BS 7508:1995 Mechanical vibration. Methods and criteria for the mechanical balancing of flexible rotors
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This International Standard classifies rotors into
groups in accordance with their balancing
requirements, describes balancing procedures,
specifies methods of assessment of the final state of
unbalance, and gives guidance on balance quality
criteria.
All rotors are classified into those which can be
balanced by rigid rotor, modified rigid rotor, or
high-speed (flexible rotor) balancing techniques.
Two methods are specified for evaluating the
balance quality of a flexible rotor in a balancing
facility before machine assembly: the first assesses
the vibration level, and the second assesses the rotor
residual unbalance. If the rotor balance tolerances
suggested herein are achieved during correction in a
balancing facility, the specified vibration limits of
the assembled machine in service (see ISO 10816-1
and parts 1 to 4 of ISO 7919) will most probably be
achieved. Accordingly, the criteria specified are
those to be met when the rotor is tested in the
balancing facility, but they are derived from those
specified for the complete machine, when installed,
or from values known to ensure satisfactory running
of the rotor when it is installed.
As in the case of parts 1 and 2 of ISO 1940, this
International Standard is not intended to serve as
an acceptance specification for any rotor group, but
rather to give indications of how to avoid gross
deficiences and/or unnecessarily restrictive
requirements. This International Standard may
also serve as a basis for more involved
investigations, for example when a more exact
determination of the required balance quality is
necessary, if due regard is paid to the specified
methods of manufacture and limits of unbalance,
satisfactory running conditions can most probably
be excepted.
There are situations in which an otherwise
acceptably balanced rotor experiences an
unacceptable vibration level in situ, owing to
resonances. A resonant or near-resonant condition
in a lightly damped structure can result in excessive
vibratory response to a small unbalance. In such
cases, it may be necessary to alter the natural
frequency or damping of the structure rather than
to balance to very low levels, which may not be
maintainable over time.
The subject of structural resonances and
modifications thereof is outside the scope of this
International Standard.
The methods and criteria given are the result of
experience with general industrial machinery. They
may not be directly applicable to specialized
equipment or to special circumstances. Therefore,
there may be cases where deviations from this
International Standard may be necessary2)
.
