fluctuations or spikes to 85° C are
not uncommon. However, when you
operate at temperatures above 85° C
for extended periods, you definitely
will shorten the life of the oil because
the oxidation process is accelerated.
Every 10° C increase in temperature
doubles the oxidation rate and cuts the
life of the oil in half.
By Rob Profilet
Lubrizol Corp.
How well do
you know
your
What does oxidation do?
hydraulic fluid?
Keep your hydraulic fluid
running healthy with
these tips
I
n this month’s MRO, Rob
Profilet from Lubrizol Corp.
answers pertinent questions
about how to keep your hydraulic oil running smooth and efficiently
over a long period of time. Profilet
is commercial manager of hydraulics and gear oils and is located at
Lubrizol’s headquarters in Wickliffe,
Ohio.
What’s different now from
20 years ago?
It’s no secret demands on hydraulic oil have increased in the past 20
years. Systems are generally smaller,
so less oil is used. Pumps have been
tasked to produce as much or more
output, resulting in higher operating
pressures. Smaller oil volume and
higher pressures result in higher oil
temperatures. Hotter oil temperatures
mean an increase in oxidation rates
and more thermal stress on the additive system. At the same time, end users want to spend as little as possible
for fluid that lasts as long as possible
and maintains a high level of protection for the equipment.
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Oxidation occurs when hydrocarbon molecules are converted to carboxylic acids. The rate of this process
increases with higher temperatures
and the presence of certain metals,
including wear particles. Over time,
oxidation byproducts can form varnish, which can result in sticking
servo valves or sludge, which can in
turn clog filters and suction strainers.
In situations where oxidation is severe, it is best to change the fluid immediately.
What types of fluids are
available?
How long should hydraulic
oil last?
The answer depends on many factors. Although the life of a hydraulic
oil really depends on many variables,
including quality of oil, operating conditions, and potential contamination, a
good quality hydraulic oil should last
at least 6 months if conditions are not
severe. With monitoring and a good
oil analysis program, the hydraulic
fluid could last much longer.
In the simplest case, some OEMs
recommend when to drain the oil. In
other situations, the answers are not
as clear.
How do temperatures
impact oil life?
The operating temperature of an
application provides good information about the potential life of the oil.
Fluid temperatures in most industrial
applications are around 60° C, and
Many different hydraulic fluids are
on the market. The base fluid comprises about 99% of the formulation in
most non-viscosity modified formulations and has a great impact on the
lifetime of the fluid. The base fluid,
in combination with antioxidants, determines the oxidation resistance of a
new, clean fluid. Hydraulic fluids are
tested for resistance to oxidation by
the Turbine Oil Oxidation Test ASTM
D943. The test measures the time in
hours for the acid number of the fluid
to increase to 2.0 when exposed to
water, heat, and oxygen with a metal
catalyst present. Some OEMs or end
users require that fluids last at least
1500 hours in this test. Some fluids
last over 5000 hours in this test.
What role does
contamination play?
Contamination in hydraulic fluids
is very common and can cause numerous problems. Water, air, dirt, fuel,
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and other hydraulic oils or lubricants
can all act as forms of contamination.
Water contamination is very common. Used oil analysis often will
show levels of 500-1000 ppm water in
hydraulic fluid taken from an industrial application. Water is detrimental
because it does not lubricate as well as
oil and can cause wear. In some cases,
it also can react with the additive
system to form acids that can cause
yellow metal corrosion. Of course,
water also leads to rust of component
surfaces and can increase the rate of
oxidation, decreasing the oil life.
Air contamination also can result
in oxidation, which can increase oil
viscosity. Over time, oxidation can
lead to varnish, a lacquer that coats
surfaces and causes servo valves to
stick. Even worse, air contamination
can cause pumps to cavitate as air in
the oil rapidly dissolves from heat
generation, which can lead to pump
loss.
Dirt, metal particles, and soot are
common hydraulic oil contaminants.
Metal particles can accelerate the oxidation process, and all of these solid
contaminants can cause abrasion or
surface fatigue on machine surfaces.
Fuel contamination introduces several changes, including lower flash
point, lower viscosity, and increased
vapor pressure. The viscosity change
alone creates lower film strength,
hampering lubrication efficiency.
How can oil life be
maximized?
Operating conditions, the type of
fluid used, and potential sources of
contamination all can influence the
lifetime of the oil. Although there may
not be a single answer to how long
your oil may last, here are three tips
on extending the life of your oil:
1. Choose a good quality hydraulic
oil from a reputable lubricant manufacturer who can provide good technical support as needed.
2. Monitor operating conditions.
Keep the fluid clean, cool, and dry.
Maintain filtration, and use clean lines
to transfer hydraulic fluids into your
equipment.
3. Analyze your used oil. How often will depend on your system. Keep
a watchful eye on your system and
change the oil when oxidation and/
or contamination become excessive.
Monitor the particle counts and make
adjustments to the filtration system as
needed to keep particle counts low.
For more information, e-mail
robert.profilet@lubrizol.com or
call (440) 943-4200.
Copyright © 2007 by Penton Media, Inc.
The Lubrizol Corporation
29400 Lakeland Blvd.
Wickliffe, Ohio 44092
www.lubrizol.com
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