Tribologie V

Vacuum distillation
This type of distillation is used for the manufacture of lubricating oils, because distillation at normal pressure (atmospheric distillation) would break (crack) the larger hydrocarbon molecules – see distillat
 

Vaseline
Natural or artificial mixtures of solid and liquid hydrocarbons made of petroleum, free of odour and taste. Natural vaseline is made of petrolatum by means of refining.
 

Viscogram
Sheet for measuring and recording the viscosity obtained for mixtures.
 

Viscometer
Device for determining the viscosity of fluids. A distinction is made between: capillary, rotation, falling-ball and efflux viscometers.
 

Viscosity
Viscosity is the characteristic of a fluid to offer resistance to the interacting laminar movement (deformation) of two adjacent layers (internal friction, shear stress): DIN 1342, DIN 51 550, DIN ISO 3104.
 

Viscosity classes
See SAE classes and ISO viscosity classification.
 

Viscosity index (VI)
It is a calculated figure on a conventional scale which describes how the viscosity of a mineral oil or synthetic oil product changes with the temperature. A higher viscosity index indicates a smaller change in viscosity with temperature than a lower viscosity index. Calculation of the VI from the cinematic viscosity: DIN ISO 2909, ASTM D 2270.
 

Viscosity index improver
Active ingredients (viscosity improver, polymers), which - when dissolved in the mineral oil - improves the viscosity temperature characteristics, i.e. they reduce the temperature dependency of viscosity; at low temperatures they improve the flow properties and at high temperatures they achieve a higher viscosity than would be the case without VI improvers. VI improvers are important constituents of multi-grade lubricating oils. The most important viscosity index improver groups are: Polymethacrylates (PMA), olefin copolymers (OCP), polyisobuthylene (PIB) and styrene-butadiene copolymers (SCB) or styrene-isoprene copolymers (SIC).
 

Viscosity temperature characteristics (VT)
The VT of a fluid is how the viscosity alters as a function of the change in temperature – see viscosity index.
 

VISCOUS FRICTION
Exists when rubbing surfaces on top of each other are completely separated by a lubricant so that there is no direct contact. No wear occurs. The viscosity, temperature, shear rate and the pressure properties of the lubricant determine the viscous friction.ion.
 

Volatility loss (NOACK)
Volatility loss of lubricating oil at higher temperatures (up to 350°C); it is particularly important for engine and cylinder lubrication. At the high temperatures that occur, a high volatility loss may be equivalent to a higher oil consumption and may lead to a change in the characteristics of the oil: DIN 51 581 (Noack Test).