Torsion: How to understand torsion

Basic knowledge Construction

There are many different types of mechanical loads that affect an object. One of them is torsion. In this blog article, 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. Torsion occurs when an object is rotated in a manner that results in an uneven deformation of the object.

Typical assembly parts in mechanical engineering affected by torsion are rotation shafts, drive shafts and motor shafts, as they have to constantly absorb torques.

These elements are often made of steel or aluminum. Under special operating conditions, carbon fiber reinforced plastic or other high-performance materials are also used.

The materials used should have a good rigidity and offer strong resistance to torsion.

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.

Since the rope is wound on a drum, a distance is created between the rope and the center of the axle, which, in conjunction with the weight force pulling on the rope, results in a torsional moment. The resulting torsional moment acts on the shaft and this is subjected to torsion. The stiffness of the shaft must counteract this moment.

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

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

T = F \times r

Twist angle

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

\theta_{t}=\frac{T}{D} = \frac{T\times L}{G \times l_{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 motors, gear units and screwdrivers. They are undesirable because these vibrations may lead to the complete destruction of an application.

Wait a second! How can torsion be used in mechanical applications?

A typical use of torsional forces is torsion springs.

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

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

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

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

Other types of torsion springs are torsion bar springs, spiral torsion springs and roller springs.

An example of an application range for torsion springs is the use as a spring hinge in self-closing door elements or as a pre-tensioning element for ratchet door handles.

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