36:1 cordless drill gearboxes.
The performance was so
good even at only 3S LiPo
( 11.1V) and on a 26 lb bot,
that I thought it might be
worth trying brushless
motors in Isotelus Rex.
An added incentive was
that the Franklin event does
not allow LiPo batteries to
be used in any bot bigger
than 3 lbs (nasty smelling
smoke from a LiPo fire is not
a good thing, doubly so in a
museum that is likely to be
hosting a marriage ceremony right
after your event!). That leaves using
LiFe batteries as the best choice, but
the selection of sizes and capacities is
much more limited than it is for LiPos.
They are also slightly heavier. I had
been using a 1,300 mAh 6S LiPo pack
and there just didn’t seem to be a
close replacement in LiFe.
If I could use brushless motors,
this would save on the weight, and
then I could use a lower cell count but
higher capacity LiFe pack and still stay
within the weight limits.
I wanted to reuse the BaneBots
16:1 P60s I had, so I would need a
motor with a lower kV than those I
had used in my test. I settled on the
Tacon 540XXL 2858 1,200 kV motor
The gear extended out 14 mm
from the mounting face on the
existing motor, so the first job was to
cut down the longer shafts on the
Tacons to the same length. I wrapped
the motors in tape to protect them
(Figure 9), and shortened the shafts
using a cutoff disk in my Dremel
clone. I then cleaned off any burrs left
on the end of the shaft with the side
of the disk.
The gears are a press-fit, but since
the exact dimension of the shaft and
the bores of the gears are unknown, I
added extra grip by adding a little
Loctite 640 in the bores of the gears
before pressing them carefully on
using my vice (Figure 10). This Loctite
is designed especially for press-fits and
gives a little extra insurance on what
already seemed like a pretty good fit.
I had ordered a couple of 500
sized motor mounting plates from
BaneBots (the mounting holes are
different from the bigger 750 motors I
had used before) and the Tacons fit
them perfectly (Figure 11). They were
now refitted to the gearboxes, and
the new brushless assemblies were
complete (Figure 12).
I then rebuilt Isotelus (Figure 13),
added a HobbyKing CAR-45A
brushless ESC for each motor, and did
a quick bench test to see that they
24 SERVO 11.2015
Figure 8. Ready to install.
Figure 9. Shortening shaft
on brushless motor.