Jim's piston seal article in AGW...

[Shed tuner]

Very interesting, particularly the o-ring bit.

So here's a Q - I get that the time that the piston carries on moving forward, between the pellet start point, and the piston subsequently stalling (immediately prior to bounce) would have quite a significant affect on power.

By why were the lightweight o-rings pistons bouncing quicker..?

Was it just the o-ring (less friction at high pressure), or was it also because of the lightweight piston with far less inertia ?

I'd like an o-ring vs parachute seal test using the same weight piston please Jim

The other thing that would be interesting, is to see the difference in landing velocity (reduced by friction, presumably moreso with a parachute seal) of the piston at the end of it's second forward stroke, as I'm pretty sure that's why those o-ringed guns feel a little harsh.


The wet lube friction reduction was also interesting - these days krytox or hot suace seem to work well for me on seals


JB
/about to convert an 28mm bore o-ringed late model airsporter, which I find unnecesarily harsh (but still good value at £25 ), to a 28mm diana seal to see how it improves

[bmp01]

Very interesting, particularly the o-ring bit.

So here's a Q - I get that the time that the piston carries on moving forward, between the pellet start point, and the piston subsequently stalling (immediately prior to bounce) would have quite a significant affect on power.

By why were the lightweight o-rings pistons bouncing quicker..?

Was it just the o-ring (less friction at high pressure), or was it also because of the lightweight piston with far less inertia ?

It's both, you know you can never simplify it down to one variable
Was there a change in the lost volume too ( lip seals are shockingly bad for this but I don't know what seal was used).

The trick would be to have an O ring piston head, low lost volume, plus a feature that slows the piston movement at peak pressure. .... I wonder if a poly piston head with the appropriate sized fit would do that Where's NickG when you need him ?

BMP01

[NickG]

I just fitted my target gun with my 22mm set up which is hands down the best reduced bore set up I have in my opinion, it has an aussie seal, I am going to build another for my MK1, identical except I am going to use an o ring, I can then give a direct comparison on shot cycle

[Shed tuner]

yeah, I like 22s with that seal.

[NickG]

jb,I sorted the 21mm with a full ally skirted piston, in the full length it makes 11ft lb, so I have it in my orange ft stock and shot it side by side with the 22, and on 20x mag at 55 yds you can see the pellets fly and land , but it moves more than the 22,
As an exercise I made a piston with a delrin skirt to go as light as possible 97grm, and in the hc it doesn't bounce so going lighter till it does , mind the bearing fell off the skirt pretty quickly, so using it to test then will make from ally, see how light we can go , as the lock time on the 21 is quicker than an R10

[Shed tuner]

tbh going smaller than 22 is just too hard with a piston body to find a spring that actually fits. 23 is also very good for this reason - loads of springs fit.
So I have one 22, one 23, and a 25mm BAM-TX test bed

[coburn]

I've often thought that a progressively wound spring like those on motorcycle forks might help control piston bounce, obviously with the tighter wound end towards the piston?

Would this help?

[hmangphilly]

I've often thought that a progressively wound spring like those on motorcycle forks might help control piston bounce, obviously with the tighter wound end towards the piston?

Would this help?

to add weight to the piston ...right ..?

[NickG]

tbh going smaller than 22 is just too hard with a piston body to find a spring that actually fits. 23 is also very good for this reason - loads of springs fit.
So I have one 22, one 23, and a 25mm BAM-TX test bed

The spring from Protec will fit though its a little strong at 3mm guage, I am using the Titans No 11 I think, I have a 20mm bore in the piston so plenty of room.

[NickG]

The piston is going to bounce any how , I think as I remember jim decided that the seal grips the bore more so controls it , my approach is to minimise the friction as much as possible , and any "control " is going to add friction, I think the trick is to build less bounce into the design ie use the pellet weight or transfer port diameter to adjust , I have an Idea for losing even more weight from the piston, though it may start to lose power , swings and roundabouts , all good fun

[BTDT]

Very interesting, particularly the o-ring bit.

