United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-12: Overgreasing of Electric Motor Bearings

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

                                 April 12, 1988


Information Notice No. 88-12:  OVERGREASING OF ELECTRIC MOTOR BEARINGS


Addressees:

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

Purpose:

This information notice is being provided to alert addressees to problems 
caused by accumulation of grease on electric motors used to drive rotating 
equipment in nuclear power plants.  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:

NRC personnel observed accumulations of grease on the air vent screens of 
electric motors used for driving such rotating equipment as fans and pumps at 
the Millstone and Calvert Cliffs nuclear power plants.  The grease appeared to 
have come from overgreasing of the electric motor bearings.  Grease was forced 
out of the bearing seals, onto the stator windings and rotor, from where it 
either fell or was thrown onto the inside of the motor housing.  Because of 
these observations, the NRC began an investigation into problems that have 
been caused in the past, or could be caused in the future, by the over-greasing 
of electric motor bearings.  

Several electric motor overgreasing events were identified that have led to 
failures of nuclear plant components:

�    At a nuclear plant in 1975, overgreasing of the motor bearings caused the 
     failure of the containment cooler fans when a bearing failed and seized.

�    At Susquehanna Unit 1, on July 24, 1984, excessive grease buildup in the 
     electric motor of a motor generator set caused the set to shut down and 
     resulted in the spurious emergency actuations of the standby gas 
     treatment system and the control room emergency outside air system (LER 
     387/84-036).






8804060370
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                                                            Page 2 of 4


�    At Palisades Unit 1, overgreasing of the motor bearings caused the 
     containment cooler fans to fail on three different occasions:  On July 7, 
     1986, overgreasing of the motor shaft bearings caused the bearings to 
     skid in-stead of roll.  This caused the motor to overheat and 
     automatically shut down.  On August 8, 1986, and again on August 11, 
     1986, overgreasing caused the motor bearings to deteriorate which, in 
     turn, caused the motor and fan to vibrate, and bolts on the equipment to 
     loosen and fail (LER 255/86-031).

In addition to these overgreasing events, a number of cases have been reported 
in which excessive motor bearing grease caused problems that were caught 
before the systems actually failed.  For example, at a nuclear plant in 1978, 
excessive grease in a motor bearing in the service-water system caused the 
bearing to become abnormally hot and noisy.  At another nuclear plant in 1986, 
grease from a bearing in the component cooling pump motor ran into the motor, 
causing it to smoke.

Discussion:

The NRC staff has solicited technical information and operating experience on 
the problems caused by the overgreasing of electric motor bearings from motor 
and bearing manufacturers, as well as from other licensees.  Their responses 
are summarized in this discussion.

Two basic mechanisms involving overgreasing of bearings can lead to the 
failure of electric motors: 

(1)  The excess grease will leak out from the bearing seal and be deposited on 
     the stator and rotor windings.  The grease deposits create a thermal 
     barrier between the windings and their cooling air, leading to an 
     increase in motor temperature.  For motors designed to use outside air to 
     cool the windings, the temperature is increased still more by the buildup 
     of dirt, dust, and other foreign materials that adhere to the grease.  It 
     is estimated that for every 10�F rise in motor temperature, the 
     insulation life of the windings is reduced by half.  The deterioration of 
     the winding insulation then causes the motor to "short out". 

(2)  The excess grease causes a "churning" action of the ball bearings within 
     their housing, increasing the temperature of the grease and bearings.  As 
     the temperature approaches the upper functional design limit of the 
     grease, viscous or hard residues form, causing a breakdown of the lubri-
     cating characteristics of the grease.  The bearing temperature then rises 
     higher, and eventually causes a reduction in bearing hardness and a sub-
     sequent breakdown of the bearing.  A rule of thumb, based on certain
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                                                            April 12, 1988
                                                            Page 3 of 4


     manufacturers' tests, is that grease life is halved for every 25�F 
     increase in temperature and is doubled for every 25�F decrease in 
     temperature. 

