Might have blown this part, can I safely use it?

WobblyHand

WobblyHand

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I have a stock high pressure air probe used to fill an air gun. I made a drawing from it. I modified the part to help me install some o-rings, by drilling a blind hole into one end. It is unpressurized. However on the left side, the exterior (with the red) shows the area that can be pressurized up to 35 MPa. (~5000 PSI). Due to a brain fart, I used a 2.5mm drill mistakenly for the blind hole. So the thin 0.82mm wall is pressurized so that it would want to collapse the area inward.

I think Barlow's formula is for internally pressurized tubing. Is there some other formula for external pressurization? Can someone help me out?
PXL_20240916_205743669.jpg
If I have to destroy the part to prevent its use, I will. This question is for a stock part. I have no idea what kind of steel it is. It is NOT 1144. The steel is quite hard, harder than HSS. I have two damaged HSS drills.

Thanks for any and all insight.
 
Ulma Doctor said:
My pea sized brain says that sub-mm thickness and 5000psi should not be used together
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That's my gut reaction as well. But it would be helpful to find out how how close to the limit it is (or beyond the limit). Might as well learn something from my blunder, besides "don't do that again". I'd think there is a bit of force trying to push the end off to the left. Maybe ~200 lbs worth.
 
I can't help with the math for external pressure and I am pretty sure to do a proper analysis you would need to know the exact properties of the metal in question.

But I have a $.02 idea of: Can you make an interference fit plug to fill the hole? That would support the inside of the thin walled material and prevent it from collapsing?
 
Hoop stresses, especially compressive, are pretty amazing, My first gut hunch says that before it collapses, the pressurized air is pushing out (left) on the o-ring with a significantly higher force, and in tension, not compression, and no bonuses for the hoop stress. I'd be more worried about the whole end of that piece coming off. (Or out completely as a projectile? Is it otherwise restrained?

Do you happen to know the PSI (KSI) yield strength for the material that piece is made of? If so, the square inches of the cross sectional area of the "thin" part under the o-ring needs to be compared to the EFFECTIVE area of the "outside" vertical edge of that o-ring groove. That is the area of a "theoretical" shape, the flat vertical surface on the left of the o-ring groove, with it's inside diameter measured from the piece in the drawing, and it's outside diameter measured as the actual bore that it will reside within.
 
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Jake M said:
Hoop stresses, especially compressive, are pretty amazing, My first gut hunch says that before it collapses, the pressurized air is pushing out (left) on the o-ring with a significantly higher force, and in tension, not compression, and no bonuses for the hoop stress. I'd be more worried about the whole end of that piece coming off. (Or out completely as a projectile? Is it otherwise restrained?

Do you happen to know the PSI (KSI) yield strength for the material that piece is made of? If so, the square inches of the cross sectional area of the "thin" part under the o-ring needs to be compared to the EFFECTIVE area of the "outside" vertical edge of that o-ring groove. That is the area of a "theoretical" shape, the flat vertical surface on the left of the o-ring groove, with it's inside diameter measured from the piece in the drawing, and it's outside diameter measured as the actual bore that it will reside within.
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All I know for sure is that the material is strongly magnetic and hard. Harder than HSS. That doesn't give me yield strength, I realize. Whether this matters or not, the pressure tube of the air gun is titanium, so it may be unlikely that the probe is made from A36 or worse steel. But that's conjecture.

That end with the blind hole is not restrained. It is placed in an open ended hole to pressurize the titanium tube via a check valve.

It would be nice to FEA model this. I tried but had trouble meshing the part. I may truncate the model and just try the end piece. The stock part doesn't have that blind hole.
 
You might have got me a little with that metricky stuff. I know what a 7 is, but how big is 7 millimeters? it's actually kinda little. So much easier to work with, but some days it just doesn't register. Working out the static force, and PRESUMING that this thing stuffs into a 7mm bore, the outward pressure pushing outwards (leftwards) on that last O-ring groove face is roughly 200 pounds. The tension to the left in the material under the o-ring (the thin spot) works out to just plain nuts. I got 32,000 pounds per square inch. Those are rough numbers, this changes a BUNCH depending on how I respectfully "tolerance" those numbers. And a bit on me doing math past my bedtime The rooster's gonna crows at 3am for most of this month... But anyhow... Being a pressure vessel, the low end of a safety factor is 3 and a half, because there's other variables in that as well... That puts you in the 90 to 100 KSI range, which is a lot (for me) to expect from mystery steel, even if I do suspect it's capable.

I got these twice, but my eyebrow still twitches when I look at them. Probably wouldn't hurt for somebody to spot check that... The shapes you're looking at (the flat surface on the left of the groove, and the cross section of the thin spot, that shape is called an annulus. Area of the big circle minus the area of the "missing" small circle. Force is pounds per square inch times the number of square inches being acted upon, and the tension is the force divided by the square inches acted upon.
 
Think you have a decent concept of the basic problem set. It's in the range of making one squeamish. Yes, the probe is inserted into a 7mm hole. Can't have much o-ring exposed or the pressure will extrude it.

Based on the difficulty of drilling a small hole in the material, I suspect that the steel is a kind of stainless, since it appeared to work harden quite readily. The HSS drill bit squealed when I attempted to drill it, which surprised me. I don't have spare material to test, and it's not worth bringing it to be analyzed, since new it only cost $18. If it's stainless, I think the properties are a bit more favorable, but it's tough flying blind.

This is where having a home brew XRF material analyzer would be nice. Maybe it could narrow down what the material is. Maybe not.

Think I'm back to the point of scrapping the part, rather destroy it, then scrap it. If unsafe I don't want anyone to use it, including me. I simply won't remember if the part is NFG a year or so into the future. So it's safest to make it inoperable.

Now this brings me to thinking about the same part I made in 1144 with identical dimensions, but with a 2mm blind hole. Yield strength of 1144 is 620MPa if I recall correctly. If it's marginal, I can remake the part and use a 1.6mm blind hole since I have some 1.6mm pins. I also can make the hole a little less deep, as it is merely an alignment feature for the tapered o-ring tool.
 
When it comes to safety, It's just not worth taking any chances.
 
Harry Knutz said:
When it comes to safety, It's just not worth taking any chances.
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Can't disagree with you.

That being said, I need to have a probe that I can replace the o-rings somehow. I had thought that using an alignment pin and a taper tool would work. Think it still will, if I get the dimensions correct. I'm trying to determine what they might be. I'm a bit reticent designing for high pressure air, so I'm trying to balance getting it done and having it safe.

I have done FEA before, so I have some basic ideas what to look out for, but have no formal mechanical training. That being said, I like having all my body parts, so I will always take the less risky approach. Intellectually it's an interesting problem - it's making me think, which is safer than just blindly doing something.

May have to create a simplified model and noodle on it to understand the basics. Once I do that, I may simulate it to see or validate what I thought. Can't sim the whole model, there's something in it that's preventing meshing. Probably need to simplify it.
 
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