What Are Moment, Torque, And Rotational Torque? Their Differences

What Are Moment, Torque, and Rotational Torque? Their Differences

In engineering and physics, moment, torque, and rotational torque are frequently used terms. They share the same dimension (Newton-meter, N·m) but differ in emphasis and application scenarios. Moment is a general mechanical concept; torque focuses on torsional effects; rotational torque is mostly used in rotating machinery and motors.

1. Moment of Force (Moment)

Definition

Moment is the physical quantity describing the rotational effect of a force on an object, equal to the product of force and the moment arm (the perpendicular distance from the line of action to the axis of rotation). It follows the right-hand rule for direction.

Core Features

Basic physical concept, not limited to rotating shafts

Emphasizes the geometric relationship between force and reference point/axis

Static, no actual rotation required

Formula

M = F × d

M: Moment (N·m)

F: Applied force (N)

d: Length of moment arm (m)

Typical Applications

Lever balance, prying heavy objects, opening doors, structural mechanics, statics calculation.

2. Torsional Moment / Torque

Definition

Torque specifically refers to the moment that causes an object to twist or rotate around its own axis, commonly used in shafts, transmission systems, and fasteners. It reflects the torsional load on shafts during power transmission.

Core Features

Focuses on torsional effect, directly related to shaft strength and stiffness design

Vector with clear direction

Widely used in mechanical and automotive engineering

Formula

T = F × r

T: Torque (N·m)

F: Applied force (N)

r: Perpendicular distance from force to rotation axis (m)

Typical Applications

Engine output torque, torque wrench setting, drive shaft verification, bolt pre-tightening torque.

3. Rotational Torque

Definition

Rotational torque is similar to torque but emphasizes the moment that sustains continuous rotation around an axis. It is widely used in motors, rotating machinery, and automation, often coupled with angular velocity and power.

Core Features

Describes output characteristics of rotating power sources

Closely related to kinematics and dynamics

Often paired with terms like rated torque, starting torque

Formula

T = I × α

T: Rotational torque (N·m)

I: Moment of inertia (kg·m²)

α: Angular acceleration (rad/s²)

Typical Applications

Motor rated torque, wind turbine main shaft torque, robot joint drive torque, rotating stage output torque.

Core Differences

Dimension Moment Torque Rotational Torque

Definition General rotational effect of force Moment for torsional transmission Moment for sustained rotating drive

Focus Force–point/axis relationship; static/dynamic Torsion, shear stress, power transmission Rotation stability, power matching, dynamic response

Fields Theoretical mechanics, statics, structures Automotive, fasteners, shaft design Motors, robotics, rotating machinery

Unit N·m (same dimension) N·m N·m

Why Are They Often Mixed?

Same physical essence and dimension (force × length)

Translation history: English torque is translated as both “torque” and “rotational torque”; moment is translated as “moment”

Industry habits:

Mechanics textbooks: moment

Automotive / mechanical engineering: torque

Motor / automation: rotational torque

Usage Guide

Lever balance, structural force → Moment

Drive shaft, bolt tightening, engine → Torque

Motor parameters, rotating mechanisms, robots → Rotational Torque