LDCs that screw directly into the Airacuda Standard shroud

[jackssmirkingrevenge]

Probably should distill that into PCP applications, or someone may take offense.

Naturally we're talking about airguns here, just wanted to show some historical examples of the principle at hand.

The design seems simple enough; and a rubber sheet can stand repeated pounding, without cracking the parts it bangs into.

I understand it better now, basically the air accumulates in the chamber via a check valve.  It's not quite blocking the muzzle but I can see there being a benefit, I guess it would be relatively easy to do a comparison with and without the rubber to see what difference it makes.

[subscriber]

Think of it as a contactless wipe, Jack.  No projectile dodging required.

[subscriber]

Here is a hybrid design:  A flapper valved primary blast chamber; with conventional baffles to strip the remaining air going down the central passage.

Again; not a production design.  Missing many fillets and other improvements, including supports to prevent the free end of the central tube from vibrating.

[avator]

Ok, I may be just an uneducated redneck yankee who migrated to the south with the misconception that I might have less competition in the area above the neck and between the ears but, what affect does stripping and reversing the direction of a nearly 3k blast of air have on the motion/harmonics of the barrel and ultimately the entire gun? Particularly the grub screws securing the breech, the LDC to the barrel/shroud or the shroud to the receiver/barrel?
Would repeated shooting be like tap tap tap of a hammer trying to force things forward?

[jackssmirkingrevenge]

Think of it as a contactless wipe

That's essentially what I call the "shut-off valve suppressor" designs are looking to achieve, allowing the projectile exit while trapping most of the gasses much like wipes, but without physical contact that affects power and accuracy.



With this design you will trap a significant amount of air at high pressure while the projectile is "plugging" the central tube but of course there's still the gas inside the tube behind the projectile some a hybrid with baffles is a good idea, but I would say maintaining volume in the accumulator side is more important for effectiveness so I would see a hybrid looking more like this:

[subscriber]

Depending on the design, Bill there would be some thrust on the LDC trying to pull it off.  Depending on the power that force may not be as large as you imagine.  Consider how common slip on LDCs are.  Even when the thrust is significant, it is delivered as if from a rubber mallet, rather than a steel hammer.

I think that the recoil reduction seen with a powerful PCP gives a sense of the force now taken up inside the LDC (and its barrel mount, and barrel to receiver mount) , compared to a bare muzzle.  I don't think this is an exact equation, but it would be fair to state for a conventional open muzzle brake.    I would also offer my opinion that for a 30 FPE .22 PCP, the recoil is barely felt and that if all of that force was applied to the barrel mounts, it would be of no consequence.  If we are talking about a 1000 FPE PCP then the forces become more interesting.

LDCs can mess with barrel harmonics, or improve them.  The air stripping function will often reduce groups noticeably.

Also, just because the air tank is pressurized to 3k PSI does not mean that is what the LDS sees.  Not unless the PCP is unregulated and the valve has stuck open, would it see that much pressure.  For your typical 30 FPE .22 PCP with a 20 inch barrel the muzzle pressure is more like 400 PSI.

The force that is applied as a thrust on the barrel is not the LDC endcap diameter x the air pressure.  Neither is it the baffle OD, because the same air pressure is acting on the opposite baffle wall, cancelling that force.  It is the baffle bore area x the pressure differential across that baffle chamber that generates the trust you are concerned about.  Yes, successive chambers' forces overlap to some degree, but the first one or two near the muzzle are probably the most significant.

Counter intuitively, the lower the reg pressure is set for a given FPE, the higher the muzzle pressure will be.   You can get a sense of this with an unregulated PCP that you shoot from 3kPSI down to 1kPSI.  You will often notice that the same PCP, shooting the same pellets, at the same tune, is quieter at 3000 PSI, and a lot louder at 1000 PSI.  The reason for this is, to make the same power at lower air reservoir pressure, the valve has to stay open a lot longer.  At high tank pressure, the valve duration is much shorter - or it would make much more power.

When I do custom LDC designs for 100+ FPE PCPs, I calculate the average pressure required to generate the power from that barrel length.  Then I assume the air tank pressure is near that average pressure, such that the average air pressure down the barrel is the same as the muzzle pressure.  Then I do strength calculations based on that.  In practice, the muzzle pressure is half to a quarter of the average pressure to achieve a given FPE.   It depends on how close the PCP is to producing it maximum power.  I add an assumed friction value to the equation, because that increases the assumed average air pressure. 

Now, this may seem to be too many assumptions.  If I were actually selling these things commercially, I would make them thinner until they blow; then reinforce the failure points.  Certainly one cannot go overboard with strength, just in case.  It is like reinforcing an aircraft so that the fuselage would survive a crash at 45 degrees to the ground at 600 MPH.  Make the plane strong enough and it would be too heavy to fly.  Rather avoid conditions that lead to crashes.

