United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-10: Failure of Motor-Operated Valve Electric Power Train due to Sheared or Dislodged Motor Pinion Gear Key

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               February 4, 1994

                               GEAR KEY


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for motor-operated valve (MOV)
electric power train failures caused by structural failure or improper
installation of the motor pinion gear key.  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 are not NRC requirements; therefore, no
specific action or written response is required.


The motor pinion gear is located on the shaft of the electric motor and
transmits motor torque to the worm shaft gear.  The motor pinion gear is
connected to the motor shaft by means of a key which transfers rotary force
from the motor shaft to the gear.  A set screw is used to prevent axial
movement of the motor pinion gear on the motor shaft.

In 1989, Limitorque Corporation issued Maintenance Update 89-1 which made
recommendations for securing the motor pinion gear key when replacing the
motor or the gears.  The update stated that some users had not adequately
secured the key, permitting it to subsequently slide out of the keyway and
decouple the motor from the actuator.  The update gave a procedure for staking
the motor shaft at the open end of the keyway to mechanically lock the key in

In 1992, Limitorque issued Maintenance Update 92-2 which stated that the
material for motor pinion keys supplied with SMB-000 through -2 actuators had
recently been changed from American Iron and Steel Institute (AISI) 1018 to
AISI 4140 steel because of customer requests for the higher strength material.

Limitorque stated that all SMB-3 through -5 actuators with motors rated in
excess of 13.8 kg-m [100 ft-lbs] and serial numbers greater than 12160 were
supplied with motor pinion keys of AISI 4140 steel.  In the maintenance 

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                                                            February 4, 1994
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update, Limitorque recommended that, if not already done, the motor pinion key
be replaced with AISI 4140 steel for SMB-3 through -5 actuators with motors
rated in excess of 13.8 kg-m [100 ft-lbs].

Description of Circumstances

On August 30, 1993, the high-pressure coolant injection system pump discharge
isolation valve at the Cooper Nuclear Station failed to open during the
performance of a surveillance procedure, causing the valve to be declared
inoperable.  Subsequent investigation revealed that the SB-3 Limitorque
actuator motor pinion gear key had migrated out of its keyway, allowing the
motor shaft to spin without transferring torque to the motor pinion gear.  The
cause of this event was attributed to the failure to stake the motor shaft
keyway following a motor replacement in 1990.

The licensee for Cooper Nuclear Station incorporated the recommendations of
Limitorque Maintenance Update 89-1 in a 1991 revision to its MOV maintenance
procedures.  However, at that time, the licensee did not investigate the
potential consequences of maintenance activities that had been performed
before the procedure was revised.  During that time, staking of the motor
shaft had not been a requirement.  Based on vendor contacts and independent
sampling, the licensee determined that the particular actuators at Cooper
initially procured from Limitorque were properly staked.  Therefore, the
potential extent of the condition at Cooper was limited to actuators that had
received onsite maintenance (affecting the motor pinion gear set) before the
procedures were revised.  The licensee further narrowed the scope of its
investigation based on a Limitorque assertion and industry information that
only larger size actuators (SMB/SB-0 and above) are susceptible to this

On October 8, 1993, the licensee of the Waterford Steam Electric Generating
Station, Unit 3, notified the NRC that the motor pinion keys in both MOVs used
for isolating the shutdown cooling system suction line were not staked to the
motor shaft.  The actuators were size SMB-0.

On October 8, 1993, the licensee of the Clinton Power Station, Unit 1,
notified the NRC that the SB-3 Limitorque actuator motor pinion key to the
high-pressure core spray injection MOV had sheared.  The actuator was attached
to a motor rated at 11 kg-m [80 ft-lbs].  The licensee determined that the key
material was AISI 1018 steel.  This motor was rated lower than the threshold
for replacement recommended by Limotorque Maintenance Update 92-2 and is a
different actuator type.


The NRC staff has reviewed information suggesting that a small, but not
inconsequential, number of MOV motor shaft keyways may be unstaked at various
nuclear power plants.  Although a low incidence of motor pinion gear key
dislodgements has been observed, it is possible that an extended time period
may be necessary for axial migration of the key in a horizontally mounted
motor, suggesting the potential for additional failures of this type.

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                                                            February 4, 1994
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The incidence of motor pinion key failures may be increased as a result of the
higher thrust and torque found to be necessary to operate some valves under
dynamic conditions.

Related Generic Communications

NRC has issued other generic communications on MOV motor pinion keys,
including NRC Office of Inspection and Enforcement Information Notices (INs)
81-08, "Repetitive Failures of Limitorque Operator SMB-4 Motor-to-Shaft Key";
85-22, "Failure of Limitorque Motor-Operated Valves Resulting From Incorrect
Installation of Pinion Gear"; and 85-67, "Valve-Shaft-to-Actuator Key May Fall
Out of Place When Mounted Below Horizontal Axis"; and NRC INs 88-84,
"Defective Motor Shaft Keys in Limitorque Motor Actuators"; and 90-37,
"Sheared Pinion Gear-to-Shaft Keys in Limitorque Motor Actuators." 

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                    /s/'d by BKGRIMES

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Thomas F. Westerman, RIV   Thomas G. Scarbrough, NRR
                     (817) 860-8145             (301) 504-2794

                     John M. Jacobson, RIII     
                     (708) 829-9736

Page Last Reviewed/Updated Friday, November 15, 2013