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

[lloyd-ss]

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.

[MustangMike]

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..

[amb5500c]

I dont know how much clearer it could be. High pressures are not a joke, and can become deadly serious real quick.
Richard

[gene_sc]

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.

[rsterne]

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

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

[Ribbonstone]

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

[rsterne]

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

[lloyd-ss]

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

[Ribbonstone]

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.

[rsterne]

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

[rsterne]

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

[lloyd-ss]

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,
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

[lloyd-ss]

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

[rsterne]

I'd just be happy with screws that would give me a 2:1 safety factor at 2500 psi....   

Bob

[lloyd-ss]

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

[rsterne]

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

[lloyd-ss]

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

[lotsofair]

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

[rsterne]

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

[aznboi3644]

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.