I do not design an LDC to stand 3000 PSI, or whatever full fill pressure is.  One might say I should, but it would mean the reg has failed (if there is a reg), and the poppet valve has stuck wide open on firing.  This is like worrying about brake failure after the wheels have fallen off your vehicle.  I do think about what would let go first, and in which direction that part is going to move.  I want a "safety valve" of sorts, to achieve a "fail safe" system.  Blowing off the LDC or the barrel should not do much harm, as nobody should be in front of you when you shoot.   Throwing fragments backward would be very bad. 

I recently did a design for a GTA member for a 9 mm Condor PCP.  He did not want to place printed parts inside an aluminum tube.  So, I made a declaration about how he needed to test the LDC before he could trust it.  I did several version of the design, with a range of wall thicknesses,  In all cases the area near the barrel muzzle was thicker than the rest.  This guy is not in the US, so while I trust he won't injure himself, people in that country are not as ready to file a law suit as they are in the US.

IF we get back to the Airacuda, the end thrust generated by the stock shroud endcap is not zero.  I think we can stand to double or quadruple that, without launching the barrel.  Especially since we are attaching to the shroud, rather than the barrel. 

If you look at all the designs above intended to be screwed into the Airacuda shroud, the male thread that is part of the LDC is the weakest link in the system (I much prefer the male threaded part to be metal).  So, if that plastic threaded section holds, I think the barrel retention screws etc are safe.

[subscriber]

Jack,

Your image editing ability is impressive.  Piece of cake compared to your animated mechanism gifs.

I agree that the initial uncorking of the projectile from the barrel muzzle is the most significant event from an air management perspective.  My goal is to generate concepts that would be fine tuned by actual testing.  This includes the relative lengths and volumes each functional zone should occupy.

I wonder if 3D printers with dual extruders could print the whole thing in one go, as a closed system.  An elastomer material would be used for the rubber seal.    I say that, while having seen 3D printed elastomers, and being less than impressed with their tear strength.   So, having the casing made in at least two parts would probably be the best strategy, to enable installation of the rubber flap valve.

[jackssmirkingrevenge]

Your image editing ability is impressive.  Piece of cake compared to your animated mechanism gifs.

The magic of copying and pasting in MS Paint 

In the interest of generating more concepts, here's a half-baked variation that would use synthetic spheres somehow lightly sprung that would act as a check valve in order to avoid relying on elastomers: 

[subscriber]

Individual spheres will return to their seats more reliably than a single O-ring.  The little balls may tend to bounce on opening and closing, unless they impact a slightly soft surface.

Plenty of precision plastic ball options to choose from:  https://www.mcmaster.com/products/bearing-balls/material~plastic/

[luge007]

Seemingly always late to the party.
I WAS going to suggest that "wipes' have been used in 'devices' for decades.  They are very effective at increasing the manner in which gasses are trapped and reducing the sound signature of the host weapon.  However it also comes at the cost of accuracy, it is not a precission device, but it was effective for what it was initialy designed for.


Also mechanical valves were tried on 'devices' decades ago, but found to get dirty quickly,  only be moderately effective as well as contributing to the noise.  Of course these two concepts I speak of are a historically referenced from a published book, that I'm not sure I can mention on this forum.


The 'conical flapper valve' is an interesting concept, if it could be made to seal well and hold up to getting slammed shut repeatedly, yet reliably release the air slowly, you might have something there.  Biggest issue may be getting a good sealing surface with a 3D printerd part.  A secondary machining opp may be needed to smooth up that surface, or the sealing material would need to be very soft, almost gummy. 


Along for the ride, love to see where this ends up.......
M

[oldpro]

     5 hours later print is done but unfortunately the threads are undersized in the model and it wont screw on the shroud. The thread od on the silencer is the exact size as the thread Id of the shroud so there's no interface between them. ill see if I can change the stl to fix this. It did however print very well with no blobs or stringing so thats a huge plus.

[subscriber]

Sorry about that Travis.  The thread size in the CAD model were developed with TorqueMaster Bob and his printing setup.   The nominal sizes are larger than what you printed, as Bob used a die to clean off fluff and the layer start-stops for a slop-free thread.  See images of thread CAD model, below. 

I have taken material out from the full form theoretical thread form, because 3D printed thread are usually to tight to screw in without chasing them with a die.  So I "shave" the thread form in CAD with the goal of having a usable thread as printed.

If we look at the nominal dimensions from the link below, you should be able to correct the problem by scaling the X and Y axis by 22/20.917 = 1.0517.  This assumes the root diameter of the shroud thread is at 20.917 mm.  If it is larger, use the measured diameter instead.

Don't scale the Z axis, as that will change the pitch; unless you have measured it as off.  In which case you can apply an appropriate correction factor.

Speaking of measurements, I would appreciate an an ID measurement for the shroud beyond the threads.  Also, a measurement of shroud face to barrel muzzle face with stock baffle removed.  I am designing a hybrid flapper valve blast chamber to fit into the shroud, with an 80 mm long extension, that matches the assumed shroud OD of 25 mm.

