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 Post subject: Springer piston dynamics
PostPosted: Sun Jan 11, 2015 9:49 pm 
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Location: Calgary, Alberta
Would converting a piston to aluminum (to reduce weight and recoil) affect performance?

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PostPosted: Sun Jan 11, 2015 9:54 pm 
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Yes, you will likely lose power.... and it will specialize it towards lighter pellets....

Bob

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PostPosted: Sun Jan 11, 2015 10:09 pm 
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rsterne wrote:
Yes, you will likely lose power.... and it will specialize it towards lighter pellets....

Bob


So, increasing the weight of the piston could potentially increase power?

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PostPosted: Sun Jan 11, 2015 10:12 pm 
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Not necessarily, but it will more the preference towards heavier pellets.... Most manufacturers seem to have a pretty good handle on what they are doing.... The biggest factor governing power is swept volume....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Jan 11, 2015 10:23 pm 
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rsterne wrote:
Not necessarily, but it will more the preference towards heavier pellets.... Most manufacturers seem to have a pretty good handle on what they are doing.... The biggest factor governing power is swept volume....

Bob


Many thanks.

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PostPosted: Mon Jan 12, 2015 1:58 pm 
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I wonder when the pellet starts moving in a springer. I imagine as you start increasing the power, early departure of the pellet might be a negative factor. I wonder if a valve that prevented the high pressure air reaching the pellet until the piston reached the end of it's travel could produce a higher peak pressure pulse and greater efficiency...


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PostPosted: Mon Jan 12, 2015 2:23 pm 
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Possibly, if it didn't restrict the flow and didn't lose heat, which is a primary source of power in Springers (it's largely an adiabatic cycle).... My bet is both the above would apply....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jan 12, 2015 6:01 pm 
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EverHopeful wrote:
I wonder when the pellet starts moving in a springer. I imagine as you start increasing the power, early departure of the pellet might be a negative factor. I wonder if a valve that prevented the high pressure air reaching the pellet until the piston reached the end of it's travel could produce a higher peak pressure pulse and greater efficiency...


I would hazard a guess that for the most part the pellet probably starts to move when the piston is near half of its
stroke. Reminds me of being a youngster with a syringe and plasticine, you would compress the 'piston' and when you'd
get to about the half way mark the plasticine would eject with a 'pop'. IIRC it was always at about the half way mark.

A similar concept is used in lasers. A method called q-switching. Albeit different physics, same idea. Would be a neat
experiment for sure.


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PostPosted: Mon Jan 12, 2015 6:13 pm 
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All the details are in the two Cardew books, Airguns, Trigger to Muzzle and Trigger to Target....Required reading for those modding Springers, IMO....

Bob

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Airsonal;
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Proud Member of the 2000+fps Club!


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PostPosted: Tue Jan 13, 2015 4:46 am 
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Hey Bob is there anything about air rifles you don't know? :)

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PostPosted: Tue Jan 13, 2015 5:03 am 
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I don't know Bob. never met the man. But I'm always fascinated with his posts and replies to other peoples posts. I think Bob knows everything about everything.

