In this article, we answer the common question “what is torque?”
Although torque refers to rotary motion, it is a fundamental concept in linear motion applications. Rotary motors produce torque, and when that torque is transmitted to a drive system such as a screw, belt and pulley, rack or pinion, or chain and sprocket, it is converted into linear motion.
What is exerted torque?
Torque is the rotational equivalent of force. Specifically, it is a force exerted at a distance from an object's axis of rotation . In the same way that a force applied to an object will cause it to move linearly, a torque applied to an object will cause it to rotate around a pivot point. The point of rotation is known as the axis of rotation and the perpendicular distance of the force to the axis of rotation is known as the moment arm. This is why torque is also called moment of force.
In the simplest scenario, the applied force is perpendicular to the axis of rotation. In this case, torque is simply the product of the force and its distance from the axis of rotation:
But in many applications, the force does not occur perpendicular to the axis of rotation. In this case, the angle of the force must be taken into account to find the length of the moment arm:
The pair is commonly referred to with a capital “T,” but the correct symbol is the Greek letter tau, “τ.” When torque is referred to as moment of force, the symbol “M” is used.
Torque is a vector quantity, which means it has magnitude and direction. The direction of torque can be found using the right-hand rule. Curl the fingers of the right hand in the direction of d (or, for the radius) to the direction of F. When this is done, the thumb points in the direction of the pair.
When sizing a linear motion application, it is important to understand the torque requirements and limitations of all system components. The required torque is one of the key parameters for sizing a motor. In most applications, you need to consider peak torque (which typically occurs during acceleration or when holding a load) and continuous torque, or RMS. Additionally, shafts and other components that transmit torque throughout the transmission, such as gear shafts, couplings, and worm shafts, must also be capable of withstanding the maximum torque of the application.
The terms “torque” and “moment” are often used interchangeably. Although the two are similar, torque produces rotation about an axis, while moment is a force applied over a distance that does not produce rotation. For example, a screw has an axis of rotation; therefore, a force applied some distance from this axis will cause the screw to rotate. A profile rail, on the other hand, is fixed and does not rotate. A force applied perpendicular to the rail will produce a moment but will not induce rotation.
