ML20078S335

Evaluation of Abnormal Wear at Limerick

ML20078S335
Person / Time
Site:	Limerick
Issue date:	02/10/1994
From:	COLTEC INDUSTRIES, INC.
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References
NUDOCS 9502240088
Download: ML20078S335 (5)

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- E, ENGINEERING REPORT sur 1 or 79 mo.

_1 Fpirt enke Morse H2 mm R-5.15-8660 Engine civision "U

Pump Drive Gears 8-1/8 x 10 0.P. Engines r"

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Evaluation of Abnormal Wear at Limerick sr G. Gutoski 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'. The greatest wear was on the driven, lube-oil-pump gear (16300443). At that time, the engine had experienced 474 hours0.00549 days <br />0.132 hours <br />7.837302e-4 weeks <br />1.80357e-4 months <br /> of operation. The flex-drive gear, its mating gears and the lube-oil-pump, governor-drive bracket (16403129) were removed and replaced. 'ihe 1

parts were examined and reports written by PECO, Technicon Enterprises, and 2

Philadelphia Gear Corporation. They concluded that the abnormal wear was due to misalignment of the driven, lube-oil-pump gear resulting from an out of square tube-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 shaf t bone. The result of this error was loading on ends the gear teeth of the flex-drive gear Ond 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 foilows.

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 f atigues 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 shif ts 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 9502240088 950215 PDR ADOCK 05000352 P

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PAGE ENGINEERING REPORT snar 2 or 79 mo.

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FEI m8ER R-5.15-8660 Engn. olvi ion s m ect Pump Drive Gears 8-1/8 x 10 0.P. Engines February 10, 1995 rmEPARED 8Y D.P. Beck BEPORT Evaluation of Abnormal Wear at Limerick

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r 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 f ace.

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'. The driven, lube-oil-pump gear saw 45 million cycles of high stress and was wellinto step 5. It had lost 0.025" of material from the tooth profile *. Neither gear was near failure since we have seen such gears loose 0.045" of material without breaking '.

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 5

8 to 18 months. Between inspections, up to 238 hours0.00275 days <br />0.0661 hours <br />3.935185e-4 weeks <br />9.0559e-5 months <br /> of operation are possible. 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 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> 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 8

described above and initial or corrective pitting. 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.

me PAGE ENGINEERING REPORT sm 3 or 79 mo.

3 Folebenks Corse rug NtW BER R-5.15-8660 Engine Divlelon Pump Drive Gears 8-1/8 x 10 0.P. Engines February 10, 1995 N

sY D.P. Beck BEsoff N

Evaluation of Abnormal Wear at Limerick APPaoVED Greg Gutoski Visualinspections were performed at Limerick at the required intervals. The first time 2

signs of this problem were reported was during their August 1994 inspection. 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 1993, 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 visualinspection 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 f ace 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 cf 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 visualinspections.

ammes PAGE ENGINEERING REPCRT sunT 4 or 79 wo.

4 Fairbanks Corse TILE msER R-5.15-8660 Engine Olviolon February 10. 1995 Pump Drive Gears 8-1/8 x 1- 0.P. Engines r ErrsEn nr D.P. Beck N

Evaluation of Abnormal Wear at Limerick

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y Greg Gutoski CONCLUSION The excessive gear tooth wear observed on diesel generator D22 at Limerick was due to misalignment caused by improper machining of the tube-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

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ENGINEERING REPORT 8mf 79 or 79 No.

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" 'C' PREPARED sr D.P. Beck REPORT tmz Evaluation of Abnormal Wear at Limerick APPROVED 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 Analvsis and Prevention', or Handbook of Practical 2

G93LQeSign_by Dudley or Fundamentals of Gear Desian by Drago.

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 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br /> the pits should heal over, that is they should smooth out slightly. However, if the pitted area continues to Orow af ter 200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br />, 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 0.012".

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 D_esian; McGraw-Hill Book Co.; New York: 1984

3. Drago, Raymond J.; hindamentals of Gear Desinn; Butterworth-Heinemann; Stoneham MA; 1988

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