FIRST air cannon robots PVC??

[Andrew Schuetze]

When I get a bit more time, I'll add links to the various threads on C.D. on this topic.

My question is how / where does one get a definitive answer as to the pneumatic pressure rating of schedule 40 PVC? I was intrigued by the team fusion air canon last year and then decided to add one to our 2005 robot this August. I'll post a picture of the prototype mounted on my classroom workbench. It works okay. Based upon three weeks of development, what I perceive to be limiting the performance is the restriction of air flow from the storage tank via the 0.5 inch fitting. Everything else is the same 1" diameter as the sprinkler valve and then expands to the 3" diameter at the back of the muzzle.
I've seen about three cannon systems which are using PVC pipe as the pressurized air storage system here on C.D. People have commented that PVC pressure ratings are for water systems and in fact the fine print at the store indicates use for water only.

My fear is that a bump into a doorway or drop on the floor may start a crack in the PVC pipe and eventual lead to catastrophic failure.

How do I know if I am being overly cautious or just being prudent?

APS


Toltech prototype

[KenWittlief]

the reason its only rated for water is pressurized water does not store up energy. Pressurized air does. If a water pipe breaks then water sprays out. If a pressurized plastic air tank breaks, then fragments of plastic spray the room, along with the concussion of the released air.

FIRST has been very restrictive concerning the use of custom / plastic pressure tanks on the robots: they are strictly forbidden. I gotta believe a lot of thought went into that decision. Putting stuff on a FIRST robot after the season ends that violates rules that exist for safety makes me a little worried (even if other teams have already done so).

Id recommend one of those portable tire air tanks you can get at Walmart for $60 or so. The ones you fill up at a gas station with compressed air. They are tested well above their rated pressure (rated at 120psi?), and they are steel, so if you drop one the seams wont burst open.

[Dan Richardson]

I Believe the rating is posted on the side of the pvc? I remember that it was deffinately on the side of the pvc glue bottle for the joint strength. The pvc is usually not the concern however it is the joints. Improper usage of pvc glue can end up in caps flying off in a spectacular like fashion. Another common problem with pvc is under sunlight it can become brittle greatly reducing the strength of the pvc. If put under pressure you can have a failure to occur, and thats no good.

Regardless its still a cheap and effective option. For a freshman engineering project I used pvc to build a marsh mellow launcher. I have considered modifying it for t-shirts as it would work effectively but It would be a one shot deal and we'd like to get multiple shirts fired before pressurizing.

In my marsh mellow launcher I only had one failure and it was due to improperly gluing. In this case I actually did not let the resin settle before I tried it he he. But after regluing I was able to consistently hold about 100 lbs of pressure in a 2 ft long by 2" diameter pvc pipe. Its fun to shoot a marsh mellow at ~ 280 fps. It is however not fun to get hit by one :-)

I've seen shirt launchers being launched at approximately ~40 psi successfully and sometimes almost too far.

Without testing however fatigue could be a concern, it may be something you'd want to consider rebuilding every so often.

[Cody Carey]

We have had our air tank pressurized to 80psi and run the end cap into a wall at about half speed of our robot (6-7mph)... It was an accident, and as soon as it happened I was sure that the tank would explode, but nothing seems to have come of it. The tank also tends to be manhandled when it is taken off of the robot, and that has never had an ill effect. We test the safety of the tank by isolating it from people and pumping it up to 110psi, which is 30psi higher than we use to actually shoot the t-shirts and footballs.

The main thing that I would say be careful about is what glue you use. Someone in a physics class at our school tried to emulate our cannon, and used glue that wasn't rated for pressure... he broke a window out of one of the doors.


Sched 40 pvc holds up fine for our uses, and we have never had a problem.

[artdutra04]

The smaller the diameter of PVC, the more pressure it can stand. In this table, you can see the maximum operating pressure and the minimum burst pressures for various grade and diameters of PVC and CPVC pipe. A 2" Sch 40 PVC pipe has a maximum operating pressure of 166 PSI. If need arieses, instead of using a larger diameter pipe to store more air, I'd just use multiple 2" diameter accumulators clustered together. Also, try to use Schedule 80 whenever possible; the extra wall thickness will certainly help lessen the danger of the accumulator rupturing.

