Springer anti bounce experiment

[Loki_79]

Awesome stuff!

I once tried to model how much friction would be needed in the last inch of travel to make a difference to the peak recoil, and I forget the numbers now, but it was a lot!!

I'm fairly convinced that the parachute seal feeling softer is probably just because it is creating a better seal, rather than adding enough extra friction to slow down the piston at peak pressure.

I wondered about adding a tiny choke/taper at the end of the comp tube, but when it came down to it I think to have any noticeable effect the forces involved were so large that the piston would have just got stuck there and made it imposible to cock the rifle!

I've always fancied the sliding weight idea. I'll try to add that into my model to see how much mass you need and how critical the spring rate is in terms of timing. I think one issue will be that to have a significant effect the added weight will need to be comparible to the mass of the piston, so kind of like doubling the piston mass. You can't get away from the fact that more moving mass has to equal more movement/recoil, but if you can delay the movement until the pellet has gone then who cares!

[Shed tuner]

I've always fancied the sliding weight idea. I'll try to add that into my model to see how much mass you need and how critical the spring rate is in terms of timing. I think one issue will be that to have a significant effect the added weight will need to be comparible to the mass of the piston, so kind of like doubling the piston mass. You can't get away from the fact that more moving mass has to equal more movement/recoil, but if you can delay the movement until the pellet has gone then who cares!

Long time mate

I think the point of the anti bounce mechanism is to slow down the acceleration at the point the piston changes direction, giving more time for the peak air pressure to flow purposefully... but if more of the % of the piston weight is inertia, all that will happen is the piston stalls earlier in the stroke, then is held a fraction longer by the arriving weight before reversing... But in that scenario maybe if would have been better to just have the weight added as static and the piston would stall later in the first place ?

[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.

[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.

[Loki_79]

Long time mate

I think the point of the anti bounce mechanism is to slow down the acceleration at the point the piston changes direction, giving more time for the peak air pressure to flow purposefully... but if more of the % of the piston weight is inertia, all that will happen is the piston stalls earlier in the stroke, then is held a fraction longer by the arriving weight before reversing... But in that scenario maybe if would have been better to just have the weight added as static and the piston would stall later in the first place ?

Yeah - it has been a while! I also can't make this Bash as it is my wife's birthday on Sunday.

I'd like to know from Nick what sort of weights are achieveable for both the piston and the sliding mass, and how they compare to the mass of his 'standard' 21mm piston. It still seems to me like you would need to be approaching double the overall weight, assuming the main piston is a similar weight, but that is just gut feeling, no actual basis at all.

I think it is the same argument for both parachute seals or for sliding weights - we can potentially increase the time that the pressure is applied, but always at the expense of a lower peak pressure.

Definitely watching Nick's results with interest!