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What is Step Motor Stack Length?

by Jeff Kordik

In our post What Do NEMA Sizes Mean?, we examined the NEMA frame sizes in which step motors are made. Larger frame sizes produce more torque. But this is not a one dimensional process: within a given frame size, the motor length can vary and that also affects torque. Because step motors require expensive tooling in order to be produced economically, a fixed rotor length is chosen, as is the stator that surrounds it.

step motor diagram

In the next photo, a single stack rotor, with bearings, is shown.

step motor rotor

To produce longer, more powerful motors, multiple rotors and stators are “stacked up”. The NEMA 17 motors below incorporate one, two, three and four stacks.

NEMA 17 step motors

The chart below shows the holding torque (in Newton-meters) for motors of varying frame size and stack length. This allows engineers flexibility in matching a motor to an application. Sometimes there is available space for a longer motor and other times it is advantageous to use a shorter motor with a larger frame size. High quality step motors are available in many frame sizes and stack lengths and with other options like encoder feedback, IP65 harsh environment ratings and shielded cables.

Holding Torque (N-m)
Frame Size One stack Two stacks Three stacks Four stacks
NEMA 11 0.05 0.11 N/A N/A
NEMA 17 0.22 0.37 0.55 0.80
NEMA 23 0.54 1.12 1.90 2.50
NEMA 24 0.87 1.25 2.50 N/A
NEMA 34 2.80 6.00 8.90 11.96

Just for fun, we plotted the relationship of stack length and frame size on a 3D chart.

step motor torque