Information Notice No. 89-84: Failure of Ingersoll Rand Air Start Motors as a Result of Pinion Gear Assembly Fitting Problems

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                              December 12, 1989


Information Notice No. 89-84:  FAILURE OF INGERSOLL RAND AIR START 
                                   MOTORS AS A RESULT OF PINION GEAR 
                                   ASSEMBLY FITTING PROBLEMS


Addressees:

All holders of operating licenses or construction permits for nuclear power 
reactors. 

Purpose:

This information notice is intended to alert addressees to the potential 
malfunctioning of Ingersoll Rand air start motors caused by the inadequate 
fitting of the pinion gear, the pinion gear retainer and the rotor shaft.  
It is expected that recipients will review the information for applicability 
to their facilities and consider actions, as appropriate, to avoid similar 
problems.  However, suggestions contained in this information notice do not 
constitute NRC requirements; therefore, no specific action or written 
response is required.

Description of Circumstances:

On February 1, 1989, during the performance of a surveillance test at Diablo 
Canyon Unit 1, the number one emergency diesel generator failed to start.  
Each diesel generator at this plant is started by four Ingersol-Rand air 
start motors, two of which are controlled by the primary dc power source and 
the other two by the backup dc power source.  In accordance with the normal 
test procedure, the backup dc power source was disabled during the test.  
After repeated tests showed that the diesel generator could not be started 
with the primary dc power source but could be started with the backup dc 
power source, the two primary start motors were disassembled for inspection.  
Both motors were found to have broken pinion gear retainers that allowed the 
start motor rotor shaft to turn without driving the pinion gear (see Figure 
1).  In addition, both motors had broken and bent retainer bolts and cracks 
in the rotor shafts.

During normal operation, the diesel generators are started using both dc 
power sources, and in an emergency the number one diesel generator would 
have been started by the still functional backup start motors.  However, 
many related problems were found on other air start motors as a consequence 
of this event.  The Diablo Canyon site has a total of 20 air start motors, 
four on each of its five emergency diesel generators.  All of the motors 
were disassembed and 




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                                                       IN 89-84
                                                       December 12, 1989
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inspected, and a total of 10 were found to have cracks in their rotor 
shafts.  Many motors had pinion gears or pinion gear retainers and retainer 
bolting that were loose.  All of the motors with cracked shafts had 
well-developed wear patterns on the retainers and on the tangs at the ends 
of the rotor shafts that engage slots in the retainers, indicating that the 
pinion gears had been loose for a long time.  In two cases, the retainer 
bolts had backed out until the retainer was on the point of becoming 
disengaged from the rotor shaft tang.

Discussion:

When a diesel generator receives a start signal, solenoid valves open to 
admit air pressure (normally 150 psi) to the air motor rotor, which turns 
the shaft and pinion gear in question.  This pinion gear meshes with and 
turns a larger drive gear that provides torque through a clutch to a Bendix 
drive.  The rotation through the Bendix drive forces a second pinion gear to 
engage the ring gear on the diesel engine flywheel, causing the diesel 
engine to turn.  The pinion gear retainers, which were broken during this 
event, consist of a disk bolted to the end of the rotor shaft (Figure 1).  
The retainer has a transverse slot that engages both a flat-sided tang on 
the end of the shaft and lugs on the pinion gear.  Thus, the shaft cannot 
turn relative to the pinion gear without breaking the retainer.  However, 
this retainer is not designed to transmit the main torque from the drive 
shaft to the pinion gear.  The torque is intended to be transmitted directly 
from the shaft to the pinion by means of a press fit between the tapered 
shaft and the matching tapered pinion bore.  The retainer is only designed 
to maintain the press fit by keeping a clamping force on the outside end of 
the pinion.

The basic cause of the air start motor problems at Diablo Canyon was loss of 
the pinion gear to shaft press fit as a result of an inadequate fitting of 
the shaft, pinion gear, retainer, and bolting.  This loss allowed the pinion 
gear to slip on the shaft, forcing the retainer to transmit the full torque 
of the motor.  This situation caused abnormal wear of the retainer and of 
the tang on the end of the shaft, cracking and deformation of the tang, and 
the loosening and backing out of the retainer bolts.  Eventually two 
retainers for the start motors on diesel generator 1-1 failed completely, 
preventing the diesel generator from starting.

The loss of contact was attributed to a number of subsidiary problems, 
including poor fit between the pinion gear and the shaft surfaces, dirt on 
the surfaces, lack of "setting" pressure on the pinion gear during assembly 
onto the shaft, and incorrect torquing of the retainer bolts.  However, the 
main problem appears to have been improper fit between the retainer and the 
pinion gear.  As shown in the detail on the lower half of Figure 1, there is 
a sharp angle at the transition between the retainer inner surface and its 
slot wall.  On the mating surface of the pinion gear, however, there is a 
radius between the side of the lug and the outer flat surface of the gear.  
As a consequence, the sharp edge of the retainer slot presses on the 
curvature of the radius at the base of the pinion gear lug.  This small 
contact area eventually degenerates as a result of shock loading, relieving 
the pinion retainer clamping force.  This problem was identified on 13 of 
the air start motors at Diablo Canyon.

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                                                       IN 89-84
                                                       December 12, 1989
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A possible contributory cause to the failure of one of the two air start 
motors that failed on February 1 was the fact that the air supply to this 
motor was higher than normal on that day.  Records indicate that the 
pressure supplied to this motor was 190 psi compared to a normal value of 
150 psi.  The vendor indicated that a pressure in excess of 160 psi would 
begin to be of concern.  It has been suggested that the unusually high 
pressure caused the first motor to fail, which left the second motor to 
carry the full load, causing it to fail also.  However, vendor tests show 
that only one air start motor is required to start a diesel generator within 
the required time limit.  Furthermore, the clutch in the start motor drive 
train should have limited the torque in the train to the design limits.  

The corrective actions for this problem included the replacement of all of 
the damaged parts, using revised reassembly procedures.  The reassembly 
procedures included lapping and blueing checks to ensure the correct fit.  
The procedures also specified the correct retainer bolt torque and the use 
of lock washers and lock wiring to maintain the correct retainer force on 
the pinion gears.  In addition, measures were taken to ensure that the start 
motor air header pressures would be maintained below 160 psi.

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate NRR project manager.




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation


Technical Contact:  D. C. Kirkpatrick, NRR
                    (301) 492-1152

Attachments:  
1.  Figure 1
2.  List of Recently Issued NRC Information Notices
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