BT 65 valve and poppet questions?

[mann]

No they don't care        depending on what size your peek seat is  you could make the poppet head smaller decreasing opening force to but need to know the size of the seat and throat and poppet head size

[customcutter]

FYI

A stem .195” od develops a closing force of roughly 60# at 2000psi

A stem .125” od develops a closing force of roughly 25# at 2000psi

Dave

Thanks.  Are closing force and sealing force different?  I would think that the area of the pressure side of the poppet would be 200-300#?  I've never used the formula although I've seen it posted a few times in some of the threads I've read on balanced valves, etc.

The black disk is actually steel, I checked it a few minutes ago with a magnet.  I thought it was to hard for plastic and it didn't deform at all when I used pliers to remove it from the stem.  It might still flex slightly when struck by the hammer, but I would think minimally.

[customcutter]

No they don't care        depending on what size your peek seat is  you could make the poppet head smaller decreasing opening force to but need to know the size of the seat and throat and poppet head size

For now I've given up on removing the delrin seat and replacing with a peek seat.  If I can come up with a way of reducing the cocking force needed I'll pursue the peek seat.  Then again my only option may be to try and make a balanced valve system.

I just used a bathroom scale and tried to measure the force needed to cock the hammer.  I measured 4 times and got readings from 14.8 to 16.4#, and that is without the hammer spring adjuster being inserted.  I only had the hammer tube section resting on the scale and used a wooden dowel to push the hammer down.  It feels like a whole lot more when trying to use the small bolt handle.  

[customcutter]

The Hatsan BT65 is an old platform and if I had handled one before buying I probably wouldn't have.  It is extremely heavy for me with my lower back issues so I spent a lot of time making a bullpup stock (much easier to handle), so I don't want to sell it.  I also know that it won't get much use with the hard cocking issue.

If you want to learn more about the gun/valve set up here is a YT video explaining most of the mods that were done by him.  I usually play YT videos from 1.5-2.0 speed.

Here is a link to rsterne's mod's he did to his AT44 many years ago also.  It is very similar design in the valve body, but the hammer is much lighter and strikes  the poppet stem instead of the black disk.  Also the hammer spring appears to be much smaller and the adjuster is mounted in the hammer instead of in the rear of the spring.  https://www.gatewaytoairguns.org/GTA/index.php?topic=33903.0

[nervoustrigger]

Are closing force and sealing force different?

Very different.  Let’s take an example with a 1/8” stem and a 1/4” valve throat operating at 2000psi.
 
The sealing force (force holding the poppet closed) is a product of the throat area (0.049 sq in) and the pressure (2000psi) = 98lbs
Actually something more than that depending on the sealing margin—the overlap between the poppet and valve seat—but at least 98lbs.
 
The closing force (force acting to close the valve after it is knocked open) is a product of the stem area (0.012 sq in) and the pressure (2000psi) = 24lbs
 
So we are talking about something in the neighborhood of a 4 to 1 ratio.  Note we are ignoring the effects of the valve spring and the sail effect but those are comparatively small in most cases.

[customcutter]

Are closing force and sealing force different?

Very different.  Let’s take an example with a 1/8” stem and a 1/4” valve throat operating at 2000psi.
 
The sealing force (force holding the poppet closed) is a product of the throat area (0.049 sq in) and the pressure (2000psi) = 98lbs
Actually something more than that depending on the sealing margin—the overlap between the poppet and valve seat—but at least 98lbs.
 
The closing force (force acting to close the valve after it is knocked open) is a product of the stem area (0.012 sq in) and the pressure (2000psi) = 24lbs
 
So we are talking about something in the neighborhood of a 4 to 1 ratio.  Note we are ignoring the effects of the valve spring and the sail effect but those are comparatively small in most cases.

Thanks for the explanation.  I thought they had to be different.  However, I don't understand using the radius of the stem for the sealing pressure?  I would think it would be the widest part of the poppet pushing against the seat?  Which is .367" or .1057 sq in generating 211# of force at 2000PSI?  Or is my math incorrect or my thinking that the back of the poppet would generate the sealing force?

[nervoustrigger]

Yes, the widest part of the poppet that is actually forming a seal is what will determine the force holding it closed.  That is what I was trying to describe when I brought up the sealing margin (overlap).
 
Your math checks out perfectly.

[sb327]

Jason always explains things quite well, thanks.

Consider this though, although the holding force and closing force may be a 4 to 1 ratio, the distance the hammer needs to travel against the larger force is significantly less than the distance it has to travel against the smaller force. I don’t have the formula handy but you can figure out what that ratio is of fpe to crack vs fpe to open/dwell.

My suggestion would be to try a different poppet with a small diameter stem. Of coarse a sleeve with a step to keep it from being blown back toward hammer would need to be made. This would be a couple pieces you could make, insert in valve and do no harm to existing parts. And still have reliable sealing. If you don’t like the results, simply remove and put old parts back. No loss other than time spent making the parts.

Dave

[customcutter]

Yes, the widest part of the poppet that is actually forming a seal is what will determine the force holding it closed.  That is what I was trying to describe when I brought up the sealing margin (overlap).
 
Your math checks out perfectly.

Thanks Jason.  That's why I asked for the clarification.  Been almost 50 years since my high school geometry and physics classes.  I kept telling myself PiRsquared, but somehow I was doing the actual math PisquaredXRadius.     Couldn't figure out why I was getting different numbers from you and Dave.  Palm slap to forehead DOH!

[customcutter]

Jason always explains things quite well, thanks.

