Super Bolt and Low pressure upgrades

[Bad Dave]

Good post bjjb99

I believe the shape of the power tube is very important in this, I know different shapes have been tried in the past but why not try it again but with different bolt/spring combos.

[FatMan]

Hmmpf!

Sheesh. I've heard a lot of stuff, but some of this takes the cake. Surely Kilaueakid is pulling our leg - and trying to see if anyone is fool enough to hose his mag!

There's no way the bolt/power tube is acting like a regulator. To do that the bolt would have to sit close to it rearmost position, the only place an air-tight fit occurs. In a closed system, the change in pressure will only result from change in volume, or change in temperature. The strength of the spring or weight of the bolt has no effect on the pressure other than some small amount of loss due to temperature.

Where does the 350 psi go? Well, to start with a lot of it goes right where Bad Dave said it goes. Expand the chamber to include the powertube and bolt and the pressure goes way down. But its not enough you say? No its not. Think about the relationship of the bolt, the bolt plunger (not sure if that's the right term) and the power tube when the bolt is fully extended. There is a HUGE air space between the power tube and the body, and the air flow is open to that space.

But that STILL isn't where ALL of it goes. Who ever said that the pressure in the entire chamber/power tube/bolt system is evenly distributed. Sure, if you leave it long enough it will be, but you can rest assured that a pressure wave runs down the powertube, bounces around as it tries to avoid the bolt plunger, and then heads down the bolt to find the ball. By the time the REST of the air pressure gets there, the ball has begun moving down the barrel, further enlarging the volume so the bulk of the air doesn't seem to produce as much pressure at the site of the ball.

The weight of the bolt and stiffness of the spring will only affect the speed of the bolt. That might have a small effect on the inital movement of the ball, but apparently not enough to be noticed. In truth, there must be SOME effect on energy transfer, but the efficiency effect pales in comparison to the blast of air out the barrel after the ball leaves. In other words, the efficiency might work out differently in theory, but probably doesn't make much useful difference in practise.

I'll close by saying I haven't tested all of this, but I tend to trust what Tom says - if it doesn't affect efficiency that is most likely why.

FatMan

[kilaueakid]

Like I said, there was no fact behind what I said, only thoughts.

Bjjb99....just out of curiosity, you say the valve would need to be increased by a factor of 6x. If that is the case, than how do some of these lp guns do it without having a dump chamber the size of pop can?

Another thing that is apparent on the mag is that when chamber volume doubles, the operating pressure is NOT cut in half. It will go down, but is not a 50/50 relationship.

I guess I am just a little confused on this subject, to me it would seem that a bolt return spring with much less tension would not need 400psi to get the bolt past the end of the powertube tip to release the 60-80psi burst on the ball. Maybe it would only need 200psi? What is the stock spring tension...35 lbs? what if the spring were 10 lbs tension? Sorry for any confusion and don't go hacking up your guns!!! Interesting subject though guys!

kila

[athomas]

It only takes about 50-60 psi to get the bolt out past the end of the power tube. That is why the residual pressure in the dump chamber of the mag is about 50 psi. Below this pressure the spring is able to push the bolt back against the pressure.

As for the pressure vs volume, a lower pressure and higher volume reduces efficency because the ball cannot be accelerated to a high velocity nearly as quickly. The acceleration lasts longer, the pressure variance from beginning of the barrel to end of barrel is less, but the residual pressure in the barrel as the ball exits is higher. This "exhaust" out the end of the barrel is wasted and the effect of the air flow out the end causes a negative pressure in the barrel as well. The only place that the low pressure guns gain some of the efficiency back is in the fact that they can use more of the gas available in the bottle.

An example would be:

1) A 400psi mag set to use a 800psi input gets 800 shots off a 3000psi bottle. It equates to 2.75 psi per ball fired.

2) A low pressure gun set to 200psi input pressure gets 900 shots off a 3000psi bottle. It equates to 3.1 psi per ball fired.

The mag operates at about 60 psi max in the barrel as the ball is fired. This is achieved as the 400 psi is forced through a smaller ventrical called the power tube. This acts as a restrictor. As the air exits the other end of the tube into the barrel it pushes the ball forward causing the barrel volume to increase. Due to the increasing volume and the restricted flow, the pressure never gets above 60.

As for low pressure guns having a dump chamber the size of a pop can. The mag has a dump chamber of only .55 cubic inches. Many low pressure guns have dump chambers in excess of 2 cubic inches. Thats about 4 times the size.

[bjjb99]

A very, very long post...

