Helicopters are highly dependent on their transmission systems, which provide the vital
links from the engines to the rotor and ancillary systems. Components are highly loaded
and must be manufactured to a high degree of accuracy; the lack of redundancy implies
that this is a `series-chain' system. Existing techniques for calculating expected life are
based upon historical data from different gearbox and helicopter types, thus limiting the
confidence of the results. Design techniques may be conservative in some areas, whilst
neglecting to consider different load patterns, usage, maintenance and environmental
factors.
This work describes the development of probabilistic models that represent damage
accumulated by fatigue, wear and corrosion of the key components with an Intermediate
gearbox (IGB). The parameters of these models represent geometrical, load and material
data at the design stage, and produce an output in terms of failure probability against
operating hours. This allows the influential parameters to be identified before building a
prototype helicopter gearbox.
The results from these models are then used to predict the upper and lower bounds of
system reliability. This enables the combination of diverse failure mechanisms to be
viewed to determine the relevant significance of each failure mechanisms. The
effectiveness of the gearbox monitoring systems has been incorporated in the computer
model by considering the probability of detection (POD) of each failure mechanism.
The work to develop models found that there is a large body of work available to
describe damage accumulation due to fatigue, but far less in regard to wear and
corrosion. Fatigue models are very sensitive to load and material variability, particularly
tooth root bending fatigue, for which many loads are considered `non-damaging'. Wear
models are mostly affected by changes in material hardness, wear coefficient and slip
amplitude; changes in load are less influential on the predicted time to failure. The
results for galvanic corrosion are dominated by the corrosion rate and time to initiate. In
the system reliability model, reducing gear load appears to be the simplest means to
increase life; increases in material strength and reduction in material variability are less
achievable without significant improvements in manufacture and/or material technology
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