Gear oil performs the following functions:
- lubricates parts (bearings, seals, synchronizer couplings, etc.);
- creates a protective film on the working surfaces of gears that reduces friction and prevents the formation of scoring;
- removes wear products from friction surfaces;
- cools the parts.
Gear oil is classified by viscosity and performance properties.
SAE J306 classification (from English. Society of Automotive Engineers - Association of Automotive Engineers, USA) determines the viscosity properties of gear oil. This standard has international recognition and is used by all oil manufacturers. Transmission oils are divided into winter, summer and all-weather. The viscosity of winter oil is indicated by a number with the letter W (what does winter mean? «winter»). The viscosity of winter oils is determined for four classes - 70W, 75W, 80W and 85W. Class 75W allows temperatures down to -40°C.
The viscosity of summer oil is marked only with a number without a letter. Classes 80, 85, 90, 140 and 250 are defined. The figure characterizes the viscosity of the oil at 100°C. The larger the number, the higher the viscosity.
In everyday use, it is more convenient to use all-weather grades of gear oil. The viscosity grade of multigrade oil is marked with an alphanumeric index consisting of two values indicating the viscosity of winter and summer oil, for example, 75W-80.
API classification (from English. American Petroleum Institute - American Petroleum Institute, USA) divides gear oils into groups depending on performance.
This standard is also recognized by the international automotive community. Currently, there are six groups of gear oil, four of which (GL-1, GL-2, GL-3 and GL-6) are not used in cars. The GL-4 group is used in transmission units with a spur gear. Group GL-5 is used in transmission units with hypoid gear. Oil manufacturers in catalogs or on labels often list in detail the characteristics of components and assemblies where the use of each grade of gear oil is acceptable. You should not think that GL-5 oil is better than GL-4, since it is designed for heavy loads. Moreover, the use of GL-5 oil is contraindicated in some types of gearboxes, where the operation of synchronizers is disrupted as a result.
Gear oils are produced that meet the requirements of both the fourth and fifth groups. The packaging of such oil is labeled GL-4/5.
These classifications define generalized requirements for the performance properties of gear oil. When selecting oil, car manufacturers conduct comprehensive tests, as a result they choose the most suitable oils for use in their car units.