hectmarr wrote:On the left goes the air pump yes? and go through the small channel?
Yes, it's important to have a pin filling in the port in order to reduce dead volume as much as possible and not limit the maximum pressure and pump efficiency.
Another question. How much pressure do you have to shoot, under normal conditions?
These sort of rifles typically operate between 1000-2000 psi.
Chamber body done in the meantime:
hectmarr wrote:You have to make many weapons, because this field is long and short life
Quite a lot of pressure accumulates this type of weapon. I am interested in the design of the air pump, to apply it in my things. It would be useful to know the volume of air involved in each shot, and how many strokes are needed to achieve, say, 1500 psi. Your work looks very neat. I was looking for some of this information on the web, but I found almost nothing.
This type of weapon, (simplified), could be my first self-constructed weapon of compressed air.
Are you going to install this valve on your weapon?
Don't worry about asking questions, if I minded answering them I would have never joined this forum!
There is absolutely no difference between the o-rings, I just have a massive inventory of o-rings from a bunch of different sources and pick the ones that fit the best.
hectmarr wrote:You have to make many weapons, because this field is long and short life
I made a new pump rod, here it is compared to the original:
The original link is threaded for 5/16-24 which I did not have to hand so I made a brass adapter to allow the use of a 3/16-18 rod. As shown the seal end is 5/8" diameter, same as the pump tube ID, and it was binding so I turned it down by 1mm and this solved the issue.
Now that I had a way to pressurize I was able to do some testing. The first thing that I found out is that I had very much underestimated the unbalance required to shift the piston. With a 19/64" to 5/16" diameter, even with 5 pumps the piston stayed put even without anything holding it in place because the pressure imbalance was less than the o-ring friction. This imbalance represents around 27% of the force from the original cup piston.
I therefore increased the larger diameter to 21/64" and made a new piston, this time the imbalance represents around 50% of the original. With this new piston, a single pump stroke was sufficient to move it without it being held in place.
With the trigger assembled however, the return spring (part #10 here) alone will keep the piston from moving until 4-5 pump strokes are applied.
I was able to film the valve opening in high speed, here is a video with the first shot at 5 pumps and the second shot at 10 pumps. Each shot is shown twice, first at 1000 frames per second and then slowed down further to 8000 frames per second:
[youtube][/youtube]
Here are the 8000 fps segments in gif form for clarity:
At 5 pumps, the piston opens and re-seats normally
At 10 pumps however, the return spring attempts to re-seat the piston while is still being blown by the escaping air, resulting in the edge of the o-ring jamming against the edge of the second step diameter.
Naturally I want to be able to go to 10 pumps and more, so this issue needs to be solved.
I did some velocity testing this morning and the results are a little disappointing.
The best result was at 16 pumps, I got 690 feet per second with 14.3 grain Crosman hollowpoints, while an example with the standard valve tuned up was able to achieve just over 800 fps with the same projectile and number of pumps.
That being said, the concept works and trigger pull is still light at 16 pumps when 10 seems to be the recommended maximum for the stock rifle.
There's clearly a lot of things that can be optimized, particularly around the piston area to prevent the o-ring from snagging on the edge of the transfer port when re-seating. I can also revisit the o-ring spacing to have closer limits, so the seal is broken almost as soon as the piston starts to move.
Another point is that beyond drilling out the barrel port to 0.170" I haven't really touched the barrel and bolt area in terms of increasing the available flow, so as ever, watch this space.
hectmarr wrote:You have to make many weapons, because this field is long and short life
It is a good solution, even more, if the transfer port is large with respect to the diameter of the o'ring.
Ami gives me good results this system of passing the o'ring through the transfer hole, in my piston chambers for hybrids, because the hole is 4 or 5 mm and the o'ring 28 - 30 mm. When the hole is well polished and with the chamfered edge, it works. In your case, you have a larger transfer hole, calculated by the images of about 8 or 10 mm ??
The transfer port is 5mm in my case. I tried the modification in the last drawing and it was even worse, the o-ring is completely blown out of its groove even at lower pressures.
Someone on another forum suggested Back-up Rings which would be an interesting option. For now however I am trying to have an o-ring in a groove instead of on the piston, where the hope is that the pressure will tend to keep the o-ring seated:
hectmarr wrote:You have to make many weapons, because this field is long and short life
I do not believe that the o'ring comes out, if it is, as you drew, housed in the body of the valve and not in the piston. You have to try it. I should be excelling very little, it's my opinion
I tried out the o-ring in the chamber design and what an adventure.
Turns out to be a lot more susceptible to friction, I tightened the screw part until the o-ring was squeezed enough not to leak, and after that the piston wouldn't budge, even at 15 pumps.
This left me with the problem of a pressurized chamber, hence the quick tool you can see on the top right in order to let the air out via the check valve.
Of course this turned it into a de facto hammer valve, launching the rod I was using to push it out of the tube with considerable haste. Fortunately I had it pointed in a safe direction
hectmarr wrote:You have to make many weapons, because this field is long and short life
watch this space!
What difference do the blue o'ring and the black one have?