Jim's piston seal article in AGW...

[davep]

I know there's no 'one size fits all' answer but where abouts in the stroke would you expect a 25 mm piston to bounce ? ....1-2 mm from the end ? more ?

Cardew in Trigger to Target suggests 0.1 inch so about 2.5mm. I think he was measuring pressure in the cylinder so probably fairly accurate

[Loki_79]

I know there's no 'one size fits all' answer but where abouts in the stroke would you expect a 25 mm piston to bounce ? ....1-2 mm from the end ? more ?

From my computer model, I would say anywhere between about 0.1mm and 0.5mm.

edit: I should say that most of the variation depends on the resistance of the pellet, a completely blocked barrel with a 3mm transfer port and minimal other lost volume I get something like 1mm.

[hmangphilly]

so depending on the fit of the pellet ,
we've got , let's say an 85mm long x 25mm dia column of air plus tp /etc, compressed into 0.1-0.5mm x 25mm dia plus tp/.?

[TonyL]

From my computer model, I would say anywhere between about 0.1mm and 0.5mm.

edit: I should say that most of the variation depends on the resistance of the pellet, a completely blocked barrel with a 3mm transfer port and minimal other lost volume I get something like 1mm.

Blimey! I thought that it was sooner than that, more like 5mm or so, but I can't remember if I've read something in Jim's articles or elsewhere. I, of course, could me miles off (I often am!).

Now, some of us have mused for years about this and wanting to uncover some more of the springer's innermost secrets, but we really need a see-through cylinder and a camera with 1000 frames per second capability. This would reveal SO much......

[hmangphilly]

Blimey! I thought that it was sooner than that, more like 5mm or so, but I can't remember if I've read something in Jim's articles or elsewhere. I, of course, could me miles off (I often am!).

Now, some of us have mused for years about this and wanting to uncover some more of the springer's innermost secrets, but we really need a see-through cylinder and a camera with 1000 frames per second capability. This would reveal SO much......

thats a brilliant idea

but you could get a good idea of whats going on by filming the piston through the cocking slot .
suitable graduations marked on the piston would work .

trouble is them pukka high speed cameras are ££££££££££££££££££££££

[Rickenbacker]

Blimey! I thought that it was sooner than that, more like 5mm or so, but I can't remember if I've read something in Jim's articles or elsewhere. I, of course, could me miles off (I often am!).

Maybe you're thinking of how much it bounces rather than when, Tone?

The question of 'how much', 5 - 10mm is it?

[TonyL]

thats a brilliant idea

but you could get a good idea of whats going on by filming the piston through the cocking slot .
suitable graduations marked on the piston would work .

trouble is them pukka high speed cameras are ££££££££££££££££££££££

Well, bugger me with the blunt end of a bus!!

A few of us had previously debated about likely materials for the see-through cylinder and its expected poor longevity.

But there's the answer, crystal clear, in your post....nice one!

As you say, some markings on the piston (and also on the cylinder) at the cocking slot. I know those high speed cameras cost fortunes, but if only this could happen.....I wonder if a laboratory at a university would be up for some experiments? We could have a few rifles readied, with different bore, stroke, pistons, TP set up etc. if the equipment worked, just one or two filmings per gun would do.

[TonyL]

Maybe you're thinking of how much it bounces rather than when, Tone?

The question of 'how much', 5 - 10mm is it?

Mmm....to be fair, lovely Phil, I think I was maybe referring to a figure I have in my head of 95% of the stroke from some of Jim's earlier articles.....But the 95% possibly refers to (approximately) pellet release point and not onset of piston bounce?

I did say that I'm very often wrong!

[Shed tuner]

from various models, experiments and other and sources, headspace at the point of bounce can be anwhere from 0.5mm for the most extreme, short stroke beasts to around 10mm. for a well set up gun (for 12 FP), I'd go with around 1.5-2mm.

The amount of bounce can be anything from 10 to 20+mm

TopDog on here (who we had the pleasure of meeting at the bash) has measured piston displacement directly using a potentiometer arrangement, to get real time piston position graphs. Very interesting

[RobF]

I've got the camera... quality sucks at 1200 fps but it allows you to see things. We could see the hammer bounce on the Walther when we were working on that.

[hmangphilly]

just had a chat about the peak pressures .

seems people are thinking any where between a few hundred and a few thousand psi .

can any one narrow that down abit

[BTDT]

TopDog on here (who we had the pleasure of meeting at the bash) has measured piston displacement directly using a potentiometer arrangement, to get real time piston position graphs. Very interesting

My friend Topdog's is the only credible attempt I'm aware of to directly measure piston position. Since then, I've made the first attempts to measure piston seal crush, which must be subtracted from the piston body position for any calculations regarding the volume of air in the cylinder. I have only been able to measure crush up to 150psi at present, so what follows is an educated guess (albeit supported by experiments last year with transfer ports that extended back into the cylinder, and consequent reductions in muzzle energy).

I think the seal face is probably in the region of 1mm - 1.5mm from the end wall at piston bounce for the rifle set ups I favour. Go short stroke or light piston, and it will have to be less for any given muzzle energy.

[BTDT]

I've got the camera... quality sucks at 1200 fps but it allows you to see things. We could see the hammer bounce on the Walther when we were working on that.

Just checked on a recent test, Rob, and in the last millisecond of the compression stroke the piston travels around 8.5mm. 1,000 fps (or near offer) is woefully inadequate, I'm afraid.

[Shed tuner]

just had a chat about the peak pressures .

seems people are thinking any where between a few hundred and a few thousand psi .

can any one narrow that down abit

Depends a lot on pellet release pressure, and also (less so) on cylinder dia (smaller allows higher pressures) and cal.

Jim's article in AGW (was it last month ?) has some pellet release pressures. I think a good working range is 400-800 psi (.22 lower than .177)

[BTDT]

Depends a lot on pellet release pressure, and also (less so) on cylinder dia (smaller allows higher pressures) and cal.

Jim's article in AGW (was it last month ?) has some pellet release pressures. I think a good working range is 400-800 psi (.22 lower than .177)

For .177"

Falcon Accuracy Plus ~120 psi.
Air Arms Express ~ 150 psi.
Air Arms Field ~ 200 psi.
The rest ~380 psi to >600 psi.

Piston bounce displacement is way less with low pressure start pellets, and all the available evidence points toward peak cylinder pressure being less, as well.