How to understand torsion.

Basic knowledge Construction

There are many different types of mechanical loads that affect an object, one of which is torsion. In this blog we will look at the basics of torsion and explore some examples.

What is torsion?

Torsion is a type of load that acts on an object when a torsional force is exerted on it. The torsion occurs when an object is rotated in a manner that results in an uneven deformation of the object.

Typical torsionally stressed components in mechanical engineering are rotation shafts, drive shafts and engine shafts, as they have to constantly absorb torques.

These elements are usually made of steel or aluminium. Carbon reinforced plastic or other high-performance materials are also used for special operating conditions.

The materials used should have good rigidity and high torsional strength.

Example of torsion

A common example of torsion occurs in cable winches (e.g. a crane lifts a load). As soon as the cable winch rotates, it pulls on the cable but the cable counteracts the pulling by weight force.

As the cable is wound onto a drum, there is a gap between the cable and the centre line of the shaft, which results in a torsional moment in conjunction with the weight force pulling on the cable. The resulting torsional torque acts on the shaft and this is subjected to torsion. The rigidity of the shaft must counteract this moment.

- (a) - Cable winch with shaft
- (b) - Cross-section of the shaft with torsional torque F

Theory of torsion

The theory of torsion states that an object subjected to a torsional force will deform in a manner proportional to the torsional force. This means that the deformation of an object is directly proportional to the size of the rotational force.

Torsional torque

The torsional torque is a measure of the rotational force of an object. It is specified as a product from the radius of the object and the rotational force. It can be calculated in many ways, but the most common method is the use of torque reaction arms. The torque reaction arm is a measured value that indicates to what extent the torque works on an object.

The torsional torque T is calculated by multiplying the force F acting on the lever by the length r of the lever.

The load leads to a twisting of the bar, which is specified as the angle of twist (θ_t)

T - Torsional moment
D - Directional moment
L - Bar length (torsion bar)
G - Shear modulus
I_T - Torsional moment of inertia, describes the size and shape of the bar cross section

Torsional vibrations

Torsional vibrations are a type of mechanical vibration triggered by the rotation of a solid body around its longitudinal axis. These vibrations can occur in many mechanical applications such as engines, gear units and screwdrivers. They are able to convert rotational movements into vertical movements and vice versa.

Just a moment! How can torsion be used in mechanical applications?

A typical use of torsional forces is torsion springs.

Torsion springs are springs that are loaded by a torque. They are often used in applications where a torque or rotational force is required.

Leg springs are a form of torsion spring. They consist of a wire with a circular cross-sectional area and can be either right- or left-wound.

The load-bearing capacity of the leg spring depends on the material as well as the material cross-section and leg length.

A higher spring force can be achieved with torsion springs by using several torsion springs on one axle.

Other types of torsion springs are torsion bar springs, spiral torsion springs and torque coil spring.

A typical application for torsion springs is as a spring hinge in self-closing door elements or as a tensioning element for ratchet catches.

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