Motor torque terminology can easily become confusing because different torque types describe different physical conditions. To make the distinctions clearer, several practical tests were conducted on spur and helical gear motors and combined with engineering principles. This article summarizes what each torque term truly means and how motors and gearboxes behave in real-world situations.
Why Motor Torque Terms Are Often Misunderstood
Similar-sounding names—stall torque, cogging torque, holding torque, backdrive torque, rated torque—often refer to entirely different operating conditions:
- Whether the motor is powered or unpowered
- Whether the gearbox is attached
- Whether the torque is short-duration or continuous
- Whether motion is forward or reverse (backdriven)
Once these conditions are identified, each torque definition becomes straightforward.
1. Stall Torque
Definition: Stall torque is the maximum torque a motor produces when powered at zero speed. According to Maxon Motor, stall torque represents the theoretical peak torque the motor can produce at zero speed.
Characteristics:
- Determined by current and torque constant
- Represents short-term peak torque
- Generates maximum internal heat
- Scales strongly with motor size, winding design, and magnet strength
During testing, motors with higher rated torque consistently showed higher stall torque, consistent with the expectation that stronger motors produce higher peak torque.
2. Rated Torque
Definition: Rated torque is the continuous torque a motor can sustain without overheating. Maxon defines rated torque based on thermal and continuous operation limits.
- Always substantially lower than stall torque
- The most relevant value for long-duration applications
In practical operation, rated torque—not stall torque—determines whether a motor will overheat or achieve long service life.
3. Cogging Torque (Detent Torque)
Definition: Cogging torque is a small, periodic torque ripple present when a permanent-magnet motor is unpowered. Faulhaber explains that cogging torque originates from the magnetic interaction between rotor magnets and stator slots.
Key Insight: Cogging torque does not necessarily increase with higher motor torque. Its magnitude depends on:
- Stator slot geometry
- Magnet strength
- Pole-slot combination
- Whether skewed slots are used
Bench tests confirmed that motors with significantly different rated torque can exhibit similar cogging torque, showing that cogging is governed by magnetic geometry rather than motor size.
4. Holding Torque
Stepper Motors: Holding torque refers to the torque an energized stepper can resist without losing position.
DC / BLDC Motors: The term is used inconsistently; some manufacturers define it as mechanical holding capability (often gearbox-related), while others refer to electromagnetic holding with current applied. Interpretation always depends on context.
5. Backdrive Torque
Definition: Backdrive torque is the torque required at the gearbox output to force the system to rotate backward. It indicates the torque needed to turn the motor and gearbox from the output side.
Primary Influences:
- Gear ratio
- Gear type (spur, helical, planetary, worm) Harmonic Drive
- Internal friction
- Lubrication viscosity
- Motor cogging and bearing friction (amplified by the gearbox) MIT OpenCourseWare
Backdrive torque determines whether the output shaft can be manually rotated when the system is unpowered.
What Real Bench Tests Reveal
Test 1: Helical-First-Stage Spur Gear Motor (HT-SOG37D2 with 545 DC Motor)
- Gear ratio: high multi-stage reduction
- First gear stage: helical
- Condition: unpowered motor attached
Observation: The output shaft could not be rotated by hand. After separating the motor from the gearbox, the gearbox output rotated smoothly and easily.
Interpretation: Helical gears introduce extra sliding friction and axial load. Combined with motor cogging + bearing friction, the high gear ratio amplified small motor-side friction into very large output resistance. This configuration behaved almost self-locking despite using spur gears after the first stage.
Test 2: Spur Gear Motor (HT-SOG30 series, high ratio)
- Pure spur gear stages
- High overall reduction ratio
- Condition: no motor attached
Observation: Even without the motor connected, the gearbox resisted backdriving at very high ratios.
Interpretation: Accumulated friction across multiple spur stages, load on shaft bearings, and lubrication drag made manual rotation difficult at high reduction ratios.
What the Tests Prove
- A high-torque motor does not guarantee a high cogging torque. Cogging torque is driven by magnetic geometry.
- Backdrive torque is usually dominated by the gearbox, not the motor. Gear ratio and gear type have a larger effect than motor friction.
- Small motor cogging torque becomes significant only after gearbox amplification.
- Helical gears significantly increase backdrive torque due to sliding friction and axial loads.
Practical Engineering Takeaways
- Stall torque and rated torque scale with motor strength; cogging torque does not.
- Backdrive torque is mainly a gearbox property.
- High gear ratios magnify every small internal friction source.
- Helical gears increase friction and reduce backdrivability.
- Spur gears backdrive more easily, but still resist reversal at very high ratios.
- Applications requiring manual release should avoid high-ratio or helical gear trains.
- Applications requiring self-locking can use high ratios or helical/worm gear structures.
Quick Comparison Table
| Torque Type | Powered? | Gearbox Influence | Main Purpose |
|---|---|---|---|
| Stall Torque | Yes | No | Peak torque capability |
| Rated Torque | Yes | No | Continuous loading |
| Cogging Torque | No | Slight (amplified by ratio) | Smoothness / startup behavior |
| Holding Torque | Yes / depends | Varies | Position holding |
| Backdrive Torque | No | Strong | Manual override / self-locking |
Conclusion
Understanding torque definitions becomes simpler once operating conditions are separated into powered vs. unpowered, and motor vs. gearbox contributions. Real-world testing demonstrates that cogging torque is governed by magnetic geometry, while backdrive torque is dominated by gearbox friction and ratio. Accurate interpretation of these torque terms leads to better motor selection, more reliable designs, and fewer surprises during integration.
