Tx200 zero transfer port

[bigtoe01]

Dave, what is your feeling about the zero transfer port setup....worth it or not?

I personally feel we need a port,although it looks to be short is best but wondering on your thoughts.

[wonky donky]

Dave, what is your feeling about the zero transfer port setup....worth it or not?

I personally feel we need a port,although it looks to be short is best but wondering on your thoughts.

I'm with you on this one Tony, I feel a short port smaller in diameter than barrel bore is needed. I think a certain amount of control is gained by having a port, I've always thought this & am even more convinced now. I don't think there's anything to be gained except information from this exercise, I certainly don't think it's the way to go.

The firing cycle is too slammy it doesn't show on the chrono, you can feel the shot cycle does change, not every shot but randomly. I suspect this is from variation in pellet fit/size minute though it might be. I have no idea what accuracy is like. I wouldn't mind betting the rifle is not hold sensitive?

I can see the argument coming in favour of lighter piston etc etc etc to cure the slammyness. I'm not convinced, I may make one to try just for completeness?

I will do the 1mm Jon suggested & the 6mm then push the sized pellet back to the breech Mick mentioned & anything else they or others may request as well as some accuracy tests.

We will have to see what happens.

[Black Beard]

You might find the slammyness is from the second forward stroke, after the piston has bounced and the pellet has left the barrel. Try it with heavier pellets with wider skirts. These will resist the push of the air harder, so the piston bounces more and there will be more energy in the second forward stroke. So it would be eveb more slammy with heavier pellets (which is backwards to being slamm on the first forward stroke).

[Shed tuner]

Good point BB - or conversely try adding some piston weight to help compression on the first stroke, 'cos that's a very small space to compress air into....
PS The SCR is also very high with that stroke... worth either shortening the stroke 10mm or adding in the 25mm washer to get a 1mm or 2mm transfer port - that could yield a practical spec ?

[Orlando D.]

I'm with you on this one Tony, I feel a short port smaller in diameter than barrel bore is needed. I think a certain amount of control is gained by having a port, I've always thought this & am even more convinced now. I don't think there's anything to be gained except information from this exercise, I certainly don't think it's the way to go.

The firing cycle is too slammy it doesn't show on the chrono, you can feel the shot cycle does change, not every shot but randomly. I suspect this is from variation in pellet fit/size minute though it might be. I have no idea what accuracy is like. I wouldn't mind betting the rifle is not hold sensitive?

I can see the argument coming in favour of lighter piston etc etc etc to cure the slammyness. I'm not convinced, I may make one to try just for completeness?

I will do the 1mm Jon suggested & the 6mm then push the sized pellet back to the breech Mick mentioned & anything else they or others may request as well as some accuracy tests.

We will have to see what happens.

I get the feeling a very tight fitting 4.53 or a true Crosman single box Premiere may help as they will fit very snuggly in my rifles.. This should allow a bit more pressure to develop initially.... Now if it is a second slam as BB said, then the short tiny port may be the only answer... I think the RWS Superdomes were also large..

You could also puposefully flare the skirts on a couple pellets for testing right now.. Who knows maybe this system may work great, but needing a larger pell then is currently available to work.. say a 4.54-4.55..
O.

[wonky donky]

I was thinking you could also do the same test 1mm into the bore, the difference between the two being the "short port effect", rather than the "no lead-in effect"

Velocity drop of around 60fps......if i discount one shot spread was 24fps.....included it was 66fps

That's what I was thinking as well Jon

Another source of data is Jim's old article from Airgunworld where he increased the transfer port length on a 77 bit by bit.

Another way for us to get a baseline off Dave is for him to push a pellet 6mm up the barrel and then push it back to the breech and then take the no transfer port velocity of the sized pellet.


All the best Mick

100fps plus velocity drop & variation of 44fps

You might find the slammyness is from the second forward stroke, after the piston has bounced and the pellet has left the barrel. Try it with heavier pellets with wider skirts. These will resist the push of the air harder, so the piston bounces more and there will be more energy in the second forward stroke. So it would be eveb more slammy with heavier pellets (which is backwards to being slamm on the first forward stroke).

