Viscosity is the measure of internal resistance to motion of a fluid due to cohesion between its own molecules it is measured in mill poise and centipoises. The viscosity of lubricants is measured by standard instruments like redwood, Engler or saybolt viscometers. If the viscosity of the lubricants is too low then the lubricant film will not be maintained between the two moving surfaces leading to wear and tear machine parts. On the other hand, if the viscosity of the lubricant is too high then it leads to excessive friction.
The viscosity of lubricant also varies with temperature. It increases with a decrease in temperature which leads to poor lubricating property due to thickening of lubricant in temperature which leads to poor lubricating property due to the breaking of lubricants film leading to wear and tear machine.
The viscosity of good lubricants as the should not vary with temperature.
Viscosity index (V.I) is defined as the rate at which viscosity of the lubricant oil changes with respect to temperature.
The viscosity of lubricants with temperature. The rate at which viscosity of lubricants changes temperature is indicated by assigning a value (from 0 to 100) for liquid lubricant in the arbitrary viscosity index scale. A lubricant whose viscosity changes rapidly with temperature has value in viscosity index scale. Similarly, lubricants index scale.
The viscosity index of mineral or petroleum oil is increased by adding certain high molecular weight substance
The significance of viscosity index
- Lubricants function over a wide range of operating temperature.
- If viscosity index of the gets squeezed out. hence, high viscosity index lubricants are required for proper working at high temperature or high load.
- If low viscosity index lubricants are used in low-temperature condition then they solidify and will resist the free movement of the machine lubrication.