Springer anti bounce experiment

[Adam77K]

... how about combining the frictional braking seal with the inertia weight?

My thinking: if the weight has a tapered front, and instead of hitting the end of the piston cavity it hits a tapered collet made of a slightly elastic material* which pushes out beneath an o-ring. The o-ring is slightly undersized so as to add no extra friction during the power stroke, and only comes into play when the inertial weight has run forward.
The taper would have to be steep enough so the weight doesn't get stuck inside the collet.

* The collet could alternatively be metal but in sections like a drill chuck.

Result: the inertial damping AND sudden frictional braking combined, without any extra friction.

I don't have the skills to test this; that's in the hardware domain and I'm more a software guy.

[NickG]

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

I don't think so John, I have tried facing pistons with polyurethane, and even faced a comp tube end plug, and it just doesn't achieve the same feel .

[NickG]

... how about combining the frictional braking seal with the inertia weight?

My thinking: if the weight has a tapered front, and instead of hitting the end of the piston cavity it hits a tapered collet made of a slightly elastic material* which pushes out beneath an o-ring. The o-ring is slightly undersized so as to add no extra friction during the power stroke, and only comes into play when the inertial weight has run forward.
The taper would have to be steep enough so the weight doesn't get stuck inside the collet.

* The collet could alternatively be metal but in sections like a drill chuck.

Result: the inertial damping AND sudden frictional braking combined, without any extra friction.

I don't have the skills to test this; that's in the hardware domain and I'm more a software guy.

Good luck making that , and fitting it all in , leaving enough room for a spring.

[Shed tuner]

Good luck making that , and fitting it all in , leaving enough room for a spring.

might not be so crazy, sliding tapered inertia weight, pushes on say 4 evenly spaced brass pins passing through the piston wall, that align to an o-ring sitting in a groove on top of them.. weight slides fwd, pins push o-ring out... Shouldn't take up any more room than any other inertia design... and has the advantage that the inertia weight can be "relatively" light...

Having the inertia weight bring in the friction, means it's tied to actual piston deceleration, rather than being tied to pressure ahead of the piston.

[BTDT]

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.

7.5N, Jamie.

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

It's not the only factor, but it plays a part in the shot cycle feel, Jon.

[Loki_79]

might not be so crazy, sliding tapered inertia weight, pushes on say 4 evenly spaced brass pins passing through the piston wall, that align to an o-ring sitting in a groove on top of them.. weight slides fwd, pins push o-ring out... Shouldn't take up any more room than any other inertia design... and has the advantage that the inertia weight can be "relatively" light...

Having the inertia weight bring in the friction, means it's tied to actual piston deceleration, rather than being tied to pressure ahead of the piston.

It makes a lot of sense when I think about it in a handwavy sort of fashion, but it still feels like the forces involved are so large that to have any significant effect the weight will need be so heavy that it will cause more problems than it solves.. As ever, more than happy to be proven wrong - I'm still thinking in terms of 5 or 6 N/mm springs in normal setups, so everything could be different with much lighter springs.

Talking about the seal compression also got me thinking about this kind of thing. I always think in terms of rigid shapes, and forget just how much stuff will deform with large forces at play. See also e.g. slow motion videos of balistic gel impacts, etc.

[BTDT]

It makes a lot of sense when I think about it in a handwavy sort of fashion, but it still feels like the forces involved are so large that to have any significant effect the weight will need be so heavy that it will cause more problems than it solves.. .

I agree with Jamie on this.

Good luck with the o ring idea. Webley said the neoprene backing washer behind the PTFE piston seal of their Vulcan was squashed at piston bounce, gripped the cylinder wall and tamed piston bounce. I tried it with a neoprene washer large enough to actually contact the cylinder wall, and it was ripped out during the first shot.

The forces involved are indeed large.

[Shed tuner]

OK, forget the o-ring... a delrin ring expanded by the pins.. should be tough enough, but still give a bit of braking... but how much friction would one need to make a useful difference.. if we are still talking 500N, then I can't see how the weight could ever exert enough force...

Maybe my transfer port one way valve idea has more mileage after all...

[BTDT]

Maybe the cumulative effect of a brake, a weight and transfer port valve could work, Jon.

Unless someone tries it, we'll never know.

[TonyL]

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.

I remember this coming up in a similar, historical thread, Jim. And we deduced then that the smaller cross section and smaller amount of material employed helped in this regard? Think we also mentioned, tied in with this, how 'O' rings were affected less than parachute seals. And Nick's been an advocate of 'O' and 'X' rings for years. And now his very clever sounding synthetic rings.

[NickG]

might not be so crazy, sliding tapered inertia weight, pushes on say 4 evenly spaced brass pins passing through the piston wall, that align to an o-ring sitting in a groove on top of them.. weight slides fwd, pins push o-ring out... Shouldn't take up any more room than any other inertia design... and has the advantage that the inertia weight can be "relatively" light...

Having the inertia weight bring in the friction, means it's tied to actual piston deceleration, rather than being tied to pressure ahead of the piston.

Didn't say it was crazy just a lot of work to build one ,and probably need to build several to get it working , I like the idea of the braking effect being tied to the inertia, with the one I have built with two o rings as a buffer they expand too easily and it appears to brake the piston on the compression stroke robbing power, so the compromise is one ring with less braking effect on the bounce .Nothing worthwhile is ever easy.

[NickG]

OK, forget the o-ring... a delrin ring expanded by the pins.. should be tough enough, but still give a bit of braking... but how much friction would one need to make a useful difference.. if we are still talking 500N, then I can't see how the weight could ever exert enough force...

Maybe my transfer port one way valve idea has more mileage after all...

O ring with delrin band underneath, better friction in the o ring material so less force needed, it would survive, being in a proper groove.

[Shed tuner]

O ring with delrin band underneath, better friction in the o ring material so less force needed, it would survive, being in a proper groove.

yeah, perfect.. if you could please just knock one up and test it, the R&D department is awaiting the results !?

I do enjoy this board

[NickG]

😂 afraid my enthusiasm qouta is nearly used up on this one , straight after the hw95 .
Once the boingerati have tested it's going in the ft gun to see if it's really any better !

[Loki_79]

Just as a thought, I added all of the seal compression force to the friction, so effectively a maximum case. Then set it up for a small piston impact to give maximum force (from the impact rather than just chamber pressure). The friction goes off the scale to about 2000N, BUT only for 0.1ms. Power went up from 10.5 to 10.6, piston bounce reduces from 18.7mm to 18.2mm, so probably not ground breaking..

To put into context, simply doubling the pellet friction is worth about a 25% reduction in piston bounce (18.2mm vs 13.3mm), a much softer landing and power goes from 10.5 up to 11.9 ftlbs. (edit - this probably suggests that the port is too big, but you get the idea!)