I was not planning to show this shroud insert yet, as it not finished.  The flapper valve shape and how it is retained are likely to change.  I may extend the blast chamber into the "nose".  Also, I intend to add support ribs to help stiffen the long internal cones against vibration.  But, as you popped up, I thought I would try to whet your appetite, and increase the odds of you providing the dimensions I need to make the part fit.  I have the in-shroud depth at about 4", but can make that match reality to maximize function. 

I believe the flapper valved blast chamber will work most efficiently when it is right close to the barrel muzzle; as opposed to tacked onto the front of the shroud.  That said, if the internal depth is short, extending the blast chamber beyond the shroud face, and perhaps increasing its diameter may be required to prove the concept.

It would be useful if you could measure your print OD in a few places to see if it is also small by 5%.  If  it is close to nominal 1.75", then I can create a larger thread form for STL for you;  Else scaling should X and Y work.


M22 x 1 mm thread specs:
https://www.machiningdoctor.com/threadinfo/?tid=2041

Nominal Diameter   22
Pitch (Distance)   1
Pitch Diameter   21.35
Minor Diameter   20.917
Thread Height   0.541

 

[subscriber]

As an afterthought, Travis, is it possible that later Airacuda versions that came with a 1/2 20 thread adaptor have shroud cap threads larger than M22 x 1?

If the threads are not M22, then I can make a version with whatever thread size they have.  What is the thread OD of your stock shroud endcap?  If that is over 22 mm then there is a split in endcap thread specs; and possibly shroud diameters for different Airacuda models.

[subscriber]

Improved design of flapper valve Airacuda insert:  Larger volume primary blast chamber, therefore fewer conventional baffles.

Of course, the nose part could be made larger in diameter than the 25 mm shown.  It could also be made longer than the 80 mm ahead of the shroud front face.

[oldpro]

Thread OD is 21.2mm same as the tread ID in the shroud. Identical

[subscriber]

Thread of is 21.2mm same as the tread ID in the shroud. Identical

Thanks Travis, but you need to be more specific about which thread you are talking about.  The printed thread OD measures 21.2 mm?  What does the stock endcap male thread OD measure?

What does the 1.75" OD measure?  This will tell us if there is a percentage error that can be corrected with a percentage correction factor.

GTA forum member Robert 5 mm has several inserts with these threads, and they screw into his Standard Airacuda shroud.  They were printed by GTA member TorqueMaster.  He chased the threads with a M22 x 1 die, but that was not really necessary, as next to no material was removed.

My suggestion is that you print only the threaded end, until you have confidence it will fit.  I have attached an STL for just the thread test piece shown below.  Printing the whole part before the thread has been figured out will be an exercise in frustration.

My CAD thread OD and root diameters are shown in the image below.  I have not changed that for the latest parts.




Meanwhile, I would appreciate your help, so I can better help you:

[oldpro]

OD of new silencer thread and ID of shroud is the same at 21.2mm and Stock end cap thread ID is 21.70 mm. I see this happening when people model threads instead of using the thread size under the threading tool in the application. Not a big deal for me I just adjusted the model but could be problematic for others. JFYI

[TorqueMaster]

Travis,
Looking at subscriber's CAD dimensions, his designed thread OD is 21.72mm (22 - 0.14 -0.14)
When I printed it in PETG, it was expected to be slightly smaller due to PETG shrinkage (est 0.5%) ->> (21.72 x .995 = 21.61mm.)
The resulting print was chased, but almost nothing removed.  It was reported to fit well on an Airacuda with an endcap measured to have 21.6mm OD threads.

I'd suggest something in your setup is causing undersized prints if you are getting 21.2mm OD on those threads.  Did you use PETG?  Other materials shrink differently.

[subscriber]

My basic version of Solidworks does not have a thread generation tool.  The only way I can generate 3D threads is with a spiral path of nominal diameter as a guide curve for the thread form I specify.  As I go to that much trouble, I can fine tune the diameter and thread form for 3D printing.

A 5% diameter error in the printing system pipeline is outrageous.  It looks like I need to create a part version with an M23 thread and let you try that...

[subscriber]

Travis,

The attached STLs are for the LDC you wanted to print, and a thread test print.  The latter was "cut off" from that LDC.  The thread diameter of this LDC is 4.5 percent larger than the one that printed too small.

M23 is 4.5 % larger than M22, so that is what thread the LDC and test print have; both at 1 mm pitch.

Now, if all diameters in your printed part are 4.5 to 5% under CAD dimensions, then the pellet path is also going to be too tight.  In which case you may be better off scaling up the X and Y axis in your slicer, until the thread is large enough for a good fit.  I have effectively scaled up the thread on the attached part print files, while leaving all other dimensions unchanged.