FAILURE ANALYSIS AND EXPERIMENTAL STRESS-ANALYSIS OF A THREADED ROTATING SHAFT

The occurrence of a fracture of an actuator wormshaft, used for openin g and closing a valve in Koeberg's Nuclear Power Station cooling water system, during routine testing, was cause for concern. Two such fract ures occurred in a particular type of actuator shaft and another 40% o f such shafts exhibited fatigue cracking. Conventional fractographic f ailure analysis indicated that there was a signifcant bending stress c omponent in the fatigue failure, the origin of which was unclear. The actuator had two torque limiting devices once the valve had seated, th e last of which was a disc brake system, and it was suspected that ina ppropriate setting of the disc brake contributed to the high cyclic be nding stresses and hence the fatigue failure. In this paper, an experi mental stress analysis was undertaken by strain gauging the actual sha ft of an actuator in situ and measuring the bending, tension and torsi onal stresses in operation during rotation, and valve closure. It tran spired that the brake disc location and setting was not the prime caus e of the high bending stresses, but rather that a single, ''thin'' loc k 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 wit h the design, and variable setting up were sufficient to cause substan tial bending, and ultimately fatigue, of the shaft, under straight-for ward, low, nominally tensile loading. This simple nut on a threaded sh aft fatigue failure scenario has wide application in a variety of simi lar 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 incorpora ted a system not unlike this long nut solution, in that the recessed n ut did not exhibit any canting over. This, together with improved shaf t processing, effectively solved the problem. (C) 1998 Published by El sevier Science Ltd. All rights reserved.