Efficient Plant April 2021 - 23

department | on the floor

A landmark study taught us that time-based maintenance is not always effective and is frequently wasteful. To be most effective, seek out the best mixture of maintenance practices for your operation.

enough maintenance.
2.	 Run-time maintenance (RTM), also
called metered maintenance:
	 Based on asset utilization, such as
number of hours running, number of
cycles, or number of miles
	 More efficient than TBM.
You can also do a combination of the two.
For example, every two months or 500 hr.
of run time.
There are situations when time-based
maintenance is needed, especially when there
are critical safety functions or regulatory
requirements. Examples include checking
fire extinguishers, boilers, and elevators. If
you have parts with a known MTBF (mean
time between failure), TBM can be considered
if higher levels of maintenance are not cost
effective.
A basic model for servicing is to ask if the
asset/issue is high risk or a critical asset. If it's
neither, then you need to analyze what should

APRIL 2021	

EP2104onthefloor.indd 23

be breakdown maintenance (very low risk) or
preventive maintenance (low risk). If it's high
risk or a critical asset, you should be using
predictive and condition-based maintenance.
Those assets in the middle (medium risk)
should be decided based on a ROI basis.
As I work with companies, I've found
time-based maintenance ranging from 25%
to 80% of total maintenance. I use 20% as a
best-practice recommendation. Time-based is
better than reactive maintenance and is easy
to implement into a CMMS. If you have assets
that continuously run, there are some possible
benefits. Primary disadvantages are:
	 increased overall costs in labor, parts,
and downtime (if it affects production)
	 maintenance too often introduces
greater human error (lack of precision
maintenance)
	 as mentioned previously, there is no
accounting for asset usage and condition
	 maintenance, if not done often

enough, leads to more downtime.
The airline study and following efforts
resulted in huge reductions in maintenance,
coupled with improved safety. As stated in
the report, the initial study by special teams
of industry and FAA personnel developed
the first scheduled-maintenance program
based on the principles of reliability-centered
maintenance.
Time-based maintenance cannot alone
be an effective way to manage maintenance
with the many non-age-related failure modes
(89% per Nolan and Heap). It should be used
as part of a comprehensive reliability and
maintenance process.
Reliability-centered maintenance (RCM)
is also more difficult to perform outside of the
airline industry (or similar highly regulated
organizations) without mature and robust
processes in place. The best-practice target
should be about 10% reactive maintenance,
25% predictive technologies and condition-based maintenance (finding potential
problems), 65% predictive maintenance (30%
performing the work orders to fix what you
find with PdM/CBM, 20% time-based maintenance, and 15% all other PMs).
The ultimate goal is design-in reliability
and maintainability and ongoing continuous
improvement, because you want to eliminate
the reasons for all unnecessary maintenance.
Learn how to decide when to use what type
of maintenance to enhance your competitive
advantage. EP

Based in Knoxville, Dr. Klaus M.
Blache is director of the Reliability &
Maintainability Center at the Univ. of
Tennessee, and a research professor in
the College of Engineering. Contact him
at kblache@utk.edu.

EFFICIENTPLANTMAG.COM |

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Efficient Plant April 2021

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