FatMan wrote:
> There's no way the bolt/power tube is
> acting like a regulator. To do that the
> bolt would have to sit close to it
> rearmost position, the only place an
> air-tight fit occurs.

Regulators do not have to be air tight to do their job in a flow through system. All they must do is restrict the flow sufficiently, causing the downstream pressure to drop. This can be demonstrated with a garden hose and a couple of valves. Since the plunger in the center of the Mag's bolt does not completely exit the powertube, the bolt plunger restricts airflow. The amount of restriction is dependent on the location of the plunger relative to the power tube tip. This position is controlled by the strength of the return spring, the diameter of the bolt plunger, and the differential pressure in front of and behind the bolt. Hence, the bolt and return spring act as a regulator. The spring limits the travel of the bolt, which determines how much the airflow through the powertube is restricted.

> In a closed system, the change in
> pressure will only result from change
> in volume, or change in temperature.

Agreed, in the ideal gas case which is what we're assuming.

> The strength of the spring or weight of
> the bolt has no effect on the pressure
> other than some small amount of loss
> due to temperature.

I believe you're likely correct with regards to the bolt weight, but I'm not so sure about your claims regarding the spring. The only way to find out for sure would be to perform a test using different spring strengths.

> Where does the 350 psi go? Well, to
> start with a lot of it goes right where
> Bad Dave said it goes. Expand the
> chamber to include the powertube and
> bolt and the pressure goes way down.
> But its not enough you say? No its not.
> Think about the relationship of the
> bolt, the bolt plunger (not sure if
> that's the right term) and the power
> tube when the bolt is fully extended.
> There is a HUGE air space between the
> power tube and the body, and the air
> flow is open to that space.

The dump chamber in a Mag is around 0.55 cubic inches in volume, as stated in the following thread:

http://www.automags.org/forums/showthread.php?threadid=18939

I do not have my Mag in front of me, so I'm working from memory here. The outer diameter of the powertube is approximately 0.5 inches. The tube is approximately 1.5 inches long. The inner diameter of the powertube is approximately 0.25 inches. The interior volume of the tube would therefore be around 0.07 cubic inches.

The bolt consists of a cylindrical volume with a piston in the middle. The piston diameter is approximately 0.25 inches, and it's about 1.5 inches long. The inner diameter of the bolt is around 0.5 inches, and again, is around 1.5 inches long. The total volume inside the bolt (minus the portion occupied by the piston) would therefore be around 0.22 cubic inches.

I don't necessarily agree with your statement that the air can flow out of the powertube and back behind the bolt. It seems to me that the bolt plunger remains in the powertube; thus, the rear of the bolt itself remains around the exterior of the powertube, restricting airflow into the space behind the bolt. However, I'll assume that you're correct for this calculation.

The inner diameter of the Mag's body is around 1 inch. The region between the dump chamber and the bolt's rear face when the bolt is in its forward position is about 1.5 inches long. The total volume of this space (minus the portion occupied by the powertube) would therefore be around 0.88 cubic inches.

0.88 + 0.22 + 0.07 = 1.17 cubic inches.
Including the dump chamber volume brings this up to 1.72 cubic inches, a factor of right around 3 increase in volume. By your argument (and my estimates of sizes), the behind ball pressure would be around 130 psi. We've still got a factor of two somewhere that isn't taken care of yet. Even if we increase the lengths of the powertube, bolt, etc to 2 inches, we're still significantly on the low side in terms of required volume.

> But that STILL isn't where ALL of it
> goes. Who ever said that the pressure
> in the entire chamber/power tube/bolt
> system is evenly distributed. Sure, if
> you leave it long enough it will be,
> but you can rest assured that a
> pressure wave runs down the powertube,
> bounces around as it tries to avoid the
> bolt plunger, and then heads down the
> bolt to find the ball. By the time the
> REST of the air pressure gets there,
> the ball has begun moving down the
> barrel, further enlarging the volume so
> the bulk of the air doesn't seem to
> produce as much pressure at the site of
> the ball.

In the thread whose URL I listed above, Tom posted a picture of an Angel's pressure curve, showing when the ball began its motion relative to the peak pressure in the barrel. He has stated that the ball begins its motion pretty darn close to the top of the pressure curve, regardless of the type of gun being fired.

To reach the total volume (chamber + powertube + bolt + space behind the bolt + volume between bolt and ball somewhere down the barrel) which would drop the pressure down to around 60 psi, the ball would have to be around 5 inches down the barrel. This conflicts with the above paragraph in terms of when the peak pressure occurs.

