Part 21 Report - 1995-063
ACCESSION #: 9502240086
Coltec Industries Fairbanks Morse
Engine Division
701 Lawton Avenue
Beloit, WI 53511-5492
608/364-4411
Fax: 608/364-0382
15 February 1995
Document Control Desk
U. S. Nuclear Regulatory Commission
Washington, DC 20555
Subject: Final Report - O.P. 38TD8-1/8 Lube Oil Pump Drive Bracket, part
number 160705716 (part of pump flex drive, part number
1669073): Report Number 94-004.
Reportable as a Potential Safety Hazard under the requirements
of 10 CFR Part 21.
On August 1994, Philadelphia Electric Company notified Coltec Industries,
Fairbanks Morse Engine Division (FMED) that the subject item "displayed
abnormal ware and pitting which is believed to have been caused by a
mismatched bracket".
The subject item is manufactured by Fairbanks Morse Engine Division. All
U. S. Nuclear power stations with FMED 8 1/8 Opposed Piston Emergency
Diesel Generator Sets are affected.
An investigation into the nature and extent of this problem, and the
action necessary to assess the impact of the failure was conducted; the
results are detailed in the attached Engineering Report; file no.: R-
5.15-8660, dated 10 Feb 95.
If there is any further need for clarification, please contact Fairbanks
Morse Engineering, or Quality Assurance.
Sincerely,
Scot A. Fratianne
Quality Assurance Manager,
Systems and Supplier Quality
A Division of Coltec Industries Inc
*** END OF DOCUMENT ***
ACCESSION #: 9502240088
ENGINEERING REPORT SHEET 1 OF 79 PAGE
NO. 1
Fairbanks Morse FILE
Engine Division NUMBER R-5.15-8660
SUBJECT Pump Drive Gears 8-1/8 x 10 DATE February 10, 1995
O. P. Engines
PREPARED
BY D.P. Beck
REPORT APPROVED
TITLE Evaluation of Abnormal Wear BY G. Gutoski
at Limerick
Introduction
Coltec Industries, Fairbanks Morse Engine Division manufactured, tested
and delivered a skidded engine-generator set serial number 879060 to
Limerick Generating Station in 1980. In August 1994, during scheduled
maintenance of the diesel generator (unit D22), excessive wear was noted
on the flex-drive gear (16200747) and its mating gears 1_/. The greatest
wear was on the driven, lube-oil-pump gear (16300443). At that time, the
engine had experienced 474 hours of operation. The flex-drive gear, its
mating gears and the lube-oil-pump, governor-drive bracket (16403129)
were removed and replaced. The parts were examined and reports written by
PECO, Technicon Enterprises, and Philadelphia Gear Corporation 2_/. They
concluded that the abnormal wear was due to misalignment of the driven,
lube-oil-pump gear resulting from an out of square lube-oil-pump,
governor-drive bracket. In their judgment failure of the gears was not
imminent. The parts were returned to Fairbanks Morse for further
evaluation and possible part 21 report.
Evaluation by Fairbanks Morse
Fairbanks Morse concurs with the evaluation by PECO and their
consultants. Our inspection indicates that the lube-oil-pump, govenor-
drive bracket does not meet the drawing requirements for squareness of
the bracket bolting face to the centerline of the gear shaft bore 3_/.
The result of this error was loading on ends the gear teeth of the flex-
drive gear and driven, lube-oil-pump gear. This caused high contact
stresses and eventual pitting as was observed on these gears. If the
gears had not been replaced the wear would have continued to the point of
failure.
The steps leading to the observed pitting and then to the projected
failure are as follows.
1. At the start, the misalignment reduces the size of the tooth
contact area and moves the area away from the center of the
tooth resulting in high contact stresses.
2. Repeated cycles of high contact stress fatigues the material
causing pits to form in the loaded area.
3. The loss of material due to pitting shifts the contact area
away from the loaded side of the tooth and toward the unloaded
end of the tooth.
4. The newly loaded area of the tooth is also highly stressed so
in time it fatigues and pits and shifts the load further toward
the unloaded end of the tooth. In this manner the pitted area
of the tooth walks across the length of the tooth.
