Educate me on Piston Weight

[sd5782]

I have been learning a bit about tuning, and have found the posts on SCR very interesting. I have sleeved a couple of transfer ports down with brass tubing in the past, as well as drilled some out, and now see what the science is behind it.

I have searched, and haven't found discussions specifically on piston weight. I know that my Mercury S will have to be lightened, and perhaps my 26mm HW77. I want to experiment on some less costly stuff first; a Baikal 512M, and an older HW30. The Baikal has a very heavy solid guide that I haven't weighed yet, and surprisingly the Macarri kitted HW30 wants to make the Beeman peep sight move around.

I would guess that there is perhaps a general weight range for power levels maybe? Perhaps the weight is more related to spring wire size?
I have read that the later HW77 piston is heavier than the first, so I would guess that a 6, or 8fpe gun would be lighter yet. Are there any general starting points, or directions to go from? It is all very fascinating.

[tinbum]

I agree this would be a good subject to get the more knowledgable talking about.

I lightened my 25mm 77 piston which worked well in conjunction with expanding the TP.
As I understand it a lighter piston reduces inertia and accelerates faster, but if the escape route for the air is too restricted the lack of weight results in piston bounce. That in turn increases felt recoil.

I look forward to some replies from people who aren't just guessing!

[Shed tuner]

I agree this would be a good subject to get the more knowledgable talking about.

I lightened my 25mm 77 piston which worked well in conjunction with expanding the TP.
As I understand it a lighter piston reduces inertia and accelerates faster, but if the escape route for the air is too restricted the lack of weight results in piston bounce. That in turn increases felt recoil.

I look forward to some replies from people who aren't just guessing!

That's about the size of it. Heavier pistons tend to be more efficient, but will take more energy to accelerate, however they need less peak velocity as they decelerate slower. Balancing piston weight, spring, port and stroke is all part of the fun

[Black Beard]

There are too many variables to make general rules except:

Heavier pellet needs heavier piston to work well.
Efective piston weight is piston weight plus a third of the spring weight. A compressed coil at the spring guide end adds no weight but one at the top hat end adds all its weight (it acts like iit is part of the piston/top hat).
Heavier pistons and pellets can get away with smaller transer ports.
Heavier pistons mean more power but also more recoil.

[Olas]

Here's an animation and some writing that fills in some details

http://www.arld1.com/images/swfs/pis...etdynamics.swf

[TonyL]

Pretty much as per posts 2, 3 and 4.

Also from my understanding, whilst a heavier piston may result in higher initial recoil, if a lighter piston was to induce more piston bounce, it's this surge recoil that is the most detrimental to accuracy.

It's far too easy to generalise, as said above, and each individual set up, in terms of bore, stroke, TP volume, spring etc will have a different "balance point". Some will have pistons which are too heavy for any configuration.

And then you remember the reports of how good those ally-piston TXs are!!!!

I suppose that, if you go too light, this can always be easily addressed by fitting internal weights.....

Gotta go out now.......

[Shed tuner]

So a question for Mr Blackbeard if I may....

If there any point at which a reduction in piston weight cannot be compensated for by more spring ?

E.g. today I had a 75mm stroke, 25mm bore TX with a 5.5 x 3.8 port, full length (13") .177. Set to about 11.6, but more initial recoil than I wanted. No slam, just right in that respect.

I reduced the piston weight (by est around 35-45 grams), and power dropped to around 9 FP. I then stuffed in a spring (had to be trimmed 'cos it was initially oversprung) and got back to exactly the same power, (which is believe it pretty close to the limit for this stroke in this gun) - also without slam. Noticeably less recoil and faster, but (obviously) more cocking effort. However as the spring is longer, with more preload (essential to counteract the bounce with the slighter piston), I'm not sure whether the total energy delivery by the spring is that much higher then before (i.e. the work done in compression), or if it's just a perception due to the higher initial cocking effort.

The question then being, beyond hitting practical limits of how much spring you can actually fit in the gun before it gets coilbound, is there some point when power becomes much harder to compensate for with a stronger/longer spring when reducing piston weight ? I'm guessing it's non-linear in some fashion, but you are the man for the models

Cheers - JB

[Black Beard]

Set up model to your initial spec. Only noticable thing is a low cylinder gap.

