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ByBill Dimitrakis andRob Profilet

The Lubrizol Corp., Wickliffe, OhioDESIGN BASICS

But there are a few steps you can follow to help

make your choice. First, determine the type of

equipment where you will use the fluid. Second,

consider the severity of the duty cycle. And third,

determine what temperatures the equipment will

experience during use.

Viscosity, antiwear levels are key in

industrial use

Industrial hydraulic systems in normal ser-

vice indoors generally use a conventional an-

tiwear-type hydraulic fluid with appropriate

viscosity. Choosing the right viscosity fluid is

important because it

ensures t he syst ems

best overal l perfor-

mance and tha t the

components are pro-

tected. The antiwear

agents contained in

the oil protect the hy-

draulic pump compo-nents from premature

wear.

F igur e 1 summa-

rizes ISO 32, 46, and

6 8 , t h e m o s t c o m -

monly used viscosity

grades because they

have the ap propriate

viscosity at the tem-

peratures usually seen

in industrial hydraulicsystems. The tempera-

ture operating range

is based on the lowest

Wouldnt it be great if there were a single list

you could consult to help you pick the right

hydraulic fluid for your equipment? The closest

thing is the National Fluid Power Associations

recommended practice T2.13.13-2002, Fluid

Viscosity Selection Criteria for Hydraulic Motors

and Pumps*. It offers guidance but not definitive

answers.

Unfortunately, fluid selection is not as simple as

wed like. Every application is different, and you

need to pay attention to equipment manufacturers

requirements, specifications, and recommenda-

tions as well.

Go with the flow

Operating temperature ranges for

conventional hydraulic fluids

200

180

160

140120

100

80

60

40

20

0

-20

-40

220

Temperature

F

10 15 22 32

ISO viscosity grade

46 68 100

90

-27

111

-9

131

5

147

18

163

28

183

39

201

50

Consider the equipment it will be used in and viscosity

when choosing your hydraulic fluid.

Figure 1. ISO 32, 46, and 68 offer the most appropriate viscosity levels for industrial

hydraulic systems.

in selecting the right fluid

CourtesyLubrizolCorp.


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Different fluids

for changing needs

The challenge is different with

mobile equipment used for off-

road construction, timber harvest-

ing, municipal waste and utilities,

for example. Mobile equipment can

see high pressures,

severe duty cycles,

a n d w i d e t e m -

perature extremes

during year-round

use in areas with

seasonal climates.

Their fluid needsto work well at low

temperatures for

winter time start

up, whi le main-

t a i n i n g e n o u g h

viscosity at high

tempera tures to

protect during se-

vere service or hot

weather.

The idea l hy-draulic fluid for

and highest temperatures at which

the viscosity remains between 860

cSt and 13 cSt, the range recom-

mended by many equipment man-

ufacturers for full load operation.

Figure 2 provides mo re detail on

ISO VG 46 fluids.

Operating temperature ranges of

ISO VG 46 hydraulic fluids220

180

140

100

60

20

-20

Temperature

F

100 150 175 200 245Viscosity index

Comparison of multigrade

to conventional fluids

Hydraulic fluid B

Hydraulic fluid A

Multigradehydraulic fluid

-20 30 100 150

Temperature F

Log

viscosity

Figure 3. Viscosity index for multigrade fluids shows how

they offer both low-temperature fluidity and high-temperature

viscosity.

Figure 2. ISO VG 46 operates in a wide range of extreme temperatures.

CourtesyLubrizo

lCorp.


mobile applications is one that flows

like a lighter oil at low temperatures

but protects like a heavier oil at high

temperatures: a multigrade hydrau-

lic oil. This is similar to using SAE

5W-30 or 10W-30 multigrade en-

gine oil in our cars, which can be

used year-round without problems.

In Figure 3, note that the multigrade

fluid has the low-temperature flu-

idity of the lighter Fluid A, while

maintaining the high-temperature

viscosity of the heavier Fluid B.

Multigrade and conventional

fluids

What differentiates multigrade

hydraulic fluids from conventional

fluids is that they contain polymer

additives, called viscosity modifi-

ers (VM) also known as viscosity

index improvers in addition to

the hydraulic performance package.

These VMs keep the fluid viscosity

more consistent over the tempera-

ture range encountered during use.

Multigrade fluids are also known

as high viscosity index (VI) fluids.

VI is a measure of how much a flu-

ids viscosity changes with tempera-

ture. A higher VI number means the

viscosity changes less as the fluid

heats and cools, which is desirable.

Conventional fluids typically have

VI around 100. By comparison, a

multigrade fluid should have VI of

at least 140. There are fluids with VIhigher than 200 for use in such tem-

perature extremes as the Arctic or

where viscosity must remain steady

across a wide temperature range.

See Figure 3.

Perks of multigrade fluids

Five of the benefits of using a good-

quality, high VI-multigrade fluid are

that it:

1. Maintains a more consistent viscos-ity as temperature changes, so the system

continues to perform properly.


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DESIGN BASICS

2. Flows better at low temperatures

for improved cold-weather opera-

tion. The equipment will experience

fewer problems with cavitation, slug-

Operating temperature range

Recommendedoperating

range Volumetric efficiency

Mechanical efficiency

Overall efficiency

Viscosity

Efficiency

Figure 4. Operating within the recommended viscosity range helps

ensure that equipment is protected and that it operates mostefficiently.


gishness, drift, or

shudder.

3 . Mainta ins

viscosity at high

temperatures to

protec t sys tem

components and

maintain overall

efficiency.

4 . P r o v i d e s

greater mechani-

cal and volumet-

ric effectiveness,

which improves

energy efficiency

and reduces fuelconsumption.

5. Can be used

year-round and over wide temperature

ranges.

Operating within the recommended

viscosity range helps ensure that

equipment is protected and that it op-

erates most efficiently, as shown in the

graph in Figure 4.

When choosing a hydraulic fluid,

remember to ask your supplier if it

is approved by your equipment

manufacturer and for performance

information. Make sure that the fluid

that will be delivered to you is clean

and water free so it will perform at its

best. Finally, be sure to conduct used

oil analysis as recommended by your

supplier to understand when your oil

may need attention.

For more information, contact

[email protected].

*National Fluid Power Association

recommended practice T2.13.13-2002

Fluid Viscosity Selection Criteria for

Hydraulic Motors and Pumpsmay be

purchased at www.nfpa.com.

Copyright 2008 by Penton Media, Inc.

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