flow of air inside a PCP

[subscriber]

Sebastian,

According to my calculations, reducing "H" to 1 mm should be safe considering a safety factor of four:

My calculation assumes 200 BAR acting across the weak point.  In reality, that 200 BAR will also act inside the 1 mm "tube" to the manometer.  As such, there is no net force trying to burst that 1 mm thick wall.  Anyway, I calculated as if there was a 200 BAR pressure differential across the weakest section; and it is safe, even then.  Also, the manometer stem is threaded in and offers support to this this section via the delrin washer.

I did a simple shell calculation, as if your weakest area was a thin walled tube (see crude image explanation below).  I considered the projected internal area of an inch long section of this thin walled tube, multiplied by 200 BAR, to generate a force of 579 pounds, trying to split (burst that tube).  I then assumed aluminum with a yield strength of 35,000 PSI (from and engineering source). 

For the same inch long section, that 579 lb force has to be borne by two walls, trying to resist splitting of the shell.  Using the yield strength and factor of safety, I calculated the minimum area that would resist that 579 lb.  This was then reduced to a wall thickness of 0.033". 

As 1 mm is 0.039", you are safe. Especially considering that wall in your design is thicker than 1 mm everywhere except right at the edge of the 1 mm diameter vent hole.

The very crude sketches below show the projected area, trying to split the shell due to the 200 BAR (red area).  And how the aluminum wall area (yellow) has to resist the force generated by the air pressure.  As the metal area is much smaller, the stress (also load/area) is much higher than the air pressure (load over area).

I hope this helps, as I need to go do something else.  If you want to see how I calculated this in detail, PM me your email address for the Excel spreadsheet...

[rumbleUU]

Im having hard time following calulations in your units, I understand how you do it, just have to put my units in, then I can follow 

I guess the safest way is to drill a hole next to the 3mm, lest say one on each side 2mm big,  this way there will never be any thinner material then 1.7mm..... I have to calculate that first, will post it here then..

I have not considered the grooves for the orings, they are pretty deep to, have to measure and see it the material is thinner on that side....

This is both fun and edjucational 

[rumbleUU]

on the other side of the 3mm hole near the o-ring grooves, there is 1.9mm material left..
I guess it seems the engineers have thought this threw 

-----------------------------------------------

Does the mechanical rule also apply high air pressure where it says something like you need to have the length of threads same as diameter of the bolt to get 100% safe to hold up... I think it is 1:1 ratio with steel bolt into a threaded steel hole, and steel into a threaded hole of aluminum its 1:1.5 ratio.....

[rsterne]

RE the question about the force holding the poppet closed.... The area used in the calculation is the outside diameter of the sealing surface.... If the poppet is flat or concave, that would be the OD of the poppet.... If the poppet face is convex or tapered, it would usually be the OD of the valve seat.... In the case of a raised seat in the valve body, and a flat poppet, you would use the OD of the seat....

If you use a tapered poppet and seat, and the poppet material is rather soft, it can wedge into the throat and make the valve more difficult to open.... I general, the softer the poppet material, the more hammer energy is required to open the valve....

Bob

[Atom]

I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm  on mine PR900W 5.5mm
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...

I didnt see yesterday when I looked quick, but it is a tapered hole right threw the gauge now where the manometer is going in, how do you make that leakfree, teflontape?

Yes teflontape I use for sealing, thread depth is 8.5 mm and thread is M8x1mm the material is aluminum I suppose for high pressure.
My pr900w 5.5mm it will be intended for slugs (Altaros slug 0.13 BC which is likely to be produced by JSB soon)
And of course I'll put SSG in it 

[rumbleUU]

I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm  on mine PR900W 5.5mm
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...

I didnt see yesterday when I looked quick, but it is a tapered hole right threw the gauge now where the manometer is going in, how do you make that leakfree, teflontape?

