Magnetic Plug Inspection Enhances Condition-Based Maintenance
from
Practicing oil analysis
Posted 3-22-04
In early 2001, the average rebuild cost for J.R. Simplot Co.’s
Caterpillar 785 haul truck differentials and final drives was
$2.11 per hour of operation. By 2003, the average cost had
dropped to $1.36 per hour of operation and the mine’s
maintenance personnel set a goal of further reducing the cost
to $1.17. At the same time, the mine’s reliability team
successfully extended the average oil drain intervals on the
differentials and final drives from 2,000 hours to 6,000 hours
and set a goal of 10,000 hours.
These impressive improvements, which were detailed in Practicing
Oil Analysis magazine’s July-August 2003 issue, were
realized through a program that includes off-line kidney loop
filtration, contamination control training for lube technicians
and mechanics, use of high-quality oils, improved oil analysis
and inspections at 500-hour intervals.
In addition to the reduction in rebuild costs and increase
in oil drain intervals, the mine is enjoying a much longer
duration between differential and final drive rebuilds; from
an average of 8,000 hours in 2001 to an average of 16,000 hours
today, with several rebuilds extending beyond 17,000 hours.
Cleaner oil has certainly had a positive impact on extending
the differential and final drive life, but magnetic plug inspections
have also played a key role in increasing the duration between
differential and final drive rebuilds.
While off-line filtering and particle analysis were key to
Smoky Canyon Mine’s reliability improvements, the mine’s
reliability team found that these techniques did not do enough
to warn about catastrophic failures in the mine’s fleet
of 785 Caterpillar haul trucks. Therefore, the team decided
to include magnetic plug inspections in the 500-hour interval
inspections. Smoky Canyon Mine’s magnetic plug inspections
have raised its reliability maintenance program beyond contamination
control to the next level. It has enabled the reliability team
to identify problems and replace several bearings before the
equipment failed, eliminating what could have turned into lengthy
down-time and substantial repair costs.
The inspections have enhanced Smoky Canyon Mine’s condition-based
maintenance program. In fact, the knowledge gained through
this program is not limited to just those individuals involved
in oil and wear debris analysis. Knowledge gained through the
plug inspection program has extended to the mechanics and lube
technicians who break down the trucks during the inspection
process. These members of the reliability team have become
so familiar with the normal and abnormal appearances of the
magnetic plugs, they can often identify a problem before the
actual laboratory analysis is performed. It is not unusual
for these team members to bring a magnetic plug to the laboratory
and alert the lab technician about possible problems in the
785 haul trucks.
It is easy to see that when mechanics and lube technicians
have this much knowledge about the equipment and this much
ownership in the processes, the company will benefit immensely.
Being able to identify problem components before they cause
complete equipment failure results in big maintenance cost
savings. So far, the mine estimates that it has saved more
than $135,000 in 2002 and 2003 on rebuild and lubrication costs
alone.1
The plug inspections have also allowed mine personnel to obtain
valuable wear debris photographs and information that will
serve as a basis to the mine’s magnetic plug inspection
program for years.
It is important to point out, that plug analysis is credible
and accurate only when used in conjunction with effective contamination
control (filtering and contamination control). Simplot’s
successful off-line kidney loop filtration and particle analysis
program has kept the oil clean enough for the reliability team
to be confident that excessive wear debris is a result of component
wear.
Plug Inspection Basics
The drain plugs in the differential and the oil level (fill
plugs) in the 785 Caterpillar haul truck final drives are
magnetic. As the filter cart is hooked to the truck, the
magnetic plugs are preserved and the wear debris on these
plugs is analyzed. Smoky Canyon mine personnel analyze the
wear debris for not only the amount or concentration of the
particles, but also for the sizes and shapes of the particles.
Small changes in wear debris are used to forecast the extent
and nature of a failure. Using a handheld microscope and/or
a typical tabletop microscope and camera, maintenance personnel
are able to inspect and identify bearing and gear wear. It
should be noted however, that the high-powered microscopes
used at Smoky Canyon Mine are not necessary for a successful
inspection program - any simple, 100-power microscope will
do the job.
First Success Story
Fairly early into the magnetic plug inspection program, the
reliability team discovered a premature bearing failure.
