Question about valve return spring

[JPSAXNC]

We see that a heavier valve return spring can help flatten the peak of the bell curve, but why? What is the mechanism that causes that reduction of the peak?

[jackssmirkingrevenge]

Intuitively a valve that recloses quicker will use less air per shot.

[Motorhead]

A stiffer poppet spring decelerates the hammers momentum quicker resulting in a tad less lift as well dwell, while also increasing the poppets willingness to return to the seat a tad quicker

[JPSAXNC]

Thanks for the replies, I agree with what your saying. I have 4 different valve return springs, all made from the same original springs. 3 are shortened to different lengths. One of the springs, the third longest gives the flattest string, the two longer springs don't. So it seems like there is (more) to it than just closing the valve faster, I'd like to know what else might be going on, if anyone has any thoughts?

[sb327]

What gun is it in?

Dave

[JPSAXNC]

A Jet One pistol, with a 40cc reservoir. I'm using a flat faced valve seat with a 5 degree Peek toilet plunger poppet with a metering ring on the poppet to control air flow. I've increased the metering ring diameter, and I've made it smaller. I've increased the hammer strike and lessened it. Faster strings and slower strings, the valve just likes that one return spring and it lowers the peak of the bell curve under all these conditions. Just not as much as I'd like. So I'm wondering if there is anything else I can do to lower the peak further.

[rsterne]

A stronger valve spring contributes a larger portion of the closing force.... This means that a given change in air pressure causes a smaller total change in closing force, which will flatten the curve, and for a given ES therefore give a few more shots.... Note that spring force is a combination of spring rate (lb/in) and length compressed.... which means valve lift becomes a factor as well.... As Motorhead said, it will usually reduce the lift and dwell as well, causing a flatter curve and increase in shot count from the lower FPE (and associated lower air use).... However, when you add hammer strike to get back to the original FPE, you still usually see a flatter curve and slightly more shots.... The price you pay is higher cocking force on the hammer spring....

I usually measure valve spring force "on the seat", by measuring the force necessary to crack the valve by pushing the stem against a scale while holding the valve body.... IIRC, a Disco is about 7 lbf....

Bob

[JPSAXNC]

Thanks Bob, I never looked at lift and dwell as being separate functions, I thought they were describing the same function.

[JPSAXNC]

I made a new valve return spring, about 5lbs. Old one 2-3lbs. I used a slightly stiffer hammer spring, and I got a faster string than I wanted, with good shot to consistency and low shot count. So my plan is to make the metering lip on the poppet about .002 bigger, to lower the velocity and see what happens.

[rsterne]

Lift and dwell are interrelated, yes.... but the lift is proportional to the hammer energy while the dwell is proportional to its momentum.... These are the residual energy and momentum, after the poppet has been cracked from the seat, which uses up energy to achieve, slowing the hammer and also reducing it's momentum.... Energy is proportional to velocity squared, whereas momentum is proportional to the velocity (no V^2 factor).... Changing the hammer mass changes the relationship between energy and momentum, whereas the energy varies only with the hammer spring (rate, preload and travel), and (given those spring conditions) is independent of the hammer mass....

Bob

[JPSAXNC]

Thanks Bob, Tuning options for the Jet pistol are limited by it's small reservoir, and more so by limited space, and basic design. (no hammer change options), (not much hammer spring options, the polymer cocking lever and trigger pivots, would suffer, with more spring force). So the only tuning options I could see were the valve return spring, metering lip on the poppet, and changing out the short stiff free flight hammer spring for one with pre-load, and the same cocking force. Some times frustrating, but a fun gun to fiddle with.

[JPSAXNC]

I ended up using about an 8-9 lb. valve return spring, because that was a spring I could find. It worked to flatten the curve. Thanks

[Back_Roads]

 Cool, Defiantly a one trick pony sort of gun I figured regulating and a bottle, using the tube as plenum was my easiest option. I need to look into the trigger pull weight yet