CHAPTER 17 ■ DC MOTORSStacking layers of planetary gears builds a full gearbox (see Figure 17-29). Just like a spur gearbox, the
planetary gearbox is then attached to the top of a plain DC motor. A planetary gearhead motor remains as
simple to control as a plain DC motor.
Because multiple smaller gears work in parallel for each larger gear, the planetary gearbox can operate
at higher torque that would otherwise damage the solitary smaller gear in a spur gearbox. Alas, the increased
maximum torque limit comes with reduced efficiency due to friction.
The output shaft comes out of the center of a planetary gearhead, rather than set off-center like most
spur gearheads. Sometimes this is desirable, sometimes not, depending on where you want to place the
wheels on your robot. A centered shaft on a gearhead motor usually indicates a planetary gearbox.
Choosing a Gearhead Motor
The same criteria that apply to selecting a plain DC motor also apply to selecting a gearhead DC motor.
Check out the voltage, dimensions, weight, and final RPM.
For heavy-duty torque applications, look for planetary gearboxes with metal gears and metal gearbox
frames. However, plastic gears are preferable for lower-torque, quick-action applications, with significantly
lighter weight.
Examine the gearbox to see that the output shaft is located at the angle and placement desired. Some
gearboxes have unusual shapes, such as rectangles, which add to the overall height of the motor.
Depending on your robot’s needs, look for the word “reversible.” Most gearheads are designed to allow
the motor to continue to operate forward and reverse. However, some gearboxes only allow forward rotation.
Non-reversing gearheads are acceptable if you want the robot’s wheels to refuse to be pushed backwards.
Figure 17-28. Planetary gears
Figure 17-29. Exposed cross-sections of a Tamiya planetary gearhead motor