Maximum Hydraulic Component
Life
Defining Fluid Temperature &
Viscosity Limits for Maximum Hydraulic Component Life
By Brendan Casey
Many factors can reduce the service life of hydraulic
components. Incorrect fluid viscosity is one of these factors. To prevent low
(or high) viscosity from cutting short component life, an appropriate fluid
operating temperature and viscosity range must first be defined and then
maintained on a continuous basis. Before I discuss this in detail, let me
explain the interrelationship of fluid temperature and viscosity, and how they
impact upon hydraulic component life.
Temperature/Viscosity Relationship of
Hydraulic Fluid
The viscosity of petroleum-based hydraulic fluid decreases as its temperature
increases and conversely, viscosity increases as temperature decreases. This is
why limits for fluid viscosity and fluid temperature must be considered
simultaneously. Low fluid viscosity can result in component damage through
inadequate lubrication caused by excessive thinning of the oil film, while
excessively high fluid viscosity can result in damage to system components
through cavitation.
Manufacturers of hydraulic components publish permissible and optimal
viscosity values, which can vary according to the type and construction of the
component. As a general rule, operating viscosity should be maintained in the
range of 100 to 16 centistokes (460 to 80 SUS), however viscosities as high as
1000 centistokes (4600 SUS) are permissible for short periods at start up.
Optimum operating efficiency is achieved with fluid viscosity in the range of 36
to 16 centistokes (170 to 80 SUS) and maximum bearing life is achieved with a
minimum viscosity of 25 centistokes (120 SUS).
Hydraulic Fluid Viscosity Grades
ISO viscosity grade (VG) numbers simplify the process of
selecting a fluid with the correct viscosity for a system's operating
temperature range. A fluid's VG number represents its average viscosity in
centistokes (cSt) at 40°C. For example, an ISO VG 32 fluid has an average
viscosity of 32 centistokes at 40°C. Note that the average fluid viscosity of
ASTM and BSI viscosity grade numbers are measured at 100°F (38.7°C). This
means that fluids of a given ASTM or BSI grade are slightly more viscous than
the corresponding ISO grade.
Determining the Correct Viscosity Grade
In order to determine the correct fluid viscosity grade for a particular
application, it is necessary to consider: " starting viscosity at minimum
ambient temperature; " maximum expected operating temperature, which is
influenced by maximum ambient temperature; and " permissible and optimum
viscosity range for the system's components.
In most cases, the machine manufacturer will specify the correct viscosity
grade. It is important to understand that the machine manufacturer's recommended
viscosity grade should change as the ambient temperature conditions in which the
machine operates change.
I say this because several years ago I was involved in the analysis of
several premature component failures from a mobile hydraulic machine. The
machine was designed and built in the Northern Hemisphere, but was operating in
high ambient air temperatures in the Southern Hemisphere. The components had
failed due to inadequate lubrication, because of low fluid viscosity.
Investigation revealed that the fluid in the system was ISO VG 32. While this
viscosity grade is suitable for cooler climates found in parts of the Northern
Hemisphere, it was not suitable for the high ambient temperatures in which this
machine was operating. The machine owner confirmed that the manufacturer's fluid
recommendation was indeed ISO VG 32.
The machine manufacturer had not altered their fluid viscosity recommendation
to take into account the higher ambient temperatures in which this particular
machine was operating. This oversight resulted in several premature component
failures because of low fluid viscosity.
The machine manufacturer's viscosity grade recommendation can be checked
using the viscosity/temperature diagram shown in exhibit 1, assuming the minimum
starting temperature and the hydraulic system's maximum operating temperature
are known. For example, let's consider an application where the minimum ambient
temperature is 15°C, the system's maximum operating temperature is 75°C, the
optimum viscosity range for the system's components is between 36 and 16
centistokes and the permissible, intermittent viscosity range is between 1000
and 10 centistokes.
From the viscosity/temperature diagram in exhibit 1 it can be
seen that to maintain viscosity above the minimum, optimum value of 16
centistokes at 75°C, an ISO VG 68 fluid is required. At a starting temperature
of 15°C, the viscosity of VG 68 fluid is 300 centistokes, which is within the
maximum permissible limit of 1000 centistokes at start up. If the machine
manufacturer's recommendation was ISO VG 32 fluid under the same conditions, I
would question it.
A word of warning here - do
not change the fluid viscosity grade in a system without consulting the
equipment manufacturer. Doing so may void the manufacturer's warranty and/or
cause damage to the system's components.
Defining Operating Temperature Limits
Having established that the fluid in the system is the correct viscosity
grade for the ambient temperature conditions in which the machine is operating,
the next step is to define the fluid temperature equivalents of the optimum and
permissible viscosity values for the system's components.
By referring back to the viscosity/temperature curve for VG 68 fluid in
exhibit 1, it can be seen that an optimum viscosity range of between 36 and 16
centistokes will be achieved with a fluid temperature range of between 55°C and
78°C. The minimum viscosity for optimum bearing life of 25 centistokes will be
achieved at a temperature of 65°C. The permissible, intermittent viscosity
limits of 1000 and 10 centistokes equate to fluid temperatures of 2°C and 90°C,
respectively.
Going back to our example, this means that with an ISO VG 68 fluid in the
system, the optimum operating temperature is 65°C and maximum operating
efficiency will be achieved by maintaining fluid temperature in the range of 55°C
to 78°C. If cold start conditions at or below 2°C are expected, it will be
necessary to pre-heat the fluid to avoid damage to system components.
Intermittent fluid temperature in the hottest part of the system, which is
usually the pump case, must not exceed 90°C.
Note that fluid temperatures above 82 C (180 F) damage seals, reduce the
service life of the hydraulic fluid and in most cases, will cause the viscosity
limits of the fluid to be exceeded. This means that the operation of any
hydraulic system at temperatures above 82 C (180 F) is detrimental and should be
avoided.
Preventing Damage Caused by High
Temperature Operation
To prevent damage caused by high fluid temperature and/or low fluid
viscosity, a fluid temperature alarm should be installed in the system and all
high temperature indications investigated and rectified immediately. The
over-temperature alarm should be set to the temperature at which the minimum,
optimum viscosity value is exceeded. As already explained, this will be
dependent on the viscosity grade of the fluid in the system. In the example
discussed above, the fluid temperature alarm would be set at 78°C.
Continuing to operate a hydraulic system when the fluid is over-temperature
is similar to operating an internal combustion engine with high coolant
temperature. Damage is almost guaranteed. Therefore, whenever a hydraulic system
starts to overheat, shut down the system, find the cause of the problem and fix
it!
About the Author: Brendan Casey has more
than 15 years experience in the maintenance, repair and overhaul of mobile and
industrial hydraulic equipment. For more information on increasing the uptime
and reducing the operating cost of your hydraulic equipment, visit his web site:
http://www.InsiderSecretsToHydraulics.com
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