Also to note: PVC loses a lot of strength as the temperature rises. If you ever plan on using your air cannon in 100^oF weather, PVC's maximum operating pressure drops to only 60% of it's original strength. (A 2" PVC accumulator would only have a maximum operating pressure of 99.6 PSI. In hot climates, this may become a major issue.)

[DonRotolo]

Originally Posted by Andrew Schuetze

My fear is that a bump into a doorway or drop on the floor may start a crack in the PVC pipe and eventual lead to catastrophic failure.

How do I know if I am being overly cautious or just being prudent?

You are in fact being prudent.

As Ken noted, a catastrophic failure in a pneumatic system can be dangerous.

[EDIT] I agree with the next post, don't use PVC for air pressure over lung power, ever.

Don

[Gdeaver]

PVC pipe fragments do not show up on X-rays. Doctors are forced to use the hamburg method of fragment removal. PVC pipe is not rated for air for good reason.

[ChuckDickerson]

Originally Posted by KenWittlief

Id recommend one of those portable tire air tanks you can get at Walmart for $60 or so. The ones you fill up at a gas station with compressed air. They are tested well above their rated pressure (rated at 120psi?), and they are steel, so if you drop one the seams wont burst open.

Another real cheap and SAFE option is to make your own steel air tank using an old freon tank. Go to a local HVAC service place and ask them for and empty freon tank. They always have one and you can probably get it for free. Then head to your local auto parts store and get a Freon tank conversion kit like this: http://www.redboxtools.com/detail.cfm?productID=1616. I have bought kits like this locally at NAPA for ~$10 but haven't bought one in a while. You just screw the adapter on the existing fitting on the freon tank and bam, instant safe steel pressure tested air storage tank. Freon tanks come in a couple of different sizes so if you prefer get a small one for the robot. Paint the tank to suit you needs.

BTW: My office is in a building next to a wet lab. The wet lab needs compressed air. Nobody wanted the air compressor near their office because of the noise so they installed the compressor out behind the building and ran the air feed line through the ceiling of the building right over my office to the lab. The installers used 3/4" PVC for the air line which is pressurized to about 120 PSI most of the time. I voiced my concerns to the installers about using PVC rather than some other pipe like copper tubing or black pipe or something and they said "don't worry about it, it is fine, we do it all the time, the side of the pipe says 600 PSI" or something like that. So far, the pipe hasn't blown up over my head but I still worry about it. I don't know why but I still have visions of the pipe exploding and showering me with a million little shards of white plastic one day.

[Dan Richardson]

Often plummers that need to run pvc on the oustide the house will cover the pvc coat the pvc with some sort of plastic polymer. The reason being is so that the pvc is not as exposed to the sun. Additional reason it adds additional support.

But the glue/resin as some have pointed out is important. I remember the "blue" pvc glue was rated for something like 180-200 psi max operating pressure.

When applying pvc glue make sure you are careful to first clean the ends of your pipe that you are applying. Bevel the edges slightly with paper or a file. Then proceed to place a thin layer on both the inside of the cap and the pvc you are using. When insert turn the pvc slightly, about a little less than a quarter of a turn.

Most important is beveling the edges and applying to both sides, often when cutting the pvc it becomes very sharp and jagged. There is often a lip on the edge and when inserting into a connector the bevel can push the glue out. So instead of an even gluing you have it bunched in certain spots. Not good for adhesive properties.

[Donut]

Another easy alternative to using the PVC for storage is to use the storage tanks FIRST gives in the kit (and if you've been around for a few years, you've probably accumulated a few of them).

Our team is developing a cannon that uses almost nothing but KOP pneumatics, with the exception being our barrel and its mount.

One question regarding the Festo Single Solenoid valves; where is the documentation for the maximum operating PSI? The FIRST documents only list the minimum, and I cannot find the specs sheet for it through google or on Festo's site.

[Doug G]

Originally Posted by Donut

Another easy alternative to using the PVC for storage is to use the storage tanks FIRST gives in the kit (and if you've been around for a few years, you've probably accumulated a few of them).