Yes teflontape I use for sealing, thread depth is 8.5 mm and thread is M8x1mm the material is aluminum I suppose for high pressure.
My pr900w 5.5mm it will be intended for slugs (Altaros slug 0.13 BC which is likely to be produced by JSB soon)
And of course I'll put SSG in it 

but the valve-seat inside the valve where the stem goes into is only 4.5mm hole, and the steam is 3mm in diameter wich goes inside that hole, thats less then 3.5mm effective hole area.... Did you enlarged that valveseat hole also? otherwise there is no point of making the other holes bigger...

[rumbleUU]

RE the question about the force holding the poppet closed.... The area used in the calculation is the outside diameter of the sealing surface.... If the poppet is flat or concave, that would be the OD of the poppet.... If the poppet face is convex or tapered, it would usually be the OD of the valve seat.... In the case of a raised seat in the valve body, and a flat poppet, you would use the OD of the seat....

If you use a tapered poppet and seat, and the poppet material is rather soft, it can wedge into the throat and make the valve more difficult to open.... I general, the softer the poppet material, the more hammer energy is required to open the valve....

Bob

got it, thanks

[Atom]

 Valve inlet at 6 mm and outlet at 5 mm...

[rumbleUU]

Valve inlet at 6 mm and outlet at 5 mm...

 

but how does the stem(valvepin) keep airtight? did you use same valvepin with the white delrin poppet?
valveseat is 4.5mm hole, if you have 6mm? then the valvepin will fall threw, no stop

[Atom]

Poppet is 8 mm...

[rumbleUU]

Poppet is 8 mm...

yes . but the steel pin and the brass part is 6mm, that will fall right threw, no stop, the brass and steel pin isnt attached to the white delrin poppet,,,, Im sure in your case when the valve is assambled together you can move the steelpin forward, when the gun is cocked then the brass and steel pin is free to move away from the delrin poppet.... not good. Maybee not plausible to happen, but it could..

if the hole was 5.5mm instead of 6mm wich you have then the brass part wich is 6mm could not be pulled/drop forward threw that hole..

But maybee you got lucky and the brasspart i 6.1mm and cant fall threw that 6.0mm hole??

[Atom]

Poppet is 8 mm...

yes . but the steel pin and the brass part is 6mm, that will fall right threw, no stop, the brass and steel pin isnt attached to the white delrin poppet,,,, Im sure in your case when the valve is assambled together you can move the steelpin forward, when the gun is cocked then the brass and steel pin is free to move away from the delrin poppet.... not good. Maybee not plausible to happen, but it could..

if the hole was 5.5mm instead of 6mm wich you have then the brass part wich is 6mm could not be pulled/drop forward threw that hole..

But maybee you got lucky and the brasspart i 6.1mm and cant fall threw that 6.0mm hole??

Brass is 6.5 mm 

[rumbleUU]

Poppet is 8 mm...

yes . but the steel pin and the brass part is 6mm, that will fall right threw, no stop, the brass and steel pin isnt attached to the white delrin poppet,,,, Im sure in your case when the valve is assambled together you can move the steelpin forward, when the gun is cocked then the brass and steel pin is free to move away from the delrin poppet.... not good. Maybee not plausible to happen, but it could..

if the hole was 5.5mm instead of 6mm wich you have then the brass part wich is 6mm could not be pulled/drop forward threw that hole..

But maybee you got lucky and the brasspart i 6.1mm and cant fall threw that 6.0mm hole??

Brass is 6.5 mm 

ok , so its 6.5mm, understand...

[shorty]

I have noticed the same thing in my .177 storm rider (same gun).

The gauge port is the restriction with the current valve system. I opened mine up slightly larger than bore diameter and experienced a slight increase in power and efficiency.

There is a solution that I am waiting for to be available. It is a fill port with gauge assembly. My plans are to build a new gauge port without the hole for the gauge to act as a plug and install the new fill port with gauge.
https://www.pyramydair.com/product/diana-fill-valve-and-gauge-assembly-for-bandit-stormrider?a=9034

The valve throat is another thing:
Very hard to open up to be successful with the current valve stem design. My solution was to reduce the valve stem diameter just behind the poppet. This is basically the same thing as opening up the throat without increasing the cracking force of the poppet off the seat.

