strategy is based on monitoring and
measuring the condition of the assets
to determine whether
they will fail during
some future period and then taking
appropriate action to avoid the consequences of that failure.
Essentially, a means of continually improving the confidence interval
in our estimations of the probability
of failure (PoF) of an asset and identification of
appropriate steps to preserve the asset, if so desired.
Listed below are some of the key steps in a predictive maintenance
Maintenance (PdM) differs from Preventative Maintenance (PM) in that it
bases maintenance needs on the actual condition of the assets rather
than on some preset schedule. This approach offers cost savings over
time-based preventative maintenance because tasks are performed
only when warranted.
- The process begins with the identification of a sample for
physical data over time, such as vibrations or particulate matter in
condition of an asset is
either condition monitored continuously or at
- Measurements detect the onset of a
degradation mechanism, thereby allowing causal stresses to be
eliminated or controlled prior to any significant deterioration
in the asset. Analysis is then performed on the collected data to
develop an appropriate maintenance schedule that is tailored to the
assets and its condition.
The asset’s performance is compared to an
appropriate standard to determine if it can continue in service. These
appropriate standards may relate to, but are not limited to,
cleanliness, cracks, deformation, corrosion, wear, pressure or
temperature limits, looseness or even missing parts. When
the condition gets to a predetermined
unacceptable level, the equipment is shut down to repair or replace
damaged components so as to prevent a more costly failure from
- This philosophy
consists of scheduling maintenance activities only if and when
conditions warrant and if so desired in accordance with the owners
Monitoring & Testing
Concepts & Thresholds
Listed below are some of distress-based
metrics used to ascertain means and needs for predictive
Monitoring & Diagnostic
monitoring can be done in a variety of ways.
includes, for example, looking out for excessive vibration, temperature
changes, lubrication degradation, or by observing any other unhealthy
trends that occur over time.
A variety of technologies are used to help diagnose
the condition of assets using
techniques such as:
Listed below are
some of the key attributes of predictive maintenance:
The Predictive Maintenance (PdM) maintenance
strategy lends itself well to some electrical and mechanical systems
and assets with the following attributes:
The alignment of
maintenance styles to individual assets is discussed under maintenance mix.
Assets - Assets with high consequences of
failure (CoF) such as significant impact
to the owners’ operations if there is any downtime.
Assets - Assets with random failure patterns.
Assets that are not subject to
Assets - Assets with quantifiable performance
thresholds. In other words, empirical data can be
collected on the distress metrics.
Some of the advantages of the
predictive maintenance approach are as follows:
- Empirical Data -
The process helps
to determine the in situ condition of in-service assets in order
to predict when maintenance should be performed.
- Asset Reliability -
Improved reliability of the assets can result in a decrease in
- Cost Efficiency -
time-based preventative maintenance because tasks are performed only
of the disadvantages of the predictive maintenance approach are listed
Cost - An upfront capital cost associated with
an increased investment in diagnostic equipment.
Training - Increased investment in staff training
to be able to navigate the P-F interval.
- Some assets do not exhibit measurable distress metrics or ones that
cannot be discerned through available diagnostic technologies.
Conditions - Some conditions are concealed conditions
and cannot be observed without destructive
- Some distress metrics do not exhibit leading indicators and therefore
the diagnostic process will not be able to detect and avert these types
Listed below are some of the asset management
principles that can be applied to this form of maintenance in pursuit
of a maintenance
- Usually best suited during the P-F Interval on some
- The maintenance
mix will include an appropriate balance of assets on a
predictive maintenance program.
article on maintenance can be found here.
Fig. Left - thermographic scan of electrical equipment. Right - Thermographic scan of mechanical equipment.
maintenance detects leading
indicators and lagging indicators along a PF Curve.
Alignment of the four different types of maintenance strategies across
the asset portfolio on decisions that are risk-based and consider the
whole-life of assets will result in a maintenance mix that is in
conformity with ISO 55001 standards.
Fig. The risk
spectrum extending along the P-F interval.
Fig. Predictive maintenance technologies along the P-F Curve in relations to Potential Failure (P) and Functional Failure (F).
Fig. The maintenance plan organized into seasonal maintenance tasks.
I. Care is trying to use a crystal ball to make forecasts about his
assets rather than rely upon the scientific method behind predictive
maintenance (PdM) technologies..
Fig. Major maintenance tasks mapped to the P-F Curve.
Fig. Different types of maintenance tasks distributed across the four seasons of an annual maintenance program.
Fig. Some of the different applications of thermographic scans to various assets.