T is torque
F is force (usually measured in oz.)
r is radius (usually measured in inches)
All stepper motors have a torque rating, and this should be the first rating you look at. This is also known as holding torque. If the motor is powered on but not rotating, the holding torque is the amount of torque necessary to turn the shaft against the motor’s will. Stepper motors also have a maximum current rating, meaning that the holding torque number is usually given assuming the maximum rated current is flowing into the motor.
Visualizing how much torque you need
Torque is a difficult measurement to estimate. Imagine you have a stepper motor shaft, and a one inch stick is attached to the center of the shaft pointing outwards. Now imagine that the motor is bolted onto a table, and you can put your finger on the table to stop the motor, since your finger comes between the one inch stick and table. A 16 oz. in. stepper motor would apply a force of one pound on your finger, since 16 oz equal one pound and the radius is one inch.
Maximum current rating
Stepper motors also have a maximum current rating you should absolutely pay attention to before buying. This rating means that if you apply more than the maximum amount of current to the motor for a long period of time, you risk damaging the motor. Yes, you can run motors below the maximum rated current, but if you do, you’ll basically be leaving performance on the table since you won’t be using your motor to its maximum potential. So if you have a 2 amp stepper motor driver, you shouldn’t pick a stepper motor that has a maximum rated current of 4 amps. That would just be a waste of money.
You will also want to pay close attention to degree rating. For most stepper motors, a rating of 1.8 degrees is given, meaning that for every full step, there is a 1.8 degree difference. This means that if you half step the motor, it will take 400 pulses to rotate the motor shaft exactly once. Beware that cheaper and smaller stepper motors have far fewer steps. Again, a 1.8 degree rating is very common.
Stepper motors usually specify a NEMA number. This rating doesn’t necessarily indicate the strength of the motor, but rather, the general size and shape of a motor. For example, stepper motors usually have four holes at the front of the motor for mounting purposes. Each NEMA number has different characteristics that dictate dimensions of the motor such as these mounting holes. There are some things that NEMA does not indicate, such as the length of the motor. So you could have two NEMA 23 motors, but one of them might be .5 inches long while the other could be 3 inches long. Of course, the 3 inch long motor would be vastly heavier and more powerful.