Hello,
CAN anyone tell me is there any allowed limits (min max) for Viscosity index of hydraulic oils?

Most mineral based hydraulic oils I have come across typically have a VI from 95 to 100. Others with Viscosity Index modifiers will have VI ranging from 150+ (depending on the amount and type of polymer used as a modifier). Certain biodegradable hydraulic oils will have VI of 200 or greater.
The VI of a hydraulic oil is determined by the type of base oil used, if it contains a viscosity index modifier, what type of VI modifier and how much is present. It also depends on how much this polymer is sheared in service.

Confirming all that ReneWA has said.
Most mineral oil, not VI improved, has VI circa 95, VI improved oils VI 150 -200.
VI improved oils generally necessary where use in a wide ambient temperature range is required, however this improvement is not permanent,as the VI additive is subject to a shearing process and its efficiency is reduced with use. Furthermore at high shear rates(found in pumps, some valves) does the additive provide any real benefit(tempory shear)?
Synthetic oils PAO, Esters and some hydrotreated mineral oils(not VI additive treated) overcome this problem at a significant cost penalty.

Dear Andja,

I would sugest the question should be what is the intended service for the hydraulic oils. If the service is normal 0 to 100 Deg C, then a very low VI, usually associated with Group I mineral oils 95 - 110 is usually appropriate. When the service is more severe, then a VI Improver is appropriate to maintain the required viscosity over the service range. Good luck.

Is the higher the VI, the better the oil is able to adapt to temperature changes?

yes!
Consider multigrade engine oils for summer and winter use (SAE5W-30). These have higher VI than monogrades(SAE30)
If you plot viscosity v temperature on a Refutus Chart a straight line is obtained, the VI merely gives the slope of the line(flatter for High VI)

quote:

If the service is normal 0 to 100 Deg C, then a very low VI, usually associated with Group I mineral oils 95 - 110

Huh?
a LOW VI oil would be like a 0-40
a Mid VI would be 50-80
a High Vi would be 90-100
a Very High VI would be 110-130

bruce

Peggy,

The higher the VI, the better the oil's capability to adapt to change in temperature. Hydraulic oils are generally considered high VI if the VI is above 150 or so.

Because the rate of change of viscosity with temperature is not as great with high VI hydraulic oils, it allows for better flow at start up (low temperatures) which provides better wear protection and mechanical efficiency particularly for pumps. At high operating temps, the oil does not "thin out" as much (compared to a hydraulic oil with VI of say 100) therefore again providing better wear protection.

Some cases where high VI products are used are where higher than normal operating temperatures are an issue.

In other cases not only is the VI important but the actual shear stability of the polymer used is also important particularly in very high speed application. Shear stability meaning the ability of the polymer to stay in one piece rather than chopped up into smaller bits.

I hope this helps.