Simplified Balanced Valve

[rsterne]

I have no idea if this valve will have the same "sweet spot" in terms of balance that the SS valve has, I just guessed that it might.... The successful SS Valves I have made had the small end between 2/3 and 3/4 the diameter of the big end (ie 67-75%).... The valves I built where the small end was less than 2/3 the diameter of the big end were so easy to open, they were subject to machine-gunning if the hammer strike was fractionally too hard, and was able to bounce at all.... At 3/4, the valve was getting too hard to knock open, IMO, but remember I was comparing it to a conventional valve that required a 1/2 lb. hammer with 2" of stroke that took over 28 lbs. to cock.... I know that Travis settled on 80% for his SS Valves, and that is probably just fine for the limited power PCPs he is producing them for.... Even at 80% diameter, he is reducing the hammer strike required by 36%....

Travis stated that he did get a valve of this design to seal up and work with a 3/16" small end.... He didn't state the diameter of the big end, but I'm guessing it was 0.350", the same as his SS valves have always been.... That is a ratio in diameters of 54%, and the valve would require 73% of the opening force it would as a 0.350" conventional valve.... ie a 27% reduction.... Remember, this valve works "backwards" compared to the SS Valve for the assist provided by the small end.... larger means MORE assist, not less as in the SS valve.... If the small end in this valve is 80% of the diameter of the big end, the opening force will be reduced by 64%, ie the valve will only require 36% of the opening force of a conventional valve with the same seat diameter....

My drawing in the first post of this thread suggests that the area of the small end be 50% of the big end.... ie the diameter would be ~71%.... That is right in the middle of the range I found worked well in my SS Valves.... Both the valves built so far are at 80-84% diameter on the small end, one of them sealed up (actually the one at 84%), the other one didn't.... If my 71% ratio doesn't work (which should get rid of 50% of the force holding the valve closed), then I'll try a smaller one.... We know that Travis got it to work by going smaller.... How much smaller should be the only question remaining, IMO....

but I've certainly been wrong before, and will be again.... I can tell you one thing, though.... If this valve doesn't work, that fact alone won't be good enough for me.... I will need to understand WHY it won't.... 

Bob

[lloyd-ss]

There are so many subtleties to the balanced valves that it is easy to end up chasing your tail trying to get a new design to work. I know, I've got a box of failures, and just a few winners. But when they work well, they work really well.

Doing the area and force calculations of the various piston and seat diameters, at various pressures, and open distances, is very important.
If the math says that it WON'T work, then it definitely won't work. If the math says that it WILL work, then there is a chance that it might work, but a VERY SMALL chance that it will work well.  Getting it to work well almost always requires a second or third iteration.

[oldpro]

As promised here are pics of the cut down thimbles. The original and smaller .187 version

[rsterne]

Travis, thanks for the photos.... You didn't state the diameter of the seat, but taking measurements on my screen, from the top photo enlarged....

Upper poppet.... rear = 13.5 mm.... front = 10.5 mm = 78%
Lower poppet.... rear = 13.5 mm.... front = 9.5 mm = 70%

That is assuming that the seat on the poppet is right around the very outer edge of the larger diameter.... and does not allow for the bore of the thimble being larger than the front of the poppet (for clearance).... If anything, the sealing diameter of the front part of the poppet would be a larger percentage of the sealing diameter of the rear portion than what I have measured from the photos.... My design is 71%, so I remain hopeful....

You might like to consider this.... To have the same 36% reduction in opening force as your SS valve, with it's 80% front section on the poppet.... a valve to this design would require only a 60% front section.... As I said, the effect of changing the diameters is reversed.... At 70.7%, both systems have the same effect... half the opening force is cancelled out by the balancing portion....

Bob

[oldpro]

.282x355 and .185x355 respectively same lower sections and valve internals.

[rsterne]

Travis, sorry, but those dimensions do not match the photos.... .185x.355 is only 52%.... Measure it yourself.... If the front portion of the lower popper in the upper photo is 0.185", the rear portion is about 0.265".... Also, since both poppets are in the same photo, the front portion of the upper poppet is only about 0.205"....

Bob

[oldpro]

 Bob your absolutely correct I gave some bad info the .187 was on the Prod version I measured it and its .240 on the Mrod version. SORRY!!! Was going from memory so that was mistake number 1 lol.

