SpudFiles

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SpudBlaster15 wrote:Area, of course.

EXACTLY what I was looking for. Thanks!

Using that I get .29" as my minimum piston travel. Awesome.

Whoops, forgot this was in a Tee, I was thinking co-ax

Oxbreath wrote:The steel hammer is likely to do some damage.

It's very hard to swing both with the same force.
Your "real world" assumption overlooks the mass of your own arm being a heavy factor in such acceleration. This means that actually, even if you think you're swinging both equally hard... you're actually swinging "your arm + hammer" equally hard, so the overall reduction in system mass is not enough to add the needed velocity to the plastic hammer to have the same energy as the steel one.

And if the two do not have the same energy after the same acceleration distance, the actual applied accelerative forces were not the same.

But it is still a fact that steel will be more likely than plastic, to do damage. And plastic will also be more likely to absorb some of the blow, itself.

I'm assuming people aren't idiots who'll pick a bumper which isn't at least reasonably elastic.

After that, hardness of the piston is irrelevant, and only the energy involved is important. And as that energy is identical in both cases, piston weight, provided at least a vaguely sensible bumper, does not affect how hard it will hit home.

Care to explain why a miniature marshmallow doesn't punch holes in a washing machine, but a AA battery does?

Both launch using the same acceleration distance from the same barrel on the same launcher. There is no mistake one causes more damage on impact.

Technician1002 wrote:Care to explain why a miniature marshmallow doesn't punch holes in a washing machine, but a AA battery does?

I already have. Page 2 of this very thread - the post was even directed at you.

I'll also add that the relative sectional densities of the target and projectile dramatically affect how momentum and energy are redistributed in an inelastic collision like this. To quote from a PM I wrote a while ago (The member I sent it to can name themselves if they wish):

...increasing sectional density ensures that the projectile spends less energy accelerating the displaced pieces of target, and thus can spare more actually doing the work necessary to punch through.

Assuming a simplified and completely inelastic collision, a projectile with the same sectional density as the projectile (for the purposes of this, I'll call this ratio X), would use up 75% of its energy accelerating the target.
A projectile where X is 5 (i.e. it has five times the sectional density of the target), only uses up about 30% of its energy. If X is 10, then only 20% of the energy is lost.

Of course, this is not a completely inelastic collision (so some energy is recovered in the collision), but for the purposes of penetrating the target, we want to bring it as close as possible to one (because we don't want to give the target the time to deform to and absorb the blow).

But the energies in the piston are however too low (well, hopefully too low) and the launcher to piston mass ratio too high to make such a thing a concern here.

Well I was going to bump the thread as soon as my extra set of hands left, since I forgot to bump it last night when I edited the first post for all the additions, then this happened:

Suffice to say I'm not impressed with UMHWPE.

Section of UHMWPE and threaded rod to be replaced with large bolt hopefully within the day. I love holidays!

Well something quite interesting happened today. I just finished machining the bolt that goes on the back of the piston, and did some basic testing, and was too excited to wait until morning to do some dry-fire testing. So I did two shots out my back door, and was very satisfied with performance. I was about to call a friend to boast that it finally works, when he called me. A horrible accident just occurred within line-of-sight of the person sighting and discharging something that looks remarkably like a sniper rifle out their back door.

saefroch wrote:A horrible accident just occurred within line-of-sight of the person sighting and discharging something that looks remarkably like a sniper rifle out their back door.

... did someone call the fuzz?

Nobody called the fuzz on me, but the "fuzz" was certainly about 100ft down the street from me, en masse. I couldn't see any of the emergency vehicles, but the townhouses along the road were lit up like day.

... but presumably the accident had nothing to do with your pneumatic?

saefroch wrote:Suffice to say I'm not impressed with UMHWPE

Wow, I've never had that problem. Did you make it too thin?

From the shape of the o ring, it looks like it got caught on the edge of the port. Did you round the port edges? Is there guides to retian the o ring in the gland? Does the bumper prevent the piston from excess over travel?

I have a feeling the high pressure along with a o ring that was able to lift out of a groove and bind had a lot to do with the failure.