My next plans are to just a make a new valve the way i want it.

The gun has alot of potential being powerful....

[rumbleUU]

I have noticed the same thing in my .177 storm rider (same gun).

The gauge port is the restriction with the current valve system. I opened mine up slightly larger than bore diameter and experienced a slight increase in power and efficiency.

There is a solution that I am waiting for to be available. It is a fill port with gauge assembly. My plans are to build a new gauge port without the hole for the gauge to act as a plug and install the new fill port with gauge.
https://www.pyramydair.com/product/diana-fill-valve-and-gauge-assembly-for-bandit-stormrider?a=9034

The valve throat is another thing:
Very hard to open up to be successful with the current valve stem design. My solution was to reduce the valve stem diameter just behind the poppet. This is basically the same thing as opening up the throat without increasing the cracking force of the poppet off the seat.

My next plans are to just a make a new valve the way i want it.

The gun has alot of potential being powerful....

how big hole on the gauge hole did you make, stock is 3mm and there is only 1.7mm "meat" to either the hole where the manometer goes or to the grooves where the oring is.... what is "safe" to drill, if you make the hole 4mm, then there will be only 1.2mm meat left

How much extra power did you gain only  open up that gauge hole?

[subscriber]

Sebastian, why not make another two 3 mm diameter through holes, like you suggested already?

[shorty]

Just under 4mm (3.85mm).

Not much more power added but some. I am just under 25fpe with 10.5 grain maxed out but, I run 20fpe on it using a different hammer spring.

The reason for not power modding more right now, is that the gun on the top half is not rigid enough to keep accuracy (even with an additional barrel band).

[rumbleUU]

Sebastian, why not make another two 3 mm diameter through holes, like you suggested already?

There is only room for 2 holes 1.5mm big(if you dont want to make the wall thinner where they allready are thin), not sure the extra holes would anything , Im only looking for more efficiency, power isnt my goal, its just interesting knowing how it all works inside a pcp...... And im not sure that even with a 8mm hole the efficiency would go up?

@ shorty, I did ask wrong last time, what I meant more, did you notice any efficency gain when increasing that hole, maybee you didnt do a full shotstring and compared to before?  Also why is your gun not rigid enough?

[rumbleUU]

This is my theory: practicly on any unregulated pcp, If im wrong, correct me: I think there is a big chance of gaining efficiency by making all the ports the same size, so the air can flow smooth, then you regulate power by only the transfer port AND hammer tension, you can also play with hammerweight and different hammersprings of course.


I want to make this PR900W as efficient as possible.... thats my goal, to begin I need to understand how it all works, even if I think I know its always better asking twice so I dont make misstake, I also dont like to step on security, I dont want to risk any failures when making wall thinner and so on and risking getting hurt if a wall burts..

[shorty]

I didn't do a before and after.

I did just run a string with 7.9 grain. 170 bar to 125 bar (2500psi to 1800psi). 27 shots. 980fps up to 1041fps and back to 982fps. Big spread but 1.64 efficiency at 18fpe is ok in my book.

If your looking for efficiency, you also need to know what power level you want.

I can tell you for sure, porting this gun without regulation is not going to get you alot of shots within in a 3% spread with it's tiny air reservoir.

My gun is set up for backyard shooting under 30 yards. When I first got this gun I was thinking power mods but, the small air reservoir and the overall mechanical strength of the gun is beginning to force me to turn mine into a 12/15fpe regulated gun.

Unfortunately, it's very hard for me to invest in such a cheap gun. Yes it's fun but so many things need attention. My main focus now and has been accuracy and i feel as if I am as close as I can get out of it.

Besides the valve and gauge block, I use a short stiff spring behind the hammer. The short stiffer spring is what gives me the efficiency. I am sure an SSG system would be better but I typically use short stiff springs.

Good luck and be safe. There's alot of information here on GTA for power and efficiency modifications. All our valves are basically the same except for the balanced ones.