[rsterne]

I will assume that the stem is 0.125".... It measures 5 mm on my screen.... Based on that, the rear of the poppets, which both measure 13.5 mm would be (13.5 / 5 x 0.125") = 0.338".... The front of the poppets would then be....

(10.5 / 5 x 0.125") = 0.263" for the larger one.... 0.263" / 0338" = 78%

(9.5 / 5 x 0.125") = 0.238" for the smaller one.... 0.238" / 0.338" = 70%

Is it possible that the small front end is 1/4" in that photo, not 3/16" ?

*LOL*…. Looks like 0.240" is the correct answer.... I should have scaled everything from the stem in the first place.... 

Bob

[oldpro]

I started thinking back and if I recall correctly I did end up using a .187 to 350 ratio to be able to get any real tuning effect I have since moved all my equipment and data up to JSAR so I will when time is available go back and find the data from over a year ago and that alone won’t be easy. I have lost data that I had no use for that is now relevant so I will go look threw it ASAP and find the chrono data also.

[oldpro]

 I just remeasured the above SMALLER poppet its Metric 6mm x 9mm.

[rsterne]

The smaller poppet in the above photo is (13 mm / 25 mm) = 52% at the small end, compared to where the seat is on the big end.... 

Doesn't really matter anyways, I'll get back into the shop in a day or two.... suffering from "Motelus Interuptus" right now.... 

Bob

[lloyd-ss]

I'll get back into the shop in a day or two.... suffering from "Motelus Interuptus" right now.... 

Bob

Darn guests. Can't they just pay their money and take care of themselves! Jeez! 

[rsterne]

New Years guests have checked out, and we have a few days respite now.... however, that means it's time to do our "off-season" maintenance.... This week I will be spackling any dents in the walls and repainting them (fortunately we still have some of the original paint, and once dry it matches perfectly)…. There are a few other jobs we do annually, in order to keep up our "5-star" rating with Trip Advisor.... the only one in this area, I might add.... 

https://www.tripadvisor.ca/Hotel_Review-g1169466-d1167372-Reviews-Mozey_On_Inn-Coalmont_British_Columbia.html

Hey, it's a shameless plug, but you only have another 2 years to come and visit us before we close permanently on Feb. 28. 2021.... (yayyyyyyyyyy, freedom dayyyyyyyyyy)….   

Bob

[Hobbyman2007]

New Years guests have checked out, and we have a few days respite now.... however, that means it's time to do our "off-season" maintenance.... This week I will be spackling any dents in the walls and repainting them (fortunately we still have some of the original paint, and once dry it matches perfectly)…. There are a few other jobs we do annually, in order to keep up our "5-star" rating with Trip Advisor.... the only one in this area, I might add.... 

https://www.tripadvisor.ca/Hotel_Review-g1169466-d1167372-Reviews-Mozey_On_Inn-Coalmont_British_Columbia.html

Hey, it's a shameless plug, but you only have another 2 years to come and visit us before we close permanently on Feb. 28. 2021.... (yayyyyyyyyyy, freedom dayyyyyyyyyy)….   

Bob

Bob,
I might just have to take you up on the offer for a stay at the Mosey. We had a local watering hole of the same name in North Bay years ago . Lots of fun to be had . I’d love to give your .257 mono a try .

[rsterne]

Yeah, we get the occasional phone call asking if we still have the peelers.... *LOL*…. guys looking for the pub in North Bay.... 

Bob

[PakProtector]

It looks like there are two tuning quantities you can mess with. One is the volume of the balance chamber, and the other is the holes feeding it. they're pretty closely related; so adding wee plastic blocks to fill it up or by removing, make it bigger you'll be able to adjust the time it takes to develop closing pressure as the chamber fills with HPA during the shot.

For the usual, close while pellet at barrel midpoint I think you will want some variation of Lloyd's spreadsheet...driven by the barrel pressure output. For sure, with test data, tuning up the spreadsheet and flow values ought to be possible.
cheers,
Douglas

[rsterne]

If the valve is not tunable with hammer strike, then we definitely need changes.... I am not willing to accept another Cothran style valve where you must change pressure to change velocity, the valve is either "cycling" or "not".... Don's valve does a great job of developing big FPE from minimal hammer strike, which was what it was made to do.... It works well in regulated PCPs, but you can think of it as having a set "dwell", regardless of hammer strike.... You hit it hard enough to open, it stays open for "X" time (eg. 1 mSec) and then closes.... If you are launching a light bullet you get high velocity and low efficiency.... The heavier the bullet, the less it has travelled down the barrel when the valve closes, and the higher the efficiency.... The lower the velocity, the higher the efficiency....

