Author Topic: Too much pressure in a Disco? Will you become a statistic? (Read 17668 times))

RatRacer · « **Reply #20 on:** January 27, 2012, 01:10:59 PM »

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.

rsterne · « **Reply #21 on:** January 27, 2012, 01:16:21 PM »

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

lloyd-ss · « **Reply #22 on:** January 27, 2012, 01:20:37 PM »

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

North Pack · « **Reply #23 on:** January 27, 2012, 01:21:22 PM »

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.

Zoomies · « **Reply #24 on:** January 27, 2012, 02:26:19 PM »

Quote from: lloyd-ss on January 27, 2012, 08:18:13 AM

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.

lloyd-ss · « **Reply #25 on:** January 27, 2012, 03:04:38 PM »

Quote from: North Pack on January 27, 2012, 01:21:22 PM

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.

Lloyd

rsterne · « **Reply #26 on:** January 27, 2012, 07:39:00 PM »

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....

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....

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

lloyd-ss · « **Reply #27 on:** January 27, 2012, 08:31:51 PM »

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

Ribbonstone · « **Reply #28 on:** January 27, 2012, 09:47:31 PM »

Crosman 180:

The 180 (and some othere old-time Crosmans) seperated the bolt from the striker cocking. Were ytwo diffenet actions.

Gun at rest.

Cycle the bolt to load the pelllet.

Cock the stricker. The plunger is reverse spring loaded to pull itse3lf back down flush (where is started):

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.

lloyd-ss · « **Reply #29 on:** January 27, 2012, 10:13:24 PM »

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

Ribbonstone · « **Reply #30 on:** January 27, 2012, 10:21:44 PM »

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.

CO2 Freak · « **Reply #31 on:** February 09, 2012, 12:47:20 PM »

I think this information should be posted somewhere permanently.
Definitely good reference material. Thanks Lloyd & Bob

Snaab9-2 · « **Reply #32 on:** February 09, 2012, 08:35:44 PM »

So Bob, you going to start making them and selling them.

rsterne · « **Reply #33 on:** February 09, 2012, 08:42:51 PM »

Naw, I'll leave that to some clever guy with a CNC.... I only work on my own stuff....

Bob

wll2506 · « **Reply #34 on:** March 18, 2016, 11:51:51 AM »

Quote from: rsterne on January 26, 2012, 11:11:50 PM

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

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

rsterne · « **Reply #35 on:** March 18, 2016, 12:43:04 PM »

I thought I answered this question already for you.... but here you go.... http://www.gatewaytoairguns.org/GTA/index.php?topic=72672.

All the information you need is there.... I prefer Alloy steel screws over Stainless....

Bob

wll2506 · « **Reply #36 on:** March 18, 2016, 12:53:28 PM »

Quote from: rsterne on March 18, 2016, 12:43:04 PM

I thought I answered this question already for you.... but here you go.... 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

PakProtector · « **Reply #37 on:** March 18, 2016, 07:45:30 PM »

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

Nomadic Pirate · « **Reply #38 on:** March 18, 2016, 07:53:29 PM »

So, is my Daughter Disco safe ? ...we have been filling it to 3000 psi

( Norm did the mods )

rsterne · « **Reply #39 on:** March 18, 2016, 11:13:45 PM »

Ask Norm.... 10-32 screws, done properly, not an issue, IMO....

Bob

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Too much pressure in a Disco? Will you become a statistic?

Author Topic: Too much pressure in a Disco? Will you become a statistic? (Read 17668 times))

Zoomies

CO2 Freak

Nomadic Pirate