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PostPosted: Tue Jan 13, 2015 5:41 am 
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The pellet will move before the piston reaches the breech, but exactly when I can't say. I'm guessing the last 5%, but it also varies on the pellet weight, the pressure needed to force it into the bore (like a cork in a pop gun, it has to pop the pellet into the bore), piston speed, spring strength/rate, bore/stroke, transfer port diameter/length, dead space in the piston face/seal, your altitude, flaws and friction, temp, and maybe some other misc stuff but probably not very important.
So the piston makes peak pressure as it impacts the breech, or at least it would if the pellet didn't move before that. So somewhere in there is the point where peak pressure is generated. So the piston compresses air, but pellet starts moving which drops pressure, then the piston bounces off the breech which also drops pressure. Like a firearm, it isn't just about max pressure, but the pressure and duration which people often refer to as area under the curve.
A lighter piston builds pressure more quickly, but also drops more quickly. A lighter pellet accelerates more quickly so it's better match for that shorter window. A heavier piston is slower to build pressure, but a heavier pellet is slower to move so again a good match. A piston too light for the pellet would build peak pressure, then rebound too soon and drop pressure before the pellet can make the most of it. A piston too heavy would be too slow building pressure so the pellet would move down the bore too soon and drop pressure before it ever peaked
Like gunpowder in a gun; if you have powder that burns at x rate, then there's a certain bullet that will work best with it. If you use slower burning powder then you need a heavier bullet if you want peak power, and a lighter bullet needs faster powder. It's a balancing game, so change one thing you have to change another or power suffers. Same with gearing in a car; top speed can only be reached with one gear ratio. Increase power or reduce drag and top speed can go up, as long as you change your gearing to match.
Typically the mfg will make the piston weight work best with light to mid weight pellets. I imagine this is for two reasons; one is it works great with the lightest pellets so they can claim max fps which sells guns, and two is a lighter piston is cheaper. So as-is your gun will no doubt shoot light pellets about as fast as possible, so no real point in lightening the piston further. If you shoot only medium weight pellets then you're probably just fine, but you can experiment by adding weight to see if it helps. Heavier pistons should be more consistent because they're less influenced by variables. Heavier pellets are on avg more accurate and have better BC so they're the better choice for virtually everything, but as mentioned the pistons are generally not heavy enough so they lose power. They'd probably lose power anyway, but a heavier piston would reduce the losses. Imo there is no need for lighter than avg pellets, so I use avg and heavier. Most consider avg as 7.9gr in .177, and 14.3gr in .22. I prefer ~8.4 to 10.5 in .177, and 14.3 to 18.1 in .22. Adding weight is easier than removing, so add some and see what happens. If you gain power then you know you're in the right direction. If you lose power then you can consider lightening it, but before I do that I'd try a heavier spring. If you still want to try lighter then I'd price a new piston first, they're often quite expensive if you need to go back. If you want lighter recoil/reverse recoil then simply lighten the spring which will help greatly, and/or add weight to the gun.


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PostPosted: Tue Jan 13, 2015 6:36 am 
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Thanks Chevota, that makes a lot of sense. I hadn't even thought about piston bounce, let alone worked out how piston weight came into it. And there was me thinking that springers were simple beasts compared to CO2/PCP. :oops:

Jim


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PostPosted: Tue Jan 13, 2015 7:17 am 
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Thanks again, Bob, for pointing out those books. I started reading one last night and now I'm fixated, very interesting info.

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PostPosted: Tue Jan 13, 2015 9:37 am 
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I recently bought a refurb phantom directly from crosman. I received the new "A" version with a larger diameter and shorter compression tube (34 vs 30mm). Cheaper stock too. The piston is thin guage stamped steel with a large top hat for weight and a very short, stiff mainspring with a heavy preload. The gun was set up for lightweight pellets to reach that magic 1200 fps.(Lighter weight piston, shorter stroke and almost the same swept volume. Less inertial resistance from the lightweight pellets would respond well to the shorter, faster piston). It almost jumped out of my hands the first time I shot it. I made a heavier tophat and shortened the spring to give better performance with lead pellets. I cut off two coils and re-assembled (lube-tune and polish too). Better, but still pretty squirrely. Cut off two more. Better. Cut off two more leaving only about 1/2" of preload. Sweet. Now it chronos 780 fps with 7.5 gr lead pellets and is a real pleasure to shoot.It will probably go 800 fps with lighter lead pellets, but I don't have anything lighter than 7.5 gr. to chrono. Surprisingly quieter too.

I have two lathes, so I played around a lot with different weights of top hats and different springs to find a balance that would make a cheap gun shoot well. I found, based on my own subjective observations, that lead pellets like a heavy piston.

Chevota's posting describes very well what happens inside a springer. Well written!

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