actuation mechanism. The cannon uses a 2” diameter
Dynaquip VAJ2.AR Series Forged Body steel ball valve. The
valve — weighing 13. 5 lbs and rated for 6,175 PSI — is a
heavy-duty unit that takes a lot of torque to actuate. Plus,
that torque increases exponentially at higher operating
During our initial testing of the steel cannon, we used
a torque wrench to measure the actuation torque for the
valve at various pressures. Based on our measurements,
we estimated that it would take at least 100 ft-lb of
torque to actuate the valve at 3,000 PSI. That, for the
record, is a lot of torque. The NPC-T64 motors that are
a popular choice for BattleBots drive trains, for example,
have a stall torque of about 69 ft-lb.
A lot of folks have asked us why we didn’t use some
other valve arrangement for Double Jeopardy. Why not
a solenoid valve, a quick exhaust valve, or some sort of
burst disk setup? The simple answer is that we did indeed
look into all of those things, but we couldn’t find anything
that met our design specifications. For our valve, we needed
something that was rated for high pressure (at least 3,500
PSI), was not too heavy, and had a large aperture.
Most solenoid valves are a diaphragm style, so they
have a very small aperture that results in significant pressure
drop over the valve. Also, most large pipe size/high-pressure
solenoid valves are ridiculously large and heavy (we’re
talking over 30 lbs and about a foot tall).
For quick exhaust valves, we couldn’t find anything
rated at high enough pressure with a large enough aperture.
That’s not to say the perfect quick exhaust valve or solenoid
valve doesn’t exist, but we
couldn’t find one, and we
were dealing with a major
time crunch. A ball valve
checked all of our boxes —
we just needed to figure out
a way to open it.
The original design that
we pitched to BattleBots
used a torsion spring to
actuate the valve. Finding an
appropriate torsion spring
was a challenge because we
needed something heavy
duty to give us over 100
ft-lb of torque. As mere
robotic hobbyists with nary
a professional engineer
among us (and with the
most sophisticated tool
in Robot Central being a
wobbly old drill press), our
tendency is always to look
for off-the-shelf parts before
considering a custom solution.
When searching for the right off-the-shelf part, we try
to think of what real world applications would require similar
specifications, and that led to trucks. Specifically, torsion
springs for the gates on large truck trailers. The springs
looked beefy enough, they claimed to provide the torque
we needed, and the price was right (a giant spring was like
$15). We ordered a few and hoped for the best.
The torsion springs, unfortunately, were just too tall. We
wanted to keep Double Jeopardy invertible, and we wanted
to keep the bot as short as possible for weight reasons. The
truck trailer torsion springs were simply too tall to fit in the
bot. It would be possible to order a custom spring with the
right parameters, but custom springs have a long lead
time and are very pricey (around a grand). There had to
be another way.
Our next thought was to look into shock cord, which
is the cooler sounding name for bungee cord. Based
on the listed specs of the cord, it looked like there were
options capable of providing the force we needed to
SIX-LAYER SPRING STEEL
AND THE LARGE BARREL.
FIVE POUND STEEL SLUGS; PUG FOR SCALE.
THE SMALLER BARREL.
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