For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
In automation design, the choice between a Stepper Motor and a Servo Motor is often decided by budget. But looking at the price tag alone is a mistake that leads to machine failure.
Steppers are excellent for holding loads stationary (high holding torque). Servos are kings of high-speed motion.
If you choose a stepper for a high-speed application, it will lose torque and "miss steps." If you choose a servo for a simple low-speed application, you have wasted $500. This guide explains the physics behind the choice.
Table of Contents
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1. Open Loop vs. Closed Loop (The Risk)
The biggest difference is not the motor itself, but how it is controlled.
Figure 1: Steppers run "blind" (Open Loop). Servos use an encoder to verify position (Closed Loop).
Stepper Motors (Open Loop)
Standard steppers run Open Loop. The controller sends a pulse and assumes the motor moved. There is no feedback. If the load jams or moves too fast, the motor "misses steps," but the controller keeps going. You won't know the part is ruined until the end of the cycle.
Servo Motors (Closed Loop)
Servos always have an Encoder (feedback device). The drive constantly compares the command position vs. the actual position. If an error occurs, the Servo Drive fights to correct it or triggers an alarm immediately.
2. The Torque Curve: Speed Kills Steppers
This is the most misunderstood concept in motor sizing.
Figure 2: Steppers have high torque at 0 RPM, but torque crashes as speed increases. Servos maintain usable torque over a wide speed range (constant torque region).
- Steppers: Have massive Holding Torque (at 0 RPM). But as soon as they start spinning, torque drops rapidly due to inductance. At 1,000 RPM, a stepper might only have 20% of its rated torque left.
- Servos: Provide Constant Torque from 0 up to their base speed. Above base speed, torque decreases as the motor enters the constant power region.
Servo motors provide constant torque up to base speed.Above base speed, torque decreases as the motor enters the constant power region.
Design Rule: Never size a stepper based on its holding torque alone. Always check the "Pull-Out Torque" curve at your target speed.
Quick Rule of Thumb: If your application exceeds 500 RPM or requires rapid acceleration under load, a servo motor is usually the safer choice.
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3. Inertia Mismatch
Servos are aggressive. They can accelerate loads incredibly fast. However, they are sensitive to Inertia Mismatch.
If the load inertia ratio exceeds roughly 10:1 (Load Inertia vs. Rotor Inertia), servo tuning becomes difficult and instability (humming or vibration) is likely without a gearbox or inertia matching. Steppers are much more forgiving of high inertia loads, making them great for heavy, slow belt drives.
4. Selection Summary
| Feature | Stepper Motor | Servo Motor |
|---|---|---|
| Cost | $ (Low) | $$$ (High) |
| Control | Open Loop (Risk of lost steps) | Closed Loop (Encoder feedback) |
| High Speed | Poor (Torque drops) | Excellent (Wide constant-torque range) |
| Holding Stationary | Excellent (No Jitter) | Good (May dither if poorly tuned) |
| Best Application | 3D Printers, Packaging, Low Speed | CNC Axes, Robot Arms |
Recommended Components
- Hybrid Steppers (Closed Loop Steppers) - The Middle Ground
- 750W AC Servo Motor Kits (Standard Automation)
About the Author:
This article is written by a mechanical design engineer specializing in mechatronics, motion control systems, and automation component sizing.
This article is written by a mechanical design engineer specializing in mechatronics, motion control systems, and automation component sizing.
As an Amazon Associate, I earn from qualifying purchases.
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