Reply With QuoteI was also wondering if the cup face provides more cushioning for the final piston landing, due to physical compression of the seal at the perimeter...
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Trying to understand how springers work, cup faced piston seals are one of the mysteries I want to understand better.
What do they really do differently than "normal" piston seals?
In my simple mind, they act as follows:
-they may reduce static compression, as they introduce lost volume?
-they increase flow, as they dont shut off the transfer port like flat faced seals?
-the compression reduction may reduce bounce, and thereby actually cause a power increase?
Does this sound logical?
What am I missing?
Too many airguns!
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I was also wondering if the cup face provides more cushioning for the final piston landing, due to physical compression of the seal at the perimeter...
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.
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It's really simple, a 'dome' pushes air aside, think of a pellet in flight, whereas a 'cup' scoops up from the edge & channels it in to the centre,
the pressure therefore builds from the centre & as it builds the thin edges will be pressed out to improve the seal within the tube, making the 'cup' far more efficient.
You can see/feel this for yourself by simply using a spoon to stir a liquid held at the very tip of the handle.
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In a nutshell, couldn't have put it better.Originally Posted by angrybear
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Thanks, thats sounds logical! is this statement based on a hunch/speculation like my own thoughts, or is it the result of something else?
Too many airguns!
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Does the edge distort over time?
Too many airguns!
P I Magnum impersonater available for hire
Long tern use of my own would suggest no, it doesn't.
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It's simply a known fact, sails have used the same principle (in reverse) for thousands of years.
the cup shape will also cushion the impact due to there being air trapped inside it to act as a buffer.
The skirt of the pellet then uses exactly the same principle in reverse just like a sail does.
Last edited by angrybear; 04-10-2023 at 06:53 PM.
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so I'm going to be difficult...
in all these examples.. spoons moving through liquids, sails moving through air, the liquid/gaseous medium can flow around the cup, so the advantage of the cup is obviously to trap the medium...
in a spring piston gun however, the air can't flow anywhere to escape.. so why is the cup shape beneficial ? Note this is seperate to "why is a thin flexible outer lip that gets pushed against the bore as air pressure increases beneficial ?" question, which is entirely different and much more easily explained without using a cup shaped seal (e.g. a regular HW style lip seal).
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.
may also be employed in conjunction with a drawn reciprocation dingle arm, to reduce sinusoidal repleneration
Dunno but the FWB 300 and FWB 65 made reasonable power with their weak springs and car-type piston-ring systems. The answer is probably far more mundane, the springer develped from the bicycle pump and it is an evolutionary hangover like the appendix. I'm sure BTDT ran a PTFE block that had no lip to great effect, at least while the temperature was with him ... Maybe thats it, it can adapt to thermal expansion and contraction in the tube where a block would have issues with tolerances and jamming?
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My exact thoughts, how is the sail/spoon shape necessary when the seal is working inside the large "cup" of the cylinder?
I'm thinking the cup shape makes more of a difference at the end of the stroke, affecting air flow and bounce.
The modern LGU/LGV seals also makes me think:
https://www.airgunspares.com/604-400...istonseal.html
https://www.pellet-guns.com/products...u-piston-seals
The central "nipple" on the LGV seal might give us a clue, it may help the flow into the transfer port?
Too many airguns!
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Until the pellet releases, the seal is just compressing air and not flowing. A cupped seal will need a longer stroke to compress the air to the pressure required due to the lost volume in the seal , the most efficient set up in my experimentation have been flat fronted piston with very little lost volume . The cup ideas may give benefits near combustion as the volume of air at the fully compressed point will be larger and maybe cooler .
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There have been a few designs trying to get the air to the TP I wonder if they worked.
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I agree with this, especially with leather washers after finding lots of damaged piston screws where the washer breaks down over time to a point that the washer has started to lose its sealing effect and the piston slams harder against the end of the cylinder. With a bit more washer in front of the piston and a inner washer that recesses the piston screw there is less damage.
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Good & valid point, but the air can flow somewhere, it can squeeze back past the piston.
The thin leading edge 'scoops' the air away from the cylinder wall & directs it in to the middle, the thin edge is also going to flex out where it will act like a piston ring to seal with very little friction,
then on cocking the seal will slip back with little to no friction,
the lower pic from Barryg you can see also has this thin lip to scoop the air in, the cup has to face forward because as the pressure builds it forces the lip out to form a better seal,
if the seal was at the base of a flat piston it would face the wrong way & increasing pressure would push it away from the cylinder wall, while drawing the piston back it would push out,
or if the whole piston had flat sides it would have to be a very tight fit for the pressure not to leak past which would cause friction, drag & loss of efficiency.
The same principle of a cupped face towards pressure is used extensively in hydraulic & pneumatic seals & systems
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