The plugs from a left rear final drive with 4,412 hours of
operation showed excessive wear debris. Figure 1-1 shows
the plug that was inspected. After the debris was washed
off, a chunky particle was discovered (Figures 1-2 and 1-3),
which led the team to believe the bearing was in jeopardy
and that a bearing inspection was needed.
Figure 1. Caterpillar 785 Haul Truck Bearing Failure
 |
Figure 1-1.
Magnetic Plug from 785 Haul Truck Left Rear Final Drive
4,412 operating hours on final drive
48,048 operating hours on truck
ISO 21/20/18 |
 |
Figure 1-2.
One of Many Chunks Discovered in Left Rear Final Drive
5/32" Long, 1/32" Thick |
 |
Figure 1-3.
One of Many Flakes Discovered in Left Rear Final Drive
5/32" Long, 1/4" High |
The decision to investigate the bearing’s condition
based on the magnetic plug inspection turned out to be the
right decision. When the team inspected the left rear final
drive assembly, they found fines, flakes and chunks (Figure
2-1), evidence the bearing was damaged. Figure 2-2 shows the
outer bearing, which had already begun to breakdown. Figure
2-3 shows that the inner bearing had started to pit, but was
still in better condition than the outer bearing. The magnetic
plug inspections led the team to investigate and replace the
bearings before the wear resulted in gear damage and equipment
failure.
Figure 2. 785 Haul Truck Left Rear Bearing
 |
Figure 2-1.
Left Rear Final Assembly Fines,
Flakes and Chunks |
 |
Figure 2-2.
Left Rear Final Drive Outer Bearing |
 |
Figure 2-3.
Left Rear Final Drive Inner Bearing |
Progression to Failure
The reliability team continued to filter the differential and
final drive oil and inspect the components on the same Caterpillar
haul truck at the scheduled intervals. The photos in Figures
3 through 5 tell the truck’s story. The pictures were
taken at each inspection (approximately 525 operating hours
between each). Figures 3-1 and 3-3 show the wear debris found
on the left rear final drive magnetic plugs after the final
drive was replaced. The wear debris found on the magnetic
plugs of the right rear final drive, which was not replaced,
is shown in Figures 3-2 and 3-4. Even though wear debris
was found on both the left and right final drive magnetic
plugs during the first few inspections, the team was not
concerned. Some wear debris is normal, especially after new
components are installed. The debris usually diminishes after
the wear-in period.
Figure 3. 785 Haul Truck Magnet Plugs
 |
Figure 3-1.
Left Rear Final Drive Magnetic Plug
525 Operating Hours on Final Drive
48,573 Operating Hours on Truck
ISO Before Filter Cart: 21/19/13
ISO After Filter Cart: 21/18/13
Plug is Covered with Heavy Fines and Many Flakes |
Figure 3-1a.
Flake from Left Rear Final Drive Magnetic Plug
5/32" long
|
 |
Figure 3-2.
Right Rear Final Drive Magnetic Plug
2,538 Operating Hours on Final Drive
48,573 Operating Hours on Truck
ISO Before Filter Cart: 21/19/13
ISO After Filter Cart: 21/18/13
5/8 of Plug is Covered with Heavy Fines |
Figure 3-2a.
Flake from Right Rear Final Drive Magnetic Plug
5/32" Long
|
 |
Figure 3-3.
Left Rear Final Drive Magnetic Plug
1,050 Operating Hours on Final Drive
49,098 Operating Hours on Truck
ISO Before Filter Cart: 21/20/14
ISO After Filter Cart: 21/18/13
Plug is Covered with Medium Fines and Many Flakes |
Figure 3-3a.
Flake from Left Rear Final Drive Magnetic Plug
3/32" Long |
 |
Figure 3-4.
Right Rear Final Drive Magnetic Plug
3,063 Operating Hours on Final Drive
49,098 Operating Hours on Truck
ISO Before Filter Cart: 21/20/14
ISO After Filter Cart: 21/18/13
Plug is Covered with Soft Fines and a Few Small Flakes |
Figure 3-4a.
Flake from Right Rear Final Drive Magnetic Plug
1/32" Long
|
However, as can be seen in Figures 4-1 , 4-3 and 4-5, the
wear debris on the left rear final drive plugs did not diminish.
Flakes and fines continued to show up and chunks began to appear.
