Page 19 - An Introduction To Predictive Maintenance

Impact of Maintenance 9

The production input is the unit of product being fed into the process or production
cycle. The quality defects are the amount of product that is below quality standards
(not rejected; there is a difference) after the process or production cycle is finished.
The formula is useful in spotting production-quality problems, even when the cus-
tomer accepts the poor-quality product. The goal for Japanese companies is higher
than 99 percent.

Combining the total for the Japanese goals, it is seen that:
90% ¥ 95% ¥ 99% = 85%

To be able to compete for the national TPM prize in Japan, equipment effectiveness
must be greater than 85 percent. Unfortunately, equipment effectiveness in most U.S.
companies barely breaks 50 percent—little wonder that there is so much room for
improvement in typical equipment maintenance management programs.

Reliability-Centered Maintenance
A basic premise of RCM is that all machines must fail and have a finite useful life,
but neither of these assumptions is valid. If machinery and plant systems are properly
designed, installed, operated, and maintained, they will not fail, and their useful life
is almost infinite. Few, if any, catastrophic failures are random, and some outside influ-
ence, such as operator error or improper repair, causes all failures. With the exception
of instantaneous failures caused by gross operator error or a totally abnormal outside
influence, the operating dynamics analysis methodology can detect, isolate, and
prevent system failures.

Because RCM is predicated on the belief that all machines will degrade and fail
(P-F curve), most of the tasks, such as failure modes and effects analysis (FMEA) and
Weibull distribution analysis, are used to anticipate when these failures will occur.
Both of the theoretical methods are based on probability tables that assume proper
design, installation, operation, and maintenance of plant machinery. Neither is able to
adjust for abnormal deviations in any of these categories.

When the RCM approach was first developed in the 1960s, most production engineers
believed that machinery had a finite life and required periodic major rebuilding to
maintain acceptable levels of reliability. In his book Reliability-Centered Maintenance
(1992), John Moubray states:

The traditional approach to scheduled maintenance programs was based on
the concept that every item on a piece of complex equipment has a right
age at which complete overhaul is necessary to ensure safety and operat-
ing reliability. Through the years, however, it was discovered that many
types of failures could not be prevented or effectively reduced by such
maintenance activities, no matter how intensively they were performed. In
response to this problem, airplane designers began to develop design
features that mitigated failure consequences—that is, they learned how to