Spring size, rate and preload on 12" .177?

[netsark]

I am building, or planning to build my own air rifle this winter, we'll see how I get along. I have access to 3d CAD, CNC equipment etc. I am currently planning on a .177, barrel length 12". I am an engineer, designing the mechanisms shouldn't be a problem, but I have a few questions.

First, with a barrel length of 12”and a piston diameter of 1”, what is the optimal or recommended spring length, preload and cocking travel? I have read up a bit on longer spring with lower spring rate/ fewer CPI vs. shorter spring with higher spring rate, more CPI, but I couldn’t find a clear consensus on which is better. That last statement is not necessarily true either of course, I know there is more to spring rate than what I laid out there. I am not looking to push the envelope on power, but I’d like to get a bit of bang for my buck here. The current design I am working on is pistol-like with the stock out the base of the handle. I imagined I would need the stock to steady the gun as the barrel is fairly long for a pistol.

Is an 8” spring, OD .875”  wire diameter .120”, 4 CPI,  preloaded  1.75” with a stroke of 2.25” adequate?
 
I do not want to make the assembly too large. I could also go with a 10” spring, but, if it doesn’t have to be, then that would be nice.

I would like to see 600-800 fps here.

Any advice on all of the above?

[rsterne]

That is a swept volume of only 29cc, which is very small for the top end of the power you are looking for.... A Diana 34 or Crosman Phantom/Quest are about 50cc, and they are ~850 fps in .177 cal with a 7.9 gr. pellet.... It may well be possible to get in the lower end of your range with that swept volume....

Bob

[QVTom]

You will find the 2.25" stroke problematic.  There are two issues that need to be overcome with such a short stroke. 1) Acceleration of the piston.  You may need more distance to get the rate up, which combined with the mass, is need to effectively compress the air without excessive rebound.  2) Piston Mass.  A short piston to conserve space has very little mass.  Light pistons tend to rebound violently, this destroys efficiency.

Take a look at some springer pistols and their power output, they will give you an idea of what can be expected.

Tom

I should add that no amount of spring force will solve the short stroke limitation.

[Scotchmo]

The 29cc would give you about 8-9fpe max.

Your spring as designed would only give about 4-5fpe.

Spring piston guns tend to run springs very close to the yield strength. That may go against everything you were taught as an engineer but it is how it's done. They fail eventually (thousands of cycles). You can keep the same spring design (about 30-31 active coils), but increase the free length to about 10" (3.2 CPI). Use 3.75 inches of preload and keep the 2.25" stroke. The combination of sufficient preload and adequate piston weight will tame the bounce.

You will have a powerplant that will launch 7.33gr@700fps.

I built a gun that closely matches your specs, though I used a gas spring instead of a wire spring.

http://www.gatewaytoairguns.org/GTA/index.php?topic=29301.0

It likes JSB 7.87gr@680fps for about 8fpe. The initial version used a very light weight piston and could launch 4.0gr pellets at over 1000fps for 9.4fpe. But for practical use (accuracy) I switched to a heavier piston and heavier pellets.

[QVTom]

Looking back at your thread you hit 30% efficiency with some combinations.  Very impressive with such a short stroke.  My short stroke designs barely topped 20% but I was operating at 3X the power level.

Tom

[netsark]

Hey, thanks guys that is really helpful, I will go back the drawing board and add make some corrections.

Doing all that math when there are people out there who know it all seems a bit silly , I’ll probably have more questions as I proceed with the project…