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PostPosted: Thu Feb 23, 2017 12:50 pm 
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Location: Stony Plain, Alberta
I'm looking at converting my QB78 to HPA and am going to use 4130 tubing rated at 90000 psi tensile strength. Barlows formula (p=2t times S divided by O) gives a burst pressure of 13,371, a safety factor of over 4 if using .065 wall thickness. I am wondering how that thickness stands up to screw tearout with .375 inch of the edge. I want to go with .065 to be able to use co2 capsules if necessary. Fill pressure would be 3000 psi maximum.

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PostPosted: Thu Feb 23, 2017 2:32 pm 
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Location: Canada
Machinery's handbook covers tear out. Short version is keep the holes a minimum of 1.5x the hole diameter from the tube end...measured from the center of the hole. I use 2x, some folks use more yet.

Fastener shear and bearing stress are two more things to keep an eye on. I like low head socket head cap screws...larger head provides more bearing area.

I also prefer to use the yield strength of materials rather than tensile.

Extra reading if you like that sort of thing...

topic35591.html

topic69553.html

Al


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PostPosted: Thu Feb 23, 2017 3:11 pm 
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Location: Stony Plain, Alberta
Thanks for the info and links. The manufacturer gives the tensile strength of 90000 psi but no yield. Going by machinerys handbook yield for that material would be around 58000 psi. I have no concern about screws shearing of. Head dia is .304 countersunk into end plug and coming out thru tube. That's 3 holes at .304 each. Using #10-32 socket head high tensile screws. Still very new at posting. Should have put those details in.

Ron

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PostPosted: Thu Feb 23, 2017 3:35 pm 
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Location: Coalmont BC
You can calculate the bearing stress on the tubing wall from the head diameter times the tube wall thickness, times the number of fasteners.... In your case, 0.304" x 0.065" x 3 = 0.0593 sq.in.... The load is the area of the ID times the pressure, of course.... at 3000 psi that would be 0.436 sq.in x 3000 = 1307 lbs.... The stress would therefore be 1307 / 0.0593 = 22Ksi.... The yield strength of normalized 4130 is 63 Ksi, so you have about a 2.9:1 safety margin to yield for the bearing load of the screw heads in the tube pockets.... Using the tensile strength of 97 Ksi, that increases to 4.4:1....

Tearout area would be wall thickness times the distance from end of tube to hole C/L times 2 (there are 2 shear planes, one at each edge of the hole), times the number of fasteners.... If your hole is 0.375" from the edge of the hole to the end of the tube, you have 0.065 x (0.375 + 0.152) x 2 x 3 = 0.065 x (0.527) x 6 = 0.205 sq.in., so the stress is 1307 / 0.205 = 6.4 Ksi.... Shear strength of normalized 4130 is 60% of 97Ksi = 58 Ksi, so your tearout safety margin is about 9:1....

I'm NOT an engineer, so you should double-check this, but that is my take on it....

Bob

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PostPosted: Thu Feb 23, 2017 6:44 pm 
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Thanks Bob. I only started seriously using HPA about a year ago. Before that was experimenting now and then. I'm no engineer either but with Al's and your info I think I have a handle on this. Worked with high pressure hydraulics for 25 years and am only to familiar with what sudden failure of hoses or cylinders can do.

Ron

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