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The performance, life and reliability of hydraulic components is acutely sensitive to the quality and maintenance of the hydraulic fluid used in the system. That is why it pays to use a high- quality hydraulic fluid, inspect fluid samples at regular intervals, and practice regularly scheduled preventive maintenance. By observing these simple precautions system downtime will be reduced and the overall life of the hydraulic system will be increased.
This document provides guidelines for the selection and care of hydraulic fluid in high-performance hydraulic systems.
How is the life of a hydraulic system shortened?
How can hydraulic system life be maximized?
How are fluid analysis samples taken?
Where can a fluid sample be analyzed?
What are the expected fluid analysis results?
What does clean hydraulic fluid look like?
Why check for iron, silicon and copper content?
What can be done to maintain the hydraulic system?
How do filters keep hydraulic fluid clean?
When does the hydraulic system need to be
flushed?
What can be done at system installation and initial start-up to prevent contamination?
Why is fluid care important? High-performance
hydraulic systems require very clean oil to maximize performance and
extend the life of system components. While
less sophisticated hydraulic systems may function acceptably with lower
levels of oil cleanliness, high-performance hydraulic equipment will
“silt up” and perform erratically when subjected to particulate
levels that are greater than an ISO 4406 rating of 16/13/9. Servovalves
must operate smoothly and predictably to deliver the tightly regulated
pressures and fluid flow for which they are designed.
How is the life of a hydraulic system shortened? Hydraulic
fluid contamination and deterioration are normal consequences for
most hydraulic systems. Failure to adequately remove contaminants, or to
change hydraulic fluid before severe fluid breakdown occurs,
will lead to poor system performance. The
most common hydraulic fluid contaminants are entrapped air and
water, along with particles of metal, rubber or dirt. To maintain clean
hydraulic fluid, samples must be taken regularly and appropriate
mitigation should be immediate. Fluid
deterioration might more appropriately be called “additive
deterioration.” Additives give the oil its particular
characteristics—and because these additives are most susceptible to
chemical and physical change, their deterioration is what leads to
fluid breakdown. Fluid
deterioration is often caused by operation at high temperatures. Fluid
reservoir temperatures are best kept below 140° F (60° C). To keep
fluid operating temperatures within the acceptable range of
100-125° F (38-52° C), standard hydraulic power units are equipped
with full-motor-horsepower heat exchangers, over-temperature interlocks
and temperature controls.
How can hydraulic system life be maximized? Regular
monitoring and maintenance of the hydraulic fluid promotes the
maximum operating performance and service life of the servohydraulic
system and its components. Two specific items must be checked
regularly—contaminants in the fluid and the fluid’s
chemical makeup. Fluid
sampling and analysis is the best way to determine whether the
fluid and filters should be changed. Fluid analysis will
provide an accurate viscosity reading while detecting specific
contaminants such as water or foreign particles. It can also be used to
check the chemical makeup of the fluid to identify whether the
additive package is still able to perform as it was originally designed.
A
correct evaluation of the contaminants in the hydraulic system is
important. When the contaminating material is identified, its
source can be investigated to prevent future contamination.
How are fluid analysis samples taken? Fluid
analysis samples should be taken with the system running at normal
operating temperatures. To insure that no foreign material enters the
sample, tools and containers used for sampling must be very clean. It is
best to use the sample bottles provided in fluid analysis kits. When
the system is installed, collect a fluid sample and label it
“first test sample” or “benchmark sample.” Have it
analyzed by a laboratory, and keep the lab report on file. Fluid
samples should be submitted for analysis twice every year. More often if
the system operates at higher temperatures or in abnormal ambient
conditions. Compare new analyses with the report from the first or
“benchmark” test sample. A schedule for changing fluid should be
developed, based on the rate of fluid deterioration.
Where can a fluid sample be analyzed? Many
filter manufacturers offer services, giving special attention to
particle counts and specific contaminants. What are the expected fluid analysis results? The
following table identifies the specifications for common
elements of hydraulic fluid, as used in servohydraulic
applications.
If
any fluid contamination indicators are borderline, the fluid
condition might be improved by cleaning. If the indicators are
unsatisfactory, the fluid must be replaced and the system may need
to be flushed. In either case, the entire system needs
attention—not just the fluid in the reservoir. What does clean hydraulic fluid look like?
Looking at and smelling hydraulic fluid
is the simplest and most effective way to determine the fluid’s
condition. Clean
fluid is amber in color. A milky, dark, or otherwise abnormal
color may indicate the presence of one or more contaminants. A milky
appearance implies contamination by water. If the fluid looks
milky, take immediate action to avoid severe damage to your hydraulic
system. Stop the influx of water and remove the water from the
system immediately. Water can be removed by passing the fluid
through water-absorbing filters, or by flushing or draining
the entire hydraulic system. A
marked change in the smell of the hydraulic fluid can indicate a
chemical breakdown. This type of breakdown is generally due to air that
has become entrained in the fluid, which creates varnish-like
nitrogen-oil compounds that contaminate the fluid. If a distinct
change in the smell of hydraulic fluid is detected, have it
chemically analyzed by the manufacturer. Also consult with the
system’s service engineer to determine if other changes or adjustments
to the system are required.
