SpudFiles

hi there,

i made a topic in the pneumatic section, for a launcher im building, and now im a bit stuck with ggdt.

i made my own hammervalve(modified coupling), and tried to use the ggdt instruction on their site to fill in the data of my hombuild valve, but for some reason the results i get are rather different from my real-life test...

according to ggdt my setup is worthless, 50fps and just around 11j muzzle-energy...

in real life i dented the steel garage door, and shot clean trough soem abs container... certainly not the result ggdt predicted.

this is my valve:



im in the dark about the valeus to be filled in now, does anybody has some ideas how to determine the correct data?

I think you need to post more information:
a screenshot of the ggdt window, some dimensions of the valve and so on

valve stem bare: 14grams
valve stem, added weight from adjustscrew: 33grams (is changable, screw weighs 18 grams)
spring tension compressed +/- 300grams
seat diameter 8mm

hammerweight and spring tension incuding travel distance is all yet to be determined.


screenshot with my guesstimates:


do these figures make sense?

quark1980 wrote:hammerweight and spring tension incuding travel distance is all yet to be determined.

Then there's very little point in running GGDT yet.

On a hammer valve, after the valve seat diameter the most important variables are the mass of the hammer, the tension of the spring, and the distance the hammer travels. Without those three asking GGDT to make predictions is about like asking a car guy how fast your car will go... Without even telling him what kind of car you have.

D_Hall wrote:

quark1980 wrote:hammerweight and spring tension incuding travel distance is all yet to be determined.

Then there's very little point in running GGDT yet.

On a hammer valve, after the valve seat diameter the most important variables are the mass of the hammer, the tension of the spring, and the distance the hammer travels. Without those three asking GGDT to make predictions is about like asking a car guy how fast your car will go... Without even telling him what kind of car you have.

isnt the point of runnig ggdt at this point for me, to know what hammer construction i should be building?

i `m not sure of at least 2 variables, namely the flow coef of my homebuild valve, and the hammer force...

the weight of the hammer, and the distance is simply on a "build as needed basis"...

isnt the point of runnig ggdt at this point for me, to know what hammer construction i should be building?

it is
What d_hall is saying is that you need to know exactly what each variable means
...and you need to know the exact dimenshions of your valve, force excerted by the spring, mass of the hammer etc. before you start your simulation

Sure you can spend some time playing with values, like increasing the mass of the hammer by 50% to see what effect it would have on performace etc., but you have to know what you're doing


ohh and as I suggested earlier - get a QEV - they are fairly easy to model in ggdt so you'd have a chance to learn some more basic stuff - like how changes in pressure, chamber volume, barrel lenght, barrel diameter etc. affect performance...

to be honest I don't think I can model hammer valves myself as they are fairly complicated... stuff that I mentioned in the paragraph above is already pretty difficult to grab at first so playing with hammer valves might not be the best idea for your first gun (or first attempt at understanding ggdt)

POLAND_SPUD wrote:

isnt the point of runnig ggdt at this point for me, to know what hammer construction i should be building?

it is
What d_hall is saying is that you need to know exactly what each variable means
...and you need to know the exact dimenshions of your valve, force excerted by the spring, mass of the hammer etc. before you start your simulation

i know, but if i knew all the exact valeus to put in there, i wouldn`t ask

Sure you can spend some time playing with values, like increasing the mass of the hammer by 50% to see what effect it would have on performace etc., but you have to know what you're doing

i have fiddled with those valeus and noticed the changes, my main reason to ask questions is if my valeus would be fairly chosen. the effect of most changes seem rather logical

i can take measurement, that`s what i did, but the harmmer force is kinda difficult to get grasp of

ohh and as I suggested earlier - get a QEV - they are fairly easy to model in ggdt so you'd have a chance to learn some more basic stuff - like how changes in pressure, chamber volume, barrel lenght, barrel diameter etc. affect performance...

i know, i can alway go that route, but for now i dont want to bin this one...

to be honest I don't think I can model hammer valves myself as they are fairly complicated... stuff that I mentioned in the paragraph above is already pretty difficult to grab at first so playing with hammer valves might not be the best idea for your first gun (or first attempt at understanding ggdt)

i see. but isn`t there a formula to determine say "hammer force"
i know its not the static spring force. othewise it would state as such right?

i`m just in the dark if my chosen valeu for hammer force resembles the truth...

so my question would be "how to determine hammer force"

i see. but isn`t there a formula to determine say "hammer force"
i know its not the static spring force. othewise it would state as such right?

You might find this useful -> http://www.thehalls-in-bfe.com/GGDT/

Valve -> Hamr Force
(Not shown in screen shot) This value represents the average force exerted by the spring in a hammer valve

POLAND_SPUD wrote:

i see. but isn`t there a formula to determine say "hammer force"
i know its not the static spring force. othewise it would state as such right?

You might find this useful -> http://www.thehalls-in-bfe.com/GGDT/

Valve -> Hamr Force
(Not shown in screen shot) This value represents the average force exerted by the spring in a hammer valve

so if i pull/push the spring i plan to use and take some measurements i can use those valeus there

that should be helpfull