     High bearing temperature may also cause the inner race of the bearing to 
     expand and slip on the rotor shaft.  The expansion and slippage of the 
     bearing's inner race leads to a misalignment between the rotor shaft and 
     the bearing; this misalignment can cause contact between the rotor and 
     stator.  Once contact occurs, the windings short and burn out.

One frequent cause of excess grease problems is the failure to provide an exit 
path for the displaced grease when new grease is added.  This can result from 
the failure to remove the grease drain plug, or from failure to unblock drain 
lines that have become plugged by hardened grease or dirt.  Another frequent 
cause is the addition of too much grease.  The correct amount must be deter-
mined for each situation.  Bearing manufacturers generally recommend filling 
the motor bearing void space from one-third to one-half full of grease.  How-
ever, the optimum amount can vary from 20% to 80% full, depending on such 
factors as the application of the motor, the environment of the motor, the 
retainer design, and the type of grease.  

The following actions can be taken to correct or prevent motor overgreasing 
and related problems:

(1)  Review motor lubrication procedures to ensure that they identify the type 
     and quantity of grease to use, the specific fill and drain nozzles to 
     uncap, and the length of time motors should be run with drain plugs off 
     after greasing the bearings.

(2)  To prevent foreign materials from contaminating the grease, ensure that 
     grease containers are covered during periods of storage and that nozzles 
     and grease fittings are cleaned. 

(3)  Determine the optimum quantity and correct type of grease required for 
     each motor by examining the manufacturer's recommendations and by 
     monitoring the behavior of grease added to motors.

(4)  Consider using prelubricated sealed bearings in applications where re-
     lubrication is difficult, where contaminants can adulterate the grease, 
     or where overgreasing might damage safety systems.

It should be noted that the vast majority of the reported lubrication-related 
incidents have been the result of underlubrication rather than 
overlubrication.  This notice is not intended to discourage the adequate 
lubrication of motor bearings, but to prevent overgreasing problems by 
encouraging a thorough understanding of correct lubrication procedures.
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                                                            Page 4 of 4


No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office.  




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


Technical Contacts:  James A. Prell, RI
                     (215) 337-5108

                     Donald C. Kirkpatrick, NRR
                     (301) 492-1152


Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment
                                                            IN 88-12 
                                                            April 12, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-11          Potential Loss of Motor       4/7/88         All holders of OLs
               Control Center and/or                        or CPs for nuclear
               Switchboard Function Due                     power reactors. 
               to Faulty Tie Bolts 

88-10          Materials Licensees:  Lack    3/28/88        All NRC licensees 
               of Management Controls Over                  authorized to use 
               Licensed Programs                            byproduct 
                                                            material. 

87-44,         Thimble Tube Thinning in      3/28/88        All holders of OLs
Supp. 1        Westinghouse Reactors                        or CPs for nuclear
                                                            power reactors 
                                                            that employ a 
                                                            Westinghouse NSSS.

88-09          Reduced Reliability of        3/18/88        All holders of OLs
               Steam-Driven Auxiliary                       or CPs for nuclear
               Feedwater Pumps Caused                       power reactors. 
               by Instability of Woodward 
               PG-PL Governors 

88-08          Chemical Reactions with       3/14/88        All NRC licensees 
               Radioactive Waste                            generating or pro-
               Solidification Agents                        cessing low level 
                                                            radioactive waste.

88-07          Inadvertent Transfer of       3/7/88         All NRC broad 
               Licensed Material to                         licensees and 
               Uncontrolled Locations                       licensees 
                                                            authorized to 
                                                            possess byproduct 
                                                            material as sealed
                                                            sources in 
                                                            teletherapy units 
                                                            or "self-
                                                            contained" 
                                                            irradiators. 

88-06          Foreign Objects in Steam      2/29/88        All holders of OLs
               Generators                                   or CPs for PWRs. 

88-05          Fire in Annunciator Control   2/11/88        All holders of OLs
               Cabinets                                     or CPs for nuclear
                                                            power reactors. 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013