Springer anti bounce experiment

[NickG]

Hi Loki, I don't think parachutes actually seal better, I have compared like for like tunes , with parachute o ring X ring and piston ring, the piston ring makes the most power, followed by the X ring ,

[Loki_79]

Hi Loki, I don't think parachutes actually seal better, I have compared like for like tunes , with parachute o ring X ring and piston ring, the piston ring makes the most power, followed by the X ring ,

If parachutes don't seal any better, then I don't see how they can have greater friction either? To increase the friction enough to slow down the piston must surely improve the seal? Not convinced either way, just seems logical to me.

It would be interesting to measure the actual air volumes (e.g. captured in a balloon / water tank) to confirm that any difference in power correlates to leaking air rather than friction, or rather dynamic friction.

This is all from memory, but taking a chamber pressure around 700psi, 25mm piston seal and 2mm lip, the peak piston friction force for nylon on steel is about 50N, so barely anything compared to the spring forces involved. I think it needs to be around 500N to start having any noticeable effect on piston position and compression volume. Could all be completely different of course with your lighter springs!

[NickG]

If we consider a piston ring , it rely's on pressure getting inside the ring expanding it to seal , there is going to be leakage through the gap though minimal, the seal comes from the better surface to surface fit, rather than the pressure exerted , so I believe the extra efficiency comes from the lower friction, all guess work big course as I can't measure anything other than fps.

[TonyL]

Also my view (which I'll have mentioned a few years ago in a similar thread - and touched on here again up above) and from my point of view as one who can't undertake any of the engineering / testing work required - if anything was to work as an "add-on" rather than a more "integral re-engineering" (like altering the piston weight / TP etc.), I'd feel more comfortable with the inertia idea than the idea that the extra friction from the seal delivering the "brake". As has been mentioned, we can't argue with physics and I feel that the "grippy" stalling device would transfer other undesirables into the action itself? The inertia idea, especially if inside the piston, is self-contained. Again, any energy has to go somewhere and, as has been said, we're just looking to postpone any negative effect from surge rather than trying to get rid altogether.

Still watching with great interest and can't wait to see the results.

But then you also wonder how much more tiny little gains can be made by just tweaking what is already there by playing with those different seals, the piston weight, its sectional density, spring force, preload, and TP diameter and volume. And that perfect tune will still only be 100% perfect (or as near as can be hoped) for one pellet and its release pressure. So we'd better be hoping it's the rifle's favourite pellet (and then even that could change once the tune is altered with different release and barrel exit timings). And that's just tweaking what's already there from the (seemingly) uncomplicated power plant.

God, I love springers.

[Loki_79]

If we consider a piston ring , it rely's on pressure getting inside the ring expanding it to seal , there is going to be leakage through the gap though minimal, the seal comes from the better surface to surface fit, rather than the pressure exerted , so I believe the extra efficiency comes from the lower friction, all guess work big course as I can't measure anything other than fps.

I'm only guessing too, but lower friction seems the most obvious explanation.

One of my biggest 'doh' moments in develpoing the model was when I tried to add a constant friction force for the whole length of the piston travel, which is of course to a first order approximation *exactly* the same as reducing the force delivered by the spring (since the force on the piston equals the force from the spring minus friction). It gets more complicated when friction is proportional to velocity and the spring force is not constant with low preloads, but still it is obvious when you actually write it down.

In earlier posts, I was thinking more of the reason for using a parachute seal vs a solid one (not o-rings or piston seals), and trying to explain/contest the idea that you may get more friction at the end of the stroke. Would be great if it were true, but I can't get the maths to add up. If there is historical/anecdotal evidence for softer stroke with a parachute, then I guess that is vs a solid seal or maybe even leather?

[NickG]

It was Jim Tyler that summised the seal expansion theory , he compressed one in a lathe and measured the growth, and it was the body of the seal that expanded not just the lip.
I arrived at the piston ring in the search for a cure for temp shift, suffered by parachute seals ( and achieved it ) now trying to get the best of both worlds.

