Page 376 - Intro Predictive Maintenance

A Total-Plant Predictive Maintenance Program 367

when obtaining readings, which makes the couplings an extension of the shaft
centerlines, whose irregularities will not affect the readings.

Although alignment operations are performed on coupling surfaces because they are
convenient to use, it is extremely important that these surfaces and the shaft “run true.”
If there is any runout (i.e., axial or radial looseness) of the shaft and/or the coupling,
a proportionate error in alignment will result. Therefore, before making alignment
measurements, the shaft and coupling should be checked and corrected for runout.

Balancing Practices
Mechanical imbalance is one of the most common causes of machinery vibration and
is present to some degree on nearly all machines that have rotating parts or rotors.
Static, or standing, imbalance is the condition when more weight is exerted on one
side of a centerline than the other; however, a rotor may be in perfect static balance
and not be in a balanced state when rotating at high speed.

If the rotor is a thin disc, careful static balancing may be accurate enough for high
speeds. If the rotating part is long in proportion to its diameter, however, and the un-
balanced portions are at opposite ends or in different planes, the balancing must
counteract the centrifugal force of these heavy parts when they are rotating rapidly.

This section provides information needed to understand and solve most balancing
problems using a vibration/balance analyzer, a portable device that detects the level
of imbalance, misalignment, and so on in a rotating part based on the measurement
of vibration signals.

Sources of Vibration Caused by Mechanical Imbalance. Two major sources of vibra-
tion caused by mechanical imbalance in equipment with rotating parts or rotors are
assembly errors and incorrect key length guesses during balancing.

Assembly errors. Even when parts are precision balanced to extremely close toler-
ances, vibration caused by mechanical imbalance can be much greater than necessary
because of assembly errors. Potential errors include relative placement of each part’s
center of rotation, location of the shaft relative to the bore, and cocked rotors.

Center of rotation. Assembly errors are not simply the additive effects of tolerances,
but also include the relative placement of each part’s center of rotation. For example,
a “perfectly” balanced blower rotor can be assembled to a “perfectly” balanced shaft
and yet the resultant imbalance can be high. This can happen if the rotor is balanced
on a balancing shaft that fits the rotor bore within 0.5mil (0.5 thousandths of an inch)
and then is mounted on a standard cold-rolled steel shaft allowing a clearance of more
than 2mils.

Shifting any rotor from the rotational center on which it was balanced to the piece of
machinery on which it is intended to operate can cause an assembly imbalance four