Page 214 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 200 I 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 3 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 signifcant bending stress
component in the fatigue failure, the origin ofwhich 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
valvc 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. 0 1998 Published by Elsevier Science Ltd. All rights reserved.
Keywo* Fatigue, mechanical connections, power-plant failures, strain gauging, stress analysis.

1. INTRODUCTION

The premature development of cracks, apparently through fatigue, in the first couple of threads
of some Rotork 30 NBAT actuator wormshafts of Koeberg Nuclear Power Station (KNPS), near
Cape Town, had focused attention on what could have been a potentially serious problem. Indeed,
on two occasions complete fracture occurred from these fatigue cracks under test bench loading
conditions at Koeberg, and over half of the shafts of this type had developed (fatigue) cracks.
The actuator that failed at Koeberg, together with others that developed fatigue cracks, was very
similar to a type shown in Fig. 1. In this figure, the worm gears, torque switch and the wormshaft
with the end locking nuts are clearly visible. Because it was considered that it must be possible to
operate the actuator under emergency conditions, for example in the event of a depressurisation
leak in the containment area, the valve was required to be operable even if the electrical supply was
below that at which the actuator motor was rated.
For this reason, the actuator armature was rewound for use at Koeberg in order to make it
capable of providing enough torque to close the valve even when the voltage had dropped from the
rated 380 V. This created the problem that the maximum output torque of the motor under normal
operating conditions (approximately 950 Nm on the valve) was now sufficient to cause damage to
the valve seat and/or actuator itself.
When the motor is energised, it rotates the wormshaft causing the valve to turn via the wormgear
arrangement. Once the valve has seated, the wormshaft begins to translate axially, compressing a
set of Belvel washers (Fig. 2) which resist the axial movement. As the shaft translates. at a certain

* Author to whom corrcspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (2), 79-89 (1 998)