While the right rear final drive plugs did have some wear debris
on them at the same inspection intervals (Figures 4-2, 4-4
and 4-6), the debris consisted of fines with a few small flakes
and no chunks. The inspections indicated another failure (the
second) in the left rear final drive of the same truck. Although
this was not good news, it did allow the reliability team to
collect a lot of information about wear debris in a short time.
Figure 4. 785 Haul Truck Magnet Plugs
 |
Figure 4-1.
Left Rear Final Drive Magnetic Plug
1,863 Operating Hours on Final Drive
49,911 Operating Hours on Truck
ISO Before Filter Cart: 21/20/14
ISO After Filter Cart: 21/18/11
Plug is Covered with Medium Fines 1/16" Deep and Some Flakes |
Figure 4-1a.
Flake from Left Rear Final Drive Plug
4/32" Long |
 |
Figure 4-2.
Right Rear Final Drive Magnetic Plug
3,876 Operating Hours on Final Drive
49,911 Operating Hours on Truck
ISO Before Filter Cart: 21/20/14
ISO After Filter Cart: 21/18/11
Plug has Medium Fines and No Flakes |
Figure 4-2a
Fines from Right Rear Final Drive Plug |
 |
Figure 4-3.
Left Rear Final Drive Magnetic Plug
2,542 Operating Hours on Final Drive
50,590 Operating Hours on Truck
ISO Before Filter Cart: 20/15
ISO After Filter Cart: 15/13
Plug has Many Large Flakes |
Figure 4-3a.
Flake from Left Rear Final Drive Plug
4/32" X 6/32" |
 |
Figure 4-4.
Right Rear Final Drive Magnetic Plug
4,663 Operating Hours on Final Drive
50,599 Operating Hours on Truck
ISO Before Filter Cart: 20/15
ISO After Filter Cart: 15/13
Plug has Small Flakes |
Figure 4-4a.
Small Flakes from Right Rear Final Drive Plug
2/32" Long |
 |
Figure 4-5.
Left Rear Final Drive Magnetic Plug
3,380 Operating Hours on Final Drive
51,428 Operating Hours on Truck
ISO Before Filter Cart: 22/20/10
ISO After Filter Cart: 22/20/8
Plug has Medium Fines and Many Large Flakes |
Figure 4-5a.
Flakes from Left Rear Final Drive Plug
7/32" and 11/32" |
 |
Figure 4-6.
Right Rear Final Drive Magnetic Plug
5,501 Operating Hours on Final Drive
51,428 Operating Hours on Truck
ISO Before Filter Cart: 22/20/10
ISO After Filter Cart: 22/20/8
Plug has Medium Fines and Some Small Flakes |
Figure 4-6a.
Small Flakes from Right Rear Final Drive Plug
4/32" Long |
Figures 5-1 through 5-3 show three different plugs that were
inspected at different intervals prior to a left rear final
drive failure. The photo in Figure 5-1 was taken 500 operating
hours after the oil was filtered and the plug cleaned. As the
photo shows, the left final drive plug was covered with wear
debris. Figure 5-1a shows the debris - flakes and chunks -
that was washed off the plug. Figure 5-2, which shows a different
plug from the same system, was taken 100 hours after the first
photo. Again, the oil was filtered and the plug was cleaned
prior to the 100 hours of operation. While the plug collected
less wear debris than the first plug, it still contained a
high density of fines and was mostly covered with large flakes.
The photo in Figure 5-3, which was taken 150 hours after the
second picture (250 hours after the oil was filtered), shows
yet another plug. This plug had no fines, but a lot of large
flakes and chunks.
Figure 5. Three Different Left Rear Final Drive Magnetic Plugs
from 785 Haul Truck
 |
Figure 5-1
Left Rear Final Drive
Magnetic Plug
4,223 Operating Hours
on Final Drive
52,271 Operating Hours on Truck
Plug Covered with
Flakes and Chunks |
Figure 5-1a and 5-1b.
Flakes and Chunks from Left Rear
Final Drive Magnetic Plug |
 |
Figure 5-2.
Left Rear Final Drive
Magnetic Plug
4,323 Operating Hours
on Final Drive
52,371 Operating Hours on Truck
100 Hours After Filter Cart, Plug has Heavy Fines and
is Mostly Covered with
Large Flakes |
Figure 5-2a.
