Efficient Plant April 2021 - 22

Answers to questions
facing today's reliability
& maintenance
professionals

department | on the floor

TBM Can't Q
Stand Alone A

What's wrong with timebased maintenance?

Dr. Klaus M. Blache
Univ. of Tennessee Reliability
and Maintainability Center
(RMC)

22

| EFFICIENTPLANTMAG.COM	

EP2104onthefloor.indd 22

Time-based maintenance (TBM)
is running out of time. Maintenance thinking has gone through many
changes since the 1978 Nolan and Heap
report for United Airlines. The report
compiled information on the conditional
probability of failure versus operating age
for numerous electrical and mechanical
airline-industry components. Its findings
challenged the commonly held belief that
component age and likelihood of failure
were directly related. Instead, as we now
know, only 11% of items reviewed were
age related and 89% were random. It also
pointed to two mistaken beliefs about
how we traditionally scheduled PM. Most
assumed that:
	 Components start with an inherent/
as-installed reliability that will deteriorate
with age.
	 Likelihood of occurrence (statistics) can
be used to predict when things fail to enable
timely interventions.
As stated in the report, the earlier
belief was that, because mechanical parts
wear out, the reliability of any equipment
is directly related to operating age. It was
incorrectly thought that the more frequently
PM was performed, the better protected
assets were against likelihood of failure.
The study found that many types of failures
could not be prevented, regardless of the
amount of maintenance performed. This led
to numerous aircraft designed-in redundancies in key functions.
Because of these findings, high levels
of time-based maintenance do not make

sense. This kind of maintenance uses time
as the main trigger for replacing or fixing
an asset at a set interval or usage frequency,
regardless of its condition. If you replace a
component, or do a complete overhaul every
year, it at least is on the schedule, but most
likely is done too frequently. Examples include changing a filter every three months,
replacing batteries in smoke alarms, seasonal checks on HVAC, lubricating a pump
every 3,000 hr., and tightening screws/bolts.
The time-based maintenance goal, of
course, is to prevent failures before they
occur. But since it's carried out, whether it's
needed or not, it's not cost effective. Keep in
mind that failure is a process over time, i.e.,
wear, misalignment, contamination. Once a
failure begins, it can take days or years until
the actual failure event occurs. In principle,
you would want to do any time-based maintenance before the P (point where you can
identify a potential failure) on the P-F curve.
In practice, because of caution or because
you always did it that way, maintenance is
usually performed too often. This also
introduces new " infant mortality " likelihood. Once you get past point P, it's time
to use technologies such as ultrasound,
vibration or oil analysis, lubricant sampling,
thermography, motor-circuit analysis, or
corrosion monitoring.
There are two ways to approach your
PMs.
1.	 Time-based maintenance (TBM):
	 Easily scheduled based on set-time
interval
	 Minimal training needed
	 Doesn't consider asset condition
based on use
	 Usually results in too much or not

APRIL 2021

3/26/21 8:41 AM


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Efficient Plant April 2021

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