Consider this though, although the holding force and closing force may be a 4 to 1 ratio, the distance the hammer needs to travel against the larger force is significantly less than the distance it has to travel against the smaller force. I don’t have the formula handy but you can figure out what that ratio is of fpe to crack vs fpe to open/dwell.

My suggestion would be to try a different poppet with a small diameter stem. Of coarse a sleeve with a step to keep it from being blown back toward hammer would need to be made. This would be a couple pieces you could make, insert in valve and do no harm to existing parts. And still have reliable sealing. If you don’t like the results, simply remove and put old parts back. No loss other than time spent making the parts.

Dave

If I can figure out how to get the existing delrin seat out, that would be a great option. 

Another question, if I decide to make another valve should I stick with brass, or could I possibly use Al (not familiar with the tensile strengths of either one)?  I still haven't looked in the scrap pieces to see what I have on hand.  My lathe is capable of SAE or Metric threads although I've only tried cutting them a few times.  I know 200bar is nothing to play around with, which is the recommended high pressure rating on the reservoir.

[sb327]

I don’t think you would need to remove the delrin seat to try what I suggested but again, I’m not 100% familiar with the valve.

Many valve bodies are made from aluminum but there may be something about the design that warrants brass. I’m not sure.

Dave

[customcutter]

I don’t think you would need to remove the delrin seat to try what I suggested but again, I’m not 100% familiar with the valve.

Many valve bodies are made from aluminum but there may be something about the design that warrants brass. I’m not sure.

Dave

Thanks, I hadn't thought of leaving it in, other than fewer seams would possibly be mean less chance of leaks. 

I need to check, but IIRC the existing cylinder is Al.  I guess I could measure it for minimum thickness if I decide to use Al.

[customcutter]

Disassembly of the breach and hammer tube/trigger assembly isn't to hard getting it back together and functioning properly has been a PITA.  I've probably taken it apart and re-assembled it 15-20 times today.  I couldn't get the trigger sear to catch the hammer.  It would go back far enough to catch the anti-bounce lever, but wouldn't catch the sear.  I thought there was something wrong with the trigger mechanism so I worked on it several different times thinking it needed to be in the fired position, then the cocked position, but no matter what I tried the hammer wouldn't go back far enough.  Finally I took the hammer out and inserted it into the trigger assembly to confirm that it was supposed to go in and which position fired/cocked the sear needed to be in.  I realized that there was a guide on the hammer that wasn't lining up properly with the grove on the trigger assembly. (You can see it in the #2 reply in this thread). 

Finally I started putting the breach back on the lower assembly, no matter what I tried it didn't want to seat properly.  Then I realized that the hammer was caught by the anti-bounce lever.  I compressed the spring enough to release the anti-bounce and success.

Thought I would add this for anyone else working on a BT65, and also for myself.  If it happens again hopefully if I don't remember the "tricks" I'll find them in a search.

[customcutter]

OK, another question about poppet design.  I know from reading probably 1000's of post on power mod's that the rule of thumb for transfer ports, etc. is that sizing should be 75-80% of the pellet size.  I think that is the actual diameters not surface area.  I guess the poppet stem major diameter less the minor diameter should yield an area equal to 75-80% of the pellet size? 

.25X.8=.2, so .1x.1=.01, .01x3.14=.0314sq inches.

The original poppet major diameter was .195"

.0975x.0975=.00950625x3.14=.0299sq inches.

The minor diameter was .157"

.0785x.0785=.00616225x3.14=.01935sq inches.

.0299-.01935=.01055sq inches.  Wow, approximately 1/3rd the area of 80% of a .25cal pellet, or is my math off again? 

Is there a "rule of thumb" for poppet stem and throat measurements based on pellet size???   

[sb327]

No rule of thumb on the stem diameter in the ‘guide’ area (your .195 area). This only effects closing, not flow.

 The stem diameter (area) in your throat area is subtracted from the throat area to determine flow in that area.

Dave

[customcutter]

No rule of thumb on the stem diameter in the ‘guide’ area (your .195 area). This only effects closing, not flow.

 The stem diameter (area) in your throat area is subtracted from the throat area to determine flow in that area.

Dave

Thanks, I simply assumed that the throat area was the same size as the "guide" area.     Time to pull out the bore gauges.

Just remembered I found a delrin seal in a bag of o-rings last night.  It measures .265" throat.  Time to re-do  the math.

[nervoustrigger]

I'm trying to follow along but am not sure what is meant by the guide area.

Is it represented somewhere on this diagram?  I know the Hatsan valve design deviates from this a bit...

[nervoustrigger]

Ken, if it would help you play with the numbers, I'm attaching a copy of my porting worksheet...

[sb327]

Hey Jason,

I’m referring to the portion of valve body the stem rides in as the guide. The part that is a close fit to stem immediately in front of hammer.

Dave

[customcutter]

Jason, Yes, the guide area is the section of the valve where the stem exits toward  the hammer.  On mine that section of the stem has an o-ring on it.  Dave explained to me that it stops air escaping into the hammer area.

On my poppet it is threaded and has a flat disk that threads onto the stem and then a nut to lock it in place.  The hammer has a large slot down the middle of it, and there is a spring loaded arm/lever that is pushed back by the bolt when cocking.  That arm rotates and slides in the slot on the hammer.  I'm not sure why it's there other than excessive cocking force.  Hatsan did away with that style hammer and arm in their next model the AT44.  Also made it a side lever cocking.

Thanks for the porting sheet.  I will definitely down load and save.  When I re-calculated based on the "throat" in the spare delrin seat I came up with 110%+ based on the 80% porting of transfer ports to pellet size.