Page 13 - An Introduction To Predictive Maintenance

Impact of Maintenance 3

inventory cost, high overtime labor costs, high machine downtime, and low produc-
tion availability.

Because no attempt is made to anticipate maintenance requirements, a plant that uses
true run-to-failure management must be able to react to all possible failures within the
plant. This reactive method of management forces the maintenance department to
maintain extensive spare parts inventories that include spare machines or at least all
major components for all critical equipment in the plant. The alternative is to rely on
equipment vendors that can provide immediate delivery of all required spare parts.

Even if the latter option is possible, premiums for expedited delivery substantially
increase the costs of repair parts and downtime required to correct machine failures.
To minimize the impact on production created by unexpected machine failures, main-
tenance personnel must also be able to react immediately to all machine failures. The
net result of this reactive type of maintenance management is higher maintenance cost
and lower availability of process machinery. Analysis of maintenance costs indicates
that a repair performed in the reactive or run-to-failure mode will average about three
times higher than the same repair made within a scheduled or preventive mode. Sched-
uling the repair minimizes the repair time and associated labor costs. It also reduces
the negative impact of expedited shipments and lost production.

1.1.2 Preventive Maintenance
There are many definitions of preventive maintenance, but all preventive maintenance
management programs are time-driven. In other words, maintenance tasks are based
on elapsed time or hours of operation. Figure 1–1 illustrates an example of the sta-
tistical life of a machine-train. The mean-time-to-failure (MTTF) or bathtub curve
indicates that a new machine has a high probability of failure because of installation
problems during the first few weeks of operation. After this initial period, the proba-
bility of failure is relatively low for an extended period. After this normal machine
life period, the probability of failure increases sharply with elapsed time. In preven-
tive maintenance management, machine repairs or rebuilds are scheduled based on the
MTTF statistic.

The actual implementation of preventive maintenance varies greatly. Some programs
are extremely limited and consist of only lubrication and minor adjustments.
Comprehensive preventive maintenance programs schedule repairs, lubrication,
adjustments, and machine rebuilds for all critical plant machinery. The common
denominator for all of these preventive maintenance programs is the scheduling
guideline—time.

All preventive maintenance management programs assume that machines will degrade
within a time frame typical of their particular classification. For example, a single-
stage, horizontal split-case centrifugal pump will normally run 18 months before it
must be rebuilt. Using preventive management techniques, the pump would be
removed from service and rebuilt after 17 months of operation. The problem with this