For decades, conventional wisdom suggested that the best
way to optimise the performance of physical assets was to
overhaul or replace them at fixed intervals. This was based
on the premise that there is a direct relationship between the
amount of time (or number of cycles) equipment spends in
service and the likelihood that it will fail, as shown in Figure
2. This suggests that most items can be expected to operate
reliably for a period "X", and then wear out.
Classical thinking held that X could be determined from
historical records about equipment failure, enabling users
to take preventive action shortly before the item is due to
fail in future. This predictable relationship between age and
failure relationship is indeed true for some failure modes.
It tends to be found where equipment comes into direct con-
tact with the product. Examples include pump impellers,
furnace refractories, valve seats, crusher liners, screw con-
veyors, machine tooling and so on. Age-related failures
are also often associated with fatigue and corrosion.