Nope, the accident had nothing to do with me (or at least that seems a reasonable assumption), just a creepy coincidence.

The UHMWPE at that area was "thin" meaning about 1/16". Keep in mind though, that's still the thickness of the o-ring. The port edges were rounded, it wasn't catching on there. I do not understand your last two questions...

And what does that picture have to do with what I'm talking about?

In reverse order, The gap in the coupler just behind the brass nipple in the photo to the right is the likely cause of the o ring and then piston damage. The piston moved back and the o ring left the brass nipple and partialy blew off in the iron coupling behind it. Piston rebound sliced the o ring on the end of the nipple and tore the end of the piston.

Tensile strength is not the same as compression strength. The o ring gets pushed against one wall of the gland and the cylinder in a compression force while the end of the piston receives high tensile forces. Between damage from pounding on the bushing and tensile forces, this appears to have come apart.

The piston over travel would be the piston going so far into the pilot the o ring exits the pipe the piston slides inside and the o ring catches on an edge as it pops out of the groove due to pressure. Then piston and bumper rebound simply slices the o ring into pieces and finishes tearing the end off the piston.

If I see this photo correctly, I see the cross, a pipe nipple for the piston to slide in, a coupling, and then a nipple to a ball valve. The piston exiting the nipple into the coupling with high pressure on it can lift the o ring from the groove (gland) and then get caught on the end of one of the pipe nipples unless you have a way to keep the o ring in the gland as the piston enters this area.


Using a long nipple instead of the 2 short ones and a coupling can remove this hazzard from the piston o ring. Installing a custom made piston stop to support the bumper and a good bumper can prevent this type of damage.

Here is an example of a piston that is in a 10 inch long nipple with a bumper and a piston stop behind that. With extensive use, there is no piston groove or o ring damage. I bought extra o rings when I made this, but never used them.


I'm going out on a limb here and taking an educated guess from the failure photo. This failed when it was fired, not when it was simply filled with air. I am taking this guess because of the o ring damage. The ring got caught and pinched in something. A simple blowout does not do that type of damage. If it blew out instead of firing, the piston failure would have prevented it from firing as the chamber would blow out into the pilot. I think it fired which caused the damage when the o ring exited the nipple and got caught on the edge inside the coupler.

Technician1002 wrote:In reverse order, The gap between the cylinder for the pilot and the barrel can be a source of o ring damage. The size of the pilot orifice through the bumper can provide a narrow edge for the piston to land on causing areas of high stress and failure. The bumper and associated structure should provide an even solid base for the piston to land on without points of high stress.

The bumper is an o-ring identical to the one on the front of the piston, the fat one. The piston cannot force itself into an uneven landing on the bumper. However it is possible that there were regions of high stress. This has now been fixed, along with added durability.

Technician1002 wrote:Tensile strength is not the same as compression strength.

I'm fully aware. UHMWPE has awesome compression strength, but not so good tensile strength.

Technician1002 wrote:The piston over travel would be the piston going so far into the pilot the o ring exits the pipe the piston slides inside and the o ring catches on an edge as it pops out of the groove due to pressure. Then piston and bumper rebound simply slices the o ring into pieces and finishes tearing the end off the piston.

Very possible. Again, this is now fixed.

Technician1002 wrote:If I see this photo correctly

Sadly, you don't. I do apologize, I'm just such a BA at chasing threads. There is a cross, a brass nipple, a coupling with the bumper inside, a bushing, a nipple, then the pilot valve. The bushing is steel, and therefore looks like a part of the nipple (of which very little is showing). Earlier on in this thread I tried to explain that there is a piston stop ahead of the pilot valve, and that was what destroyed earlier (I exchanged a brass nipple for a steel one). I'm feeling generous:


The transition from the coupler back to the brass bushing was probably the source of the wear on the back of the piston, though there is almost no wiggle at all possible for the piston inside the brass nipple.

And just to clarify, the picture above, re-posted here, is with the piston fully retracted, and thus illustrates the reduced piston travel. Previously it could retract almost all the way into that nipple.