Heavy Fines and Large Flakes from Left Rear Final Drive Magnetic Plug
Flakes 6/32" X 6/32" |
 |
Figure 5-3
Left Rear Final Drive
Magnetic Plug
4,473 Operating Hours
on Final Drive
52,521 Operating Hours on Truck
ISO Before Filter Cart: 22/21/14
ISO After Filter Cart: 22/21/13
250 Hours After Filter Cart,
Plug has no Fines but Many Flakes and Chunks |
Figure 5-3a and 5-3b.
Chunks from Left Rear Final
Drive Magnetic Plug |
The team knew this much wear debris was not normal and that
there was another problem with the left rear final drive.
The final drive was taken apart and inspected. It was discovered
that the hub guide had not been properly inspected when the
first problem was discovered and corrected, and that slag
from a rough edge on the hub guide (Figure 6) caused during
the first bearing failure had resulted in the excess wear
debris. Once again, through diligent plug inspections, the
reliability team discovered the problem and corrected it
before the equipment failed completely.
Figure 6. Hub Guide Slag from Bearing Failure at 48,048 Hours
In addition to plug inspections, the reliability team also
performed oil analysis at each inspection. The data collected
on the haul truck’s differential and rear final drives
(Figure 7) indicated a problem. As Figure 7 illustrates, the
ISO codes were up slightly before the failure.
Figure 7. Haul Truck Differential and Final Drive.
ISO Codes Before and After Filtering.
Normal Wear
While it is expected that the ISO numbers will increase when
excessive wear debris is found in the oil, it is important
to note that as the Caterpillar haul trucks’ final
drives and differentials age, it is normal for the final
drive ISO cleanliness numbers to increase slightly (Table
1).
Filtering will decrease the numbers some; however, they are
not likely to revert back to the original ISO cleanliness levels
because wear debris does increase as the components get older.
Because some wear debris is common during normal operation,
the Smoky Canyon Mine reliability team felt it was important
to record the ISO code data and photograph the magnetic plugs
during normal operation. Mine personnel can compare ISO data
and magnetic plugs from other haul trucks with this information
when determining if the wear is normal or excessive.
Figures 8-1 through 8-6 show photos of plugs from final drives
and differentials that are experiencing normal wear. These
photos verify that even under normal operating conditions,
a small amount of wear debris will be found on magnetic plugs.
Figures 9-1 through 9-3 show photos of magnetic plugs during
a typical break-in period, after a final drive and differential
had been rebuilt. As the photos show, medium fines and a few
flakes were present during the earlier inspections at 951 hours
on the final drive. The inspection at 1,428 hours on the final
drive shows that no flakes are present, and there are fewer
medium fines. Less than 600 operating hours later - at 2,000
hours - only soft fines are seen. These photographs verify
that the plugs will contain some wear debris early in the break-in
process, but it will diminish as the equipment continues to
operate.
Failure Photographs
In addition to the photographs of plugs from systems incurring
normal wear, the Smoky Canyon Mine reliability team felt
it was important to photograph plugs before equipment failures.
In addition to the failures that have already been discussed,
the team has collected photographs on other bearing failures.
Figures 10-1 through 10-3 show the condition of the same
plug at three different inspection intervals prior to a haul
truck bearing failure. At each interval, more wear debris
was found on the plug, which initially had collected fines,
but was covered with large flakes at the final inspection.
Figure 10-4 shows the bearing, which was damaged significantly.
Figure 11-1 shows
a plug that was removed from a haul truck’s
left rear final drive when the bearing failed. The plug was
covered with fines and the damage to both the inner and outer
bearing (Figures 11-2 and 11-3) was significant.
Conclusion
What started as a contamination control program with the goal
of extending drain intervals and differential and final drive
rebuild intervals, has blossomed into much more. The Smoky
Canyon Mine reliability team has successfully implemented
a program that has not only met these goals, but has exceeded
the original differential and final drive rebuild goals and
has helped the team avoid several catastrophic failures.
In fact, no catastrophic gear failures due to component wear
have occurred since the team began inspecting the magnetic
plugs. The reliability team’s success has lowered maintenance
expenses on the mine’s mobile equipment, one of its
largest cost centers, and has helped the mine stay competitive
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