Piston vs. Piston [Update: video comparison]

MRR · Mon Feb 23, 2009 1:18 pm

Okaaaayyy, I said let's burry this topic but on second thought I had a plausible idea why the two pistons behave so different.

When you use a barrel sealing, solid piston in a tee it doesn't benefit from the pressure in the chamber.

When you set the piston a little back into the housing and the sealing onto a bolt for example, it behaves like a projectile. All pressure of the chamber pushes on the surface of the piston (minus the bolt).

Sticky_Tape · Mon Feb 23, 2009 1:21 pm

Great idea I actually thought of that but I still think the solid piston would be better due to (see my last post in this topic).

ALIHISGREAT · Mon Feb 23, 2009 2:27 pm

The pressure pushing forward on the sealing face section of the piston nullifies any gain.

MRR · Mon Feb 23, 2009 2:28 pm

Even when the surface is bigger than the sealing face?

The diameter of the piston is 21mm and the O.D. of the barrel is 20mm, so you need pretty much negative pressure on a solid piston to actuate.

If you have a "piston on a stick" the pressure is not only on the sealing face but also on the surface of the piston (when piloted). That makes the sealing come away from the barrel much easier. The result is that you need less negative pressure.

If I'm totaly wrong, then could someone please explain it to me !?

...for dummies

john bunsenburner · Mon Feb 23, 2009 2:59 pm

No i am fairly sure you a right and was just about to point it out when you made your post, great find MRR!

ALIHISGREAT · Mon Feb 23, 2009 4:15 pm

I hope you don't mind me butchering your diagram... but...

the back of the sealing face is being pushed by the same pressure as the area inside the two yellow lines... so the green lines show the only area where there will effectively be pressure acting backward when the piston is piloted.

john bunsenburner · Mon Feb 23, 2009 4:31 pm

Look ali the surface that pushes against the barrel is smaller than his piston surface and so the pressure on latter is smaller than the pressur ein the other direction!

MRR · Mon Feb 23, 2009 5:35 pm

When you look at the first diagram you can see the way the pressure goes
with a solid piston (red arrows). The air push against the side of the piston.

The other design has the advantage of a little more surface (piston surface -minus- sealing face) and a straight orientation in one direction.

sputnick · Mon Feb 23, 2009 5:51 pm

What about something like this...

would the change in angle affect it at all, since the force it not necesarily pushing it forward?

Just a thought.

Ragnarok · Mon Feb 23, 2009 7:32 pm

Even though you wanted it buried, watching the video, I think it's simply less flow past the piston - note that when using the "washer piston" it takes considerably longer to fill. 2 or 3 times longer at least I'd guess.

That's indicative of slower flow past the piston - which will also result in more efficient triggering.

In other words, regardless of what you think about the fit being similar, the "washer piston" is considerably tighter in the housing, which means it can be piloted more easily.

EDIT @ Sputnick: No, because although the pressure is not pushing it forwards, there is now a larger area for it to act on (the area of the "cone" is higher than just a flat disk), which exactly cancels that out.

Leonard · Mon Feb 23, 2009 9:02 pm

I agree with Rag. Judging by the pics in the first post, the equalization hole on the hot glue piston is HUGE. That's your problem.

MRR · Tue Feb 24, 2009 1:35 am

This was only one piston of more than 5 that I made out of hot glue. I had one you needed 30 sec. to pressurize with. Also other materials didn't work on the solid piston setup. I also made a solid piston with a build in check valve. They all didn't work with a blow gun.

ALIHISGREAT · Tue Feb 24, 2009 2:43 am

john bunsenburner wrote:Look ali the surface that pushes against the barrel is smaller than his piston surface and so the pressure on latter is smaller than the pressur ein the other direction!

yes thats correct... but the effective amount of surface useful for pushing the piston back is exactly the same as the solid piston because the area on the back of the sealing face cancels out an equal area on the main piston body, leaving the green areas in MY diagram, MRR hasn't placed them correctly.

daxspudder · Tue Feb 24, 2009 2:52 am

I think this has no more effectiveness than a regular piston, other than the fact that it can inherently be lighter due solely to less volume... the area acted on when piloting is the same in each, if anything sputnick's little green arrow attributes for performance loss in your design(not just the conical design he made...)

MRR · Tue Feb 24, 2009 9:31 am

Perhaps it's just one of these "bumblebee cases". None knows how they do it, but they fly.

Right now I feel like a creationist in front of a science tribunal, trying to proof something without facts.

At least there was a discussion about it and that's what forums are about.

And yes, I know that scientists solved the bumblebee problem by now.