CHAPTER 17 ■ DC MOTORSI find gf-cm the most graspable torque unit for small robot motors. Table 17-4 lists other common
torque units and how to convert them to gf-cm. (You can reach the same results by converting a unit to N·m
with Table 17-2 and then converting N·m to gf-cm with Table 17-3.)
Sliding Torque
A full can of soda is about 380 grams. A motor with 380 gf-cm of torque can rotate a 380-gram mass
connected 1-centimeter away (380 gf × 1 cm = 380 gf-cm).
Torque isn’t restricted to a 1-centimeter distance. A 9 V battery is a little over 38 grams. That same motor
could rotate 38 grams connected 10 centimeters away (38 gf × 10 cm = 380 gf-cm). Torque allows for less
mass farther out or more mass farther in, as long as the numbers multiplied together are less than or equal to
the motor’s torque.
This has practical implications. If you build a robot arm that the motor can’t move, you can either get a
motor with higher-rated torque, you can shorten the arm (the length), or you can reduce the mass (the weight).
Table 17-3. Table for Converting Between the International Standard N.m and Other Common Torque Units
Known Unit Multiplier Desired Unit
N·m x 0.101 971 621 = kgf-m
N·m x 0.737 562 15 = lbf-ft
N·m x 8.850 745 795 = lbf-in
N·m x 10.197 162 1 = kgf-cm
N·m x 100 = N.cm
N·m x 141.611 932 7 = ozf-in
N·m x 1000 = mN.m
N·m x 10 197.162 1 = gf-cm
N·m x 10 000 000 = dyn-cm
Table 17-4. Table for Converting Other Common Torque Units to gf-cm
Known Unit Divider Desired Unit
kgf-m / 100 000 = gf-cm
lbf-ft / 13825 = gf-cm
N·m / 10197 = gf-cm
lbf-in / 1152 = gf-cm
kgf-cm / 1000 = gf-cm
N·cm / 102 = gf-cm
ozf-in / 72 = gf-cm
mN·m / 10.2 = gf-cm
dyn-cm / 0.001 = gf-cm