Information Notice No. 81-08: Repetitive Failures of Limitorque Operator SMB-4 Motor-to-Shaft Key

                                                            SSINS No.: 6835 
                                                            Accession No.: 
                                                            IN 81-08       

                                UNITED STATES
                               March 20, 1981

Information Notice No. 81-08:   REPETITIVE FAILURES OF LIMITORQUE 
                                   OPERATOR SMB-4 MOTOR-TO-SHAFT KEY 

Description of Circumstances: 

During normal surveillance testing of motor-operated valves at the Cooper 
Nuclear Station, it was discovered that the RHR "B" loop suppression pool 
cooling inboard throttle valve would not operate. The valve did not operate 
because the key between the motor pinion gear and the motor shaft had 
failed. The operator for the valve has been identified as being Limitorque 
model SMB-4. Analysis of the failed key revealed that a standard type 1010 
steel key had been installed instead of the required special type 4140 steel 

Further review of the problem established that similar failures with 
motor-to shaft keys had also taken place at the Pilgrim, Hatch, and 
Fitzpatrick nuclear power facilities. 

Limitorque Company has performed an evaluation of the problem, and they have
concluded that the problem is confined to operators having a torque rating 
in excess of 100 ft/lb. The standard type 1010 steel possesses adequate 
strength to accommodate motor operators having a torque rating that is less 
than 100 ft/lb. 

Recommended Action for Holders of Operating Licenses and Construction 

It is recommended that licensees having Limitorque Operator SMB-4 installed 
in their facilities verify that the special type 4140 steel keys have been 

This can be easily accomplished by performing a hardness check of the 
motor-to-shaft keys during the next scheduled surveillance testing. 

No written response to this information notice is required. If you need 
additional information with regard to this subject, please contact the 
Director of the appropriate NRC Regional Office. 

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