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Lubrication is a science and it is intended that «TRIBOLOGY FROM A TO Z» will provide you with a thorough scientific yet easily understandable glossary of all the relevant terms. We hope that it will prove to be an informative yet fascinating read!


Association des Constructeurs Europeéns d’Automobiles.
Successor organisation of CCMC and CLCA. Represents the European automobile industry in respect of operating supplies (fuel and lubricants). The specifications originally issued by the CCMC (organisation that proceeded the ACEA) are still valid: G 4, G 5, D 4, D 5 and PD 2. (See CCMC.)

Active ingredients (additives)
Additives which are added to mineral oils, mineral oil products and synthetic oils
as substances soluble in oil. As a result of a chemical and/or physical effect, they change or improve the characteristics of lubricants, fuels, heating oils, etc. in respect of characteristics such as oxidation stability, flow characteristics and anti-knock properties.

High-molecular hydrocarbons and highly polymerised compounds, aluminium soaps, etc., the purpose of which is to increase the cohesion inherent in the oil. Boosts adhesion properties of oil films on metal surfaces through structural viscosity characteristics, mostly linked to an increase in viscosity.

Oil-soluble substances added to lubricants to obtain and optimise certain characteristics. For example, through their chemical or physical action they influence: viscosity temperature characteristics, pour point, flow characteristics,
oxidation stability, foam formation, detergent action, etc.

Undesired chemical change in mineral and synthetic products during use and storage.

Ageing characteristics
These are used to assess the length of time for which lubricating oil can be used for the specific application.

AGMA Specifications
American Gear Manufacturers Association. Lubrication recommendations for open and closed industrial gear units. Classification into nine viscosity grades.

Air filter oils
Odour-free and ageing-resistant oils for cleaning the air intake
(20 – 100 mm2/s40). White oils must be used for special systems.

Aluminium complex lubricating greases
are highly resistant to water, are easy to handle and hardly exude any oil. Have a high dropping point (to over 230°C) and good high-pressure characteristics. Application temperatures up to approx. 160°C.

Coolant, which in its concentrated form contains approx. 90% ethylene glycol and propylene glycol, inhibitors, additives, colorants and a very small amount of water. Specifications for coolant are what are termed company specifications stipulated by the individual vehicle manufacturers. They also specify the mixing ratios.

Anti-knock resistance
It specifies the performance (resistance) of a spark-ignition engine in respect of detonation during combustion in the engine: DIN 51 756 P 1 to P 6, RON (Research Octane Number), MON (Motor Octane Number), FON (Front Octane Number) and ROON (Road Octane Number).

Chemical additives that slow down the ageing of oil considerably.

Anti-wear additives
Active ingredients which aim to reduce wear in mixed friction areas; a distinction is made between:
a) Mild-acting additives such as fatty acids, fatty oils (highly polar, surface-active substances), metal dithiophosphates, etc.
b) High-pressure additives made of lead, sulphur, chlorine, phosphorous connections, etc.
c) Solid lubricants such as graphite, molybdenum disulphide, etc. – see EP additives.

The American Petroleum Institute classifies the requirements of various American engine manufacturers into a system of service categories (classifications). These differentiate between gear oils for passenger cars, engine oils for spark-ignition engines and diesel engine oils.
The American Petroleum Institute uses the following service categories:

API classification for engine oil (commercial categories)
Engine oils for diesel engines are classified into CA to CG categories. For these lubricant categories, too, the same evolution can be seen as for the service classes. Today we are particularly interested in CD categories for heavy duty diesel engines with and without turbocharging, CE for very heavy duty and high-speed diesel engines with and without turbocharging and sharply fluctuating loads. After 1994 CD was superseded by CF. The newly created CF-2 for two-stroke diesel engines and the CF-4 specification, which supersedes CE and defines additional values such as those that relate to volatility loss and piston cleanliness, are of interest. Since June 1994 the CG specification has also included emissions limits for the latest generation of engines.

API classification for engine oil (service categories)
These specifications are very common.
Engine oils for spark-ignition (petrol) engines are classified in the categories SA to SH. SA generation products hardly used to contain any additives (non-compounded). The subsequent SB and SC generations contained mild compounds with active substances to prevent carbonisation, cold sludge, ageing, corrosion and wear. In 1968 the SD category satisfied the increased demands of the vehicle manufacturers. The next generations of SE, SF and SG engine oils for the toughest requirements met the requirements for more powerful engines and new traffic situations (stop-and-go traffic) in the 1970s and 1980s. Since 1993 the SH specification has been used. This also specifies the volatility loss (NOACK), filterability, foaming behaviour and flash point.

API classification for passenger car gear lubricants
Gear oils rated GL 1 to GL 5. A product with the required rating is used depending on the application, design of the gear mechanism and the load. For instance, GL1 is a non-compounded oil for gear trains and worm gears as well as helically toothed and spiral-cut final drives with minimal loads. Nowadays the greatest demand in the market is for GL 4 and GL 5 specifications. The GL 5 rating covers gear oils for highly loaded hypoid axle drives and, in some cases manual and special gearboxes as well. GL 6 oils for very highly loaded final drives have now been withdrawn.

Hydrocarbon compounds with cyclic molecular structures. They produce viscosity temperature characteristics (see VI) in lubricants and have a bad effect on resistance to oxidation. Added to fuel for spark-ignition engines to prevent combustion knock. The percentages added are increasingly being limited on account of their carcinogenic effect.

Mineral deposits that remain after lubricants have been burnt, in the form of oxides (oxide ash) or sulphate (sulphate ash after sulphuric acid has been added).
A mixture made of bitumen and mineral building materials (sand, etc.).

ATFs are special lubricants with specific and high requirements for automatic transmissions. What are required are very good temperature characteristics and shear stability, high oxidation stability and excellent anti-foaming properties and air release characteristics. The specifications currently used are very extensive.
Here are the most recent ones: GM Dexron III, FORD Mercon, MB 236.8, etc.

Aviation lubricant which is adapted to the special requirements of the aircraft engine through its processing and composition. Mainly synthetic oils.

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