Page 219 - Failure Analysis Case Studies II

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brake unit was removed, even though the full stall torque was not insubstantial. The results of this
test programme are described in the following section.

3. RESULTS
3.1. Bending test series
For the bending test series five tests were initially undertaken with the trip limit switch overridden,
so that the shaft rotation was nominally arrested by the action of disc brake against the brake pads.
The output from the strain bridge was recorded on the digital storage scope which also had the
capability of producing a hard copy of the stored image. The amplitude settings were 0.5 s/cm
horizontally and 0.2 V/cm vertically. It was expected that these tests would yield an oscillating
signal about the x axis which would gradually increase in stress amplitude, as the brake engaged,
until the wormshaft was arrested.
This type of strain trace was indeed obtained, of which test number 3 (Fig. 4(a)) is typical, but
with one major variation-the trace exhibited a substantial offset strain and did not oscillate
about the mean position. These traces may be regarded as composed of 3 regimes which may be
characterised as follows (refer to Fig 4(a)). Region I corresponds to the free rotation of the shaft
under full speed conditions. Small (noise) oscillations about the mean indicate the gauges were
responding but there was no bending in the shaft. As the torque limiter brake was applied (region
11) individual oscillations in the trace at the frequency of rotation (period 0.042-0.05 s) were
observed. These increased in amplitude as the brake engaged, until the brake effectively arrested the
shaft and load was maintained. The curious observation, however, was the large offset strain of
approximately 178 p (microstrain) in this case (Fig. 4(a)). This will be discussed in greater detail
later. Region I11 is the appropriate constant load condition, once the shaft had been arrested. When
the power was dropped completely the strain trace returned to zero and the x axis.
When the limit switch system was engaged (Le. not overridden) then the disc brake torque limiter
was not put under load. The recording of the bending stress for this case indicated minimal stresses
(less than 5 MPa) (Fig. 5(a)) as expected.
Although the number of bending tests was limited due to time constraints, it is nonetheless useful
to interpret these bending strains in terms of (i) the measured offset; (ii) cyclic amplitude; as well as
(iii) peak strain, together with the consequent derived stresses. These stresses, with the torque switch
limiter overridden, are effectively equivalent whether derived from a modulus viewpoint or from a
calibration curve approach and the data is summarised in Table 1. The data in the table is considered
to be accurate to within approximately 8% and from this data it can be inferred that, under
nominally “normal” bench test loading conditions it is possible for significant peak bending stresses
of approximately 170-200 MPa to occur. Such cyclic stresses are not trivial, especially when one
takes into account the stress concentrating effects of the threads, from which it would appear fatigue
at the thread roots is highly likely. The inherent cyclic stress is, however, typically less than 50 MPa,
which seems reasonable from a design viewpoint.

3.2. Tension test series
The strain gauges were now connected to the strain gauge amplifier using the TW leads in a
“tension bridge” configuration as opposed to a “bending bridge” configuration. The loading tests
against the disc brake torque limiter with the limit switch disconnected, just as in the bending test
series, were repeated. A typical strain time trace result is shown in Fig. 4(b) from which, it is clear
that the cyclic stresses were very low as the disc brake engaged but the wormshaft, once arrested or
stalled, showed a net tensile stress, as might be expected. The magnitude of this tensile stress,
however, was not excessive, typically 45 5 MPa.
Again for completeness a tension test was conducted where the limit switch detector was employed
(so that the disc brake torque limiter did not come into operation) and the measured tensile stress
was low (< 12 MPa) (Fig. 5(b)). Thus under safe operation of the limit switch the wormshaft tensile
stresses were low and the performance quite satisfactory.