somewhere permanently.
Definitely good reference material. Thanks Lloyd & Bob
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So Bob, you going to start making them and selling them. ;D
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Naw, I'll leave that to some clever guy with a CNC.... I only work on my own stuff....
Bob
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I think I may have found a solution for those who wish to run a bit more pressure in a Disco.... McMaster Carr sell a Stainless Steel pan head 8-32 x 3/16" machine screw, part # 91400A189.... It is made from 300 series Stainless to MIL spec 51957 which requires a minimum of 80,000 psi tensile strength.... However, it appears that the typical tensile strength of 300 series Stainless in small fasteners is more like 125,000 psi.... Here is some data I found....
http://www.marfas.com/mechanical.html (http://www.marfas.com/mechanical.html)
There isn't any specific data for screws under 1/4", but the trend is clear, smaller is stronger.... Although these screws meet the MIL spec, they are listed in the McMaster Carr catalogue as "not rated".... Your thoughts, Lloyd?....
Bob
Bob, did you have a chance to use these 300 series screws. I just bought both the 10x32 low head set screws and these. These 8x32 would be a lot easier for me to do, but how much extra security do you think they have ... enough for 2200 psi ?
I'm just trying to get a little more air in the tube for an extra 5-8 shots ?
wll
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I thought I answered this question already for you.... but here you go.... http://www.gatewaytoairguns.org/GTA/index.php?topic=72672. (http://www.gatewaytoairguns.org/GTA/index.php?topic=72672.)
All the information you need is there.... I prefer Alloy steel screws over Stainless....
Bob
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I thought I answered this question already for you.... but here you go.... http://www.gatewaytoairguns.org/GTA/index.php?topic=72672. (http://www.gatewaytoairguns.org/GTA/index.php?topic=72672.)
All the information you need is there.... I prefer Alloy steel screws over Stainless....
Bob
I'll go the extra mile then and put in 10x32's
wll
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If you overfill a Disco, you're a 'statistic' either way...either one of the large number that have(so far) lived to tell the tale, or if it fails you become a statistic for that occurrence.
cheers,
Douglas
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So, is my Daughter Disco safe ? ...we have been filling it to 3000 psi
( Norm did the mods )
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Ask Norm.... 10-32 screws, done properly, not an issue, IMO....
Bob
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Manny,
You asked a simple yes-no question but go ahead and read through this whole answer. Your daughter will be glad you did.
If Norm did the work of installing the 10-32 screws, you are ok in that department, but you should do a visual inspection of the gun to help ease your mind.
This visual inspection procedure can be used for the special 10-32 screws like I assume you have, and for folks who have the factory 8-32 screws, even if they never accidentally overfill their Disco.
For this first part of the inspection leave the air in the gun. Make sure the gun isn't cocked. Remove the action from the stock. Remove the trigger assembly.
There will be 3 screws located 90 degrees apart, located underneath the breech, that secure the valve into the main air tube. Here's a good picture of the 3 special high strength low profile socket head cap screws that we are referring to as the 10-32 screws.
SAFETY NOTE TO EVERYONE- If your gun only has the standard Crosman 8-32 Phillips head screws, never ever ever fill your Disco above the recommended 2000 psi max fill. The 8-32 screws are the limiting mechanical factor in the fill pressure. This picture shows the special 10-32 screws that many people use to increase the safety factor in the Disco.
Find these 3 screws and look at them very carefully in good light.
(http://i226.photobucket.com/albums/dd79/loyd500/Valves/valve%20screw%20shear%20testing/Disco%20valve%20screws-A_zpsi3n6269a.jpg) (http://s226.photobucket.com/user/loyd500/media/Valves/valve%20screw%20shear%20testing/Disco%20valve%20screws-A_zpsi3n6269a.jpg.html)
With the air still in the gun, look very carefully at the 3 screws. You will notice that the screws will be pushed all the way to the back edge of the holes (toward the rear of the gun) because the air pressure is always trying to push the valve out the rear of the air tube. Look carefully at the tube where the screws are pushing on it. Look for any bulged or puckered up areas where the metal in the tube has distorted. This picture, and the next picture show the tiniest bit of distortion in the tube from the screw bearing hard against it. (BTW, these are test pieces that I made to do valve screw failure testing.)
(http://i226.photobucket.com/albums/dd79/loyd500/Valves/valve%20screw%20shear%20testing/8-32-3x4000-3_zps90e83027.jpg) (http://s226.photobucket.com/user/loyd500/media/Valves/valve%20screw%20shear%20testing/8-32-3x4000-3_zps90e83027.jpg.html)
Here is the same tube and you can see the distortion more clearly. If you see this with the factory Phillips head screws, you've got serious trouble. If you have the 10-32 screws and have been filling to 3,000 psi, you might see this distortion at ONE screw only. The reason is because the holes in the valve and the holes in the tube aren't normally going to line up perfectly, and some movement is going to occur as the 3 screws settle in to the point that they all bear an equal portion of the load. Again, if you see a tiny bit of distortion on only one 10-32 screw, you are ok. If you see it on all 3 screws, bad news.
