Understanding Linear Stepper Motor Structures: External vs. Non-Captive vs. Captive

When selecting a linear stepper motor for your automation project, you will encounter terms like “External,” “Non-Captive,” and “Captive.” While they may all use the same NEMA frame size (e.g., NEMA 17 or NEMA 23), their mechanical operation and integration requirements are fundamentally different.

At AKT Motor, our HSL Series primarily focuses on the External configuration, which offers the greatest flexibility for machine designers. However, understanding the differences between these three types is crucial for choosing the right component.

1. External Linear Actuators (The AKT Standard)

This is the most common and versatile configuration, found in the majority of our 20HSL, 42HSL, and 57HSL series.

• How it works:The motor shaft is replaced by a lead screw that is integrated directly into the motor’s rotor. When the motor spins, the lead screw rotates. A separate nut (typically flanged) is attached to the load (your carriage or gantry). As the screw turns, the nut travels back and forth along the screw.
• Key Advantages:
  • Customizable Stroke: Since the screw extends outward, manufacturers like AKT Motor can cut the screw length (L2) to your exact specifications, allowing for very long strokes.
  • High Efficiency: Direct coupling eliminates the need for belts or external couplers, reducing backlash.
  • Design Flexibility: You can choose different nut materials (like anti-backlash nuts) to suit your precision requirements.
• Best For: 3D Printers (Z-axis), CNC routers, and linear rail systems where the motor is fixed, and the carriage moves.

2. Non-Captive Linear Actuators

In a Non-Captive design, the mechanics are inverted.

• How it works:The nut is integrated inside the motor’s hollow rotor. The lead screw passes through the motor body. When the motor spins, the internal nut rotates, causing the lead screw to move linearly through the motor (provided the screw is prevented from rotating by an external mechanism).
• Key Advantages:
  • Compact Stroke: The screw can retract fully into the motor, saving space.
  • Unlimited Travel (Theoretically): As long as the screw is long enough, it can pass through the motor body.
• Disadvantages:
  • Anti-Rotation Required: The machine designer must provide a mechanism to prevent the screw from rotating; otherwise, the screw will just spin without moving forward.
• Best For: XY tables and applications where the motor moves along with the load.

3. Captive Linear Actuators

The Captive actuator is the most “plug-and-play” solution but is mechanically complex.

• How it works:Similar to the Non-Captive version, the nut is inside the rotor. However, the lead screw is mechanically restricted from rotating by a spline shaft or an internal anti-rotation guide built into the motor housing. The shaft simply extends and retracts like a hydraulic piston.
• Key Advantages:
  • Self-Contained: No external anti-rotation guides are needed. It produces pure linear motion out of the box.
• Disadvantages:
  • Limited Stroke: The stroke length is physically limited by the length of the motor body and the internal mechanism. You cannot have a long-stroke Captive motor without a very long motor body.
• Best For: Precision push/pull applications, valve control, and short-stroke dispensing.

Comparison Summary

*Note: For Non-Captive motors, the screw only moves linearly if the user prevents it from rotating.

Conclusion

For most industrial automation, laboratory equipment, and CNC projects, the External Linear Actuator (HSL Series) provides the best balance of customizability, cost, and performance. It allows you to define the exact stroke length you need while leveraging standard linear rails for guidance.

If you are unsure which structure fits your specific machine design, the AKT Motor engineering team is here to help you select the perfect NEMA 8, 11, 17, or 23 solution.