The only explanation I can come up with is the plunger is restricting the airflow through the powertube, thus limiting the peak pressure. Since the bolt return spring strength is what is limiting bolt travel, we've got a regulator of sorts.


kilaueakid wrote:
> Bjjb99....just out of curiosity, you
> say the valve would need to be
> increased by a factor of 6x. If that is
> the case, than how do some of these lp
> guns do it without having a dump
> chamber the size of pop can?

An interesting question... here's my take on it. Many lp guns operate at a lower chamber pressure than the Mag (250 psi versus the Mag's 400 or so). Tom has stated that the Mag has one of the lowest behind-ball pressures in the paintgun world. His plot of the Angel's pressure curve shows a peak behind-ball pressure of around 100 psi for that gun. If you start with 250 psi and only drop to 100, you don't need nearly as large a volume increase as you would going from 400 psi down to 60. Thus, the lp guns that start with a lower operating pressure have no need for soda can sized volumes into which the pressure can expand. Athomas has already stated that initial chamber sizes in lp guns can be 4 times that of a Mag.

> Another thing that is apparent on the
> mag is that when chamber volume
> doubles, the operating pressure is NOT
> cut in half. It will go down, but is
> not a 50/50 relationship.

I'm pretty sure I disagree with this statement. Can you point me to a reference which explains what you're saying? Maybe it's the way it's worded, but it sounds like you're darn close to violating a thermodynamic law or two.

Athomas wrote:
> The mag operates at about 60 psi max in
> the barrel as the ball is fired. This
> is achieved as the 400 psi is forced
> through a smaller ventrical called the
> power tube. This acts as a restrictor.
> As the air exits the other end of the
> tube into the barrel it pushes the ball
> forward causing the barrel volume to
> increase. Due to the increasing volume
> and the restricted flow, the pressure
> never gets above 60.

That's a good concise description of the Mag's behavior. The only thing I might add is that the bolt plunger does not completely exit the powertube, further restricting the airflow.

BJJB

[bbqchicken8]

the pressure at which the bolt is moved and the pressure that is released firing the ball are the same in the automag. To be act gently on the paint, you want the bolt to run at a lower pressure. if you want to lower bolt pressure, your valve pressure(burst of air firing ball) is lowered too. this means that your automag is going to have decreasing efficiency when you lower the pressure. if you were to have a duel regulator in the AIR assembly, one that controls bolt pressure and one that controls valve pressure then you could possibly reduce pressure. this is most likely not feasible with the current valve assebly design. you would have to create a new gun. low pressure reduces efficiency. i get about 1250 shots per 20oz. with my impulse. i'd say that's respectable and i run at about 150psi with no lpr.

[Bad Dave]

RT has a 10ms bolt movement time, that is an absurd cyclic rate. The simple answer to the mags faults is that it cycles too fast.

Need to look at force not pressure. if they decrease the peak bolt movement speed, increasing the time then the mag will be easier on paint. so cocker has 4 fps and mag has 22fps bolt movement.

need to equate a max cyclic rate of 16bps with a bolt speed then make the spring/bolt combo to do this.

[kilaueakid]

Bjj99.........

Take a look at the new aluminum valves Tom is working on. Dump chamber is twice as big but pressure is still running at above 300psi.


kila

[FatMan]

Hmm, something funny.

bjjb99:

OK, well I'm not really clear on all of that. My definition of a regulator is not the same as yours. The plunger (among other things) limits the flow of air out of the dump chamber. I had accounted for that. The spring isn't going to have much effect on that, as the bolt will travel its length in any (working) case and the only real difference would be plunger in the tube versus plunger out of the tube. The airflow is restricted by geometry, not spring tension.

The dimensions you mentioned sound OK. The "pressure reduction" sounds OK. I wasn't expecting more than that (which is what I meant when I said "that STILL isn't where ALL of it goes").

What you missed is the part that comes after that. The bolt is only open for a few milliseconds. When the bolt closes again there is still pressure in the chamber. In other words not all of the pressure in the dump chamber is vented to the ball and out of the barrel.

So, I think we're saying almost the same thing, only I don't call that a regulator, I call it restricting air flow - and I don't believe spring tension has much of an effect on it. Some, but not enough to really make a big difference on efficiency. I also think you'll find that the plunger DOES exit the power tube for all practical purposes (the tip of the plunger may not extend past the tip of the tube, but it extends well past the beginning of the flared portion of the tip).

Call it what you want - that's fine with me, but that still doesn't add up to bolt mass or spring tension having a significant effect on efficiency - which I think was the point of the discussion.

