FAILURE ANALYSIS AND EXPERIMENTAL STRESS ANALYSIS OF A THREADED ROTATING SHAFT

Abstract

The occurrence of a fracture of an actuator wormshaft, used for opening and closing a valve in Koeberg's Nuclear Power Station cooling water system, during routine testing, was cause for concern. Two such fractures occurred in a particular type of actuator shaft and another 40% of such shafts exhibited fatigue cracking. Conventional fractographic failure analysis indicated that there was a signifcant bending stress component in the fatigue failure, the origin of which was unclear. The actuator had two torque limiting devices once the valve had seated, the last of which was a disc brake system, and it was suspected that inappropriate setting of the disc brake contributed to the high cyclic bending stresses and hence the fatigue failure. In this paper, an experimental stress analysis was undertaken by strain gauging the actual shaft of an actuator in situ and measuring the bending, tension and torsional stresses in operation during rotation, and valve closure. It transpired that the brake disc location and setting was not the prime cause of the high bending stresses, but rather that a single, “thin” lock nut was canting over slightly against some Belvel spring washers and applying significant bending stress, via the actuator housing, to the shaft. The conventional tolerances on this ordinary nut, together with the design, and variable setting up were sufficient to cause substantial bending, and ultimately fatigue, of the shaft, under straight-forward, low, nominally tensile loading. This simple nut on a threaded shaft fatigue failure scenario has wide application in a variety of similar bolted shaft applications. A substantially longer recessed nut was used and reduced the offset bending stresses significantly (from 180 to 25 MPa), vindicating the interpretation. The final design incorporated a system not unlike this long nut solution, in that the recessed nut did not exhibit any canting over. This, together with improved shaft processing, effectively solved the problem. © 1998 Published by Elsevier Science Ltd. All rights reserved.