2.3.
2 Viscosity Index
This value shows how emperature affects the viscosity of an oil. The viscosity ofoil
Areases with increase in temperature, and vice versa.
The rate of change of viscosity with temperature is indicated on an arbitrary scale
called Viscosity Index (V).
The lower the viscosity index, the greater the variation in viscosity with changes
in temperature and vice versa.
Viscosity index is computed by comparing the change of viscosity of the oil between
the temperatures at 38C and 100°C, with the two reference oil havinga viscosity index of
0and 100 respectively. The viscosities ofunknown oiland two reference oils are the same
at 100°C. The viscosities of the three oils are measured at 38°C.
L-U
Viscosity index VI = -x100
L-H
L-SSU viscosity of reference oil at 38°C with a viscosity index of 0.
H-SSU viscosity of reference oil at 38°C with a viscosity index of 100
U-SSUviscosity of oil at 38°C whose viscosity index is to becalculated.
2.3.3 Oxidation Stability
The most important property of an hydraulic oil is its oxidation stability. Oxidation is
caused by achemical reaction between the oxygen of the dissolved air and the oil. The
oxidation of the oil creates impurities like sludge, insoluble gum and soluble acidic
products. The soluble acidic products cause corrosion and insoluble products make
the operation sluggish.
The rise of system temperature accelerates the oxidation. Below 55°C, the oxidation
is low. But above 65°C, for every 9°C rise in the system temperature the life of oil is
decreased by 50%. So the safest upper temperature is 50°C and the best working
temperature is 38°C. Ifthe temperature goes above 65°C, an oil cooler must be installed.
Indiana oxidation test is used for finding the oxidation stability of oil. It is carried
out by heating 300 cm² of oilto 172°C in aglass tube and bubbling air through the oil
at the rate of litres /hour. To find the sludge present at a given time, a 10gram sample
of the oil is diluted with 100 cm' of petroleum naphtha. When the oil goes into the
solution, the sludge or oxidised oil precipitates. It may be filtered and measured. The
amount of sludgeis repored in milli grams per 10 grams of oil. The test is normally run
tor 150 hours. Sludge determinations are run on samples taken after 50,100,150 hours.
Upon conclusion of the test the sludge figures are plotted against time in hours.
2.3.4 Demulsibility
he abilityof a hydraulicfluid to separate rapidly from inoisture and successfully
resist emulsification is known as Demulsibility".
Toil emulsifies with water, the emulsion will promote the destruction oflubricating
value and sealant
properties.
Highly refined oils are basically water resistant by nature.
2.3.5 Lubricity
Wear results in increased clearance which leads tooall sorts of operational
including tall of etticiency. Atthe time of selecting athydraulic
2.3.6 Rust Prevention
l oil,
care
dif cultiex
to selectone which willbe able to lubricate the moving parts efficiently must taken
be
The moisture entering intothe hy draulic system with air causes the parts made
materials to rust. This rust if passed through the precision made pumps and
nay scratch the nicely polished surfaces. So, additives named 'inhibitors are valves fer ous
the oil to keep the moisture away from the surface. added to
2.3.7 Pour Point
The temperature at which an oil willcongeal is referred to t as the pour
lowest temperature at which the oil is able to flow easily. It is of point, i.e., the
great
cold countries where the systems are exposed to very low temperatures. importance in
2.3.8 Flash Point and Fire Point
Flash point is the temperature at which a liquid gives off vapour in sufficient quantity
to ignite momentarily or flash when a flame is applied.
The minimum temperature at which the hydraulic fluid willcatch fire and continue
buming is called the fire point.
2.3.9 Neutralisation Number
The neutralisation number is a measure of the acidity or alkalinity ofa hydraulic fluid.
This is referred to as the PH value ofthe fluid. High acidity causes the
oxidation rate in
an oil to increase rapidly.
Aweighed amount of sample in titration solvent is titrated with a standard alcoholic
potassium hydroxide solution to a definite end point. The indicator is alpha-naphthol
benzein solution. The colour change is from orange to green or bluegreen.
Neutralisation number = Total ml of titrating solution
Weight of sample used -x5.61
Asatisfactory hydraulic fluid must have the following properties.
a. Chemical stability to prevent formation of sludges, gums, carbon or other
deposits which clog openings, cause valves and pistons to stick or leak.
b. Freedom from acidity so that the fluid is
non-corrosive to the metals in the system.
c. Lubricating properties sufticient to avoid
wear in moving parts.
d. Satisfactory
viscosity and
adequately within the normalviscosity index so that the oil will perform
e
Pour point well below the temperature ranges of the machinery.
f. Flash point as high as minimum temperature expected in normaloperation.
g. Minimum toxicity. possible for safety and to prevent possible evaporation.
