By Bill Dimitrakis and Rob Profilet
The Lubrizol Corp., Wickliffe, Ohio
DESIGN BASICS
Go with the flow
in selecting the right fluid
Consider the equipment it will be used in and viscosity
when choosing your hydraulic fluid.
W
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 service indoors generally use a conventional antiwear-type hydraulic fluid with appropriate
viscosity. Choosing the right viscosity fluid is
important because it
ensures the system’s
Operating temperature ranges for
b e s t ove r a l l p e r f o rconventional hydraulic fluids
mance and that the
220
components are pro200
tected. The antiwear
201
180
agents contained in
183
160
the oil protect the hy163
140
draulic pump compo147
131
120
nents from premature
111
100
wear.
90
80
Figure 1 summa50
60
rizes
ISO 32, 46, and
39
40
68, the most com28
18
20
monly used viscosity
5
-9
0
grades because they
-27
-20
have the appropriate
-40
viscosity at the tem10
15
22
32
46
68
100
peratures usually seen
ISO viscosity grade
in industrial hydraulic
systems. The temperaFigure 1. ISO 32, 46, and 68 offer the most appropriate viscosity levels for industrial
ture operating range
hydraulic systems.
is based on the lowest
Temperature – ˚F
Courtesy Lubrizol Corp.
ouldn’t 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 Association’s
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
we’d like. Every application is different, and you
need to pay attention to equipment manufacturers’
requirements, specifications, and recommendations as well.
Courtesy Lubrizol Corp.
220
mobile applications is one that flows
like a lighter oil at low temperatures
but protects like a heavier oil at high
temperatures: a multigrade hydraulic oil. This is similar to using SAE
5W-30 or 10W-30 multigrade engine oil in our cars, which can be
used year-round without problems.
In Figure 3, note that the multigrade
fluid has the low-temperature fluidity of the lighter Fluid A, while
maintaining the high-temperature
viscosity of the heavier Fluid B.
Operating temperature ranges of
ISO VG 46 hydraulic fluids
Temperature – ˚F
180
140
100
60
20
-20
100
150
175
Viscosity index
200
245
Figure 2. ISO VG 46 operates in a wide range of extreme temperatures.
Different fluids
for changing needs
The challenge is different with
mobile equipment — used for offroad construction, timber harvesting, municipal waste and utilities,
for example. Mobile equipment can
see high pressures,
severe duty cycles,
Comparison of multigrade
and wide temperature extremes
to conventional fluids
during year-round
use in areas with
Hydraulic fluid B
seasonal climates.
Their fluid needs
to work well at low
Multigrade
temperatures for
hydraulic fluid
winter time start
up, while maintaining enough
viscosity at high
Hydraulic fluid A
temperatures to
protect during severe service or hot
-20
30
100
150
weather.
Temperature – ˚F
The ideal hyFigure 3. Viscosity index for multigrade fluids shows how
draulic fluid for
Log viscosity
Courtesy Lubrizol Corp.
and highest temperatures at which
the viscosity remains between 860
cSt and 13 cSt, the range recommended by many equipment manufacturers for full load operation.
Figure 2 provides more detail on
ISO VG 46 fluids.
they offer both low-temperature fluidity and high-temperature
viscosity.
Multigrade and conventional
fluids
What differentiates multigrade
hydraulic fluids from conventional
fluids is that they contain polymer
additives, called viscosity modifiers (VM) — also known as viscosity
index improvers — in addition to
the hydraulic performance package.
These VMs keep the fluid viscosity
more consistent over the temperature range encountered during use.
Multigrade fluids are also known
as high viscosity index (VI) fluids.
VI is a measure of how much a fluid’s viscosity changes with temperature. 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 VI
higher than 200 for use in such temperature 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 goodquality, high VI-multigrade fluid are
that it:
1. Maintains a more consistent viscosity as temperature changes, so the system
continues to perform properly.
gishness, drift, or
shudder.
3. Maintains
Recommended
operating
viscosity at high
Volumetric efficiency
range
temperatures to
protect system
components and
maintain overall
Mechanical efficiency
efficiency.
4. Provides
greater
mechaniOverall efficiency
cal and volumetric effectiveness,
Viscosity
which improves
Figure 4. Operating within the recommended viscosity range helps energy efficiency
ensure that equipment is protected and that it operates most
and reduces fuel
efficiently.
consumption.
5. Can be used
2. Flows better at low temperatures year-round and over wide temperature
for improved cold-weather opera- ranges.
tion. The equipment will experience
Operating within the recommended
fewer problems with cavitation, slug- viscosity range helps ensure that
Operating temperature range
Efficiency
Courtesy Lubrizol Corp.
DESIGN BASICS
Copyright © 2008 by Penton Media, Inc.
equipment is protected and that it operates 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 Pumps may be
purchased at www.nfpa.com.