TechArchive

This article was originally written in the period 1995-2000

Lubrication is too important to approach simplistically and without understanding. This guidance from Kluber Lubrication covers the basic lubricant types

Lubricants are divided into the following groups: gaseous, liquid, cohesive and solid. Among these, the gaseous lubricants are insignificant because construction costs for gas or air lubrication are very high.

Lubricants should not only reduce friction and wear, but also dissipate heat, protect surfaces, conduct electricity, keep out foreign particles and remove wear particles.

Liquid lubricants

The advantages of a lubricating oil, as compared to a grease, are improved heat dissipation from the friction point and excellent penetrating and wetting properties. The main disadvantage is the complex design required to keep the oil at the friction point and prevent the danger of leakage. Liquid lubricants include fatty oils, mineral oils and synthetic oils

Fatty oils are not very efficient as lubricating oils. Even though their lubricity is usually quite good, their resistance to temperatures and oxidation is poor. Mineral oils are most frequently used as lubrication oils, but the importance of synthetic oils is constantly increasing. These offer higher oxidation stability, resistance to high and low temperatures And long-term and lifetime lubrication. Anti-corrosion and special release agents are special products which also fulfil lubrication tasks.

Cohesive lubricants

Cohesive lubricants are used when the lubricant should not flow off, because there is no adequate sealing and/or when resistance against liquids is required. The lubricant types play an essential role nowadays, since it is possible to achieve long-term or lifetime lubrication with minimum quantities. Cohesive lubricants include lubricating greases, lubricating pastes and lubricating waxes. Their task is to protect surfaces, conduct electricity and keep out foreign particles.

Lubricating waxes are based on hydrocarbons of high molecular weight and are preferably used for boundary or partial lubrication at low speeds. Wax emulsions also contain an emulsifier and water.

Starting with a certain temperature, lubricating waxes typically change their structure from a coherent to a fluid state. The melting point depends on the waxes’ ingredients, and their structure is reversible. If the tribological requirements are mainly about corrosion protection, a coherent structure is of advantage.

The advantages of lubricating waxes and wax emulsions over traditional lubricants are their excellent inherent lubricity and special anti-corrosion properties. In addition, they provide a non-tacky protective film when applied below their melting point.

Their main disadvantage is the lack of heat dissipation until the incorporated water has evaporated. Lubricating waxes do not flow below their melting point, which is of special importance for relubrication.

Lubricating greases are based on a base oil and a thickener, which imparts the cohesive structure and one or more additives. They can be used for elasto-hydrodynamic, boundary or partial lubrication.

Complex greases generally have a higher drop point, are more resistant to oxidation, liquids and vapours. Synthetic thickeners are most resistant to temperature.

The main advantage of a lubricating grease over an oil is that it remains at the friction point for a longer time and that less effort is required in terms of design. Its disadvantage is that it neither dissipates heat nor removes wear particles from the friction point.

Lubricating pastes contain a high percentage of solid lubricants. Used in the case of boundary and partial lubrication, especially for clearance, transition and press fits, they are mainly applied under extreme conditions and prevent fretting corrosion, stick slip and adhesive wear.

Depending upon their composition, lubricating pastes are resistant to water and water vapour and have good anti-corrosion characteristics. Metal-containing pastes may be suitable for service temperatures up to 1200degC. Lubricating pastes can be classified in terms of:

• solid lubricant types (MoS<2>, graphite, metals, PTFE, other plastics)
• base oil (synthetic oil, mineral oil and mixtures)
• application range (lubricating and assembly paste, high temperature paste, conductive paste, etc.)
• special characteristics (colour, EP properties, etc.)

Solid lubricants

Solid lubricants include tribo-system materials, tribo-system coatings and dry lubricants for tribo-systems.

Their main task is to protect surfaces. Solid lubricants also include synthetic, metallic or mineral powders, such as PTFE, copper, graphite and MoS<2>. As powders are difficult to apply, they are mostly used as additives. Solid lubricants are normally used as dry lubricants operating under boundary lubrication conditions. If liquid or cohesive lubricants are incorporated in the tribo-system materials, there can even be partial lubrication.

Solid lubricants are mainly used when the application of liquid or cohesive lubricants is not ideal for functional reasons, or when there is risk of contamination and when, at the same time, the lubrication properties of solid lubricants are sufficient.

• Kluber Lubrication
• Tel: 01422 205115
• Fax: 01422 206073