How to Choose a 42/57/86 Stepper Motor: Complete NEMA Frame Size Guide and Torque Reference Table

The most important thing to say up front: a NEMA frame size only defines "how it mounts," not "how much force it can produce." A NEMA 17 motor's holding torque can range from 0.2 N·m to 0.6 N·m — a 3× difference. Selecting a motor by frame size alone is the most common stepper motor selection mistake.

Figure: NEMA 8 / 17 / 23 / 34 stepper motors side by side — a frame size only defines mounting dimensions, not torque

1. What Is NEMA

NEMA = National Electrical Manufacturers Association.

For stepper motors, the relevant standard is NEMA ICS 16-2001 (Motion/Position Control Motors, Controls and Feedback Devices).

A NEMA Frame Size Only Defines These

1. The width and height of the square mounting flange (or the outer diameter of a round flange)

2. The diameter and position of the mounting holes

3. The diameter and depth of the locating pilot

4. The shaft diameter

A NEMA Frame Size Does Not Define These (all decided by the manufacturer)

• Torque and speed

• Rated current and voltage

• Winding inductance and resistance

• Step angle (1.8° / 0.9°)

• Body length (axial length)

• Special environmental conditions (temperature, protection rating)

• Whether a gearbox or leadscrew is included

• Connector shape and position

This explains why "two stepper motors that are both 42-frame can differ 2–3× in performance" — because their lengths and windings are completely different.

2. NEMA vs Metric Frame Size Comparison Table

Just remember this correspondence:

NEMA 17 = 42 frame, NEMA 23 = 57 frame, NEMA 34 = 86 frame

NEMA ICS 16-2001 Standard Dimensions (inches)

Practical tip: The mounting hole center distance of a NEMA 23 is typically 47.14 mm (1.855"). Measuring this distance is the fastest way to identify a frame size in the field.

Figure: NEMA 23 square mounting flange — mounting holes, pilot and shaft diameter are the core dimensions a frame size defines

3. Typical Holding Torque Ranges

Within the same frame size, the longer the body, the higher the torque. The following are typical industry ranges (for quickly establishing order of magnitude; always refer to the manufacturer's data):

Key reminder: When comparing motors from different manufacturers, you must compare at the same body length. A 2–3× difference in holding torque between two NEMA 17 motors is completely normal.

Limaisheng NiMotion Reference Parameters

4. The Correct Selection Workflow (Four Steps)

Step 1: Determine the Mechanical Interface

First check how much space the equipment can leave for the motor and what the flange hole pattern is. This step determines the upper limit of the frame size, but does not decide the final selection.

Step 2: Calculate the Required Torque (the Real Deciding Factor)

Trequired = Tfriction + Tacceleration + Tgravity + Texternal

• Tacceleration = J × β (moment of inertia × angular acceleration), usually the largest term;

• Leadscrew mechanism: T = (F × PB) / (2π × η);

• Timing belt / rack and pinion: T = F × r.

Safety factor: use 1.5–2×. Stepper motors have no overload capability; for high-acceleration or vertical-axis applications, 2–4× is recommended.

Step 3: Check the Torque-Speed Curve and Verify Actual Torque at the Operating Speed

This step is the most often skipped and the most likely to cause problems.

A stepper motor's torque decays rapidly as speed increases. Selecting with the static holding torque is equivalent to assuming the motor always runs at zero speed.

The correct approach:

1. Determine the pulse frequency (pps) corresponding to the maximum operating speed;

2. Find the torque at that frequency on the torque-speed characteristic curve;

3. Use this torque (not the holding torque) for verification.

Rule of thumb: the pull-out torque should be ≥ 1.5× the required torque; if direct starting is needed (no acceleration/deceleration), also verify with the lower pull-in torque curve.

Figure: During selection you must verify the actual torque at the operating speed with the torque-speed curve, not the static holding torque

Step 4: Choose the Length and Winding

Within the same frame size:

• Need more torque → choose a longer body (thicker lamination stack);

• Need better high-speed performance → choose a low-inductance high-speed winding (higher current at the same voltage, and also higher heat);

• Heat-sensitive → choose a closed-loop solution (outputs power on demand, reducing temperature rise by 30%–50%).

Practical rule: if the torque margin exceeds 100% of the actual requirement, you can step down one frame size. Systematically "sizing up one frame" adds weight, reduces efficiency, and does not improve accuracy.

5. Matching Voltage and Current

How to Set the Voltage

The higher the bus voltage, the better the high-speed performance — this is the most effective way to counter the winding's L/R time constant.

Conclusion: as long as the equipment runs above 600 rpm and needs to carry a load, you should consider 48V instead of 24V.

How to Set the Current

• Running current: set to the manufacturer's rated value; this is the source of torque;

• Idle current (static holding current): can be set lower than the running current to reduce temperature rise while still preventing lost steps;

• Drivers usually support 0–1.5 A (small sizes) or a wider adjustable range.

Common misconception: setting the running current far above the rated value "to get more force." The result is sharply increased heat, demagnetization of the magnetic material, and actually reduced torque. Magnetic materials generally demagnetize around 130°C; a motor housing temperature of 80–90°C is completely normal, but be alert if it exceeds that.

6. Typical Application Division of the 42 / 57 / 86 Frames

42 Frame (NEMA 17) — The Most Versatile

Features: small size, low cost, richest ecosystem.

Typical applications:

• 3D printers (extruder, X/Y axes)

• Small CNC, laser engraving

• Pick & Place mechanisms

• Dispensers, small SMT

• Camera gimbals, valve actuators

• Medical equipment (syringe pumps, analyzers)

Application boundary: torque requirement < 0.6 N·m, speed < 1000 rpm.

57 Frame (NEMA 23) — The Industrial Workhorse

Features: the balance point between torque and size; the most widely used in industrial automation.

Typical applications:

• Semi-professional CNC, laser cutting

• Positioning and conveying in automated production lines

• Packaging machinery, labeling machines

• AGV drive and steering

• Electric grippers, electric cylinders

• Logistics sorting

Application boundary: torque requirement 0.5–3.0 N·m, speed < 800 rpm.

86 Frame (NEMA 34) — Heavy Duty

Features: high torque, suitable for heavy loads.

Typical applications:

• Industrial CNC feed axes

• Presses, stamping machines

• Large conveyors and lifts

• Heavy automation equipment

Application boundary: torque requirement 3.0–12.0 N·m.

Note: the 86 frame often enters the range where "a servo is more appropriate." If high speed or high dynamics are needed, seriously compare a servo solution.

Figure: Typical application division — 42 frame (3D printers), 57 frame (automated production lines), 86 frame (industrial CNC)

7. How Integrated Motors Change the Selection

Traditional selection is "choose the motor first, then the driver, then wire it up." An integrated (drive-in-one) motor changes this workflow:

Impact on 42/57/86 selection:

• The frame size selection logic is unchanged (still based on torque and flange size);

• But you no longer need to select a separate driver, nor consider the number of pulse ports;

• You can directly choose a closed-loop type for lost-step protection, at a cost increment far smaller than a traditional "open-loop + external closed-loop driver" solution.

For example: Limaisheng NiMotion's STM57/STM86 integrated stepper servo motors integrate the driver and an absolute encoder within the 57/86 frame. Externally there are only two wires — DC power + CANopen/RS485 — with protection ratings of IP6K4 and IP65 respectively, and operating temperatures covering -40 to +85°C / -40 to +75°C.