Maintenance Technology August 2016 - (Page 26)
OVERALL VALUES ARE the most common measurements
and calculations used in vibration analysis. What's more, some
reliability and maintenance programs rely solely on them. The
goal is to remove monitored equipment from service once the
overall vibration level exceeds a certain threshold. Although this
approach would appear to be quite cost effective, in reality it
frequently isn't. In fact, overall vibration monitoring can become
extremely costly for a facility.
High vibration levels can be caused by internal and external
sources. They include, among others, imbalance, misalignment,
belt defects, mechanical looseness, bearing-related issues, gear
defects, and cavitation. Once identified, they should all be
corrected. Keep in mind, however, that equipment often experiences multiple defects at once. For example, it's possible for the
amplitudes of certain frequencies to increase while the amplitudes of other frequencies decrease. The fact that these situations
indicate a variety of specific conditions poses a problem for
those relying solely on the overall-vibration approach.
An overall vibration value is centered on the frequency range
being acquired and calculated based on a formula selected by
the manufacturer of the vibration-monitoring device. Expressed
in a mathematical representation of the energy exhibited by all
defects combined, plus the vibration currently experienced in
the machine, the overall vibration value cannot accurately differentiate among defects caused by various machine conditions.
What should you do once an overall vibration level exceeds your
target amplitude and the equipment is removed from service?
First, stop with the assumptions. They're often made about
the causes of high overall values, and work is completed based
on them. Relying solely on overall values and only making
assumptions about their cause can easily lead to incomplete
information about the health of your equipment. This, in
turn, can lead to misguided equipment repairs or detection of
problems only in the painfully late stages of failure. As a result,
extra resources and efforts are invested in determining the true
Clinging to a single approach
that made economic sense for
your plant 'back in the day'
could be an expensive strategy.
Trent Phillips, CMRP, CRL
source of elevated vibration levels-which translates as misspent
time, unnecessary equipment maintenance, increased costs, and
There are several actions you can take to ensure your
vibration program is effective, i.e., that it correctly identifies
conditional changes in the equipment and sources of vibration.
■ Make sure the most important equipment in the facility is
monitored. Don't arbitrarily assign monitoring intervals.
■ Confirm that monitoring intervals allow enough time to
identify, plan, schedule, and correct the identified findings
before unwanted equipment failures occur.
■ Verify that recommendations are implemented. Knowingly
ignoring conditional changes in equipment health will result
in downtime, extra cost, and lower capacity.
Be sure you understand the failure modes in each machine,
based on principles of FMEA (failure-mode-effects analysis).
Band alarms and analysis should be used to indicate changes in
the condition of your equipment and, at the same time, identify
their causes or sources. Specific bands can be easily created,
measured, and trended around particular failure modes in
equipment, including misalignment, imbalance, and bearings.
This information leads to more accurate alerts of impending
failure conditions than generic overall measurements-and, as
an extra benefit, actually identifies the failing component.
Who should collect routine vibration data? This is an important
issue given the fact that wasted time wastes dollars.
Operators and mechanics should be up to the task. Both can
acquire comprehensive vibration measurements on equipment
during the course of their normal work activities. They also can
make sure machines are shut down if vibration levels exceed
acceptable values and notify others regarding the need for
corrective actions. This approach allows analysts to focus on
collected data and determining root causes of defects.
What other valuable condition-monitoring data might be
missing? Tracking process information such as temperatures,
Table of Contents for the Digital Edition of Maintenance Technology August 2016
On The Floor
Accepting The Challenge
Advanced Software In The Jungle
Put Efficiency in MRO Storerooms
Rethink Overall Vibration Monitoring
Reliable Pumping Supplement
Practical Oil Analysis: Why and What For?
SAP Tips and Tricks
Compressed Air Care
Nurture STEM Learning
Heed Drive-Belt Temps
Internet of Things
Maintenance Technology August 2016