Hi pressure hydraulic piston failure.

Motorsports-X · May 18, 2017

i have this piston that goes inside of an injection molding machine. its very odd the way it broke. it almost looks like its 2 peices of steel that are somehow fastened together, but its most like "just the way it broke" that makes it look like that.

I think the problem stems from the sharp corners in the seal channel

This is like the 4th one that has broken

Im going to make one with radius'd corners and see if that works. Not sure one what steel to use. This one checks about 35 rockwell, but im thinking it might be some sort of stainless. not 100% sure on that though.

any suggestions?

Motorsports-X · May 18, 2017

im thinking h13 might be a good choice based strictly on toughness.. but im leaning towards 4340. . the main problem i have, is i cant find a chart that shows toughness/tensile at 36hrc all i can find is ultimate.

Milland · May 18, 2017

Look at gas nitriding or similar to get a harder, but as important, more gall-resistant surface. Even if you get the strength and fatigue resistance from removing the sharp edge you still want to protect the piston and bore surfaces. Might be worth investigating other surface treatments for similar benefits.

Motorsports-X · May 18, 2017

How would one go about testing a part for surface hardness? the hardness test i have, im assuming, only test the substrate.

Milland · May 18, 2017

Microindentation Hardness Testing | Microindentation Hardness Failure Analysis | Microindentation Hardness Material Analysis

Even a file test on the broken part surface and along the fracture will tell you if there's a difference.

magneticanomaly · May 18, 2017

More info may get more help. How high is "high pressure" 4000PSI? 10,000? 50,000?

What material is the bore or cylinder this piston rides in made of?

And, of course, a picture or dimensioned sketch .

muckalee · May 19, 2017

Can you post pictures of the fracture surface?

Luke Rickert · May 19, 2017

I am always looking for cool examples to do detail FEA, send me a sketch and I can probably tell you why it is failing and give an idea of the required material strength and condition. Machine parts should never fail catastrophically, there is something wrong with the design or the setup. You are probably looking at some sort of fatigue or fracture mechanics problem but without knowing the setup it is rather hard to tell.

Luke

crossthread · May 19, 2017

I spent a good deal of my career working on high pressure pistons (50,00 psi). The pistons were made of synthetic sapphire which is hard as woodpecker lips. As hard as they were the biggest culprit for premature wear was salt crystals. I had a heck of a time explaining that one to customers. In almost thirty years I never saw one break. Plenty worn out but never broken. The only time they ever broke is when I had to break them to remove them from a totally corroded fixture. Where I am going with this is that in my opinion they should be as hard as possible. If they are breaking then there is some kind of misalignment going on or something like that.

Motorsports-X · May 19, 2017

Im almost positive that it failed because of internal sharp corners. what i cant explain is why it was ok for the first 20 years and now we have broken 4 in the last couple years. I can only assume these new ones are made out of some cheap crap that cant handle the load. I'm adding radii to all the corners and making them out of h13 esr, so if these break there is something def wrong with the machine

KilrB · May 19, 2017

Did the first one have internal sharp corners?

Was it made in China?

Toms Wheels · May 19, 2017

If a part made it 20 years, and you have broken 4 in at short time. SOMETHING Changed, and I doubt it is sharp corners.
What comes mind are a number of items, in no particular order.
1. Piston rapidly hitting dead ends, with debris or uneven surface that it impacts against, causing the piston to be cocked, and fatigues failure.

2. Air in system, causing jumps and stops.

3. Excessive pressure, a cylinder rated at 5000psi, is now used in a 10,000psi system.

4. Bent cylinder rod causing fatigue failure whic hitting dead stops.

5. Something Else.

Motorsports-X · May 19, 2017

this is a very simple check valve/spring type arangement. and the spring just basically hammers it against its seat twice per machine cycle. every 30 seconds 24-7-365. we cant find any misalignment, burrs, over pressure, etc. Thats why we keep coming back to the material the new ones are made of. there is no telling if the original one was high strength steel, and these appear to be a week stainless. possibly a 316. im going to stick this h13 one in there, and if it breaks, then we def have a machine problem.

magneticanomaly · May 19, 2017

Poppet valves often have a difference between poppet angle and seat angle. The obvious purpose for this is to concentrate the seating force to get a better seal and promote "wearing in"

The less obvious effect is to make the valve head a spring, whose slight flexing reduces shock to both valve and seat. I still want to see a picture, but I imaging it is possible that if you have been replacing new poppets into old seat the seat has worn to match the poppet angle, so now each cycle imparts a greater shock load than correctly-contoured parts would see.

Motorsports-X · May 19, 2017

there is no seat. its just a flat piston on both ends. fluid pressure from one end, spring pressure plus additional external force pushes it back. its really a very basic (and not very good) design imho.

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Hi pressure hydraulic piston failure.

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