Ron Burnett (Login rgb1) Crosman Forum Member 188.8.131.52
Would it work?
July 28 2012, 5:43 PM
Using rounded values, let's run thru a few calculations. The dimensions of the pneumatic spring: overall volume 2.2 cu.in. bore .93 (.18 valve stem) length 3.4 cross sectional area .65 sq.in. During the pump stroke, the pressure in the reservoir rises from 925 (the precharged value) to 1000. The displacer piston is moved .28 in. in the process of forcing in the ~12 cu.in. of air. During firing, and assuming adiabatic expansion, the reservoir pressure drops to 895. So......the force exerted on the piston, from start to finish goes from 650 lb (1000 x .65) to 582 (895 x .65). The effective spring rate, K (change in force/change in length), for this configuration is 245 lb/in. Since maintaining high cycle pressure is our goal, K should be kept small.
What can be accomplished with a mechanical spring within the same geometric outline? From a Schnorr catalog, #009100 fits our requirements of max force and OD. Listed values of 660 lb @.016 deflection and initial height .081 will work. The length of space available, after the pump stroke, is 3.12 in. and 48 springs [3.12/(.081-.016)] will fill it. Assuming a linear relationship between force and deflection, the spring rate for such a stack would be 840 and the generated pressure would go from 1000 to 640.
Comparing average cycle pressures for the first part of the overall expansion process........950 vs 820. The spring stack would weigh .8 lb and cost upward of $50. Hope this helps.
Ron Burnett (Login rgb1) Crosman Forum Member 184.108.40.206
"Cartridge form" .........
July 29 2012, 11:39 AM
is a good way to characterize it. Here's the back end showing the thumb button used to close the exhaust valve. Also visible are the threads that secure the whole assembly (5.7 in. long) into the upper tube.
Here's the front, the end of the exhaust valve being visible. With the valve closed and the barrel locked into place, the conical tip enters the pellet skirt. No wasted space.
CO222 (Login CO222) Crosman Forum Member 220.127.116.11
Nice but is reducing pressure drop so important?
July 30 2012, 12:16 AM
Or is it worth sacrificing a bit of pressure in a longer run-up to get higher opening speed,
because this would raise the 'mean effective pressure' in the most important stage of acceleration?
Obviously with a small air charge you can't use too much for valve operation, there'd be a sweet
spot with power drop off either side.
Ron (Login oo7fuzz) Crosman Forum Member 18.104.22.168
what would you druther
July 30 2012, 1:34 PM
In a full dump scenario, given a standard volume at a standard pressure, would you rather see the the pellet accelerate quickly which will see the charge run out of steam quickly OR would you rather control the charge in a way that would conserve the charge to apply a lessor energetic force to the pellet where said charge would supply boost to the pellet for a greater timeline or simply use more barrel?
Ron (Login oo7fuzz) Crosman Forum Member 22.214.171.124
The energy stored in the piston is supposed to
July 29 2012, 11:37 AM
maintain a higher overall pressure on the charge durring the firing cycle per stored energy in the spring.
You are correct that the valve spool is blown open by a diff pressure per the diameter differences as shown. Once the valve is cracked, the entire area of the face of the spool can then recieve total force of the compressed charge.
I did not show any plans for triggering or searing the valve spool against the force it will apply. There can be varied methods of accomplishing such.
(Login Hudson12tum) Crosman Forum Member 126.96.36.199
You could even take it a step further
July 30 2012, 9:12 PM
and make it an air-conserving valve with adjustable timing. I designed one like this a few years ago. Probably lost my sketches since then.
In this pic, the blue cylinder is a lightweight piston with a non-airtight fit in the valve housing. When the valve opens (from a backwards hammer strike or maybe blow-open) the blue piston gets blown forward until it hits the shoulder on the valve needle and closes the valve, like the displacer piston from Steve's old DP392. After the valve is shut air leaks past the piston and the spring resets its position. The air inlet (not shown) from the pump or extended reservoir is behind the blue piston.
That gives you a volume-metered air conserving valve for operating at a repeatable pressure. The volume of air dumped could be adjusted by cutting a thread on the spool. Having a moveable stop for the piston or it's return spring ride on those threads would turn the valve stem (the part sticking out the back of the housing) into a power-adjusting knob. Just turn the valve stem to adjust how far the piston has to travel before it closes the valve.
I THINK it would also be self-regulating in a pumper, settling down to a predictable pressure if you left the power setting alone and gave it the same number of pumps every shot.
Gippeto (Login gippeto) Crosman Forum Member 188.8.131.52
Re: A question, Gippeto
August 1 2012, 10:33 AM
There's no easy way to adjust it without machining pieces. The spool, spool bore piece and chamber volume are the major tuning tools as I see it.
Spool travel would be set initially such that "open" area is equal to bore area + ~ 10%. Should be close enough for a start.
Bottom line is that the valve is going to remain open until the chamber pressure drops and creates the force differential to close the valve. A spring could be added to fine tune this point, but I would personally like to avoid the use of springs.
A paintball type regulated N2 tank is supposed to be threaded into the valve body on the left. Some tuning will be done by adjusting the regulator setting.