Page 216 - Failure Analysis Case Studies II

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switch tripped at approximately 110-130 Nm and the maximum torque capacity was approximately
950 Nm.
In view of the concern about the shaft cracking problem, arising only from test bench loading,
extensive analysis of the wormshaft, both overseas [I] and locally [24] had been undertaken. The
overseas analyses, involving Rotork (Bath) and Nuclear Electric (Bristol), were quite extensive and
considered inter alia analytical stress analysis, metallurgical analysis, physical component fatigue
testing, theoretical fatigue crack growth calculations, finite element stress analysis modelling, pro-
duction methods and surface hardening treatment, and finally experimental life tests. The summary
of these overseas tests effectively concluded [ 11 that the combined effect of the French requirement
for motor categories and the British Rotork shaft sizing requirement, was that the motors were
slightly too powerful for the shaft, at least for the 30 NATB series. The option of changing to
completely new motors (or shafts) was, however, unacceptable to the electricity power utility,
ESKOM .
Subsequently microscopic analysis [3] revealed that pre-existing defects and micro cracks could
exist as the result of manufacture, at the root of the threads, which would facilitate fatigue crack
growth and, ultimately, fracture, if the cyclic loading conditions were sufficiently high. It was initially
believed [l, 23 that the manufacturing technique of heat treating first, followed by finish grinding of
the threads, led to the inherent thread root defects. Thus, by reversal of this process, i.e. heat
treatment after thread grinding, it was hoped to solve or at least alleviate the problem. Despite this
change in manufacturing route the cracking problem still continued. The fractographic analysis also
indicated that there was a significant bending stress component in the fatigue fracture surface.
Despite this clear evidence of bending fatigue on thc local shafts, the Rotork report [I] indicated
from their analyses and tests, that there should not be a fatigue cracking problem, within the
projected life of the plant, yet they did not consider any vibration or “judder” effects, which can
and do occur on the local test bench-up to 20% of the time, according to the principal operator
[4]. In addition local test bench setting up details and test bench loading could conceivably have
been different from the Rotork U.K. conditions, even though the test bench was built by Rotork to
their own specifications. In addition operational procedures may have differed. For example, Rotork
[I] refer to only 10-12 stall conditions per year, whereas the author observed and counted over 15
in one single actuator test bench run on a single day! In view of the discrepancies between analysis
and testing overseas, and the local performance, the inference was that there might have been some
local phenomenon which led to the fatigue cracking.
In addition, for any meaningful life evaluation of the shafts, an essential critical set of data that
is required are the cyclic stresses and load spectrum that the shaft experiences, both in calibration
and torque setting tests as well as in service. Under high levels of cyclic stress, such as 390 MPa [l],
fatigue is bound to occur, except in cases where the surface finish is completely free of defects, Le.
a highly polished surface finish. Even then fatigue crack initiation could still form by persistent slip
band formation, but admittedly this would only occur after large numbers of initiating fatigue
cycles, beyond the normal life of the plant.
An additional key unknown in the stress evaluation was the degree of bending stress applied,
presumably due to, for example, uneven loading on the brake disc from run out or uneven disc
brake contact or brake pad wear. If the bending component was minimal the stress levels would be
significantly reduced [I]. Hence a key feature in the wormshaft component fatigue life may well be
setting up details to avoid any bending (as opposed to merely axial or torque loads).
Hence there was a move to measure, physically-rather than estimate, theoretically-the stresses
in the actuator wormshaft in operation in situ, initially on (i) the Koeberg test bench and
subsequently, (ii) the plant, to assess the likelihood of fatigue cracking. If the combination of
inherent defect size, from manufacture, and cycle stress amplitude, in bench testing (and service),
were sufficiently high, i.e. greater than threshold, then fatigue would be inevitable [4].
This paper, therefore, attempts to answer some of these questions by using strain gauge measure-
ments of the shaft in operation on the test bench, to evaluate these stresses and the consequent
potential for fatigue. Such physical stress measurements were needed to establish what the peak
stresses and range of stresses were, as well as their origins. In particular, was the critical evaluation
of the presumption that the high stresses in the threaded portion of the shaft arose from the
disc brake loading and that these were more severe from bending rather than axial or torsional
loading.