Section 9: Balancing Rotors
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Introduction
The instrument can be used to balance rotors that are rigid but not
rotors that flex significantly at their operating speeds.
An imbalanced rotor is one that has an uneven mass distribution that
causes the rotor to vibrate when it is rotated. Balancing a rotor
means correcting the uneven mass distribution by adding or
removing weight to/from precisely calculated positions on the rotor.
A rigid rotor can be balanced in one plane or two planes i.e. any
uneven mass distribution in the rotor can be corrected by
adding/removing weights to/from one or two selected cross-sectional
planes on the rotor.
For single plane balancing, the mid plane of the rotor is usually used
as the balancing plane. For dual plane balancing, usually the planes
at the extreme ends of the rotor are used, however, other planes on
the rotor can be used also.
Since the effect of a rotating weight (i.e. the centrifugal force)
increases with the radial distance of the weight, it is common to add
weight to, or remove weight from, the rim of the rotor rather than a
position close to the centre of rotation. By making weight
adjustments at the largest possible radial distance, the amount of
weight that needs to be added to, or removed from, the rotor can be
minimized.
The centrifugal force of a heavy spot on the rotor that causes the
rotor to vibrate increases with the square of the rotational speed. The
vibration level of the rotor may be acceptable at one speed but not at
another. It is thus important to always allow the rotor to settle to its
normal operating speed before taking balancing analysis
measurements.
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