more ample volume of Desperation. The lifter didn’t seem
to have a problem, and the only limiting factor was the
tendency of the VEX robot base to tip over with a heavy
When the robot was properly counterweighted, it
easily elevated the 700-plus page hardcover. If that’s what
the 40N servo could do, what would the 100N be capable
We’re Here to Build Robots and
Chunk Pumpkins, and We’re All
Done Building Robots
Our first foray into punkin chunkin (in the October
2015 issue) was with our PVC cannon that used a sprinkler
valve that had a small port size, but was easily wired up to
be remote controlled. For the sequel (in the December
2015 issue), we upped the ante with a much larger
diameter ball valve for a cannon with a lot more power,
but without a good solution for actuating the valve.
We made it a trilogy (in April 2016) with a scaled-down mini tank punkin chunker. We kept the series going
(in April 2017) with a high pressure steel cannon chunker.
What could we do this time that would take advantage of
the linear servo motor while keeping the franchise fresh?
We could go for a different type of chunker, of course.
In the big leagues of punkin chunkin, there are several
different categories of chunker other than the air cannon.
A popular design that we’ve been itching to try is the
catapult. While the most popular catapult design for
chunkin looks to be the trebuchet, we wanted to try
something a little more straightforward that didn’t involve
as many moving parts as a carefully balanced and swinging
Another classic catapult design is the mangonel, which
uses torsion instead of a counterweight to fling
pumpkins into the great beyond. We just happened
to have plenty of torsion springs lying around from
some partially successful experiments in actuation for
the large diameter ball valve on the PVC cannon.
Pulling the boom arm on a mangonel back
engages the torsion spring and stores potential
energy. The difficult part is figuring out a good way
to release that energy. What at first blush seems like
the “easiest” option is pulling a pin to release the
boom. Just one simple motion. How hard could it
Pretty hard actually — because of the shear
force. If the mangonel boom is pressing against the
pin, it creates a significant shear force that makes the
52 SERVO 10.2017
TESTING THE TRIGGER.
BUILDING A MANGONEL.