Balanced Hammer Valves

[jackssmirkingrevenge]

A good article on the subject showing the different configurations:

[wyz2285]

Tried to make one and it did work, a lot of little holes tho. Kinda impressive open a 1000 psi valve with a light press of your thumb.

[keks2033]

I tried to make a balanced valve and I did it. The flow area in the valve is now 8.5 mm, piston diameter 7 mm. Such a valve opens with a light drummer and soft spring, tested at a pressure of 70 atm. The lead ball after the shot went into the board by 30 mm, the air in the tee is enough for 2 shots.

[mrfoo]

They're missing BtB's cbhv, which is like the balanced poppet valve, but rather than a vented piston, it has a closed chamber - when the valve opens, the chamber is pressurised, as the pressure drops in the barrel it closes.

[jackssmirkingrevenge]

I tried to make a balanced valve and I did it.

I love the determination nice!

They're missing BtB's cbhv, which is like the balanced poppet valve, but rather than a vented piston, it has a closed chamber - when the valve opens, the chamber is pressurised, as the pressure drops in the barrel it closes.

Isn't the CBHV as Brian made it like the second diagram in my original post?

[mrfoo]

Nah, if you read the article, the chamber is explicitly mentioned as being vented to the atmosphere. Why anyone would bother to do that is anyone's guess.

[jackssmirkingrevenge]

Ah I see what you mean. Well if it isn't vented, then the air within that chamber would need to compress for the valve to open, increasing the resistance. Also, you could have a leak from the main reservoir into this chamber and no way of detecting it, apart from a reduction in power due to the negation of the balanced effect.

[mrfoo]

IIRC, BtB had problems with failed o-rings on that chamber, which would have removed the counterbalance and effectively turned the valve into a standard hammer valve.

However, without failed seals, it's beautiful. The opening force is defined by the counterbalance ratio, once the seal is cracked it's the HPA which does the rest of the opening, QEV style, counteracted by the pressure in the counterbalance chamber, which acts as an air spring. As pressure drops in the barrel, it's overcome by the counterbalance chamber pressure, and that, acting as an air spring combined with the piston's inertia, provides positive valve closing action.

It's the positive closing action the others are missing.

[jackssmirkingrevenge]

It's the positive closing action the others are missing.

A spring is implied in the other designs though, otherwise they wouldn't re-close.

The problem I found with a closed chamber is that the overpressure from valve opening tended to compress the air in the chamber behind the o-ring very quickly that led to cases where either the o-ring would fail or high pressure air would find itself in the chamber and stay there, leading to a reduction of the balanced effect. Venting this space eliminated the issue.

[mrfoo]

either the o-ring would fail or high pressure air would find itself in the chamber

Any idea on exactly why this was happening? It seems a bit weird if other o-ring joints subject to the same (admittedly violent) forces aren't failing in the same way. A couple of things occur to me, viz:

• Overheating caused by rapid compression, plus rapid cooling caused by the decompression of the propellant air, causing o-ring failure.
• A floating o-ring being pushed back and forth as the pressure differential goes from +ve to -ve vs the chamber, with leakage at each change of differential.

Other options might be due to overly-generous clearances around the counter-piston, roughness in the chamber tearing the ring, or any number of other other machining issues, but I'm assuming based on your history that you're good enough with machine tools to get that right

I ask because, frankly, I find the spring-less nature of the design very elegant; with some very minor tweakage it could be performance tuned by adjusting the static pressure in the counterbalance chamber (indeed, to the point that the valve faces themselves could be of identical diameter, making the machining much simpler.)

[jackssmirkingrevenge]

The o-rings in my experiments were a tight fit in a perfectly smooth chamber and should not have been able to "float" so I assume that it was the extreme forces at the moment of firing that were causing the issue.

I find the spring-less nature of the design very elegant

It is on paper but I've found it to be a case of "sounds good, doesn't work" in practice, at least in the context of my limited experimentation in this area. I'm definitely a relative newbie when it comes to this sort of valve.

to the point that the valve faces themselves could be of identical diameter, making the machining much simpler

There's a lot of tweaking in the "proper" airgun community when it comes to hammer valves, much more that I've been aware of until recently. One of the areas of interest is the use of different materials as a seat for the stem valves as some "unstick" more easily and give visibly better performance. In that regard the use of a spool with an o-ring on each side seems to be less than optimal and indeed with sufficiently high pressure there's the risk that the o-ring will be blown out. In that context you'd want a bit of pressure differential in order to help the valve seal against the seat.

[mrfoo]

I'm definitely a relative newbie when it comes to this sort of valve.

If *you're* a newbie...

In that context you'd want a bit of pressure differential in order to help the valve seal against the seat.

Yeah, that seems reasonable.

[jackssmirkingrevenge]

if *you're* a newbie...

Not to pneumatics in general of course but when it comes to hammer valves there are a lot of enthusiasts that have been playing with this sort of valve since they are the dominant configuration for commercial airguns. I've been hanging around the GTA forums lately and it was eye-opening to see the levels to which such valved are modified and tuned, and the various technologies to optimize them, like say the Daystate "slingshot hammer". We're mostly concerned with dumping chambers on this forum but for commercial airguns, getting the most shots with consistent velocity is a higher priority and tinkerers engineer accordingly.

[Brian the brain]

Mine does vent the area in front of the piston to atmosphere. A threaded rod serves as the valvestem. I filed it flat on one side to create a vent through the piston/poppet towards the barrel and hammer. So the third picture is accurate.This should also mean that once the valve opens pressure can flow the other direction..helping the valve to close..The vent fixed the O- ring problem. I can't remember having to replace it since.

[mrfoo]

Ah, that clears that up. thanks, Brian.