210.0   General information on lubricants

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Overview  

General

Functions to be fulfilled by lubricant are just as important as those of structural components. Lubricants, too, should be adapted to all the stresses occurring in a component or vehicle to meet all the technical demands imposed with regard to their lubricating functions. The base oil, the composition of the lubricants, as well as the kind and the share of the additives are subject to high demands, since the efficiency, the service life and the operational safety of a vehicle for all the parts requiring lubrication depend to a great extent on the quality of the respective lubricant.

Viscosity

The viscosity (flow resistance) is the ability of a lubricating oil to establish internal resistance (friction) against distortion (relative motion between two fluid layers). The viscosity rating indicates the flowability of the lubricating oils (e.g. in the low and high temperature range).

Dynamic viscosity

The SI unit (SI = Système International d'Unités) for dynamic viscosity is the Pascal second (Pa s).

Measuring unit: 1 Pa s = 1 Ns/m2.

Viscosity conversion: 1 mPa s = 1 cP (centipoise).

Kinematic Viscosity

The quotient of dynamic viscosity and density of the lubricant is the kinematic viscosity.

The SI unit is m2/s.

Viscosity conversion: 1 mm2/s = 1 cSt (centistoke).

Viscosity temperature response (VT response)

The VT response of a lubricating oil indicates the change in viscosity under the influence of temperature. The viscosity of lubricating oils changes with temperature and pressure, that of structurally viscous oils also under the influence of the shear rate. Generally viscosity decreases as temperature rises.

Viscosity index (VI)

The viscosity index identifies the viscosity temperature characteristics of lubricating oil. It is a calculated number on a conventional, graduated scale which indicates the viscosity change of a mineral oil under the influence of temperature. A high viscosity index identifies a relatively minor change of viscosity in relation to temperature and vice versa. The viscosity index can be calculated from viscosity values which were measured at 40 °C and 100 °C.

SAE viscosity grades

The Society of Automotive Engineers (SAE) has established a classification for the two most important lubricating oils of motor vehicles, engine and gear oils, with regard to their viscosity, which was included in the respective national standards. The SAE class indicates the viscosity at low and high temperatures. Viscosity is of importance at low temperatures for the cold start and at high temperatures, for example during wide open throttle operation, for adequate lubricating properties. The SAE-grades specified for MB vehicles can be found on Sheets 224.1/.2 and 231.1/.2/.3.

SAE-grades for motor oils

SAE J 300 defines the SAE viscosity grades for engine oils (refer to Sheet 211.0). In today's customary multigrade oils, two numbers are stated, e.g. SAE 10W-40. The number before the "W" describes the flow properties of the engine oil when cold and the number after the "W" describes the viscosity in the high-temperature range. For oils classified with "W", the cold flow behavior is defined by means of two limits to be adhered to, the "cold cranking viscosity" and the "low temperature pumping viscosity" (both dynamic viscosities in mPa s). In the high-temperature range, the classification of the engine oil in an SAE-grade depends both on its kinematic viscosity (in mm2/s) at a temperature of 100 °C as well as on the dynamic viscosity (in mPa s) at 150 °C and high shear rate 106 1/s (=HTHS high-temperature high shear viscosity).

SAE grades for gear oils

The SAE viscosity grades for gear oils are standardized in SAE J 306. As for engine oils, the SAE viscosity grades with the letter "W" (e.g. 75W) indicate a limit for the cold flow behavior of the gear oil. The limit defines the temperature at which the dynamic viscosity of the gear oil must be less than or equal to 150 000 mPa s. The viscosity characteristics at 100 °C are defined for all SAE grades by the limits for the minimum kinematic viscosity as well as for SAE grades 80, 85, 90 and 140 in addition by a limit for the maximum kinematic viscosity.