a is a wheel, so it seems appropriate that this
October we revisit one of our favorite projects: our
large pneumatic punkin chunker. Any constant
readers of SERVO Magazine might think that we
seem to write about air cannons as often as Stephen
King spins yarns about possessed machinery, but we
promise that this project is as different from our previous
efforts as From a Buick 8 is from Christine.
In the October and December 2015 issues, we built
large PVC air cannons so that we could partake in the age
old October tradition of punkin chunkin. We even built a
mini air cannon for the Agent 390 tracked base from
Actobotics in the April 2016 issue. We like to think of those
projects as just a prelude to something bigger, like Night
Surf to The Stand. We perpetrated some pulpy pumpkin
carnage with our PVC cannons, but what kind of orange
inferno could be wrought with a high pressure steel
Designing something for a high pressure application
presents a unique host of challenges: specialized parts are
required, and new design problems rear their heads like
living topiary animals that sneak up behind your back.
Could we track down all the parts we needed for a high
pressure pumpkin apocalypse? Or would the unique
demands of this extreme application leave us helpless like
victims trapped in a tiny Pinto by Cujo?
The first step to a prom night prank that will horribly
backfire is to track down a bucket of pig’s blood.
Fortunately, the first step to building a high pressure
cannon is less messy. Identifying the pipe and fittings we
needed would require us to nail down some design
specifics, but the skeleton of the design was already there.
We would go with the three pronged trident design of our
previous efforts. We want to strike a balance between a
chunker that is high pressure, large scale, and not
That’s a lot of variables to start with, so we picked a
pressure to shoot for that would be as surely destructive as
the consequences of the aforementioned prom night prank:
3,000 psi fit the bill.
PVC, needless to say, would not work for a 3,000 psi
cannon. What we needed was Schedule 160 steel pipe. We
settled on a 2. 5” pipe diameter — a bit smaller than the 4”
PVC cannons, but big enough to look appropriately
menacing. High pressure fittings come in a few flavors, and
our overriding design consideration was ease of
manufacturability. We had threaded fittings and socket
weld fittings to choose from, so we opted for the threaded.
Threaded fittings are significantly heavier than socket
weld fittings, but much easier to install. We’re not top
notch welders, but we can thread pipes together with the
best of them — sometimes, threaded is better. Though, we
did find that even the pipe sealant for a 3,000 psi
application was a little tougher than usual to track down.
And a bit more expensive.
Nailing down the valve was a trickier proposition. The
by Bryce Woolley and Evan Woolley
A SCHEDULE 160 FORGED STEEL TEE.
54 SERVO 10.2016
Go to www.servomagazine.com
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