Friction and friction coefficient - determination of friction values of materials

The friction coefficient is a physical variable derived from the field of tribology for the friction between two objects. The friction coefficient sets the force that occurs during friction (frictional force) in relation to the force with which the objects are pressed together (pressing force). The friction coefficient is thus an important parameter when examining material wear and sliding properties. This article explains the basics of the friction coefficient, its measurement methods and its applications in technology.

What is dry friction?

General friction is the resistance between two solid surfaces and delays relative movement in the opposite direction.

Dry friction is a special type of friction when there is no lubricant or liquid between the surfaces. Dry friction depends largely on the roughness of the contact surfaces.

When liquids or lubricants play a role, this is called fluid or liquid friction. In other media (e.g. air or water) however, it is referred to as air friction or flow friction.

Friction can be observed in many industrial applications and situations, such as turning a screw into an internal thread. Or when threaded nuts move along a screw drive (e.g. in 3D printing). The goal is usually to minimise friction and thus increase the wear resistance of the system.

Types of dry friction

Dry friction can be divided into two categories:

• Static friction: Static friction occurs when the two surfaces are in contact but have not yet been moved against each other.
• Dynamic friction: Dynamic friction occurs when an external force is large enough to initiate movement between two surfaces.

These two categories of dry friction show different behaviour.

Static friction

Static friction (also known as adhesive friction) occurs when the force applied is not large enough to initiate movement and the object remains static or in balance.

Calculating static friction force

The static friction coefficient (μ_s) describes the ratio between the normal force (F_N) and the resulting reaction force or adhesive friction (F_H)before the movement begins - i.e. at rest:

The following applies to the normal force in the inclined plane with friction angle:

The following applies to the normal force in the plane without friction angle:

The friction coefficient is always unitless and is determined experimentally. In most cases, the friction coefficients of various material pairings (e.g. steel on steel) have already been determined and can be found in relevant specialist literature - see also "Materials and table with friction coefficients".

Dynamic friction

Dynamic friction (also known as kinetic friction) occurs when the force applied is large enough to set the object in motion.

Calculating dynamic friction force

The dynamic friction coefficient (μ_d) describes the ratio between the friction force (F_R) and the normal force (F_N) during the movement between the surfaces:

The following applies to the normal force in the inclined plane with friction angle:

The following applies to the normal force in the plane without friction angle:

The friction coefficient is always unitless and is determined experimentally. In most cases, the friction coefficients of various material pairings (e.g. steel on steel) have already been determined and can be found in relevant specialist literature - see also "Materials and table with friction coefficients".

Experimentally determining friction coefficients and friction values

The friction coefficients for static friction and dynamic friction must be experimentally determined as they depend on various factors such as surface texture and roughness, speed of movement and environmental conditions.

Experimental determination of friction coefficients and friction values requires the precise performance of friction tests under controlled conditions.

• Design a suitable test setup that allows two material samples or surfaces to rub against each other. The setup should allow an external force or weight to be applied to initiate friction and control movement.
• Select the materials for which you want to determine the friction coefficient and ensure that the surfaces are clean and free of contamination. The surfaces should provide a representative depiction of the actual application conditions.
• Prepare the surfaces of the material samples carefully to minimise unevenness due to contamination. Clean surfaces contribute to reproducible results.
• Check the environmental conditions and keep them constant in every test conducted. Carry out the tests in a controllable environment where they can keep as many environmental factors as possible constant. This mainly affects the air pressure (Δp constant), the temperature (ΔT constant) and the humidity.
• Perform the friction tests. Measure the applied forces and the resulting reaction forces or friction forces while the movement is taking place or when you try to initiate the movement.
• Repeat the friction tests several times for meaningful data.
• Calculate the friction coefficients (μ_s and μ_d) based on the measured data. Use the appropriate formulas to determine the friction coefficients or friction values for the selected material combination. Note the environmental conditions as well.

During the test, measure the following forces:

• Measure the static friction force on the spring force meter shortly before the object is set into motion.
• Measure the sliding friction force on the spring force meter while the object is moving.

Then calculate the friction coefficients:

Measuring accuracy and sensitivity are decisive in order to obtain accurate data. The friction coefficients determined can depend greatly on the specific application conditions.

Experimental determination of the friction values can be time-consuming and expensive. Nevertheless, it is essential to improve understanding of the frictional properties of materials and to develop efficient technical applications. Careful planning, precise execution and statistical evaluation are required to achieve accurate and reliable results.

Importance of friction in industrial applications

Friction plays a central role in a variety of industrial applications and is a fundamental physical phenomenon that not only brings advantages, but also challenges.

In many technical systems, such as engines, gear units or bearings, it is necessary to control or minimize friction to reduce energy losses and wear and improve efficiency.

• Motion control and braking systems: Friction is used in braking systems to control and slow down the movement of machines. Targeted exploitation of frictional properties enables precise control and safety.
• Adhesive friction and stability: In many applications, such as standing on an inclined surface, static friction is crucial to ensure stability and prevent slippage.
• Material wear and service life: Friction can cause material wear, which can reduce component service life. It is important to understand frictional properties to minimize wear and maximize component service life.
• Material selection: Knowledge of the frictional properties of materials is crucial when selecting materials for specific applications. The friction values must be taken into account in order to select optimal material combinations for specific purposes.
• Lubrication: Efficient lubrication is crucial to reduce friction and wear in many mechanical systems and extend their service life.

Influence of friction on wear

Most industrial applications have the following objectives:

• minimize wear
• maximise the efficiency of the system
• maximize the service life of the system

Friction, lubrication, roughness and wear form a dynamic system and are mutually dependent.

The scientific background of friction and wear is being explored in the field of tribology - the teaching of friction, lubrication and wear on components. All industrial applications, where mechanical components work together or meet, can be considered a so-called tribological system.

The mutual interactions must be taken into account, particularly in long-term applications:

• Temperature and other environmental conditions can affect friction properties. At higher temperatures, materials can soften, which can lead to changed friction. On the other hand, a high temperature can also lead to lubricant failure or increased wear.
• Wear of the contact surfaces (e.g. abrasion) can influence the friction properties in the long term. If material wears off or comes loose from the contact surfaces, this can lead to a change in friction factors. Increased wear can also lead to increased friction and deterioration of performance.
• Lubrication, whether in the form of liquids or solids, plays an important role in influencing friction. Suitable lubrication can reduce friction and minimise wear. However, poor lubrication or lack of lubrication can lead to increased friction and wear.

In all industrial applications, it is important to consider the interactions and carry out regular wear checks.

Measures to increase friction

In some industrial applications it may be important to increase component friction. For example, to prevent loosening of screw connections.

To increase friction, the following measures are in place, for example:

• Increase surface coarseness or roughness: Roughening a surface can increase friction. One possibility of roughening is the so-called blasting (e.g. sweep blasting) in which the surface is changed directly. Another option is surface treatment, in which a sheathing is applied to the base material - for example by hot galvanising.
• Use friction additives: Additives can be added to certain machine oils to increase friction.
• Use adhesives or padding: Applying adhesives or pads can increase friction. Teflon tape or thread-locking fluid are suitable for example, in screw connections. These agents can also have a sealing effect.

Materials and table with friction coefficients

Below is an overview of the dry friction coefficients of typical material pairings.