shielded bearings since they protect
against dust, etc., getting into the
bearings without the drag of a fully
sealed bearing. I also checked that the
speed rating exceeded the likely max
RPM of the drum.
The dynamic load rating of 243
lbs seems adequate, but our
applications are so outside normal
use that establishing a relationship
between dynamic load ratings and
actual effectiveness for any weight
class in combat robotics would be
The first bearing I tried did not fit
onto the 1/4” titanium shaft, and it
was still a press-fit on the shaft even
after I de-burred the shaft and gave it
a quick sanding on the lathe.
Given that the ground titanium
shaft was supposed to be just under
1/4” (and the bearing just over) and
should have been an
easy sliding fit, I
decided to try
another of the
bearings and it slid
I had bought
four bearings, and
all the rest fit the
lesson to learn is do
not assume that if a
bearing won’t fit, that all
similar bearings won’t fit.
I measured the shaft and
it was not oversized, so I
should have guessed
earlier that the problem
was with that particular
bearing and not the
The bearings were a
light press-fit in the bore,
and the rotor was
secured in place using a
smear of “Goop” adhesive
(Figure 15). The stator side of
the motor is screwed to the side
wall of the chassis (Figure 16).
It is important to ensure that
none of the screws hit the wires
going into the motor or the
coils on the magnets
I used small nylon washers
to space the screws out to
accomplish this, adding two
washers under two of the
screws and one under the other
I assembled the drum onto the
axle and added a number of shims
(Figure 17) to maintain the correct
axial position of the rotor relative to
the stator. I found that two shims
resulted in a gap between the stator
and rotor similar to that of the original
The next task was to balance the
drum assembly. I mounted it on the
axle and supported the axle on two
sockets of the same height. The
shielded bearings allow the drum to
rotate freely, and the heavy side will
24 SERVO 06.2015
Figure 13. Losing weight.
Figure 14. Flanged ball bearing.
on the chassis.
Figure 17. Shims
on the axle.
Figure 15. Pressed-in bearing and glued rotor.
Figure 12. Use a vise stop and edge finder.