Simplified Balanced Valve

[rsterne]

Gary, although I showed the thimble fixed in that diagram, you are correct, it requires excellent alignment and concentricity between the two ends of the valve.... When I build my SS Valves I mount the thimble in an oversize hole in the perforated wheel at the front end of the valve.... The thimble, like Don's rod, is free to move a bit radially in any direction so that it can self-align with the poppet.... It works perfectly, and I will use the same arrangement on this valve.... I am just not "good enough" to get the fit required otherwise.... and I can see no downside in making it "self-aligning".... I have been able to get a good seal even with a PEEK poppet by this method on a SS Valve, which has 2 O-rings and requires careful machining of the poppet and thimble to get the two different diameters sliding nicely.... This valve only has a single O-ring, and only one bore that needs careful machining.... In reality, the thimble can "rock" a bit on the poppet and still seal....

Bob

[tkerrigan]

I can see right now that it will be a while before I try building one.  Sure would be nice to have some for my 6 DAQ pistols and 1 carbine, they are a bear to cock.  Good luck on the project.  Regards, Tom

[rsterne]

One more post and you hit 1000, Tom.... *grin*….

Bob

[lloyd-ss]

Bob, good work on presenting a less complicated balanced valve. Simplifying a design and maintaining its best characteristics can be difficult. And getting rid of O-rings, particularly ones that function as dynamic seals is always a help with reliability.

Stiction is always a problem with dynamic O-rings that are constantly pressurized. But certain design practices can help minimize the problem. Think of "good" regulators. The best (not necessarily airgun related) rely on diaphragms, not O-rings, but many good ones do use O-rings.
To reduce stiction with dynamic o-rings, design manuals, like the Parker bible, will suggest that the o-ring glands can be made deeper and narrower, along with harder duro o-rings. Also, the compression on the o-ring can be biased toward the static diameter of the o-ring, with the static diameter having maybe 10% compression, and the dynamic side having only 5%. But its a lot of trial and error. Polished finishes on the dynamic surfaces never hurt either. And most important, I think, is keeping the clearance between the hard parts as tight as possible, like just a few thou total ont he diameter.....unless it causes other problems, like binding.  And that does point out one final problem, making balanced valves requires more precision than conventional valves. Sigh. 

O-rings aren't really that great for dynamic seals, but they are certainly used a lot.

There was some mention of the difficulty of drilling holes in stems, and I agree, a major pain. When I can, I prefer to use hardened dowel pins for valve stems, but that's not always possible. Here's one way to avoid having to drill a hole thru the stem, by grinding a groove along the stem before pressing or threading the poppet into place. I use a thin cut-off wheel in a Dremel tool, along with plenty of water to stop burning. I've had good luck with the technique.  BTW, this poppet didn't work, as you can see by how high the o-ring sticks up out of the groove. That's exactly opposite to what I said earlier.




I hope your new design is successful. The devil will be in the details of getting the diameters and passages the right size.

[PakProtector]

Bob, I think I have a way to make things more difficult:

The air chamber ahead of the valve poppet should get a check valve to keep it pressurized longer as a bounce inhibitor. It just has to leak enough that by the time you can load another pellet, it is back to zero. The more of that ~3 second interval you use the better I think, so a 'leak' would not have to be very large at all. You will see the chamber fill with throat pressure at a rate decided by the 'jet' size and its volume, and it just has to come back to zero before you pull the trigger again.

And no, I have no idea how to execute the idea that won't have a ball bouncing around loose after the first shot...
cheers,
Douglas

[rsterne]

The whole idea here, Douglas is *KISS*….

Lloyd, thanks for the comments.... I have used the idea of a groove ground in the shaft, pretty hard to control the size (area) of it.... Any comment on the idea of the "wedging" action I mentioned in Reply #34 being in issue?....

Bob

[shorty]

There is a very elegant solution to drilling a long hole through the valve stem.

Hypo-tubing with a short piece of drill rod epoxied in or cold welded in. I am pretty sure you can harden the hypo-tubing a bit. Cheap on amazon and almost every size available along with different wall thickness's.

I used this method when working on the decompression poppet.
https://www.gatewaytoairguns.org/GTA/index.php?topic=109867.0

[lloyd-ss]

Bob,
Yes, I tend to agree with tendency of the o-ring to extrude into the gap between the piston and the tube, and then become worse when the direction of travel further pinches it. The methods I described will reduce it, but won't make it go away. Getting the o-ring onto the other side of the pressure/travel should be a good fix.

[rsterne]

Yeah, I think if the pressure is forcing it one way, and the travel on firing the other, it should push the O-ring back into the middle of the groove.... I think the problem with the SS Valve is that the two O-rings are loaded in opposite directions, so the front one, which is wedged into the rear corner, gets a double does of friction on the initial firing after sitting for a while....

