Springer anti bounce experiment

[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.

[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]

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.

[evert]

That's why I like the floating inertia option - much less to go wrong

I'm thinking that a Theoben with its floating inertia weight could be easily modified as a test bed.

[Shed tuner]

I'm thinking that a Theoben with its floating inertia weight could be easily modified as a test bed.

not really... it floated in the space occupied by a spring... so on fitting a spring you have the same problem and would be back to a solution similar to Nick's

[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.

[Loki_79]

Hi Jim, it has also been far too long!

both were in the order of hundreds of Newtons

Do you think 500N of peak static friction is achievable for a stock HW seal, or more like 200/300N?

I have not touched my model for years, but maybe time to pick it up again. I have always assumed that any extra friction comes from expansion of the parachute lip, and so limited to a small area. I had not considered expansion of the whole seal during impact, but even small differences in that case are of course multiplied over a much larger area.

As always, the tricky thing with modeling in the absence of real world data is that you can easily set non-physical values. I have always tried to fit the shape of the piston bounce curve with my limited measured data, whilst trying to stay within back-of-an-envelope type of parameter constraints, but as usual with so many parameters there is more than one way to achieve the same results.

I'd happily lend my electronics accelerometer gadgetry to Nick for a few months if he was up for it!

[BTDT]

Hi Jim, it has also been far too long!



Do you think 500N of peak static friction is achievable for a stock HW seal, or more like 200/300N?

I have not touched my model for years, but maybe time to pick it up again. I have always assumed that any extra friction comes from expansion of the parachute lip, and so limited to a small area. I had not considered expansion of the whole seal during impact, but even small differences in that case are of course multiplied over a much larger area.

As always, the tricky thing with modeling in the absence of real world data is that you can easily set non-physical values. I have always tried to fit the shape of the piston bounce curve with my limited measured data, whilst trying to stay within back-of-an-envelope type of parameter constraints, but as usual with so many parameters there is more than one way to achieve the same results.

I'd happily lend my electronics accelerometer gadgetry to Nick for a few months if he was up for it!

500N would require a peak pressure circa 1,500psi with the old style HW seal, Jamie, if my measurements were anywhere near accurate. I'd expect something certainly over 200N, possibly over 300N, but it depends on peak pressure, of course.

[Loki_79]

500N would require a peak pressure circa 1,500psi with the old style HW seal, Jamie, if my measurements were anywhere near accurate. I'd expect something certainly over 200N, possibly over 300N, but it depends on peak pressure, of course.

Thanks - that sounds like something to aim for. I was definitly below that with typical peak piston seal friction more like 50 or 100N depending on parameters.

What was the static friction at rest with no compression? I may have asked you all of this before, but seem to only have data for pellet friction.

[NickG]

Little up date , buttoned the front of the piston to keep it all aligned, the spread over 10 shots is 8 fps max to min, the piston with a spacer and no buffer gives 4 as does the identical gun I am using for comparison , so a little worse but acceptable, I have both guns running at the same power, the buffer gun has a noticeable softer cycle, stopped playing with them now, and will bring them to the bash let a few other people try them .In case I am making it up ��
https://photos.app.goo.gl/99MEJMEifLofh1Vp7

[Shed tuner]

any chance that could just be allowing the piston to acheive a softer final landing against the end of the comp tube, rather than the anti-bounce affect ?
We really need to hook you up to that accelerometer of Jamie's

[Shed tuner]

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.

Very interesting Jim...

I'd assumed it was due to (in order of relevance) 1) softer piston landing 2) slightly reduced bounce

I never figured that the lower initial acceleration would be significantly responsible for that "soft" feeling