(http://i226.photobucket.com/albums/dd79/loyd500/Valves/valve%20screw%20shear%20testing/8-32-3x4000-4_zps94c1d2ba.jpg) (http://s226.photobucket.com/user/loyd500/media/Valves/valve%20screw%20shear%20testing/8-32-3x4000-4_zps94c1d2ba.jpg.html)
Here is a picture of a test piece made from a factory tube that was severely overloaded. If you see this, again, you've got big trouble.
(http://i226.photobucket.com/albums/dd79/loyd500/Valves/valve%20screw%20shear%20testing/8-32-2x3000-6_zpsa10bed16.jpg) (http://s226.photobucket.com/user/loyd500/media/Valves/valve%20screw%20shear%20testing/8-32-2x3000-6_zpsa10bed16.jpg.html)
Here is an A513 Type 5 steel tube (commonly used) that was overloaded even more than the previous factory tube. All of the components have yielded and the actual failure pressure was higher than predicted because of the way ALL components are sharing the overload.
(http://i226.photobucket.com/albums/dd79/loyd500/Valves/valve%20screw%20shear%20testing/10-32-2500-3_zps19bf9872.jpg) (http://s226.photobucket.com/user/loyd500/media/Valves/valve%20screw%20shear%20testing/10-32-2500-3_zps19bf9872.jpg.html)
Here is the screw removed from the severely overloaded tube. The tube and valve body, and screw were all distorted and the screw was extremely difficult to remove. Failure can be a multistage process with the weak points changing between the various components until you get a big ka-pow.
(http://i226.photobucket.com/albums/dd79/loyd500/Valves/valve%20screw%20shear%20testing/10-32-2500-6_zps7b947584.jpg) (http://s226.photobucket.com/user/loyd500/media/Valves/valve%20screw%20shear%20testing/10-32-2500-6_zps7b947584.jpg.html)
So what do you do if you see some distortion and have some concerns?
Again, this is good for the factory screws and the 10-32 special screws. First off, go ahead and put the trigger assy back on the gun and shoot all the air out. Make sure the gun is totally empty. Take the trigger back off. Try removing the valve screw that shows the most distortion. Once it breaks loose, how does it unscrew? Does it come out smoothly, or does it feel bent? If it feels bent, look at the screw and the threaded hole. How bad are they? Screws that are constantly overloaded, or that are removed and tightened often, can have a short life and it is not a bad idea to periodically replace them. If it seems like something is wrong, don't take a chance. Fix the problem. Safety has to be number one.
Lloyd
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that's an excellent post Lloyd good descriptions and pics!!!
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(http://i157.photobucket.com/albums/t50/ribbonstone/springers/843dac3c-c9aa-4353-8e7a-2bb1e65c40bc.jpg) (http://s157.photobucket.com/user/ribbonstone/media/springers/843dac3c-c9aa-4353-8e7a-2bb1e65c40bc.jpg.html)
Come on...I can get one more use out of this screw, it ain't broke yet.
(OK...I cheated and it's a scope mount screw...but you get the idea.)
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I was gonna check my daughter's disco, but can't get the stock off because the safety button is in the way, how do you get that trigger unit off ? I just can't see the way.
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I was gonna check my daughter's disco, but can't get the stock off because the safety button is in the way, how do you get that trigger unit off ? I just can't see the way.
Take a jewelers screwdriver and pull the small metal bar that is behind the safety. Then the safety button will slide out. Then take off the stock and remove the rear bolt and stock lug that is holding the trigger unit on. Careful to not crank down on the stock lug when tightening back up. They are notorious for stripping.
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Did it.
Everything looks perfect on this gun, no metal imperfections, ....while I was there I cranked up the power adjuster to max,
....Maybe a .177 Hog hunt in the future ? :)
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Great post, I don't think I've had my Disco to 1900psi, and have only had about 10 fills, if that. Put a RVA in it and clipped the spring and played with it a bit, and she is shooting pretty good :- )
I will change the screws to a better quality 8x32, and after reading your post, may well rethink my up to 2100 psi plans.
Thanks Lloyd !
wll
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Great! Sounds like everyone is on the safe track.
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Quick question about pressure ..... After your done shooting should you remove all pressure from gun?
Does that matter?
Thanks
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Quick question about pressure ..... After your done shooting should you remove all pressure from gun?
Does that matter?
Thanks
Leaving pressure in the gun is fine. In fact, depressurizing the gun can be harmful simply because the pressurized gun keeps particulates out of the system for the most part.
