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Oil Off Testing
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Introduction |
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An Oil-off test protocol evaluates bearing materials
and lubricants for durability and operational life under marginal
lubrication conditions that may be experienced in service. The
commonly used aircraft bearing steel M50 is used as a baseline
for evaluation purposes. Oil-off tests generally support other
tribology tests for material and lubricant performance. Other
test protocols include attributes for adhesive wear, abrasive
wear and load-carrying (scuffing) capacity. |
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Test Rig and Test Materials |
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Oil-off tests can be conducted with the Wedeven Associates,
Inc. WAM8 or WAM9 test machines, as shown in Figure 1. The
test specimens consist of a ½ inch ball specimen (or
alternate ball sizes), which rides against a flat disc specimen.
The disc specimen represents bearing ring materials.
Final finishing of the disc specimen is conducted by Wedeven
Associates, Inc. Surface preparation consists of fine grinding,
followed by abrasive lapping to a surface finish of 2 µ-inch,
Ra. The lapping operation is done with specimen rotation about
its centerline to create a circumferential lay in the direction
of rolling motion. The finishing method consists of several
lapping stages to obtain a consistent surface texture. Care
is taken to avoid microscopic bends and folds ("leafing")
in the surface texture. The final finish is similar to a finely
honed bearing raceway.
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Test Procedure |
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An oil-off test protocol was established with the following
test conditions:
Ball M50, ½ inch dia., Grade 5 (or alternate size
or Grade)
Disc M50, finished to 2 min Ra (baseline material and finish)
Contact stress 2.49 GPa (360 ksi)
Entraining velocity 10.16 m/sec (400 in/sec)
Contact slip 5%
Temperature (disc) 200 °C (392 °F)
Lube supply drip feed until oil-off at 600 seconds
The test is initiated after the disc specimen temperature
stabilizes at 200 °C. The first 60 seconds of the test
is operated at or near pure rolling. The remainder of the
test is run at 5% slip. A computer-controlled peristaltic
pump is shut off after 600 seconds. To avoid release of residual
oil on the ball and disc surfaces, tests are conducted with
the ball thermocouple, oil shielding material and parts of
the heating equipment removed from the test area immediately
following oil shut off. Oil-off tests can be conducted with
various criteria for test termination. These include: (1)
oil-off operation until a rapid rise in traction coefficient
to an arbitrary value; (2) oil-off operation until a rise
in traction coefficient to 0.15 and (3) oil-off operation
for 220 seconds, which is beyond the time typically required
to reach a maximum traction coefficient (~0.4) and the onset
of a high wear rate.
Except where noted, the test oil is Mobil Jet II. Mobil Jet
II is classified as a standard (STD) oil under MIL-PRF-23699
specification. From our testing of qualified jet engine oil
products, Mobil Jet II has excellent lubricating ability compared
to most oil brands on the qualified products list for MIL-PRF-23699.
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Typical Oil-Off Test Results |
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To establish the role of oil lubricating characteristics
during oil-off conditions, two tests were run with oil TEL-0004,
which has known difficulties in certain applications. TEL-0004
is a qualified Grade 4 product under MIL-PRF-7808K specification.
The U.S. Air Force supplied the test oil with the designation
TEL-0004. Oil-off tests were also conducted with high load-carrying
DOD-L-85734 oil, which is used in aircraft gearboxes and demanding
engine applications.
Typical oil-off traction behavior for M50/M50 materials and
the three oil types are shown in Figure 2. With continuous
oil supply and a test temperature of 200 °C, the traction
coefficient for a full EHD oil film is on the order of 0.025.
A typical traction coefficient for jet engine oil at ambient
temperature (~23 °C) is on the order of 0.07.
The rise in traction coefficient after oil-off is associated
with local adhesion and transfer of material from the ridges
of the finishing marks on the disc specimen. The local material
transfer, smearing and oxidation create material pileup and
a loss of surface integrity. At this stage the surfaces encounter
high local friction events due to material pileup and high
friction oxides. The amount of wear is not significant. While
the surface disturbance appears to be minor, the situation
is in a run-away condition heading for gross surface failure
and a traction coefficient on the order of 0.4.
The maximum traction coefficient of 0.4 is attributed to
massive adhesion and material transfer between the surfaces.
The rise in traction coefficient is associated with the spread
of adhesive events across the operating track on the specimens.
A momentary departure from an increase in traction is believed
to be the result of lower tangential shear caused by the onset
of wear.
These tests show that oil starvation leads to local adhesion
and pileup of material on the surfaces with a corresponding
rise in traction coefficient. Once this process starts (and
without subsequent oil replenishment) adhesion and oxidation
of transferred material becomes a run-away process. The run-away
process seems to be well on its way by the time the traction
coefficient has increased from 0.025 to 0.15 -- a six-fold
increase in traction coefficient.
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Effect of Material or Oil Lubricating Quality
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The above results show that the initiation of run-away traction
is due to adhesion and oxide growth at local sites. Resistance
against adhesion depends upon the material pair, along with
protection against removal of surface films that prevent atomic
bonding between the surfaces. If this is the case, the important
attributes of the material for oil-off performance are associated
with the material's response to chemistry or "tribo-chemistry".
The oil-off test results with the three test oils in Figure
2 are summarized below with respect to the average time to
at traction coefficient of 0.15.
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Test Oil
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Average time to traction coefficient
of 0.15
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TEL-0004(4 cSt) MIL-PRF-7808K
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10.5 seconds (2 tests)
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Mobil Jet II (5 cSt) MIL-PRF-23699
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42.0 seconds (3 tests)
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DOD-L-85734
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81.0 seconds (1 test)
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Oil additive chemistry and other factors affecting the lubricating
ability of the oil are significant contributors to oil-off capability.
An eight-fold difference between the time-to-failure is evident
between a low quality oil for lubrication compared to a high
quality oil. The traction data in Figure 2 shows that the onset
of failure for TEL-0004 is a sudden event. The additive chemistry
in the DOD-L-85734 oil results in a gradual rise in traction
coefficient. The gradual rise in traction coefficient is associated
with a controlled tribological process, which results in less
severe local damage to the surfaces. There are noticeably fewer
areas of material transfer or islands of oxidized material on
the surfaces with the DOD-L-85734 oil.
These results strongly suggest that a high quality oil for
lubrication can provide greater tolerance against surface
damage during oil-off operation with better probability for
recovery after a high traction event due to momentary oil-out
conditions. It is recognized that with perhaps a few exceptions,
high performing oils for lubricating ability are almost always
low quality oils for thermal stability and coking. The significant
feature that these tests bring out is that testing for bearing
or material performance for oil-off capability can be significantly
affected by oil type.
The oil-off test protocol can also be used to evaluate bearing
materials, surface finishing processes (roughness and texture)
and effects of surface defects. The test protocol can be expanded
to include material and oil attributes and their potential
for recovery following a high traction and material transfer
event. The recovery is accomplished by repair of the surfaces
through a wear process. However, incomplete surface repair
and accumulated damage below the surface are likely to reduce
fatigue life. In any case, recovery, even with a sacrificial
wear process, has the potential to achieve survivability in
the near term. To explore this further, inspection and documentation
of oil-off bearing test hardware (or field hardware) is recommended.
It is possible that a high quality lubricating oil may allow
higher bearing torque rise and heat generation in an oil-off
event than a low quality oil.
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