Mechanical Design: What are "moment (of force)", "torque", and "turning moment"? What are the differences between them?

Has anyone else here mistakenly thought that "moment," "torsional torque," and "driving torque" all mean the exact same thing and share the same calculation formulas?

To be honest, after working in non-standard automation for so many years, I've always assumed there was no difference between them. I've used the definitions interchangeably, applied the formulas casually, and done my drafting and component selection purely based on habitual experience. But the more I dug into it, the more I realized:

If they feel so similar in meaning, why do they have different names? In some scenarios, why can't the formulas be used universally? And more importantly, using the wrong one can directly affect the safety of motor, gearbox, and shaft component selection!

I'm sure many mechanical designers, newcomers to the industry, and self-taught beginners share this same confusion. In daily drafting, selection, and transmission calculations, we casually say "moment," "torsional torque," or "driving torque" without ever really examining the differences, when they are interchangeable, and when they absolutely cannot be mixed up.

Today, I've sorted out this essential knowledge clearly. I'll explain the definitions, differences, applicable scenarios, and calculation formulas in one go. After reading this, you'll never get them mixed up again, and you'll be able to accurately apply them for future selections, load calculations, and shaft strength verifications.

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I. The Relationship Between the Three: Moment is the General Term; Torsional Torque and Driving Torque are Specific Types of Moment

Many of you probably, like me, assumed these three terms meant the same thing. I only recently learned that they do have distinctions.

In fact, Moment is the broad, general concept. It covers the widest scope. As long as a force causes a tendency for an object to rotate, it can be called a moment.

Torsional torque and driving torque, on the other hand, are specialized sub-categories derived from the general moment. They are specific terms used exclusively in mechanical transmission, rotating machinery, and shaft loading—but they differ in their application scenarios and the nature of the forces involved.

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II. What is Moment (of Force)? Application Scenarios and Formula

Moment is the effect of a force that produces rotation. It is the rotational effect of a force around a point or an axis. However, it is not limited to dynamic rotation; it also applies to static states, such as forces on levers or support points.

Common scenarios: Linkage rod forces, fixture clamping forces, hinge loads, cantilever loads, and static overturning moment calculations.

Formula:
M = F × L
(Force × Lever arm), unit: N·m.

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III. What is Torsional Torque (Strength-Oriented)? Application Scenarios and Formula

Torsional torque describes the twisting or shearing destructive force acting on a shaft. It is used to describe the torsional load on transmission shafts, ball screws, and rotating axles, with a focus on strength, load safety, and fracture verification.

Simply put: the force that twists or breaks a shaft is the torsional torque.

Common scenarios: Transmission shaft strength verification, ball screw loading, keyway checking, shaft fatigue life, and structural torsion resistance calculations.

Formula:
T = F × D / 2
Where: D = drum/sprocket diameter (m), F = linear tensile force (N).

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IV. What is Driving Torque (Power-Oriented)? Application Scenarios and Formula

Driving torque focuses on the active driving force and rotational output power. It is the amount of rotational force that a motor or gearbox actively delivers to drive a load. It emphasizes power selection and load matching.

Simply put: the "muscle" that the motor uses to turn the equipment is the driving torque.

Common scenarios: Servo motor sizing, stepper motor matching, belt drives, chain drives, and turntable lift load calculations.

Formula:
T = 9550 × P / n
(Where P = power in kW, n = speed in rpm).

Note: In everyday spoken language, "torsional torque" and "driving torque" are often used interchangeably. However, in strict engineering practice, the distinction is clear: use driving torque for power/motor selection, and use torsional torque for strength/failure checks.

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V. The Core Differences Between the Three