By Rob Profilet
Lubrizol Corp.
Are all mineral-based
hydraulic oils
the same?
These tips will help you
choose the right hydraulic
oil for your application.
n this month’s MRO feature, Rob Profilet from Lubrizol Corp. answers pertinent questions about the differences in
hydraulic oils and how choosing the right
one results in peak operation. Profilet is
commercial manager of hydraulics and
gear oils and works at Lubrizol’s headquarters in Wickliffe, Ohio.
I
Why is it difficult to choose the
right fluid?
Approximately 200 million gal of
hydraulic oils are used in the U.S.
annually. About 165 million gal of
that volume are mineral based. The
end user has to sort through a multitude of suppliers, brands, and performance levels to determine which
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fluid is best for his operation.
A fluid supplier will help the end
user by providing a product data sheet
for the hydraulic oil, which will typically list its physical and performance
properties and any original equipment
manufacturer (OEM) approvals or
specifications it meets. But even with
performance data and pricing in hand,
it can still be difficult to differentiate hydraulic oils and choose the fluid
that provides the most value.
What performance properties
are important?
To appreciate the differences in
hydraulic oils, it helps to understand
what performance properties are important. Hydraulic oil needs to last a
long time (meaning extended oxidation life) and protect equipment by
providing anti-wear properties while
preventing rust and corrosion.
Most hydraulic oils are designed
to have good demulsibility properties. This means they separate water
quickly so it can be drained from the
sump, which is important because
water doesn’t lubricate as well as oil.
Also, hydraulic oils should resist varnish formation, which can result in
sticking servo valves. The fluid’s ability to release air quickly is important
so pumps are protected from cavitation, where air in the oil is released
and then redissolved, generating heat
that can damage pumps.
What goes into hydraulic oil
and how can formulations vary?
A typical non-viscosity modified
hydraulic oil will be formulated with
99% base oil and 1% additive. There
are a number of high-quality base oils
available today. There also are some
lower quality, low-cost base oils that
find their way into hydraulic oil formulations. These base oils can cause
significant performance problems, including emulsification of water, which
can increase pump wear, corrosion,
and poor filtration. Fluids formulated
with poor base oils also may have a
shorter effective life, resulting in more
frequent oil drains. It is important for
end users to ask their lubricant suppliers about the base oil in their fluid
and its impact on performance properties such as oxidation life, water
separation, filtration, and anti-wear.
Ask if the base oil is from a consistent source. Ask if the properties of
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the base oil vary from batch to batch,
potentially influencing the finished
hydraulic fluid.
Although hydraulic oil will typically have less than 1% additive, the
composition of that additive is critical
to protecting the hydraulic system.
How do you protect against
wear and maintain thermal
stability?
The heart of the additive system is
the anti-wear component. Zinc-containing and zinc-free formulations
both exist in the marketplace, and
both provide anti-wear protection.
Most fluids will contain zinc chemistry, generally in the form of zinc dithiophosphate, for anti-wear. It can
be tempting to compare fluids based
on zinc content alone, simply choosing the fluid with more zinc. Although
zinc helps protect against wear, too
much or not the right balance of zinc
can result in corrosion of yellow metals.
There also are different types of
zinc anti-wear molecules. Some are
derived from secondary alcohols and
are less thermally stable than those
derived from primary alcohols. Often,
a supplier can provide pump test data
and thermal stability testing to validate the anti-wear performance and
thermal stability of a fluid.
What about rust, corrosion, and
filtration protection?
Another key component in protecting equipment is selecting the right
balance of corrosion inhibitors. The
fluid supplier can easily demonstrate
this with ASTM test method D665A
or the more severe D665B test for
steel and D130 for copper. Some
fluids may contain corrosion inhibitors that can lead to filtration problems because they react with potential
contaminants. Filtration test data will
generally be available for high-quality
fluids to validate performance. Filtration tests can be performed with or
without water to ensure there is not
a dramatic decrease in filtration performance when the hydraulic fluid is
contaminated with water.
Can you protect against
oxidation and formation of
varnish?
Hydraulic fluids contain antioxidants to help prevent the oxidation
of hydrocarbon molecules. Oxidation rates for hydraulic oil can double
for every 10° C increase in operating
temperature. Hydraulic oils will still
operate at temperatures above 180° F;
however, the oxidation rate will have
accelerated, decreasing the life of the
oil. As hydraulic oil becomes oxidized
and thermally degraded, varnish particles may form. Over time, varnish will
deposit on servo valves, causing them
to stick. Varnish is sticky and attracts
other wear particles in the system,
forming an abrasive residue that can
result in more wear. The soft deposit
eventually becomes a hard lacquer,
which is difficult to remove. Fluid
suppliers generally measure the oxidation performance of their hydraulic
oil based on the ASTM D943 test.
How durable is the fluid?
Hydraulic oils are carefully formulated to provide the right balance of all
performance requirements. Formulators will combine the different additives
in the laboratory with a chosen base oil
and perform tests on the new, unused
fluid. To determine durability and performance retention, tests are conducted
on used fluids after extended use. This
helps to ensure that even as oil ages,
acceptable performance is maintained
in your equipment.
What role does cost play?
When choosing a fluid, be sure to
consider the performance factors previously discussed. Some fluids may
cost more per gallon but provide a
much greater benefit than those costing less. In a machine with a 100-gal
sump, which fluid would be more economical?
• Fluid A is $5/gal, lasts three
months, and causes servo valves to
stick.
• Fluid B is $8/gal, lasts 12 months,
and doesn’t make servo valves stick.
In your selection process, be sure
not only to consider the purchase price
but also fluid longevity, protection of
the equipment, and potential downtime plus labor in case of failures.
What OEM approvals are
important?
It always is important to understand
if your equipment manufacturer designates a certain type of oil for use in
your equipment. OEMs have carefully
developed specifications and approval
processes for lubricant manufacturers
to follow to have their fluid approved.
Be sure whatever fluid you select
fits your equipment manufacturer’s
guidelines.
For more information, e-mail
robert.profilet@lubrizol.com or
call (440) 943-4200.
Copyright © 2007 by Penton Media, Inc.
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