the motor shaft. If it’s not, there are a couple of things you
can do to solve the problem:
• If the wheel hub is too small, drill it out to fit the
shaft. Obviously, this only works if the hole you drill
isn’t so large that it destroys the wheel hub.
• Use a reducing bushing, which is basically a short
length of hollow tube. The inside of the bushing is
sized to match the motor shaft; the outside is for the
wheel hub. You then carefully drill a hole through the
bushing. Use a longer setscrew, if needed, so it
extends through the bushing and makes contact
against the shaft.
What if the wheel lacks a setscrew? Or, the hub is too
large for a bushing? Or, the wheel will be wrecked if the
hub is drilled out? One solution is to use a flange that does
have a setscrew and a hub with a properly sized bore that
matches the shaft. You attach the flange to the wheel
using fasteners, then mount the flange to the shaft.
So, what’s a “flange?” It’s anything that’s smaller than
the diameter of the wheel, and has a setscrew and hub
compatible with the motor shaft you want to connect it to.
Specially made flanges are available through Actobotics,
Lynxmotion, Jameco, and several other online retailers. The
flanges come in different bore sizes. Cost is modest.
A convenient ready-made flange is a surplus gear (a
sprocket also works). Drill two or more holes through the
face of the gear and match holes into the wheel. Be sure to
keep the gear and wheel concentric. Mount the flat part of
the gear against the wheel (see Figure 7), then attach the
assembly to the motor shaft.
Mounting Wheels to R/C Servos
Servos engineered for full rotation are most often used
for robot locomotion and are outfitted with wheels. Since
servos are best suited for small- to medium-sized robots
(under about three pounds), the wheels for the robot
should ideally be between two and five inches in diameter
Servos differ in the type of spline used on their output
gear. The splines provide a positive grip of the servo horn
(sometimes called control disc or hub), so that it won’t slip
on the hub as the motor turns. The three most common
spline types are noted simply by the manufacturer that
popularized each one: Hitec, Futaba, and Airtronics. If you
are purchasing wheels for your servos, make sure the
wheels use the proper hub spline.
By far, the easiest way to attach wheels to servos is to
use a wheel that’s specially engineered for the job. Many
specialty robotics retailers such as Parallax, RobotShop,
Pololu, and others sell wheels meant for use with the
standard size Hitec and Futaba servos.
Now for the bad news: Your choice of wheel diameters
is pretty limited. You’ll find just a few sizes, with 2-1/2”
(give or take a few fractions of an inch) being the most
common. If you need a smaller or larger size, you can
always make your own wheels as detailed next.
The general approach for attaching generic wheels to
servos is to use one of the servo horns that comes with the
servo and secure it to the wheel using screws or glue (see
Figure 8). The inner portion of the horn fits snugly over the
output shaft of the servo. Here are some ideas:
Lightweight foam tires — popular with model
airplanes — can be glued or screwed to the servo horn.
Popular brands are Dave Brown and Du-Bro, and these can
SERVO 04.2014 41
Figure 7. Concept behind using a metal or plastic gear as a flange
for mounting a wheel to a motor shaft.
Figure 8. Most any kind of wheels can be converted for use with an
R/C servo by attaching a servo horn to the side of the wheel. If your
servo didn't come with an assortment of horns, you can purchase