Build Your Own Combat Robot

FIGURE 6-11
Two typical
right-angle drive
speed reducers
using the 56C
NEMA standard
face size.
Chapter 6: Power Transmission: Getting Power to Your Wheels 123
shaft and the output shaft are parallel. The other general class of speed reducers
are called right-angle drives. In this type of system, the output shaft is at a right angle
with respect to the motor shaft. Many wheelchair motors, windshield wiper motors,
and power window motors are right-angle drives.
Most commercial speed reducers use standard mounting methods defined by
NEMA (National Electrical Manufacturers Association). Two NEMA general classifications
should be considered: the NEMA Frame size, and the NEMA Face size.
The NEMA Frame size defines the standard dimensions for the motor mounting
holes to secure the motor frame and defines the height of the motor shaft from the
mounting surface. The NEMA Face size defines the bolt hole pattern on the front
face of the motor. Two NEMA speed reducers are shown in Figure 6-11.
With NEMA face mounting, the speed reducer is bolted directly to the face of the
motor. With NEMA frame mounting, both the speed reducer and motor are
mounted to the same base plate. The NEMA specifications also specify the motor
shaft diameters, shaft length, and the type of keyway. A motor and a speed reducer
with the same NEMA specification will fit together like a glove. If you are trying to
fit together a motor and a speed reducer that do not have the same NEMA specification,
then you will have to build an adapter to mate these two components together.
Right-angle drives are usually made up of worm gears, which can provide high
speed reductions. The right-angle drives that use bevel gears usually have low gear
reductions.
Speed reducers using spur gears are the lowest cost speed reducers when compared
to helical gear speed reducers and planetary gear speed reducers. Helical
speed reducers can transmit higher torque than spur gear speed reducers, and they
run quieter. But helical speed reducers are sometimes less efficient than regular
spur gear speed reducers. Planetary gearboxes offer a high gear reduction in a
small package, and Harmonic speed reducers are the most compact speed reducer.