Information Notice No. 81-08: Repetitive Failures of Limitorque Operator SMB-4 Motor-to-Shaft Key
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
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.
Recently issued IE Information Notices
Page Last Reviewed/Updated Thursday, March 25, 2021