mark.f wrote:spudtyrrant wrote:@ mobile general rule for barrel sealers is 1/2d thats what mr.crowley uses and thats i have used. i have never actually seen the math that proves this rule nor have i seen any comparisons between 1/2d piston travel and 1/4d travel would like to know how you guys know whether it is overkill or not and if it is overkill what does it matter this cannon is going to be large enough where if i add another inch to the travel it shouldn't make too noticeable a difference
@spudamine yeah my piston is not likely to come to those kind of forces it also looks like their was a lot of bubbles in the glue i will be casting mine into a mold all at once so it will have much less bubbles also look at how the pvc shattered it looks like the pvc didn't come through much better than the hot glue and that is what most people have suggested for my piston. with the glue i wouldnt have to go to the trouble of cutting peices of plywood to cover each end of a fitting plus it will take a lot less time
I generally use a little longer than 1/4 of the port diameter for my valves.
I just read your link and for the most part the math is right on, but there is a slight flaw in the theory.
The orifice limits the flow. It's the COF of the valve. Having the piston opening the bare minimum, there would be two stacked orifices. One to get into the barrel and one to get past the piston. Opening the piston a little further reduces or makes insignificant the effect of one restriction in the flow. I don't consider a piston fully open until it is 1/3 to 1/2 the diameter of the piston. As an example, the valve orifice on my 2 inch valve is 3.14 square inches. The ports into the valve body has 6 square inches of area.
Some thoughts on the piston.. It has to do with the math again.
In going from a 2 inch to 4 inch (double size) the volume and mass increase by a cube where the cross section increases by only a square.
In simple terms a 4 inch piston has 4 times the area of a 2 inch piston but 8 times the mass. Think about it carefully. In opening it moves twice as far so at the end of the stroke, it is traveling the same speed as the smaller piston, but with 8X the mass. Care must be given on a bumper for this and the forces.