Jim's piston seal article in AGW...

[Rickenbacker]

Alright fellas?




edit: Actually, all you want is a check valve in the TP. https://en.wikipedia.org/wiki/Check_valve

[NickG]

The 'one-directional' friction is the hard thing to achieve (maybe shark skin would help as well), but after the pellet has gone, it doesn't really matter what happens and it should be beneficial if the piston never actually lands again.

As discussed earlier there is no way that friction can impart force back to the rifle, since it is a closed system - essentially just two masses on a spring, where the two masses also happen to rub against each other. Think of the two extremes:
1) if there is no friction, the rifle and piston move about the combined centre of mass, just like all masses on springs.
2) if there is 100% friction, nothing moves at all.

Friction just reduces the force of the spring felt by both parts, but they still move about the combined centre of mass. 50% friction is equivalent to having a 50% weaker spring and no friction. If somehow the intermediate friction could move the rifle, then the 100% friction (jammed) rifle would also move away with 100% of the spring force!

I'm not sure what you mean by the stepped piston though, where the seal (or seals) would be? If there is no lost volume, I don't see the difference from a cylindrical piston, since all the points of contact will meet at the same time. Or are you saying that you are effectively increasing the surface area for the final contact at the end of the stroke? (sorry for the string of questions!)

Now this may be a little simplistic, but the Shark skin analogy is interesting, I went out to the shed after first reading this thread and attempted to machine some reverse lips into a polyurethane rod, this I achieved, four or five of them about 0.5 mm apart, they raise the diameter of the material and do offer up a lot more friction in one direction than the other , my concern was that they would also wipe all the lube from the cylinder when the cocking arm is returned .

[bmp01]

Alright fellas?




edit: Actually, all you want is a check valve in the TP. https://en.wikipedia.org/wiki/Check_valve

Define the spec.
Pressure rating - a couple of hundred bar, 1000 bar shold do it.
Operating speed - full flow to zero flow in under 1 micro second.

Your right we do want one.....

BMP01

[Rickenbacker]

Define the spec.
Pressure rating - a couple of hundred bar, 1000 bar shold do it.
Operating speed - full flow to zero flow in under 1 micro second.

Your right we do want one.....

BMP01

A simple ball check valve in a drilled-out TP should do the trick.
Perhaps try it out on something crap, like any Webley.
You might want to start with an ill fitting ball bearing, because it seems you don't want it to work too well...

I do have these amazing ideas from time to time. Jon Budd secretly wants one of my HW77 / TX200 custom cocking handles.

[crowbar]

What about split multi disc P.T.F.E. piston washer mounted on a reverse taper spigot, when the piston bounces the taper
will expand the rings and increase friction rearwards.

A split collet with a O ring seal could also be used.

[Loki_79]

Also, has anyone made anything out of leather yet to keep TonyL happy! Hmmm... can you get leather from sharks?...

[bmp01]

A simple ball check valve in a drilled-out TP should do the trick.
Perhaps try it out on something crap, like any Webley.
You might want to start with an ill fitting ball bearing, because it seems you don't want it to work too well...

No, you're wrong, even if it weren't perfect I'd still want a check valve. It's about the most obvious solution to the problem. .... but....but..... oh, nevermind.....

BMP01

Sidenote: no 'banging head against brick wall' icon then....

[Rickenbacker]

No, you're wrong, even if it weren't perfect I'd still want a check valve. It's about the most obvious solution to the problem. .... but....but..... oh, nevermind.....

BMP01

Sidenote: no 'banging head against brick wall' icon then....

I mean't that you might not want the check valve to be 100% efficient.

You wouldn't want the vacuum sucking the seal off the end of the piston.

Or turning the world inside-out.

I should also warn you that taking me seriously is often a mistake.

[Shed tuner]

A simple ball check valve in a drilled-out TP should do the trick.
Perhaps try it out on something crap, like any Webley.
You might want to start with an ill fitting ball bearing, because it seems you don't want it to work too well...

I do have these amazing ideas from time to time. Jon Budd secretly wants one of my HW77 / TX200 custom cocking handles.

it's a variation of this that I'm working on how to make simply / easily / reliably as referenced earlier in this thread. Had quite a few convs about how to do it at a Bash alst year.

I'm pretty much there now. Will use a TX comp tube as it's easy to replace if it all goes horribly wrong.

But it's a bit more complex than it might first appear...

[Shed tuner]

Now this may be a little simplistic, but the Shark skin analogy is interesting, I went out to the shed after first reading this thread and attempted to machine some reverse lips into a polyurethane rod, this I achieved, four or five of them about 0.5 mm apart, they raise the diameter of the material and do offer up a lot more friction in one direction than the other , my concern was that they would also wipe all the lube from the cylinder when the cocking arm is returned .

interesting... how does it shoot ?

[bmp01]

it's a variation of this that I'm working on how to make simply / easily / reliably as referenced earlier in this thread. Had quite a few convs about how to do it at a Bash alst year.

I'm pretty much there now. Will use a TX comp tube as it's easy to replace if it all goes horribly wrong.

But it's a bit more complex than it might first appear...

That will be a feat indeed. Are you planning on using the reversing airflow to operate the valve ? Tried a few times, schemes only, can't get anywhere near where I think it stands a chance.

BMP01

[bmp01]

I should also warn you that taking me seriously is often a mistake.

Ok
I dont have a no problem it that

BMP01

[Rickenbacker]

it's a variation of this that I'm working on how to make simply / easily / reliably as referenced earlier in this thread. Had quite a few convs about how to do it at a Bash last year.

I'm pretty much there now. Will use a TX comp tube as it's easy to replace if it all goes horribly wrong.

But it's a bit more complex than it might first appear...

I'm sure it is more complex in practice JB.

I guess, keeping the volume low-ish through a valve (compared to a regular TP) would be a challenge?

Be interesting to see what you come up with though.

[Rickenbacker]

Ok
I dont have a no problem with that

BMP01

And you speak Italian!!?

If I were a lady, I would *swoon*

[NickG]

What about split multi disc P.T.F.E. piston washer mounted on a reverse taper spigot, when the piston bounces the taper
will expand the rings and increase friction rearwards.

A split collet with a O ring seal could also be used.

As jb, has noted , its not as simple as that its easy to assume that the part that you want to expand when the piston bounces will stay put and obediently expand , It won.t it will just travel rearwards with the rest of the piston assembly because the cylinder that its in is lubricated .
Its one of those simple problems on the surface , that become incredibly intricate once you start to look at it properly, which sums up springers nicely