[Shed tuner]

what I can defo say is an over-tight parachute seal (roughly 10 kg of force to move from static, rather than the 1kg I normally go with) produces a very soft shot cycle, even when the extra spring is added to bring it up to the same power.

(downside is temp sensitivity I case anyone is thinking of the obvious next question)

[BTDT]

what I can defo say is an over-tight parachute seal (roughly 10 kg of force to move from static, rather than the 1kg I normally go with) produces a very soft shot cycle, even when the extra spring is added to bring it up to the same power.

(downside is temp sensitivity I case anyone is thinking of the obvious next question)

That's static friction, Jon. What that does is take the sting out of the initial piston and hence recoil acceleration, which we perceive as a soft shot cycle.

I measured kinetic (sliding) friction with an old school 25mm HW seal, and with a seal I made from 90A SHORE polyurethane, at simulated cylinder pressures up to 150psi, then extrapolated the values up to actual peak pressures. My seal produced roughly twice the friction of the HW seal (because it's a softer material), but both were in the order of hundreds of Newtons. The friction gives a gentler deceleration to piston compression stroke landing, and a gentler acceleration into piston bounce, so slightly lower peak pressure, but maintaining pellet driving pressure over a longer time period.

I gave up on my seals because they were too temperature sensitive, and converted the AA piston back to take the factory seal. A friend shooting a TX identical to mine internally shot a full four seasons of UKAHFT Nationals, Daystate Midland Hunter and club shoots in all temperatures without a single tweak to the scope.

[evert]

Has anyone ever done a comparison between the old and new style of HW seal?
The previous type had a much thinner and more flexible sealing lip than the current ones, at least the 26 and 30mm versions.
My hypothesis is that the earlier flexible lip seal should give higher friction at high pressures, and thereby slow piston bounce.

Someone on here made up a test rig (BTDT?) that was able to measure action movement and therebay document piston bounce/slap/surge.
Swapping between the two styles of HW seal and doing this measurement would be a very interesting topic for a basement mechanic like me.

[TonyL]

There's a superb article on piston bounce by Jim in this month's Airgun World, tinkery people..

And enlightening information about the magnitude of the forces involved. Wow!

For anyone who might not usually buy the magazine, it's worth it for this article alone if you're interested in spring gun mechanics.

[Weasle]

There's a superb article on piston bounce by Jim in this month's Airgun World, tinkery people..

And enlightening information about the magnitude of the forces involved. Wow!

For anyone who might not usually buy the magazine, it's worth it for this article alone if you're interested in spring gun mechanics.

My first 'go to' read when the Mag falls thru the door - Good old Jim rescues rescues my sanity regularly.

Cheers Steve

[BTDT]

There's a superb article on piston bounce by Jim in this month's Airgun World, tinkery people..

And enlightening information about the magnitude of the forces involved. Wow!

For anyone who might not usually buy the magazine, it's worth it for this article alone if you're interested in spring gun mechanics.

My first 'go to' read when the Mag falls thru the door - Good old Jim rescues rescues my sanity regularly.

Cheers Steve

Thank you kindly, young Sirs.

The article was inspired by this thread, as it happens.

[TonyL]

Thank you kindly, young Sirs.

The article was inspired by this thread, as it happens.

With pleasure, Sir.. And, in your case, Jim, the "Sir" title should really mean just that and you should receive a knighthood for your services to airgunnery. Maybe this New Year, eh?

Your articles are always the highlight for me. So rammed with information, practical, pragmatic and thought provoking. And that's not taking away anything from the many other contributors who do a brilliant job and make the magazine what it is.

When IS that book coming out, by the way?

Re this article, I did wonder as to whether this thread had anything to do with it. Take a bow, Nick.

And it's amazing to think how superbly many springers shoot when there are forces at work inside to this magnitude. I appreciate that it's all over so quickly but it's huge testament to this system which can provide this level of performance and refinement combined with longevity.

Keep the excellent work coming, Jim, and I'll have five signed copies, please.

[Shed tuner]

Jim, did I conclude correctly that using a lighter lathe will reduce piston bounce ?