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Technical data are subject to change without notice.
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• Box 4 • S-645 21 Strängnäs • Sweden
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Machine Classes
To assess machine condition, the vibration severity
measured on a specific piece of plant has to be com-
pared with a representative norm value.
The international standards group industrial machin-
ery into six different vibration classes , depending on
·
machine size and function
·
stiffness of foundation
For each class, the standards give vibration severity
levels in four bands, ranging from very good condi-
tion through average and poor to bad. Provided that
the correct class is chosen, the instrument reading
can be directly related to machine condition.
Most industrial plants belong to vibration classes II, III
and IV.
Class I refers to independent parts of machines, for
example electric motors up to 15 kW.
Classes V and VI are used for heavy reciprocating
prime movers and machines which are intended to
vibrate - for example vibrating screens (see Appendix
page 15 for precise definitions).
Motor power and types (electric, turbine, diesel), ma-
chine size and foundation stiffness (concrete base,
metal frame, etc.) will give a first indication of ma-
chine class. For example, most smaller process pumps
on a chemical plant would be Class II. A 100 kW
ventilation fan on a concrete base would be Class III.
However, the same fan fastened to the less rigid
metal deck of a ship could be considered as Class IV.
Classification of machinery is largely a matter of expe-
rience because the definitions provided by the stand-
ards are deliberately loose. Manufacturers should be
able to specify acceptable vibration levels for their
equipment, and their information can be used as a
reference.
Similarly, if it is reasonably sure that a machine is in
good condition, the actual vibration reading can be
used as a starting point for the assessment of future
changes.
in/s RMS
Limits Class Class Class Class
II
Medium size machines
without special foundations
III Large machines on rigid
foundations
IV Large machines on soft
foundations
