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maintenancec ircleWord for the day: 32-46-68

Hydraulic system very plainly referred as power packs is one of the most widely used methods of power

transmission which completes heavy and repetitive tasks with ease. Applications varying from a simple hand operated

palette trolley to wing-control system of an aero plane depend on hydraulic system for proper and reliable functioning

throughout its life cycle. Hydraulic oil or fluid is the most important element in any hydraulic system and helps in

transmitting power from the source pump for example to the end user, cylinders for example. Now let us

understand some of the properties of hydraulic oil and how it influences the performance of a system.

To perform the task successfully, hydraulic oil should possess certain characteristics, of which Viscosity is the most

important one. Viscosity is the measure of a fluids resistance to flow. If a fluid is flowing easily, its viscosity is low

and if it flows with difficulty, its viscosity is high. For instance, water flows easier than glycerin and hence has alesser viscosity. There are two types of viscosity measurements: Absolute or Dynamic viscosity and Kinematic

Viscosity.

Absolute or Dynamic viscosity is defined as the amount of force required to slide one block by

unit distance in unit time over another block separated by known thickness of the oil, at a fixed

temperature. This viscosity value is mentioned in following units: N s / m2

(Newton second per

square meter), Kg / m s (Kilogram per meter second). Since these units are very large in number,

the value is also given in Poise (p). 1 Poise = 1 g / cm-s (one gram per centimeter

second) = 1 / 10 Kg / m s. Poise is usually a large unit and hence the commonly and

commercially used terminology is Centipoise which is one hundredth of a poise.

The density and temperature of a fluid influences its viscosity, both of which needs to be considered in preciselydefining its value. Kinematic viscosity is defined as the ratio of Absolute or Dynamic viscosity of a fluid to its density.

Since m2

/s (meter square per second) is a very large unit, it is further divided into two smaller values and are denoted

in Stokes (St) and Centistokes (cSt).

1 stoke (St) = 1 cm2

/ second = 10-4

m2

/ s and 1 centistoke (cSt) = One Hundredth of a stoke. Centistokes is themost commercially used measure of viscosity and is always specified at 40C as the reference temperature. 32, 46, 68

are some of the widely used oil grades in hydraulic systems and refers to Kinematic viscosities in centistokes (cSt)and are defined as per ISO standards. Let us take 68 grade oil as an example to understand the viscosity values. The

density of 68 grade oil is 865 Kg / m3.

68 Grade = 68 centistokes = 68 mm2

/ second = 68 x 10-6

m2

/ second @ 40C

Absolute viscosity = Kinematic Viscosity x Density = 68 x 10-6

m2

/ second x 865 Kg / m3

= 0.0582 Kg / m-s =

58.2 Gram / m-s

So, when two blocks are separated by a fine layer of 68 grade oil, a force of 58.2 grams will move one block by one

meter in one second. Therefore, higher the viscosity number, higher the force required to move the block. Following

table gives a brief description of various hydraulic grades and its SAE (Society for Automotive Engineers, a standard

certifying body of USA) equivalent grades.

NNEEWWSSLLEETTTTEERR FFOORR MMAAIINNTTEENNAANNCCEE CCOOMMMMUUNNIITTYY

Figure 1

smsmKg

mKg

mKg

smKg

Density

itysAbsoluteViiscoKinematicV /2

3

3/

/cos,sity =

=

==v

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Viscosity, centistokes (mm2 / second)Density, kg / cubic

meterISO GradeEquivalent SAE

GradeAt 40C At 100C

32 10W 32 5.4 857

46 20 46 6.8 861

68 20W 68 6.7 865

100 30 100 11.4 869150 40 150 15 872

220 50 220 19.4 875

High viscosity oil is desirable for a hydraulic system since it maintains proper sealing between mating surfaces of

cylinder-piston area, valve-spool area etc. but has following disadvantages:

1. It increases the resistance to flow, resulting in increased power consumption2. High temperature caused by friction3. Pressure drop increases resulting in energy loss4. Chances of slow and sluggish operation of the actuators

On the other hand, low viscosity oil results in following disadvantages:

1. Increased internal leakage2. Excessive wear of sliding and rotating components3. Reduction in pump efficiency

So, for any system selection of hydraulic oil is always a compromise and following points should be taken into

consideration at the design stage itself.

1. Pressure and flow requirement2. Duty cycle of the equipment3.

Criticality of the equipment4. Location of the equipment (since temperature affects viscosity)

5. Product quality expected from the equipment6. Initial investment costs7. Operating temperatures

Since temperature plays a vital role in maintaining viscosity, it is important to select an oil that has almost stable

viscosity within given temperature range. Concurrently, a good heat exchanger system should be installed in place tomaintain constant temperature, generally around 40C. Following table gives range of viscosities for commonly used

hydraulic oil grades. Oil life reduces by 50% for every 10C rise in temperature after 40C.

Kinematic Viscosity Limits, at 40C

ISO GradeMidpoint Kinematic

Viscosity, cSt at40C

Minimum Maximum

32 32 28.8 35.2

46 46 41.4 50.8

68 68 61.2 74.8

100 100 90.0 110

150 150 135 165

220 220 198 242

With advancement in technology, synthetic oils are being developed which not only have wider operating

temperatures, but also eliminate the need to use different grades for different purposes. This oil will last much longer

than conventional hydraulic oils, whose average life is around 5000 operating hours.