CHAPTER 17 ■ DC MOTORS
Contrast the electric screwdriver with a computer fan. The computer fan has lots of speed, but the fan
blade can be stopped or jammed easily (very little torque). Or take the circular saw, which has a powerful
combination of speed and torque.
The Significance of Distance in Torque
It is easier to hold an object by your sides than it is to hold the object with your arms outstretched. Therefore,
torque must be measured in both how much mass can be turned and how far away that mass is.
Motor torque is specified in newton meters (N·m). The little dot between “N·m” is often omitted.
Lots of other units specify torque, although they’re all measuring the same thing. A common unit is
pound-force foot (lbf-ft). Some imperial-system terms were specified in reverse to distinguish between
torque and work, but motor advertisements specifying “foot pound” mean the same thing as “pound foot,”
and so on.
The letter “f,” for force, is often omitted from advertisements. So, instead of seeing lbf-ft, the motor
could be listed as lb-ft, ft-lb, or even ft-lbf. They all mean the same thing. To recognize a motor’s torque
specification, look for any force/mass (like N, g, lb, or oz) combined with a length (like m, cm, ft, or in).
Table 17-2 lists the other most-common torque units and how to convert them to N·m. For example,
to compare a 3.2 lbf-ft torque motor to a 4 N·m torque motor, multiply 3.2 lbf-ft by 1.355817948 to get
4.338617434 N·m. Based on that conversion, the 3.2 lbf-ft motor has a greater maximum torque than the
4 N·m torque motor.
Table 17-3 shows how to convert from N·m to the other most-common torque units. For example,
to compare a 0.4 N·m torque motor to 50 ozf-in torque motor, multiply 0.4 N·m by 141.6119327 to get
56.64477308 ozf-in. Based on that conversion, the 0.4 N·m motor has a greater maximum torque than the
50 ozf-in torque motor.
Table 17-2. Table for Converting Between Other Common Torque Units and the International Standard N.m
Known Unit Multiplier Desired Unit
kgf-m x 9.806 650 029 = N·m
lbf-ft x 1.355 817 948 = N·m
lbf-in x 0.112 984 829 = N·m
kgf-cm x 0.098 066 500 = N·m
N.cm x 0.01 = N·m
ozf-in x 0.007 061 552 = N·m
mN.m x 0.001 = N·m
gf-cm x 0.000 098 066 5 = N·m
dyn-cm x 0.000 000 1 = N·m