when the end of the stroke was
reached. The large pulley on the
left side of the photo was driven
by the small orange and silver
motor below it. This powered
the flywheel while the robot
jockeyed for position during a
The rear bars of the flipping
arm had bearings inside them so
the shaft could turn the flywheel
while they stayed still.
For this build, I allotted
extra time for testing before the
competition because I ran into so
many problems on the previous
Figure 3 shows what
happened on the first full power
test flip. The Kevlar thread failed
when the hard stop was hit at
the end of the stroke.
Unfortunately, the Kevlar
thread I used was the largest my
supplier offered. I needed to find
a suitable replacement or a new
supplier in time to order the new
thread and install it into the
Mike Jefferies — a frequent
contributor in the SERVO
Combat Zone — suggested that I
try out a kind of rope called
Dyneema. This rope was available in
many sizes, ranging from smaller
than the Kevlar thread I was using
to large enough to tow a car out of
the mud. I decided to order a few
different sizes and rebuild the
robot to fit.
The Kevlar thread I was using
was only .040” in diameter. I
ordered .075”, .095”, and .125”
thick Dyneema ropes from a
spearfishing supplier and set about
trying to fit them into the design
while I waited for them to arrive.
The trouble with using the
bigger rope was that the width
between the rear flipper arms could
not grow because I did not have
time to move the main weapon
rails. That meant I had to make
space for them by making the
flywheel and coil drum narrower.
FIGURE 4. New coil plate.
jammed against the insides of
the weapon rails. The clutch
engaged and disengaged
properly but the system did not
reset because of the rope jam.
Once I realized what was
happening, I decided to switch
down to the thinnest rope that I
had purchased to see if that
would alleviate the pressure. One
more test showed that it was
strong enough to handle the
load while leaving plenty of extra
space on the coil drum shaft.
This final version of the
weapon assembly proved to be
reliable enough to meet my goal
of three unattended flips. In the
end, the weight from the wider
coil plates and the necessity of
making the flywheel narrower
forced the flywheel mass to be
lower than the one in Reclipso.
While this meant that the
flipper did not throw other
robots as well as I wanted, the
clutch mechanism performed
flawlessly and that was a huge
step in my quest.
I decided to call the new
robot Threecoil, and its first
competition was the 2012 Bot
Blast in Bloomsburg, PA.
You can see the completed
machine in Figure 5. The robot
performed well beyond my
expectations at the competition.
The clutch mechanism never
jammed and the weapon fired
multiple times in each match. I
would have been completely
satisfied if those were the only
results but the robot also managed
to win the award for Coolest Robot
and the first place trophy.
These results were so exciting
that I decided to see if I could scale
the design up to a 30 pound
sportsman class machine for
Motorama 2013. You will have to
wait for another issue to see how
that went. SV
FIGURE 5. Ready for competition.
Once I did that, I had to replace
the rear coil plates with wider
pulleys that included a place to
attach the new Dyneema rope. You
can see one of the new coil plates
in Figure 4.
The new coil plates sported a
much wider and deeper channel for
rope up to .125” in diameter, and
an integrated pin for locking the
rope to the coil plate instead of
having it stick out the side like I did
with the Kevlar thread.
Once I installed all the new
parts, it was time for another test. I
decided to start with the thickest
rope to see if any of the sizes I
bought could survive the forces in
This time, the new rope held up
great, but it took up so much space
on the coil drum shaft that it
30 SERVO 06.2013