A technical white paper entitled The Role of Vibration
Monitoring in Predictive Maintenance has been written by
Dr Steve Lacey, engineering manager at Schaeffler (UK) and
offers engineers advice and guidance on the condition
monitoring of rolling bearings

As companies look to reduce costs,
maintenance can often become a
casualty. However, as equipment and
machinery becomes more complex and
automated, the need for a properly structured
and funded maintenance strategy is more
important than ever.

Undertaking thorough risk assessments
across the business can help
companies identify how critical
existing machines are to their
overall operation, which helps
to determine the potential
return on investment of a
properly funded
maintenance strategy.

Dr Lacey says: “Rolling
bearings are a critical
component used extensively
in rotating equipment and
machinery.When they fail
unexpectedly this can result
in a catastrophic failure with
high associated repair and
replacement costs. Vibrationbased
condition monitoring can be used to
detect and diagnose machine faults and form
the basis of a predictive maintenance strategy.”
As well as providing the reader with
information on the basic approaches to the
various types of maintenance strategy
(reactive, preventive and predictive), the
paper provides guidance on how to set up
and identify the criticality of assets in a
business, including potential return on
investment from implementing a predictive
maintenance regime. It then discusses the
technical benefits of using condition
monitoring systems and techniques to
support the predictive maintenance strategy.

This includes vibration monitoring, which
can be used to detect early signs of failure of
rolling bearings.

Detailed analysis of the various rolling
bearing vibration monitoring techniques are
appraised. Sections are included on
frequency spectrum, envelope spectrum,
Cepstrum analysis, bearing characteristic
frequencies, typical bearing defects, variable
compliance and bearing speed ratio.

Dr Lacey comments: “Rolling bearings
generate characteristic vibration frequencies
that can combine to give complex vibration
spectra, which at times may be difficult to
interpret other than to an experienced
vibration analyst. However, with rolling
bearings, characteristic vibration signatures
are often generated in the form of
modulation of the fundamental bearing
frequencies. This can be used to our
advantage and vibration condition
monitoring software is designed to identify
these features and provide an early warning
to an impending problem. This usually takes
the form of signal demodulation and the
envelope spectrum, which indicates early
deterioration of the rolling contact surfaces.”
The final chapter of the paper considers
various real-life scenarios of rolling bearing
vibration monitoring, including detailed
studies of a 250kW electric motor; an impact
crusher drive shaft; a 2MW generator on a
test bed; a vertical impact crusher; and the
gearbox of a wind turbine.

Wind power, for example, is a rapidly
growing form of renewable energy in many
parts of the world and wind turbines are set
to play a key role in future energy supply. In
the UK, there is increasing interest in placing
wind turbines offshore, which provides
advantages such as improved wind conditions
and reduced planning restrictions. However,
the environment in which such turbines have
to operate is more demanding, requiring a
higher degree of integrity and reliability if
costs are to be minimised.

As Dr Lacey puts it: “Due to the remote
location of these offshore wind farms,
accessibility for maintenance is difficult and
so it is critical that faults are detected early
and consequential damage reduced or
avoided and that repair costs are minimised.

This results in shorter downtimes and cuts
revenue losses. Detecting bearing damage
early can also mean the difference between
replacing the wind turbine gearbox at a cost
of around €250,000, compared with a
bearing replacement cost of just €5000.”
Wind turbine gearboxes are subject to
high dynamic loads, and because of the
changing wind conditions the load spectrum
varies greatly, consisting of high peak loads
and low load operating conditions. The high
static safety required for maximum load
means that bearings with high load carrying
capacity are required. On the other hand,
when there is little wind, loads are low and
this can lead to bearing damage due to
sliding of the rolling element set. As a result,
many field-operating failures originate from
gearbox bearing failure.Misalignment, poor
lubrication and maintenance also contribute
towards this trend.