Our team is developing a cannon that uses almost nothing but KOP pneumatics, with the exception being our barrel and its mount.

One question regarding the Festo Single Solenoid valves; where is the documentation for the maximum operating PSI? The FIRST documents only list the minimum, and I cannot find the specs sheet for it through google or on Festo's site.

This depends on the size and length of your barrel. Our team's CanonBot uses a 3" Barrel, 3 feet long. At first we tried the Festo valve at 60 psi, but there's just too much restriction for our application. The sprinkler valve seemed to work the best for us. We use 2 steel tanks which lead to a manual ball valve which acts as our safety. After the ball valve is the sprinkler valve and then to the barrel. We keep safety valve closed except when the robot is in operation and everyone is clear of the robot and the barrel. We usually run our pressure between 60 and 80 psi.

[Donut]

We're planning on using 2 or 4 Festos in parallel, we want to see how much can be done with using mainly FIRST parts.

I think there may have to start being "cannon contests" at off season events with how many of these are being built.

[KenWittlief]

I would look into using a pnuematic cylinder, with a mechanical trigger

if you try to use a cylinder to launch stuff the airflow into the cylinder is too restricted by the small connectors and hoses

to fix that, you put a mechanical stop, about half way down the cylinders travel, that holds the pressurized piston back

to fire the launcher you pressurize the cylinder against the trigger/ release pin, then you pull the pin out of the way, allowing the cylinder to extend all the way

in effect you have a large pnuematic spring.

I think you will find this takes much less air, than blowing Tee shirts and stuff out the end of a PVC tube. Also the thing you are launching does not have to fit snugly into the tube, you could put just about anything in there.

The plus for safety is you can build this will all standard (approved) pnuematic parts and valves, you wont have to worry about pressurized PVC failing in a room with thousand of people around.

It can still be made to look like a cannon, in fact you could release CO2 or smoke to make it look more omminous. Only you would know whats really happening inside.

[Andrew Schuetze]

Originally Posted by Donut

Another easy alternative to using the PVC for storage is to use the storage tanks FIRST gives in the kit (and if you've been around for a few years, you've probably accumulated a few of them).

Our team is developing a cannon that uses almost nothing but KOP pneumatics, with the exception being our barrel and its mount.

This was indeed prototype 0.1. I used 8 KOP tanks in four parrallel circuits. I.e. two tanks in series feeding into a manifold with four inlets. The manifold was all 0.5" black pipe tees. I then used couplings to enlarge the opening to the 1.0" diameter for the sprinkler value. Not nearly enough air flow was generated to launch the tee shirt out of the 3.0" diameter barrel.

Prototype 0.2 used three 36" lengths of 1.0" black pipe in parrallel which fed into the sprinkler value. Now we had the shirt launching but only a few feet and a really loud harmonic reverberation in the pipe ala Big Rig air horn.

Prototype 0.3 can be seen in the CD media and is now linked in my original post.

[Billfred]

Originally Posted by KenWittlief

I would look into using a pnuematic cylinder, with a mechanical trigger

if you try to use a cylinder to launch stuff the airflow into the cylinder is too restricted by the small connectors and hoses

to fix that, you put a mechanical stop, about half way down the cylinders travel, that holds the pressurized piston back

to fire the launcher you pressurize the cylinder against the trigger/ release pin, then you pull the pin out of the way, allowing the cylinder to extend all the way

in effect you have a large pnuematic spring.

I think you will find this takes much less air, than blowing Tee shirts and stuff out the end of a PVC tube. Also the thing you are launching does not have to fit snugly into the tube, you could put just about anything in there.

The plus for safety is you can build this will all standard (approved) pnuematic parts and valves, you wont have to worry about pressurized PVC failing in a room with thousand of people around.

It can still be made to look like a cannon, in fact you could release CO2 or smoke to make it look more omminous. Only you would know whats really happening inside.

I get the basic concept, but what would work for the pin? It'd have to be something strong enough to handle the force of the cylinder, yet also be able to be yanked out of place (or so I think).