5. At this point misalignment is no longer an issue since the
teeth make contact along their whole length. However, the load
is only carried on the high points
ENGINEERING REPORT SHEET 2 OF 79 PAGE
NO. 2
Fairbanks Morse FILE
Engine Division NUMBER R-5.15-8660
SUBJECT Pump Drive Gears 8-1/8 x 10 DATE February 10, 1995
O. P. Engines
PREPARED
BY D.P. Beck
REPORT
TITLE Evaluation of Abnormal Wear APPROVED
at Limerick BY G. Gutoski
between the pits and the gear runs roughly since the tooth
profile is no longer an involute. The small contact area and
the additional load due to speed variation causes relatively
rapid loss of material across the whole tooth face.
6. As the gear wears, the strength of the tooth decreases and the
bending stresses in the root of the tooth increase.
7. Fatigue cracks develop and spread in the root of the tooth
causing the teeth to break off and the gear to fail.
The gears at Limerick did not fail but the above scenario was in process
on two gears, the flex drive gear and the driven, lube-oil-pump gear.
These two gears saw high contact stresses and the resultant pitting. The
flex drive gear teeth saw 25 million cycles of high stress and was at the
start of step 5, it had lost 0.006" of material from the tooth profile
4_/. The driven, lube-oil-pump gear saw 45 million cycles of high stress
and was well into step 5. It had lost 0.025" of material from the tooth
profile 4_/. Neither gear was near failure since we have seen such gears
loose 0.045" of material without breaking 4_/.
Recommendations for Action At Nuclear Plants
Fairbanks Morse knows of no other governor drive bracket with this
machining error and thinks that it is unlikely that other engines in
nuclear plants will have the same problem. However, if such a problem did
exist and the problem was not detected early enough, serious consequences
would result.
The recommended service procedures include a visual inspection of these
gears every 12 to 18 months 5_/. Between inspections, up to 238 hours of
operation are possible 6_/. If properly performed, these inspections will
detect this problem in the first inspection after significant off-center
pitting occurs (step 2). The rate of wear from step 2 to step 4 depends
upon misalignment and therefore will vary. The rate of wear from step 5
to step 7 is independent of misalignment and is estimated to be over 300
hours 7_/. Since the operating time from first detectable indication to
failure exceeds the operating time between inspections, we can conclude
that visual inspections if properly done will prevent a failure due to
misalignment.
A proper visual inspection must be able to distinguish between the
destructive pitting described above and initial or corrective pitting
8_/. Initial pitting is a wear-in phenomena which corrects itself by
spreading the load more evenly on the tooth. Initial pitting is common on
these gears and is not a cause for concern. In fact, flex-drive gears
with initial pitting have run for 10s of thousands of hours without
problem.
ENGINEERING REPORT SHEET 3 OF 79 PAGE
NO. 3
Fairbanks Morse FILE
Engine Division NUMBER R-5.15-8660
SUBJECT Pump Drive Gears 8-1/8 x 10 DATE February 10, 1995
O.P. Engines
PREPARED
BY D.P. Beck
REPORT
TITLE Evaluation of Abnormal Wear APPROVED
at Limerick BY Greg Gutoski
Visual inspections were performed at Limerick at the required intervals.
The first time signs of this problem were reported was during their
August 1 994 inspection 2_/. At that time, the problem was progressing
through step 5. Given the wear rate calculated for step 5, abnormal gear
damage should have been evident at a prior inspection in February 1993.
However none was reported. It is likely that in February 1 993, pitting
was observed but was thought to be initial pitting. In August 1994, this
mistake was not made, the pitting was identified as abnormal and action
was taken.
If misalignment is a possibility, then the inspection must be performed
by someone expert enough to distinguish between initial pitting which is
acceptable and destructive pitting which is not acceptable. The
differences are subtle and identifying them calls for considerable
experience with gears. To help make the distinction, it is recommended
that gear lash measurements be taken and tracked to determine the rate of
wear. When using visual inspection of wear patterns, the problem must be
identified before step 5, that is before the pitting has progressed all
the way across the tooth.
There are two other methods that could be used to identify this problem.
1. The tooth contact patterns could be generated in place, under
load and evaluated to determine the degree of misalignment.
Again, the inspection must be performed by someone expert
enough to distinguish between acceptable and unacceptable
misalignment.
2. The lube-oil-pump and govenor-drive bracket could be removed
and the alignment between the bracket face and gear-shaft
centerline measured.
Once it is established that misalignment is not a problem, it is our
experience that these gears will perform reliably for 10s of thousands of
hours. Any deterioration of these gears will progress very slowly and can
be easily detected before it is dangerous by a competent mechanic taking
gear lash measurements and performing visual inspections.