FatMan

[xatle]

reducing bolt speed is easy, all you need to do is reduce the i.d. of the power tube and o.d of the bolt plunger, reduce the area of the bolt that is in contact with the pressure in the dump chamber and you reduce the total force felt by the bolt. the smaller power tube would also further restrict air flow, reducing the peak pressure behind the ball and increasing the amount of time needed to dump the chamber.
this may or may not be an improvement, but it wouldnt be too hard to experiment, just need a bolt with a removeable plunger and a handfull of different sizes of plungers and power tube assembly's, well, sounds easy anyway...

[AGD]

The thinking is on track here. Yes you can slow the bolt speed by reducing the size of the power tube but then it is too small to flow all the air needed fast enough. The other problem is that orings do not come in small enough size increments to experirment. You just cant say you want a .200 power tube and get an oring for it.

AGD

[bjjb99]

FatMan wrote:
> OK, well I'm not really clear on all of
> that. My definition of a regulator is
> not the same as yours.

Heck, for all I know, I may be calling it a regulator when it really should be called a restrictor. Perhaps a restrictor is simply a dynamic form of a static pressure regulator.

> The plunger (among other things) limits
> the flow of air out of the dump
> chamber. I had accounted for that. The
> spring isn't going to have much effect
> on that, as the bolt will travel its
> length in any (working) case and the
> only real difference would be plunger
> in the tube versus plunger out of the
> tube. The airflow is restricted by
> geometry, not spring tension.

In a properly functioning Mag, the bolt does not "bottom out" against the spring. See the following thread for reference:

http://www.automags.org/forums/showthread.php?threadid=24945

The bolt return spring is not fully compressed during a normal firing of the gun. The distance the bolt travels is limited by the strength of this spring. Put a very strong spring in there, and the pressure behind the bolt plunger can't overcome the spring force. Remove the spring, and the bolt will travel its full length. Spring strengths in between these two extremes will result in different amounts of bolt travel, particularly when air begins to work its way past the plunger and powertube tip. I agree the airflow is restricted by geometry; that geometry is determined by the force of the spring and the differential pressures on either side of the bolt. In such a complex interface as the powertube tip and bolt plunger, a few hundredths of an inch in either direction can significantly change the gas flow rate, and therefore the behind-ball pressure curve will be affected.

> What you missed is the part that comes
> after that. The bolt is only open for a
> few milliseconds. When the bolt closes
> again there is still pressure in the
> chamber. In other words not all of the
> pressure in the dump chamber is vented
> to the ball and out of the barrel.

Absolutely true. That's why I included the chamber volume in my calculation. The final pressure in the dump chamber is around 50-60 psi, which seems to indicate that the chamber gets sealed shortly after the behind-ball pressure reaches its peak.

> So, I think we're saying almost the
> same thing, only I don't call that a
> regulator, I call it restricting air
> flow

Sounds reasonable enough to me. I've been working under the premise that one can regulate pressure dynamically by restricting flow.

> and I don't believe spring tension has
> much of an effect on it. Some, but not
> enough to really make a big difference
> on efficiency.

I agree with you here. My original post was an effort to explain to kilaueakid where "the other 320-340psi goes". I am making no claim regarding efficiency, though it would be interesting to see how a Mag with an operating chamber pressure of 150 psi compares to a standard Mag.

I still think it would be an worthwhile experiment to see what the behind-ball pressure is during firing if one uses bolt return springs of different strengths. I'm guessing that the behind-ball pressure will be lower for stronger springs and higher for weaker springs, simply because of the flow restriction/regulation caused by the relative positions of the bolt plunger and powertube tip.

BJJB


kilaueakid, when I searched for it, I was not able to find a post describing the chamber volume for the aluminum valves. I believe I did see the 300 psi chamber pressure mentioned, though.

[FatMan]

OK

Originally posted by bjjb99


In a properly functioning Mag, the bolt does not "bottom out" against
the spring. See the following thread for reference:

Agreed. I've read the thread several times. I don't equate not
bottoming out with not clearing the power tube tip

The bolt return spring is not fully compressed during a normal firing of
the gun. The distance the bolt travels is limited by the strength of
this spring. Put a very strong spring in there, and the pressure behind
the bolt plunger can't overcome the spring force. Remove the spring,
and the bolt will travel its full length. Spring strengths in between
these two extremes will result in different amounts of bolt travel,
particularly when air begins to work its way past the plunger and
powertube tip. I agree the airflow is restricted by geometry; that
geometry is determined by the force of the spring and the differential
pressures on either side of the bolt. In such a complex interface as
the powertube tip and bolt plunger, a few hundredths of an inch in
either direction can significantly change the gas flow rate, and
therefore the behind-ball pressure curve will be affected.