Barracuda Gols weighed at 10.57gns 115fps velocity drop spread of 7fps

[Shed tuner]

Cheers Dave, so 60 fps due to lead in, 100 fps due to lead in plus longer port, so the port is giving ~ +40 fps

[bigtoe01]

so to conclude this, a transfer port is needed, we can not rely on the pellets to form the restriction to build the pressure consistently.

I am now thinking bore the end plug to form a 2mm long port, use a 300S conical breech seal and machine the barrel to suit, unless we can decide on a similar setup to what Dave is already running that would have zero air waste...i have one idea but will need to do a test machining on a scrap barrel to be sure it would work...it would be similar to how the HW100 barrels used to seal but with a little gripper groove for the O ring.

Boring the end plug maybe easier on the mill using an end mill, pilot drill first to size on the lathe then drop the end mill in to just flatten the bottom of the hole.

2mm is needed for strength, if we could get the barrel to sit exactly onto the face of the TP the O ring idea i have would work beautifully.

[Black Beard]

Wonky

Did the air gun feel slammyer with the heavier pellets and did the power go up?

I am guessing that:

Power up, less slammy = piston crash on first stroke forwards.
Power down and more slammy = piston stopping hard on second forward stroke.

No transfer port is going to make the air gun very sensitive to pellet weight and release point (skirt consistency). It might work better with match quality pellets.

The pellet starting down the barrel could make enough lost volume to make the piston crash or inconsistencies in its starting position could upset things.

BB

[Snooper601]

so to conclude this, a transfer port is needed, we can not rely on the pellets to form the restriction to build the pressure consistently.

I am now thinking bore the end plug to form a 2mm long port, use a 300S conical breech seal and machine the barrel to suit, unless we can decide on a similar setup to what Dave is already running that would have zero air waste...i have one idea but will need to do a test machining on a scrap barrel to be sure it would work...it would be similar to how the HW100 barrels used to seal but with a little gripper groove for the O ring.

Boring the end plug maybe easier on the mill using an end mill, pilot drill first to size on the lathe then drop the end mill in to just flatten the bottom of the hole.

2mm is needed for strength, if we could get the barrel to sit exactly onto the face of the TP the O ring idea i have would work beautifully.

Hi Tony, Assuming you have a milling cutter or flat bottomed drill of exactly the right o/d you could hold the milling cutter in a collet chuck fitted in the tail stock and save having to re-mount the plug on the mill.
Do it all in one operation then, especially if you fit a dro to the tail stock like the project in Model Engineers' Workshop May edition where they use a £10 digi vernier.
Alternatively there are some small diameter indexable boring bars, 5mm, available now that would make facing the bottom of the pocket easy.

Cheers

John

[Prof51mtw]

I’ve just caught up with this brilliant thread and, belatedly, thought it might be worthwhile putting my ‘four penny worth ‘in, because, many years ago, I did some consultancy work for a major air gun maker looking at similar issues, which threw up some interesting results. A lot, but not all, of this material has been published in AGW over the last couple of years, but not in one article.

Anyway, for what it’s worth……………………………………….

In order to obtain a consistent transfer of energy from the compression chamber to the pellet, the energy pulse in front of the piston seal has to be transferred to the base of the pellet in such a way that the pellet is always accelerated in the same way from shot to shot. In order to achieve this, two things have to happen: (1) the air must give up as much of its heat energy as possible (i.e. it must be forced to cool as rapidly as possible) so that the maximum amount of thermal energy is converted into flow energy; (2) the pressure pulse in the compression chamber must be ‘stretched’ as it transfers into the barrel behind the pellet skirt in order that pressure, and hence acceleration, is delivered to the moving pellet throughout the air’s expansion phase as the pellet travels up the barrel.

The most efficient way of converting the heat energy, which is put into the air during the compression stroke, into flow energy is to try and maximise the mass flow rate and temperature drop of the air flowing out of the compression chamber. In order for this to happen, the air flow must be induced to choke (achieve Mach 1 for the air at peak temperature in the flow path). When this occurs, not only is the energy transfer maximised, but the pressure pulse in the barrel attains a limiting value and the pulse looks like a rectangular wavelet, the length of which is set by the transfer port diameter. (There is a single article on how this works, but I don't have the reference to hand). This condition provides a virtually constant mass flow rate of air into the barrel and the shape of pressure pulse ensures consistent pellet acceleration for the duration of the pulse.