Reduced piston weight to 235g and preload up 4mm to compensate...

Cylinder gap down a little, cylinder pressure up from 107 to 123 bar. Recoil energy and movement are down but recoil forces are up. Bigest drop is in recoil movement.

Piston weight down to 200g, preload up some more

Less cylinder gap, increased pressure again, even less recoil movement.

Piston weigh down to 150g, preload up some more

Piston almost crashing, pressure up to 195 bar, recoil movement down to about half the original. Recoil will be very fast and piston has a fast impact at the end of its final stroke.

Piston weight 100g

Can't get it above 9.4ftb without piston crashing and need less preload. Pressure 236 bar but goes to stupid levels if preload is pushed up beyond crashing point. Reducing TP to 2.8mm lets me increase the power before it crashes to 10.6ftlb by reducing lost volume (gives 316 bar!). Fair bit of back flow in the transfer port when the pellet is most of the way down the barrel.

Piston weight 75g

Only getting a few ftlb before piston crashes.
But by cheating I can get it up to 11ftlb. This involves no extra lost volume (I was using 1/4 cc on the other runs) on the cylinder, 2.8mm transfer port and a pellet made from a material 1/4 the strength of steel (to stop it expanding under 512 bar).

So the model suggests that there are limits due to the piston crashing or internal pressures getting too high. Go to far and you can't get the power due to lost volume or everything blowing up.

Normal cautions apply- it isn't real and like all computer models, crap in = crap out.

BB

[thisisdonald]

I wonder if someone could eloquently try to explain the feeling or sensations of piston bounce, slam, twisting , etc.. alongside the physics of what is happening inside the gun?

[Shed tuner]

Excellent BB, really interesting, thanks for that.....

I need some time to read that through a few times and have a think, but thanks for getting back...

My initial thought from, which seems to correlate with what I'm seeing, is there is a sweet spot somewhere between 150g and 200g (but I think 200g is much more achievable) for reduced recoil movement...

Thinking/reading....

[Black Beard]

If you are going to try further mods you might want to consider trying sleeving down the transfer port. With your spec I don't think you gain anything with bigger than about 2.8 mm. This will reduce lost volume further and reduce the chance of piston crash/zero cylinder gap. Pellet weight used was 8.5 grains so you might have a bit more leeway with heavier pellets.

[Shed tuner]

Cheers BB, will give 3mm port a whirl....

I generally use 8.4 gr JSBs, so that's about right...

Thanks again,

JB

[Shed tuner]

edit... so this thing is doing 795 with 8.4s and 820 with 7.9s (so basically even power accross the two weights). Clearly I need to collapse a coil to bring the power down a bit, but it's still settling in/up, so waiting till it levels out a bit, currently only had about 75 shots through it...

**update - collapsed a coil, it's now doing 785 with 8.4s, 813 with 7.9s (both very consistent) **

But, if the power is flat with 7.9s an 8.4s, isn't reducing the port going to introduce too much piston bounce ? And if it brings the power up, I'll have to take even more off the spring, reducing preload and accentuating the bounce ? I could counteract by adding piston weight, but that's exactly what I don't want to do in terms of recoil "feel"...

Total piston weight is 213 gram inc ally top hat and seal, port length is a bit less, around 5.3 (awkward to measure). Lost vol is pretty low as seal just has a small groove...

Recoil is nice and quick.

Rgds - JB

[Black Beard]

I donn't think you will feel ay difference with a sleeved down transfer port. It doesn't start reducing the flow rate until you go below about 2.8mm. The flow is not choked. Your air rifle has a tiny capacity so not much air is moving.

What you do gain is less lost volune so an increased cylinder gap = increased safety before the piston crashes. You also get increased drag in the second part of the cycle so you get a slightly softer landing of thepiston when it finally stops. Lost volume doesn't sound like an issue for you.

If you sleeve the transfer port using locktite it would be easy to change it back. I'd go to 3mm. If you do it and you get a change in performance then let me know. More data to validate/disprove the model.

[Shed tuner]

Excellent BB, thanks again... Will give it a whirl and revert back