Several billion Euros of economic losses are caused every year through friction and wear. In order to reduce these cost extensive tribological basic research is carried out. On this basis, companies then occupy themselves with specific knowledge, such as OKS Spezialschmierstoffe GmbH with the development of high-performance lubricants.
What is friction?
Friction is the mechanical resistance to the relative movement of two surfaces. Friction is usually undesirable in technical systems, because it is associated with energy loss, friction heat and wear.
Determining the coefficient of friction
The following equation is used to determine the friction (to Coulomb).
Friction can be divided into sliding friction, pivoting friction, rolling friction and rolling resistance friction.
The tribological system
For an optimal problem solution all the influencing variables in a tribological system have to be known. Allowances have to be made for the complex interactions of these factors. Ambient influences (dust, temperature or moisture) and structural factors (material, surface or geometry of the friction bodies) play just as great a role as stress factors (speed, pressure stress or vibrations) as far as selecting the correct intermediate material (= lubricant) is concerned.
The course of the coefficient of friction of a friction bearing with oil or grease lubrication in the various friction and lubrication states can be described using the Stribeck curve as an example. During the start-up phase the static friction is followed by the solid state friction (high coefficient of friction/high wear). As speed increases a partial separation of the sliding surfaces takes place in the mixed friction phase by the lubricating film (medium coefficient of friction/medium wear). The emergency running film that is formed by solid lubricants protects at exactly this point (see red curve). At high speeds a hydrodynamic liquid film separates the sliding surfaces completely from each other (as at aquaplaning). In this phase of liquid friction the lowest wear and the lowest friction is achieved.
Multiple function of the lubricants
The functions of a lubricant can be varied and, depending on the particular application, can be necessary alone or in combination. Besides the primary demand placed on the lubricant – maximum power transfer combined with a minimum of friction and minimum wear – it is often necessary to fulfil various secondary properties such as water resistance, chemical resistance, compatibility with plastics or corrosion protection.