Basics of Tribology

What is tribology

"Tribology is a branch of science and engineering dealing with the interaction of surfaces moving relative to one another. It addresses the entire field of friction, wear and lubrication, and includes interaction between boundary surfaces of solid bodies as well as those of solid bodies, fluids and gases". (Definition acc. to former DIN 50323)

What does a tribo-system look like?

How is friction generated and how can it be prevented

Whenever surfaces adhere to one another, slide or roll against one another, friction is generated. The forces acting between the surfaces are referred to as friction forces. The most important cause of friction forces lies with the surface condition of the friction bodies. Friction forces always act against motion, impeding or preventing it.

On rough surfaces, large forces are required to move the bodies relative to one another. On smooth surfaces, small forces are sufficient. In practice, friction cannot be eliminated completely. Even on extremely smooth surfaces, some, albeit very low, friction will occur. Friction causes abrasion/wear.

A fluid, i.e. a lubricant, interposed between the contact surfaces of the friction bodies will have a smoothening effect. The fluid penetrates into the microscopically small valleys of the surface. A fluid layer separating the friction bodies forms. This is what we call lubrication. The fluid separating layer is referred to as lubricant.

Using the right lubricant contributes to separating the friction bodies and minimising wear. This leads to longer service life of the component, less energy consumption to move it and hence lower costs.

What types of friction are there?

Sliding friction is a type of dynamic friction between bodies sliding against one another (brake, plain bearing, piston in cylinder.

Rolling friction is also movement friction . The bodies that roll on each other theoretically touch each other in a point or line shape (point contact: sphere; line contact: cylinder). 
The normal forces cause elastic deformation at the contact point. When surface contact occurs, sliding occurs in the edge zones, which is referred to as deformation-related micro-sliding.

Rolling friction is another type of dynamic friction. The bodies rolling against one another theoretically have point or linear contact (point contact: ball; linear contact: cylinder). The normal forces cause elastic deformation at the contact zone. As the surfaces come into contact, sliding occurs on the edge of the contact zone, which is referred to as micro-sliding due to deformation.

Combined rolling and sliding friction may occur at the same time in the relative motion of two friction bodies in contact to one another.

Boring friction occurs between two bodies where one body rotates about an axis perpendicular to the contact point surface of the other body.

What friction conditions are there?

The friction condition has a major influence on the friction and wear behaviour in the components' contact zones. We distinguish between the following three friction conditions: boundary friction, mixed friction and fluid friction.

The Stribeck curve describes the characteristic of the friction force as a function of the friction speed under hydrodynamic lubrication. Hydrodynamic lubrication means that lubricant enters the lubrication gap due to the relative motion of two contact surfaces. The pressure of the lubricant causes separation of the two contact surfaces.

In boundary layer lubrication, the lubricant penetrates into the solid body, forming a reaction layer. The load then acts on the friction bodies' roughness peaks.

In mixed lubrication, the load acts partly on the lubricant film and partly on the roughness peaks touching each other.

In full-fluid lubrication, it is the lubricant which fully bears the load. The contact surfaces are completely separated.

What types of lubricant are there?

What are the effects of lubricants?

Why are lubricants so important for the success of your company?

Matching specialty lubricants help reduce unplanned downtime caused by friction and wear, extend the maintenance intervals in machines and installations, reduce costs (e.g. operating costs, maintenance and repair costs), thereby increasing the profitability of your enterprise.

Do you wish to acquire more in-depth knowledge of the subjects tribology, lubrication technology and lubricants?

Then the trainings offered by Klüber Lubrication might be an option for you. Twice to three times per year, we offer two-day trainings in Munich. Alternatively, you can order individual trainings tailored to your needs on the subject of "lubrication and lubricants" to be held on your premises.

Do you have any questions or need consulting?

Klüber Lubrication can look back on more than 90 years of experience in the fields of friction optimisation and specialty lubricants. We will be pleased to provide personal advice for your specific requirements regarding refrigeration compressor oils.

This is how you can reach us:
Stefan Zuber
Global Business Team Compressors
Phone: +49-89-78 76 502
Email: Stefan.Zuber@klueber.com

Website: www.klueber.de

Editor and copyright: Klüber Lubrication München SE & Co. KG
Reprints, total or inpart, are permitted only prior consultation with Klüber Lubrication München SE & Co. KG and if source is indicated and voucher copy is forwarded. The data in this document is based on our general experience and knowledge at the time of publication and is intended to give information of possible applications to a reader with technical experience. It constitutes neither an assurance of product properties nor does it release the user from the obligation of performing preliminary tests with the selected product. It constitutes neither an assurance of product properties nor a guarantee of the suitability of the product for a specific application. They do not release the user from from the obligation of performing preliminary field tests with the product selected for a specific application. All data are guide values which depend on the lubricant's composition, the intended use and the application method. The technical values of lubricants change depending on the mechanical, dynamical, chemical and thermal loads, time and pressure. These changes may affect the function of a component. We recommend contacting us to discuss your specific application. Products from Klüber Lubrication are continually improved. Therefore, Klüber Lubrication reserves the right to change all the technical data in this document at any time without notice.

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