United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-36, Supplement 1: Loosening of Locking Nut on Limitorque Operator

                                                            SSINS No. 6835 
                                                            IN 84-36 SUPP. 1

                               UNITED STATES 
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, D.C.  20555

                             SEPTEMBER 11, 1984

INFORMATION NOTICE NO. 84-36 SUPPLEMENT 1:  LOOSENING OF LOCKING NUT ON 
                                            LIMITORQUE OPERATOR 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP). 

Purpose: 

This information notice is being provided to supplement Information Notice 
84-36 for the purpose of assuring that loosening of locking nuts does not 
occur in Limitorque valve operators.  The NRC expects that recipients will 
review this notice for applicability to their facilities.  Suggestions 
contained in this notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

Information Notice 84-36 described events at the Hatch Nuclear Power Plant 
where loosening of a set screw on a locking nut on the worm gear shaft of a 
Limitorque operator led to backing out of a locknut and subsequent 
inoperability of a safety-related valve.  Recent information received from 
the vendor indicates that the specific part affected on the SMB-4 valve 
actuator was Part #83 (bearing locknut, see Figure 1), which is located on 
the actuator worm shaft--not the worm gear shaft.  An enlarged drawing 
showing more details of the bearing lock nut is shown in Fig. 2.  The 
bearing lock nut performs the function of transmitting loads from the worm 
gear (Part #16) to the worm shaft causing axial movement of the worm shaft 
followed by compression of the belleville spring and activation of the 
torque switch.  The vendor indicates that the operators at Hatch contain a 
unique feature that is not found in most other valve applications that 
resulted in the originally noted defect.  These units use a ball screw valve 
stem instead of the valve manufacturer's conventional Acme threaded valve 
stem.  The ball screw has the opposite hand (right-hand) thread to the 
standard Acme (left-hand) thread. 

The significance of this difference is that unthreading of the bearing 
locknut could affect either the closing torque switch function when the 
valve is equipped with a right-hand stem thread or the opening torque switch 
function 

8409070254 
.

                                                          IN 84-36 SUPP. 1 
                                                          September 11, 1984
                                                          Page 2 of 2 


(with obstructed or overloaded disk) when the valve is equipped with a 
standard Acme left-hand stem thread.  This is because the torque control 
mechanism is activated as a result of axial movement of the worm shaft due 
to forces generated as the valve comes to a full-close position, full-open 
position, or some intermediate obstructed position.  Figure 2 shows the 
direction of movement of the worm shaft at the Hatch facility when the valve
comes to a closed position. 

It is recommended that holders of CPs and OLs consider this additional 
information for applicability in their ongoing efforts to resolve the 
concerns addressed in Information Notice 84-36. 

No written response to this information notice is required.  If there are 
any questions regarding this matter, please contact the Regional 
Administrator of the appropriate NRC regional office or this office. 


                               Edward L. Jordan Director 
                               Division of Emergency Preparedness 
                                 and Engineering Response 
                               Office of Inspection and Enforcement 

Technical Contact:  John J. Zudans, IE
                    (301) 492-8030

Attachments:
1.  Figure 1, "SMB-0 to SMB-4 & SMB-4T"
2.  Figure 2, "Worm & Torque Spring Assembly"
3.  List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013