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Cool thanks for the info!
Lance
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Just fixed the valve body and installed 10x32's the hole I thought was bad was the air hole, so I drilled and tapped the correct one.
Major question ..... I can't seem to get answered !!!!!! My 3 drill holes go through into the poppet hole ... is that bad ? The 1/4" 10x32 fits tight and the poppet moves freely still.
http://s20.postimg.org/3zjlxpsal/10x32_in_Valve_smx.jpg (http://s20.postimg.org/3zjlxpsal/10x32_in_Valve_smx.jpg)
wll
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I don't think you will have a problem with the holes going through into the hole for the stem.... If it leaks, it will only do so during the shot.... Normally, you would shorten the 10-32 screws to 3/16".... I use blind holes and a bottoming tap....
Bob
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I don't think you will have a problem with the holes going through into the hole for the stem.... If it leaks, it will only do so during the shot.... Normally, you would shorten the 10-32 screws to 3/16".... I use blind holes and a bottoming tap....
Bob
Bob, thank you, I was worried about it blowing the screw out. I will add a touch of blue Locktite also just to make sure the air does not loosen the screw. This was my first try at this valve and I thought I measured right ?, the starting tap may have pushed through, I should have just given it a few twist and then used a bottom tap, better yet I should have ground off the tip of the starting tap to begin with, and then gone to a bottom tap.
Thank you Bob.
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William,
the blue Loctite is a good idea when you break thru into the valve. You can do a few test shots before installing the action back into the stock and feel if there is excessive air coming out around the screws when the gun is fired. Like Bob said, it shouldn't be a problem. I know a few people who, when modifying a Disco valve for high power, and drilling out the throat and installing the high strength 10-32 screws, just go ahead and drill thru into the exhaust port area rather than go to the extra trouble of not breaking thru. They can also use a little longer screw that way, and they just plan on using the gel type blue loctite to take care of the sealing. It works.
Lloyd
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William,
the blue Loctite is a good idea when you break thru into the valve. You can do a few test shots before installing the action back into the stock and feel if there is excessive air coming out around the screws when the gun is fired. Like Bob said, it shouldn't be a problem. I know a few people who, when modifying a Disco valve for high power, and drilling out the throat and installing the high strength 10-32 screws, just go ahead and drill thru into the exhaust port area rather than go to the extra trouble of not breaking thru. They can also use a little longer screw that way, and they just plan on using the gel type blue loctite to take care of the sealing. It works.
Lloyd
Great news, as one of the major problems I had was drilling and worrying about going through the poppet hole. This makes doing this job much, much easier. Since I have another Disco, this will be done for the one in the box that I have not shot yet ... yes before I air her up for the first time.
Thanks guys for all you help and testing, knowing that many airgunners including myself at times can fly by the seat of their pants sometimes, and HPA's are way to dangerous for that.
wll
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I want to thank Lloyd for starting this post and the great information.
Thanks also to Bob and others for theirs.
wll - I followed a previous post from Bob on disco valve mods for more power. I opened the throat on an otherwise brand new disco valve to 15/64 and subsequently see daylight through the bottom of all three screw holes. Like you, I was initially concerned.
I will drill and tap them for 10-32 low SHCS ( steel alloy from McMaster ) and through hole them when I do. I plan to shorten the 1/4" screws if they protrude into the chamber past the opening, cleaning up the threads so they don't gall the aluminum.
Another forum member suggested some Loctite low strength formula for the screws to help with sealing. Keep in mind this area is behind the poppet and is only pressurized when you fire it. I was initially contemplating a dab of RTV on the threads but I will instead go with the Loctite 222.
I am not planning crazy pressures but exploring what I can get with more pressure before required hammer spring and cocking forces exceed my liking. The extra safety with a little extra FPS is my goal.
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I want to thank Lloyd for starting this post and the great information.
Thanks also to Bob and others for theirs.
wll - I followed a previous post from Bob on disco valve mods for more power. I opened the throat on an otherwise brand new disco valve to 15/64 and subsequently see daylight through the bottom of all three screw holes. Like you, I was initially concerned.
I will drill and tap them for 10-32 low SHCS ( steel alloy from McMaster ) and through hole them when I do. I plan to shorten the 1/4" screws if they protrude into the chamber past the opening, cleaning up the threads so they don't gall the aluminum.
Another forum member suggested some Loctite low strength formula for the screws to help with sealing. Keep in mind this area is behind the poppet and is only pressurized when you fire it. I was initially contemplating a dab of RTV on the threads but I will instead go with the Loctite 222.
I am not planning crazy pressures but exploring what I can get with more pressure before required hammer spring and cocking forces exceed my liking. The extra safety with a little extra FPS is my goal.
We are on the same page ;- )
wll