Part 2: Wheel Assembly
by Michael Simpson
Ihave built many robots in the past and the drivetrain
always causes most of my grief. The type of drivetrain
you decide on will dictate some of the factors used in the
design of your robot, including its size and how many
batteries it can carry. Since I am going to be building two
robots, I think it’s important that I give them names to
make referencing them throughout this series easier:
sets for each wheel. These are needed because the original
servo horn does not give you enough clearance.
This is the six-wheeled RS- 64 based robot. It will be
approximately two feet in diameter and capable of carrying
a large array of devices and instruments.
This is a two-wheeled RS- 28 based robot. It will be less
than 12” in diameter and sized so that it could be used in
the Trinity Fire Fighting contest.
The Wheel Assembly
As I mentioned last month, we are using the DU-BRO
550TV wheel and the Dynamixel RX- 64 servo motor for our
wheel assembly (Figure 1). With two robot bases come two
options: you can build two wheels for the three-wheeled
robot or six wheels for the six-wheeled robot. If you plan on
building Firebot, you will need to use one of the Horn 28T
There is no way to attach the wheel directly to the
RX- 64. We need to create an intermediate plate that will
attach to the servo horn of the RX- 64 and then to the
wheel. The plate is simply a circular piece of compressed
PVC cut into a 58 mm disk as shown in Figure 2.
The goal is for the disk to fit snugly inside the center of
the tire as shown in Figure 3. I used a scroll saw to shape
mine. You can also use a band saw or possibly a small
coping saw. Cut the wheels slightly larger than 58 mm.
This way, you can use a rotary tool with a sanding drum
to trim the disk so that it fits snugly. The disk does not have
to be perfect. As long as it fits snugly, it will be okay.
You may use other materials for this disk. My local craft
store carries some 1/8” thick hobby plywood, as well as
some precut disks that only needed to be sanded to the
exact size. Originally, I used acrylic, but found that it
cracked too easily. Compressed PVC can be purchased from
various online sources, as well as most sign shops. It should
be 1/8” thick; 3 mm stock will work as well.
Once your disks are cut, you need to prep the wheel in
order to attach the disk. Take the
wheel, and using a permanent
marker, place four marks on the
wheel as shown in Figure 4. Be
sure to mark the side of the wheel
that does not have the air valve.
The mark should be roughly 11
mm from the groove. The goal
here is to drill a hole far enough
from the center of the wheel so
that it does not interfere with the
horn we are going to attach. We
can’t drill the holes too far from the
36 SERVO 09.2008