Chris Hansford,MD of Hansford Sensors, explains modern
vibration monitoring technology and offers advice on how
to successfully install and use accelerometers to maximise
machine time and enhance efficiency

Automation has become an essential
tool for industry, delivering benefits
that include increased productivity,
consistent quality and cost reduction. Yet
automation also brings a new set of
challenges, as more and more companies
become increasingly dependent on physical
rather manual assets. In particular,
machinery has to be constantly available.

Downtime costs money, which, in turn,
places ever greater emphasis on the
importance of effective maintenance.

One of the key tools for plant and
equipment maintenance today is vibration
monitoring. In some circumstances where
vibration is carefully monitored it is possible
to extend operating life beyond
recommended maintenance intervals, while
in others a rapid increase in vibration must
be taken seriously if a catastrophic failure is
to be avoided.

The accelerometers used to measure levels
of vibration are typically easy to install and
use; however, an accelerometer is only as
good as the engineer who is responsible for
it. A poorly installed or maintained
accelerometer will not offer the precision or
longevity of which it is capable and so it is in
everyone’s interest to apply best practice to
managing their performance. To achieve this,
it is worth spending a little time considering
what an accelerometer is and how to
effectively install one.

The current array of sensors or
accelerometers for vibration monitoring
offered by companies such as Hansford
Sensors can operate over a wide temperature
range, measuring both high and low
frequencies with low hysteresis
characteristics and excellent levels of
accuracy. These devices also offer robust and
reliable service, due to stainless steel sensor
housings that can prevent the ingress of
moisture, dust, oils and other contaminants.

There are two main categories: AC
accelerometers, which are used with a data
collector for monitoring the condition of
higher value assets such as wind turbines,
and 4-20mA accelerometers, which are used
with a PLC to measure lower value assets
such as fans and pumps. Both are capable of
detecting imbalance, bearing condition and
misalignment but AC accelerometers can
also identify cavitation, looseness, gear
defects and belt problems.

Accelerometers contain a piezoelectric
crystal element bonded to a mass. When the
sensor is subject to an accelerative force, the
mass compresses the crystal, causing it to
produce an electrical signal that is
proportional to the level of force applied.

The signal is then amplified and conditioned
using inbuilt electronics that create an
output signal, which is suitable for use by
higher level data acquisition or control
systems. Output data from accelerometers
mounted in key locations can either be read
periodically using sophisticated hand-held
data collectors, for immediate analysis or
subsequent downloading to a PC, or routed
via switch boxes to a centralised or higher
level system for continuous monitoring.

To correctly specify an accelerometer,
engineers need to consider the vibration level
and frequency range that is to be measured,
as well as environmental conditions, such as
temperature and whether corrosive
chemicals are present. Further considerations
follow. For example, is the atmosphere
combustible? Are there weight constraints?
Consultation with a company that has
experience in a wide range of sectors can
swiftly enable the right decisions to be made.

To correctly install an accelerometer,
engineers should mount the device directly
onto the machine surface on a flat, smooth,
unpainted surface that is larger than the base
of the accelerometer. The surface should be
free from grease and oil, as close as possible
to the source of vibration and at right angles
to the axis of rotation. By following these
guidelines, you will have already supported
your accelerometer, and thus your
maintenance regime, by enabling the device
to give the most accurate measurements of
vibration levels.

Once data has been collected in the most
efficient manner, machine reliability data can
be effectively analysed and interpreted and,
with an efficient vibration monitoring system
in place, engineers can maximise machine
performance, energy efficiency and output.