Information Notice No. 89-84: Failure of Ingersoll Rand Air Start Motors as a Result of Pinion Gear Assembly Fitting Problems
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
All holders of operating licenses or construction permits for nuclear power
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
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.
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 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.
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
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