I think a couple of truths emerge from this. We do need a transfer port to provide the choke. Within sensible limits, the diameter of the port is more important than its length, although a minimum length is required to establish choking: the shorter the stroke, the smaller is the transfer port diameter required for maximum energy efficiency.

With regard to friction losses in long transfer ports; when the experimental work on this was originally done, I did suggest to the manufacturer involved that it might be worth taking a look at port profiling, along the lines of de Laval nozzles, but this was never taken up, owing to the potential manufacturing costs involved. I still don’t know whether this would be worthwhile, but I seem to remember that John Whiscombe was keen on it at the time, although Gerald Cardew had doubts. Maybe something to look at in the future?

Mike

[wonky donky]

Wonky

Did the air gun feel slammyer with the heavier pellets and did the power go up?

I am guessing that:

Power up, less slammy = piston crash on first stroke forwards.
Power down and more slammy = piston stopping hard on second forward stroke.

No transfer port is going to make the air gun very sensitive to pellet weight and release point (skirt consistency). It might work better with match quality pellets.

The pellet starting down the barrel could make enough lost volume to make the piston crash or inconsistencies in its starting position could upset things.

BB

Power went down by about 2fpe & the firing cycle felt just the same, which I find strange? the firing cycle I mean.

[wonky donky]

Anyway, for what it’s worth……………………………………….

In order to obtain a consistent transfer of energy from the compression chamber to the pellet, the energy pulse in front of the piston seal has to be transferred to the base of the pellet in such a way that the pellet is always accelerated in the same way from shot to shot. In order to achieve this, two things have to happen: (1) the air must give up as much of its heat energy as possible (i.e. it must be forced to cool as rapidly as possible) so that the maximum amount of thermal energy is converted into flow energy; (2) the pressure pulse in the compression chamber must be ‘stretched’ as it transfers into the barrel behind the pellet skirt in order that pressure, and hence acceleration, is delivered to the moving pellet throughout the air’s expansion phase as the pellet travels up the barrel.


I think a couple of truths emerge from this. We do need a transfer port to provide the choke. Within sensible limits, the diameter of the port is more important than its length, although a minimum length is required to establish choking: the shorter the stroke, the smaller is the transfer port diameter required for maximum energy efficiency.

With regard to friction losses in long transfer ports; when the experimental work on this was originally done, I did suggest to the manufacturer involved that it might be worth taking a look at port profiling, along the lines of de Laval nozzles, but this was never taken up, owing to the potential manufacturing costs involved. I still don’t know whether this would be worthwhile, but I seem to remember that John Whiscombe was keen on it at the time, although Gerald Cardew had doubts. Maybe something to look at in the future?

Mike

Thanks Mike, the heat transfer is something that wouldn't have entered my head for a long time if ever tbh. The stretching of the pressure pulse, although I couldn't have explained it that way is the conclusion I have reached, it's in my head as constant continuous driving force.

As far as friction loss that's something I would never consider either, I know it happens but it's over my head.

As for port experimentation......well now is the time.
Thanks for your imput

[wonky donky]

Hi Tony, you could hold the milling cutter in a collet chuck fitted in the tail stock and save having to re-mount the plug on the mill.
Do it all in one operation then, especially if you fit a dro to the tail stock like the project in Model Engineers' Workshop May edition where they use a £10 digi vernier.

Cheers

John

i
If I'm reading it right, that's how I would do it John. It's how I did the compression plug.

[Snooper601]

i
If I'm reading it right, that's how I would do it John. It's how I did the compression plug.

Yes Dave, I bought an ER32 collet set with a MT3 chuck, for the Clarke lathe, and before I used them I bought a Hardinge lathe with a MT2 tail stock. another chuck to buy then

I've been looking at a Boxford Vm30 mill which iis int30 so the MT3 won't fit that.

I must make space for the Hardinge so I can experiment like everyone else.


Cheers

John