What is meant by the viscosity index or viscosity index (VI) is a number that indicates the range of changes in the viscosity of a hydraulic fluid associated with changes in temperature. Thus the viscosity index is used as a basis in determining the characteristics of the hydraulic fluid viscosity related to temperature changes. Regarding the viscosity index is specified in DIN ISO 2909. Hydraulic fluid is said to have a high viscosity index when the viscosity changes little (stable) over a relatively large temperature changes.
Or it can be said that the hydraulic fluid can be used in a range of temperature changes significantly. Hydraulic fluids, especially hydraulic oil is expected to have a viscosity index (VI) = 100. In fact, most of the hydraulic oil given additional ingredients (additives), called “VI improvers” to increase the VI becomes higher than 100. Hydraulic oil with high viscosity index also called multigrade oils , To determine the viscosity change is noticed Ubbelohde ‘s viscosity – temperature diagram below (figure 3).
Figure 3. Ubbelohde ‘s viscosity -temperature diagrams
Hydraulic fluid must have certain characteristics in order to meet the requirements in carrying out its functions. Characteristics or attributes required include:
A. Viscosity (Viscosity) pretty.
Hydraulic fluid must have sufficient viscosity in order to fulfill its function as a lubricant. If the viscosity is too low, the oil film is formed to be very thin and is unable to withstand friction.
B. Viscosity Index are good.
With good viscosity index, the viscosity of hydraulic fluid used on the system will be stable with temperature changes work quite volatile.
C. Fireproof (non-flammable)
Hydraulic systems often operate in places that tend to arise or adjacent fire with fire. It is therefore necessary fire-resistant fluids.
D. No foaming (foaming)
When many foamy hydraulic fluid will result in a lot of air bubbles trapped in the hydraulic fluid so that there will be compressable and will reduce the power transfer. In addition, the presence of foam was terjilat possibility of fire will be greater.
E. Hold cold
The meaning is that the cold-resistant hydraulic fluids not easily freeze when operating in cold temperatures. Freezing or melting point that is desired by the hydraulic fluid ranged between 100-150 C below the temperature at the beginning of the engine is operated (start-up). This is to anticipate the block (blocked) by hydraulic fluid that freezes.
F. Resistant to corrosion and wear resistant.
Hydraulic fluids should be able to prevent corrosion due to corrosion does occur then the construction will not be easy to wear in other words, the machine will last.
G. De mulsibility (Water separable)
What is meant by de-mulsibility is the ability of a hydraulic fluid to separate water from hydraulic fluids. Why water must be separated from the hydraulic fluid, because the water will cause corrosion when it comes to metal.
H. Minimum Compressibility
In teorotis fluid is uncompressible (can not be compressed). But the fact of hydraulic fluid can be compressed up to 0.5% by volume for each pressure 80 bar. Therefore, it is required that a hydraulic fluid to be relatively compressible or if compressible possibility is very small.