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piston troubles

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 1:42 pm

clemsonguy1125 wrote:I used it in an exact diameter pipe, I greased it up and it was fine, You could always sand it a little. I tried it with hot glue and had issues but its been done plenty of times before. this is the how I made the stick guns piston, ill take a picture later if you want.


its fine im doing the epoxy casting now
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Unread postAuthor: SpudBlaster15 » Mon Jul 19, 2010 1:55 pm

Technician1002 wrote:A light weight piston is faster than a heavy piston.


True, but with large diameter pistons and/or high chamber pressures (High opening forces), the effect isn't as significant as intuition would suggest. I've ran calculations for all of the high pressure copper piston cannons I've built, and in most cases, doubling the piston mass only increases the valve opening time by ~0.5-1ms. Probably not enough to significantly affect launcher performance.

urgle the danish cow wrote:ok im going to remake the piston using the idea clemsonguy brought up but i know that if i did an epoxy casting in the exact same diameter pipe that it will not fit easily into the cannon. what should i do


Apply grease to the inner surface of the mold before pouring the epoxy, and the casting will pop right out once cured. I used this process for casting hot glue pistons ages ago, and they turned out well. If you do it properly, the piston should have a rather nice fit in the valve body.
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Unread postAuthor: Technician1002 » Mon Jul 19, 2010 2:20 pm

SpudBlaster15 wrote:
True, but with large diameter pistons and/or high chamber pressures (High opening forces), the effect isn't as significant as intuition would suggest. I've ran calculations for all of the high pressure copper piston cannons I've built, and in most cases, doubling the piston mass only increases the valve opening time by ~0.5-1ms. Probably not enough to significantly affect launcher performance.


This is true for higher mass projectiles. With low mass projectiles and shorter barrels, the low mass piston does make a difference. My goal with most pistons is to have them no more than twice the mass of the projectile. As pistons become more massive, a light projectile can be much further down the barrel by the time the valve is fully open. The lighter the projectile and the shorter the barrel, the more important this is.

Below is an example setup where a light piston pays off. Projectile was a piece of candy. The barrel is short. The piston and projectile are about the same mass. The valve is fully open in the same amount of time the projectile takes to move it's length in the barrel. For the rest of the travel, the valve is wide open.

The cannon below is only moderate pressure <100 PSI. Low pressure or high pressure, the distance the projectile moves as the valve opens scales to the rate the valve opens, so high or low pressure, the mass ratio does play an important role in how far the projectile moves before the valve is fully open.
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Marshmallow cannon vs can
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Close up of the damage
Last edited by Technician1002 on Mon Jul 19, 2010 3:10 pm, edited 2 times in total.
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Unread postAuthor: SpudBlaster15 » Mon Jul 19, 2010 3:04 pm

Technician1002 wrote:Low pressure or high pressure, the distance the projectile moves as the valve opens scales to the rate the valve opens, so high or low pressure, the mass ratio does play an important role in how far the projectile moves before the valve is fully open.


Considering the piston travel required to obtain maximum valve flow (Varies from about 1/4 to 3/4 of the barrel port diameter, depending on the configuration of the valve), the mass ratio isn't too important.

For example, the 0.7" porting T valve on my high pressure copper pneumatic requires a piston travel of ~0.5" for maximum flow. Even if you assume the forces acting upon the projectile and piston are of the same magnitude (Which is not the case, as the piston is larger in diameter), and ignore the numerous other factors, a piston with 10 times the mass of the projectile could be used, and the valve would still fully open before the round had traveled 1/4 of the length of its relatively short 20" barrel.

Obviously the effect will be more substantial with longer piston travel distances, but the use of a more realistic piston mass brings it back down to the insignificant range.

Also, I see your gumball pierced soda can, and raise you one paintball pierced soda can, done with a medium length 4ft barrel, and a M<sub>piston</sub>:M<sub>projectile</sub> ratio of 7:1. :D

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Unread postAuthor: Technician1002 » Mon Jul 19, 2010 3:32 pm

Nice job. I'll have to get a paintball barrel and do some comparison shots at the same pressure.

Considering a valve is considered fully open in 1/4 the diameter, the piston can be up to 4X the mass of the projectile and still be considered fully open in less than 1 diameter of projectile travel.

With a large diameter piston, the area increases so high performance can still be reached with larger diameter and therefore heavier pistons. Attaching a 1 inch barrel onto my 2 inch cannon would be an extreme example. The smallest barrel I attach on the 2 inch is a golf ball barrel. On the 1 inch the smallest barrel is a 1/2 inch barrel for AA batteries. I did put a AA battery through the steel shell of a washing machine with only 100 PSI.
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Unread postAuthor: SpudBlaster15 » Mon Jul 19, 2010 3:54 pm

Technician1002 wrote:Nice job. I'll have to get a paintball barrel and do some comparison shots at the same pressure.


For comparison purposes, that shot was done with a 30in<sup>3</sup> chamber at somewhere between 75-85PSI. It was a few years ago, so I don't recall the exact figure, but I do remember that the $8 bike pump wouldn't make it past 90PSI without the hose connection exploding.

Considering a valve is considered fully open in 1/4 the diameter, the piston can be up to 4X the mass of the projectile and still be considered fully open in less than 1 diameter of projectile travel.


1/4 of the diameter is more suited as a model for coaxial launchers. With close ratio T valves, I typically model the inlet port as a rectangle or circle, which gives much larger piston travel values.
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