Bob

[Motorhead]

Yeah, I think if the pressure is forcing it one way, and the travel on firing the other, it should push the O-ring back into the middle of the groove.... I think the problem with the SS Valve is that the two O-rings are loaded in opposite directions, so the front one, which is wedged into the rear corner, gets a double does of friction on the initial firing after sitting for a while....

Bob

Yup .. a Bi-directional Chinese finger trap effect.

[lloyd-ss]

Years ago at work, I met with a very experienced application engineer... a guru.... for a company called Seal Science. Just in passing he said that selection of the wrong seal material was the most common problem he ran into. Unfortunately, he retired before I had a chance to privately corner him about airgun o-ring applications. Talk about a missed opportunity! Darn! 

[Hobbyman2007]

I’ve gone through some of my textbooks from trade school and some litterature from Parker , both seem to recommend the use of backup rings for pneumatic applications . Mostly due to keeping lubricant for the oring . I’m wondering since in our use of dynamic orings this might be a good practice and eliminate some of the sticksion issues were seeing . I know it’s been brought up before ,just wondering if anyone’s tried it .

[lloyd-ss]

I’ve gone through some of my textbooks from trade school and some litterature from Parker , both seem to recommend the use of backup rings for pneumatic applications . Mostly due to keeping lubricant for the oring . I’m wondering since in our use of dynamic orings this might be a good practice and eliminate some of the sticksion issues were seeing . I know it’s been brought up before ,just wondering if anyone’s tried it .

Denis, Looks like you are thinking outside the box, and in the right direction.  We rely on O-rings because of their price, availability, selection, and compact size, and they are usually very forgiving and work well.  But this stiction issue is a real bugaboo. I've used back-up rings in static applications at very high pressures, in particular when hydraulic pressure testing air tubes. The backup rings were needed to keep the O-rings from failing before the tube. And they worked! But I've never used anything smaller than a size 111, and the back-up rings, if you use 2, double the width of the o-ring gland, so that can be a problem. Spring or o-ring loaded u-cup seals work well, but small sizes are limited, gland sizes are usually large, and cost can be pretty high. I put a spring loaded glass filled Teflon seal in an air pump I made and it worked great, but golly, it was $20!

Bob's approach of eliminating/minimizing the problem by design, is good. But finding alternate kinds of seals is a good idea too. I see that McMaster has the backup rings down to size 004, and they are really cheap. You might only need the back-up ring on one side, so the groove width problem might not be too bad.  Good idea Denis!

[oldpro]

I’m sure you guys have considered this, but with that setup you might need a soft and flexible seal on the seat to get a 100% seal every time. The Cothran valve has a loose piston (can wobble and adjust to the perfect alignment) to seal the vented chamber. As such, the stem and seal are free to “find and seal" to the seat.
If the setup is fixed on both ends (three points counting the stem), you’ll need exceptional tolerance and finish control… just my opinion

Wyo

The SS valve has a floating thimble to allow for alignment. I agree with what your saying

[rkr]

I’m sure you guys have considered this, but with that setup you might need a soft and flexible seal on the seat to get a 100% seal every time. The Cothran valve has a loose piston (can wobble and adjust to the perfect alignment) to seal the vented chamber. As such, the stem and seal are free to “find and seal" to the seat.
If the setup is fixed on both ends (three points counting the stem), you’ll need exceptional tolerance and finish control… just my opinion

Wyo

The SS valve has a floating thimble to allow for alignment. I agree with what your saying

Alternatively you could copy JBs self centering valve design to let the poppet float.

[mann]

I may have to change my SS valve so the center floats may be just remove the threads and put a snap ring in the end of the valve to hold it from popping out

[rsterne]

The SS valve has a floating thimble to allow for alignment.

I haven't seen one of your new SS Valves with the threaded in one-piece thimble.... I figured they were fixed.... How do you float the thimble in them?....

Bob

[oldpro]

The SS valve has a floating thimble to allow for alignment.

I haven't seen one of your new SS Valves with the threaded in one-piece thimble.... I figured they were fixed.... How do you float the thimble in them?....

Bob

The old ones are threaded the new ones have a circlip on each side anda wavy washer to take up the slack but allow movement 

[rsterne]

Ahhhhhhh, didn't know you went back to the circlip.... I had one of your first SS valves, and it had a circlip.... The perforated mounting wheels I make on my SS valves are held in place with a circlip, and in addition the center hole is oversize and I use a shouldered screw tightened into the thimble to provide lots of radial movement for alignment.... Seems to work great, and I will do the same in the new valve.... I got the poppet roughed out and the stem made, threaded, drilled and glued into the PEEK today.... I'll make the thimble next and then finish the diameter on the poppet while holding it with the stem.... The front end of the stem is drilled with a tiny center drill so I can center it on a live center when doing the final turning and O-ring groove....

Bob

[Tack Driver 10]

Bob, great job on explaining how these balanced valves function.
I was in the middle of a valve experiment and decided to take a stab at one for a 13xx.

This one uses two springs, one for the check and the other for the stem.
Some pics of the prototype minus the springs.