Valve Springs - Heavy vs. Light

[JerryW]

I am in the middle of building a retained air pumper, the valve is done, but I need some help with the poppet return spring. I have several different weight springs and the valve is also adjustable for preload so there will be plenty to experiment with, but I don't know where to start.

In my head, I can make a case for either heavy or light spring tension, but how much does it matter compared to the air pressure behind the poppet? The opening force is probably influenced more by the air pressure than the spring weight, so do I use a heavy spring to combat hammer bounce? Do I use a light spring to keep from bouncing the hammer in the first place? Is there something else I'm overlooking? Am I over-thinking it?

Any explanation/theory/opinion/help would be appreciated.

Thanks,
Jerry

[rsterne]

I have generally found that too light a valve spring encourages hammer bounce.... Most of the closing force is the air pressure working on the diameter of the valve stem.... and yet I still found more hammer bounce with light valve springs....

Bob

[JerryW]

Thanks, Bob. I plan on starting with my medium spring, then I can try adding preload or try the heavier spring or go the other way and try the lighter spring. I still have a lot of parts to make so it'll be a while before I get to that point.

I've got a lot to learn, but that's part of the fun.

[rsterne]

Looks like a monster airtube, what is the diameter?.... and how do you plan on pumping it....

Bob

[JerryW]

It's .995" i.d. DOM tube. With a separate check valve, I'm going to set the spacing to give me 20cc of valve volume. For the pump, I'm looking at 5.5" of stroke for about 70cc of swept volume with a 22" long pump arm. If it proves too hard to pump, plan B will change it to a 4.5" stroke(57cc, about the same as a Benji 39X).

The first version will be .177 cal. I'm hoping for just one or two pumps between shots for about 14-15FPE, but I don't know how to verify it except to actually build the rifle. I've tried working through the math and I think(!) I will get about 65psi per pump(?). If I can get good efficiency I think it's possible.

Thanks to you, Bob, for being so free with all the info in your posts. It's been a big boost in my confidence to take on this project. Not to mention I've taken some design ideas from some of your parts.

Jerry

[rsterne]

That's a pretty big tube.... At 1500 psi, you will have 1165 lbs. of force on the piston.... A Benji is about the same volume as your shorter stroke version, but the tube diameter drops the force to about 715 lbs. at the same pressure.... My Disco tube was even less at 650 lbs.... It may be quite a bear to pump, depending on the pressure you need....

Bob

[JerryW]

Just as a point of reference, Bob, about how much force is needed on the pump handle on your Millenium Pumper and the Disco Pumper? I asked in the Pumpers - Design/Mod thread(http://www.gatewaytoairguns.org/GTA/index.php?topic=35237.msg409799#msg409799), but it was an old thread that was likely overlooked.

Thanks for the insight,
Jerry

[rsterne]

I've never measured it, but it's considerable.... ie it takes a fair amount of effort.... The Millenium pumper with the extended billet 39X linkage is about the same pumping effort at 1800 psi as the Disco Pumper Carbine with the 39X linkage is at 1500 psi.... They both have Disco size tubes (0.745" ID)....

Bob