What is viscosity? Viscosity
is a measure of the resistance of the fluid to flow. A low
viscosity will not provide adequate lubrication to parts, resulting in
increased wear on the parts. Many hydraulic fluids will shear or
thin out with use. A
viscosity check should be made whenever fluid samples are
analyzed. If the fluid’s viscosity is outside of the
specified flow limits, the fluid should be replaced. (NOTE:
Viscosity varies with temperature, so a numerical viscosity value is
meaningless unless the temperature is specified.) What
is oxidation? As
hydraulic fluid deteriorates over time, it oxidizes and produces
deposits that may cause servovalves to stick. Signs of this natural
process include changes in fluid color, odor, or acidity level.
Sludge, gum or varnish in the system are further evidence that oxidation
has taken place. A
fluid analysis with an ultra-centrifuge test can detect the level
of oxidization. The rate of oxidization increases significantly at
operating temperatures higher than 150° F (66° C). Oxidation is
irreversible, and fluid must be changed when oxidization is
detected.
Why check for water content? Water
is highly undesirable in hydraulic systems. It can cause emulsions to
form, and it can lead to corrosion. More than a trace of water may
indicate an improper mechanical condition, poor performance of a heat
exchanger or ingestion of water through the breather filter. During
system operation, gross contamination of hydraulic fluid is
indicated by a milky color. If the system is not in operation,
contamination can usually be detected by sampling the fluid at the
bottom of the reservoir, where water normally settles as it separates
from the hydraulic fluid. A
simple test for water contamination is the “spat” test:
Place a few drops of oil on a hot plate heated to above boiling,
but less than 350° F; if the oil bubbles or sizzles, there is an
unacceptable amount of water in the oil. If water contamination exists, fix the leakage and remove the water from the system immediately. Water can be removed by passing the fluid through water-absorbing filters, or by flushing or draining the entire hydraulic system.
Why check for iron, silicon and
copper content?
These
analyses are a valuable aid to troubleshooting. A high iron reading may
indicate wear of hydraulic power unit parts. High silicon readings may
indicate ingestion of dirt or compounds containing silicon (e.g.,
sealing compounds or defoamants). High copper readings may indicate
component wear and/or faulty heat exchangers. If fluid analysis shows high levels of iron, silicon or copper, identify the source and replace components as necessary. Fluid analysis should be a continuing process and sample results must be evaluated for trends that indicate a change in the condition of the hydraulic system.
What can be done to maintain the hydraulic
system? Regular
monitoring and maintenance of the hydraulic fluid will provide
maximum operating performance and service life for the hydraulic system
and its components. Keeping a maintenance log that records: Dates
of fluid sampling Laboratory
results
- Viscosity
- Particle Count
- Water, % by content
- Total Acid Number
-UC
-Oxidation Filter
changes Fluid
changes Operating
temperature-test results pH
test results Other
inspections or maintenance This
log will help to detect signs of deterioration and can be used to
develop a schedule for changing fluid.
How do filters keep hydraulic fluid clean?
Filters
provide the fluid cleanliness levels required by servohydraulic
systems. These sizes and efficiency ratings have proven to be
effective in controlling the silt particles that cause erratic
servovalve operation. Filters
must be cleaned or replaced during routine maintenance of the hydraulic
system. Most filters supplied are not cleanable; refer to your
manufacturer’s product information for guidance on changing filters.
System
filters must be capable of maintaining a normal ISO 4406 particle
count of 16/13/9. When
a high particle count is identified the source of contamination
should be located and corrected. Collecting samples from various system
locations will help identify the source of contamination. Clean the
hydraulic fluid by changing the system’s filters and
running the hydraulic power unit for a period of time. Filters will
collect solid contaminants larger than a specified diameter After
filtering the hydraulic fluid, check it for cleanliness. An
ISO 4406 particle count reading of 16/13/9 or better is required for
test systems. Some critical test applications may require an ISO 4406
level of 14/12/8. If the appropriate level can’t be achieved after
cleaning, a change to the filtration plan must be considered.
When does the hydraulic system need
to be flushed?
At
installation, or after any of the system’s hydraulic components have
been replaced, flush the system to remove particle contamination.
If the reservoir is very dirty, or if sludge or varnish is present, the
system must be flushed with a flushing compound. Flushing
compounds are solvent free, oil soluble cleaners designed specifically
for cleaning and flushing gummy oxidation deposits and insoluble
materials from hydraulic systems. Solvent flushing is a costly,
time-consuming process. Contact the system’s service engineer before
using a flushing compound. Flushing
the hydraulic system, with or without a flushing compound,
dislodges contaminants that are harmful to servovalves. For this reason,
it is extremely important to replace servo valves with flushing
valves before the flushing procedure. Filter elements should also
be changed immediately before adding the flushing compound to
prevent contaminants already in the elements from being reintroduced to
the system. Refer to the manufacturer’s documentation for more
detailed information.
What can be done at system installation and initial start-upto prevent
contamination? The
manufacturer should have designed, manufactured, shipped and installed
your hydraulic distribution system to manage particulate contamination
of the hydraulic fluid. After the system is installed and
flushing operations are completed, the system should be
trouble-free. Precautions
must always be taken to avoid contamination during installation or
maintenance. When a plug, cap, hose or hydraulic component is removed,
foreign matter can enter the system and eventually cause damage. On
complex systems with large fluid capacities, and in distribution
systems that use welded carbon steel pipe, contamination risk is high. A
complete system flush—before attaching actuators and servo
valves—is imperative. After
the fluid is cleaned or the system is flushed, sample the
fluid for cleanliness. The proper cleanliness level is an ISO 4406
particle count of 16/13/9 or better. If necessary, clean the fluid
continuously until this level of cleanliness is achieved. |
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Hydraulic
Fluid Care
