Machinery Lubrication India Jan Feb 14 10 | Page 29

taken into consideration by wearing eye
protection, for example.

Why Take Oil Samples from
a Failed Gearbox?
Laboratory analysis of oil samples from
a failed gearbox might answer the
following questions:
• Does the oil meet the original
equipment manufacturer (OEM)
specification?
• Was the oil contaminated?
• Was the oil degraded?
• Does the oil contain evidence useful
for finding the root cause of failure?
• Is the oil representative of the service
oil?

Does the Oil Meet the OEM’s
Specification?
Sometimes a gearbox fails because the
wrong oil was used. To prove whether
the oil meets the OEM’s specification,
the following laboratory tests should be
performed on used oil samples and
compared to laboratory test results
from samples of fresh, unused oil that
conforms to the OEM’s specification:
• Viscosity at 40 degrees C and 100
degrees C (ASTM D445)
• Spectrometric analysis to determine
elements in the oil (ASTM D5185 or
D6595)
• Acid number (ASTM D664 or D974)
• Infrared spectroscopy to determine
additive content (ASTM D7412,
etc.)

Micropitting often will have a pattern
that indicates misalignment.

Was the Oil Contaminated?
The fatigue life of gears and bearings is
adversely affected by water. For
example, as little as 50 ppm of water
reduces rolling bearing fatigue life by 75
percent. Therefore, the Karl Fischer
titration method (ASTM D6304)
should be used to determine the water
content. Other laboratory tests such as
viscosity, spectrometric analysis and
infrared analysis can help determine if
other fluids such as the wrong oil,
flushing oil or coolant contaminated
the service oil. Spectrometric analysis
might disclose contamination via
environmental dust by showing high
concentrations
of
silicon
and
aluminum.

Was the Oil Degraded?
The oil might lose its ability to lubricate
if its viscosity changes significantly or if
it is oxidized. The manufacturing
tolerance on viscosity is plus or minus
10 percent. Therefore, ISO VG 320 oil
should have a viscosity that falls within
the range of 288 to 352 centistokes at
40 degrees C.
There are many possible causes for an
increase or decrease in viscosity. For
example, some oils have additives
known as viscosity-index (VI) improvers
that might not be shear stable. With
time in service, these oils lose viscosity
because the VI improvers shear down.
In addition, overheating might cause
oxidation. Contamination by water and
wear debris accelerates oxidation. The
following symptoms are indicative of
oxidation:
• A foul odor (sour, pungent or acrid
smell)
• A dark color
• An increase in viscosity
• An increase in the acid number
• A shift in the infrared spectrum

A lubricant with inadequate anti-scuff
additives caused scuffing on this spiral
bevel pinion.

Does the Oil Contain
Evidence for Finding the Root
Cause of Failure?
Wear debris in the oil may help indicate
failure modes that occurred in the
gearbox and reveal contaminants that
contributed to the failure. Spectrometric
analysis can uncover contamination via
environmental dust by showing high
concentrations
of
silicon
and
aluminum. These results might explain
abrasion on gear teeth and bearing
surfaces. Depletion of anti-scuff
additives may confirm a scuffing failure,
and excessive water concentration
might explain corrosion.
Other test methods used to monitor
abnormal wear of gearboxes include
ferrous density, particle counting
(ASTM
D7647)
and
analytical
ferrography (ASTM D7690).
Direct reading (DR) ferrography is a
ferrous density test that measures the
amount of ferrous wear debris in an oil
sample. The results of DR ferrography
are ge