ENGINEERING REPORT SHEET 4 OF 79 PAGE
NO. 4
Fairbanks Morse
Engine Division FILE
NUMBER R-5.15-8660
SUBJECT Pump Drive Gears 8-1/8 x 1- DATE February 10, 1995
O.P. Engines
PREPARED
BY D.P. Beck
REPORT APPROVED
TITLE Evaluation of Abnormal Wear BY Greg Gutoski
at Limerick
CONCLUSION
The excessive gear tooth wear observed on diesel generator D22 at
Limerick was due to misalignment caused by improper machining of the
lube-oil-pump and governor-drive bracket. The gears were wearing rapidly
and presented a dangerous situation. Fairbanks Morse does not expect
other engines to have this problem. However, in critical installations,
the possibility of misalignment must be eliminated. This can be done
through inspection of wear patterns, inspection of tooth contact patterns
or inspection of the bracket. Whichever method is employed, it must be
performed by competent persons. In particular, the examination of wear
patterns requires experience with gear failures.
Footnotes
1_/. Drawings of this assembly can be found in Appendix A
2_/. Reports by PECO, Technicon Enterprises and Philadelphia Gear
are contained in Appendix B
3_/. The Fairbanks Morse inspection report is in Appendix C
4_/. The Gear Tooth Thickness measurements and Calculations are in
Appendix D
5_/. Letter from Tom Skinner of Fairbanks Morse to "Owners of
Fairbanks Morse EDG -Nuclear Industry" dated February 27, 1985
6_/. The expected plant operating requirements are given in Appendix
E
7_/. The wear rate calculations are given in Appendix F
8_/. Guidelines for Inspection of the Flex-Drive Gear and its Mating
Gears are given in Appendix G
ENGINEERING REPORT SHEET 79 OF 79 PAGE
NO. E5
Fairbanks Morse FILE
Engine Division NUMBER R-5.15-8660
SUBJECT Pump Drive Gears 8-1/8 x 10 DATE February 10, 1995
O.P. Engines
PREPARED
BY D.P. Beck
REPORT
TITLE Evaluation of Abnormal Wear APPROVED
at Limerick BY G. Gutoski
APPENDIX G
Inspection Guidelines for the Flex-Drive Gear and it Mating Gears
The gears in the lube-oil and water pump drives are visually inspected
every 12 to 18 months. The inspection should look for gear problems as
detailed in standard references such as Metals Handbook, Failure Analysis
and Prevention 1_/, or Handbook of Practical Gear Design by Dudley 2_/ or
Fundamentals of Gear Design by Drago 3_/.
It is particularly important to look for evidence of misalignment on the
flex drive gear (16200747) and its mating gears since these gears are not
case-hardened and misalignment has been found to cause rapid wear of
their teeth. The first indication of significant misalignment will
probably be tooth-end pitting on the mating gear. As the damage
progresses, it will be possible to classify the pitting as destructive
pitting. The pits will become larger and the pitted area will travel
across the length of the teeth. It is important to detect and replace
gears with this condition before the destructive pitting has progressed
across the length of the teeth.
Although pitting can indicate a gear problem, on the flex gear and its
mating gears a condition known as initial pitting or corrective pitting
is common and is not a cause for concern. This kind of pitting will be
located on the pitch line and the pits will be shallow and under 1mm in
diameter. Gears with initial pitting can be safely run, but the condition
should be noted and followed at the scheduled inspections. Within 200
hours the pits should heal over, that is they should smooth out slightly.
However, if the pitted area continues to grow after 200 hours, or grows
into the tooth dedendum or the pits grow in size, then the gear should be
replaced and the cause determined.
Gear lash measurements can be used to help distinguish between
destructive and initial pitting. If initial pitting is present, the rate
of change in gear lash should decrease during wear-in. On the other hand,
if destructive pitting is present the rate of change in gear lash should
remain the same or increase. Note that the maximum allowed gear lash is
0.01 2".
References:
1_/. Metals Handbook, 8th Edition, Vol 10, Failure Analysis and
Prevention; American Society for Metals; Metals Park, Ohio;
1975
2_/. Dudley, Darle W.; Handbook of Practical Gear Design; McGraw-
Hill Book Co.; New York; 1984
3_/. Drago, Raymond J.; Fundamentals of Gear Design; Butterworth-
Heinemann; Stoneham MA; 1988
*** END OF DOCUMENT ***
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Page Last Reviewed/Updated Wednesday, March 24, 2021