Tham khảo tài liệu 'failure analysis case studies ii episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Failure Analysis Case Studies ỊỊ . Jones Editor 2001 Elsevier Science Ltd. All rights reserved 199 FAILURE ANALYSIS AND EXPERIMENTAL STRESS ANALYSIS OF A THREADED ROTATING SHAFT R. B. TAIT Department of Mechanical Engineering University of Cape Town Private Bag Rondebosch 7700 Republic of South Africa Received February 1998 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 signifeant 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 .