An SS Valve with a large front vent and a small hole through the poppet acts in a similar manner.... I think both valves are "blowing open" until the pressure inside the balancing chamber builds to the point of closing them.... If you make the vent through the poppet larger in an SS Valve, it wants to close more quickly.... More hammer strike "drives" it open more, instead of it just blowing open.... so you can control lift and dwell with hammer strike.... Installing a small front jet adds a "Cobra" component, turning the front chamber into an air spring, and reducing the dwell.... It's like adding a stronger valve spring, allowing you to tune over a broader range of pressure.... but requires more hammer strike to do that....

IMO, the main reason to make a balanced valve is to reduce the hammer strike, particularly in Big Bore PCPs, where getting a hard enough hammer hit can actually be the limiting factor to the FPE.... Generally, they tend to be unregulated, so to get more than 1 or 2 shots you have to be able to produce a bell-curve.... That requires a valve where you can change the dwell with hammer strike.... So that is my goal.... I want to do it in the simplest manner, with the least parts and seals, and the easiest machining.... and I expect a few setbacks along the way.... The "stiction" problem in SS Valves that creates low velocity on the first shot after the gun sits, is such a setback.... and one we need a design change (getting rid of the wedging of the front O-ring) to overcome, IMO....

The cool thing is, that once accomplished, you should be able to use the balanced valve concept in regulated PCPs, or in lower powered PCPs, to reduce the cocking force, hammer weight and stroke, lock time, etc.etc…. The bottom line on efficiency is where you close the valve in the pellet's travel down the bore, ie the dwell as compared to the pellets weight, and hence velocity.... While different valve designs can have an effect on that efficiency by having a "squarer" flow profile.... or by resisting hammer bounce.... I still think the biggest gains in efficiency are made by changing the basics of what you can call the "shot profile" (a plot of air pressure vs. pellet position in the barrel).... which ultimately results in lowering the residual muzzle pressure as a percentage of the firing pressure.... and hence the amount of energy lost on every shot....

Bob

[Tack Driver 10]

My original valve worked initially but started leaking so I decided to use a wedging type valve stem instead of a stem that sits on top of the seat. This is similar to the check valve on the input of the valve on a pumper.  They seal with very little pressure.

Made a stem from Teflon that wedged into the seat. Worked great for a few cycles then blew the head off the end. Too thin at the glad with a .030" vent through a .100" gland. Thought about making one from Delrin but figured I'd skip that step for now.

Next made a stem from brass with a Teflon wedge insert and it is what I'm currently testing. It has a .210" thimble through a .280" throat and a .330" stem diameter. It uses a #004 70 duro poly o-ring. If this works then I may try a .210" x .250" version.

FWIW....The valve stems looks bent but it's my old camera that is distorting the image.

[shorty]

Bob,
This is just my opinion and thoughts on a few things. I hope it helps this valve design.

I have a feeling your simplified balanced valve will operate like a Cothran valve or worse just become a dump valve. Unless you make the balancing chamber so small it defeats the purpose of cracking easy.
I also have a feeling you will not get enough of pressure delta across the cross through hole in the valve stem into the balancing chamber during firing sequence due to size constraints.

In your description above about the SS valve, I think there is something else going on but similar to your description but different (just my opinion).

After reviewing one of Lloyds balanced valve designs with opening and closing forces ( very easy to understand, thanks Lloyd ), I decided to try and put things into a spread sheet with the diagram of the valve to help see and calculate just as much as I can to help myself understand what's going on. It may not be exact or perfect as I am no expert.

I couldn't calculate the delta through the valve port so I excluded it. And, I put a revision in on your simplified valve to control dwell. This should also work for the cothran valve.

I have a feeling this would work just as a simplified cobra valve without O-rings and thimble too.

[PakProtector]

I like the larger stem bearing for developing closing force. The tuning by orifice size to the balance chamber is still there. So far so good...IMO, an' all that.

Douglas