So here's a Q - I get that the time that the piston carries on moving forward, between the pellet start point, and the piston subsequently stalling (immediately prior to bounce) would have quite a significant affect on power.

By why were the lightweight o-rings pistons bouncing quicker..?

Was it just the o-ring (less friction at high pressure), or was it also because of the lightweight piston with far less inertia ?

I'd like an o-ring vs parachute seal test using the same weight piston please Jim

The other thing that would be interesting, is to see the difference in landing velocity (reduced by friction, presumably moreso with a parachute seal) of the piston at the end of it's second forward stroke, as I'm pretty sure that's why those o-ringed guns feel a little harsh.


The wet lube friction reduction was also interesting - these days krytox or hot suace seem to work well for me on seals


JB
/about to convert an 28mm bore o-ringed late model airsporter, which I find unnecesarily harsh (but still good value at £25 ), to a 28mm diana seal to see how it improves

Hi Jon. Glad you found the article interesting.

As said, it's a combination of both the very low friction of the o ring (which lowers frictional resistance during the early part of bounce when the compressed air is still accelerating the pellet) and the low mass of the piston, which has to travel faster during the compression stroke to maintain the momentum needed to compress the air, to a higher pressure necessitated by the shorter time the piston spends in the proximity of the cylinder end.

The piston final landing velocity is probably just as influenced, if not more so, by preload as the type of seal. The pellet's long gone, the air pressure is low and so the parachute seal's friction is low.

[TonyL]

I haven't managed to read the article yet, chaps. Managed only a quick look this morning before work and looking the usual JT piece of excellence.

JB....told you you'd love Jim's articles!

[coburn]

to add weight to the piston ...right ..?

Not sure, would it? at the point the piston began to bounce the tighter would coils would provide more resistance to any resultant bounce?

As stated earlier there are so many variables this is probably a moot point.

[Shed tuner]

I've often thought that a progressively wound spring like those on motorcycle forks might help control piston bounce, obviously with the tighter wound end towards the piston?

Would this help?

progressive springs work in car suspension, as they spend most of there life partially compressed. so working in the soft part of the wind, until a heavy load is put on it, causing the soft wind to go near coil bound, and bringing the heavier wind into play (with more force).

On a springer, all that would happen is the initial cocking effort would be low, then higher (once the easy wind was compressed solid, the tough wind would then need compressing, taking more force).

On the firing cycle, the tough wind would let loose faster than the weaker wind, so faster initial acceleration, then less force towards the end as ther weaker wind unleashed.

I'm not sure how this could help bounce - in fact I think the opposite - it would help reduce slam.

It doesn't matter which end is which wind - the two ends of the spring will move away from each other at the same speed regardless.

[Shed tuner]

Hi Jon. Glad you found the article interesting.

As said, it's a combination of both the very low friction of the o ring (which lowers frictional resistance during the early part of bounce when the compressed air is still accelerating the pellet) and the low mass of the piston, which has to travel faster during the compression stroke to maintain the momentum needed to compress the air, to a higher pressure necessitated by the shorter time the piston spends in the proximity of the cylinder end.

The piston final landing velocity is probably just as influenced, if not more so, by preload as the type of seal. The pellet's long gone, the air pressure is low and so the parachute seal's friction is low.

Hi Jim, let's do an identical mass experiment then... to try and strip out the mass from the equation.

Agree on preload being the major factor for second stroke... but more from the perspective of reducing the rearwards displacement IMHO. I think you are probably right about the seals in this phase though, as the pressures are so low; I guess this is why old leather (high friction) seals can feel so nice and soft (at the expense of mv). So why the o-ring harshness ? Maybe it's also due to the nose of most o-ringed pistons being essentially solid, with no flex on landing as you'd get with a conventional seal.

Clearly none of this has any affect on accuracy (the pellet long gone, as you say), but a significant affect on "niceness".