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
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
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
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
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.
Here For Exhibit 1
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
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