I'm with you on all that, except maybe the last statement. On that I'd
have to say I just don't know. A significantly stronger spring would
prevent the plunger fully exiting, and the gun wouldn't work right. A
significantly weaker spring would let the bolt bottom out and it might
work, but would have a lot more recoil. Small variations in spring
strenth I doubt have much effect on pressure to the ball. Maybe some,
but then you adjust the chamber pressure and that goes away.

Absolutely true. That's why I included the chamber volume in my
calculation. The final pressure in the dump chamber is around 50-60
psi, which seems to indicate that the chamber gets sealed shortly after
the behind-ball pressure reaches its peak.

Again I doubt that's true. I will say very loudly I'm guessing here,
but I'd bet the bolt doesn't return until after the ball is out of the
barrel. The ball is gone in about 6ms, and I have heard figures like
10ms for bolt return, though I'm not sure those are correct (Tom, do you
want to clue is in or this, or do you prefer to sit back and just watch
us stab in the dark? :-) ). I think the flow restricting issues are a
combination of the chamber to power tube to bolt system, and that the
exact position of the bolt (governed by the spring) doesn't have much to
do with pressure that meets the ball, provided the bolt at least goes to
a position within tolerance.

I agree with you here. My original post was an effort to explain to
kilaueakid where "the other 320-340psi goes". I am making no claim
regarding efficiency, though it would be interesting to see how a Mag
with an operating chamber pressure of 150 psi compares to a standard
Mag.

I still think it would be an worthwhile experiment to see what the
behind-ball pressure is during firing if one uses bolt return springs of
different strengths. I'm guessing that the behind-ball pressure will be
lower for stronger springs and higher for weaker springs, simply because
of the flow restriction/regulation caused by the relative positions of
the bolt plunger and powertube tip.

Yeah, like I said - its not a really fundamental argument, just friendly
hashing! I'm all for experiments, but for now I'll hold to my
prediction that the spring tension isn't a major factor in the design.
But, I could be all wrong.

Oh, and its sure is lot more fun to discuss this with you than someone
without a clue! You've made me think about what I'm saying!

FatMan

[bjjb99]

FatMan wrote:
>
> Agreed. I've read the thread several
> times. I don't equate not bottoming out
> with not clearing the power tube tip

That makes some sense, given that the bolt plunger is somewhat recessed inside the bolt. I'm going to have to take some accurate depth measurements for the Mag's body and compare them to the bolt length, the spring thickness when fully compressed, the power tube length, and the length of that bolt plunger. We should be able to determine how far out of the power tube tip the plunger travels in a "bottoming out" condition. That would at least put some geometrical limit on the airflow restriction.

Now where did I put my calipers...

> Yeah, like I said - its not a really
> fundamental argument, just friendly
> hashing! I'm all for experiments, but
> for now I'll hold to my prediction that
> the spring tension isn't a major factor
> in the design.

The thing that bugs me all the time is that there are all these really interesting puzzles out there, and when I come up with an experiment to take a look at one of 'em, I end up not having the resources to perform the test! I really want to stick a pressure sensor (or a series of 'em) in a barrel and fire some paint, but I lack the N-thousand dollars necessary for an accurate and precise testing system! Grrrr.

Oh, for anyone reading this, cash donations are accepted, but they're not tax-deductible. :)

> But, I could be all wrong.

Who knows... we both could be wrong and it's really just a bunch of magic elves doing the work.

> Oh, and its sure is lot more fun to
> discuss this with you than someone
> without a clue! You've made me think
> about what I'm saying!

It's always good to explore a topic with someone who makes me exercise my brain. If I ever get a testing rig set up, I'll be posting my results .

BJJB

[FatMan]

Now THERE'S an idea!

Now why didn't I think of that! If someone would just put up the funding we could put all of this to rest! bjjb99 can do the physical experiments, I can do the computational model.

I've got some experts in fluid dynamics here, and we've discussed several models of the Mag. All I need is a graduate student for the summer to get the coding done and we'd have something. Including overhead and all we'd be looking at $10K - which is REALLY cheap by industry standards if you think about it. I have the machine to do it on (500 1GHz P3 cpus) and the people with the know how. Just need the cash to pay the grad student.

Tom, you want to invest in a little research?

Now, if there was some way that the federal government would get interested in this ... hmmm ... NASA? ... DOE? ... DOD? Maybe. But they'd want to model REAL guns.

Nope, Tom, I think you're it! Where do we apply for a grant?

FatMan