Again, our preference was to have the mounting point
somewhere on the cannon to keep the entire assembly self-contained.
We envisioned some sort of large U-bolt that would
wrap around the barrel of the cannon, tightly embracing
the barrel as if the bolt were its number one fan. At first,
we envisioned something like an exhaust clamp with the
top plate curved to grip the pipe. The problem with an
exhaust clamp was that the legs of the bolt were short, and
we figured one of the legs (if long enough) would make
the perfect fixed mounting point for the other leg of the
Fortunately, our local big box hardware store came
through with a surprisingly large U-bolt with legs as long as
the Behemoth that emerged from The Mist. The store even
had a long nut that allowed us to lengthen the leg of one
side of the U-bolt to make the perfect mounting point for
the fixed leg of the spring. We lengthened the U-bolt with
another section of half inch all thread (which was quickly
becoming the MVP of this project, like Kojack to the
Boulder Free Zone).
We bent the leg of the torsion spring so that it could
better grip the all thread mount, and sandwiched it
between pairs of large nuts and washers. With the initial
mounting in place, we were ready to test the trigger as the
cannon sat on our workbench unpressurized. If we closed
the valve manually and then let go, we hoped to see the
valve spring open.
As soon as we closed the valve, we could sense that
something happened …just like the unlucky recipients of C.
Danny Jacobs’ lightning cures. We could see the U-bolt start
to slip around the barrel of the cannon and the torsion
spring began to lose strength like the True Knot after going
too long without taking steam. We would have to do a bit
more to keep everything in place.
Our form of hobbling the U-bolt was to mount it
directly to the frame on Protobot that the cannon sits atop.
The braces for holding the cannon in place happened to be
the right location and height to pick up the U-bolt legs with
a few 1x2 wooden supports. With the new supports in
place, we could close the valve without unwanted
movement from the rest of the device. Would the valve
spring open with the vigor of the Running Man, or would it
slowly creep open with the sluggishness of something just
dug up from the Pet Sematary?
Chunk of the Century
Our initial test was somewhere in between. The valve
started opening a little slowly, but then sprang open quickly
after it reached a certain point. We wondered if that would
still be effective. If the sluggish movement only happened
when the valve was still technically closed, then we might
be all right. We decided to finish up the mechanism and
see what happened.
The final component of our robotic trigger mechanism
was the latch. We initially had plans of mounting another
motor that would twist a latch open using its normal axis of
rotation. The goal was to minimize the shear force that
would impede the opening of the latch. If the latch
actuated in a different direction from the shear force, the
shear force from the cocked valve should be a lesser
impediment to the latch.
We think we’ll explore that latch design further for the
3,000 psi cannon, but our existing setup on Protobot might
SERVO 10.2016 57
MOUNTING THE TORSION SPRING. SPRING LOADED!
BOLTING EVERYTHING DOWN.