Page 378 - Intro Predictive Maintenance

A Total-Plant Predictive Maintenance Program 369

Section 16.2.1) by its counterpart in the other half of the rotor, can result. If the rotor
is slightly cocked, the small axial distance between the two very large centrifugal
forces causes an appreciable couple imbalance, which is often several times the allow-
able tolerance because the centrifugal force is proportional to half the rotor weight
(at any one time, half of the rotor is pulling against the other half) times the radial
distance from the axis of rotation to the center of gravity of that half.

To prevent this, the assembler should tighten each setscrew gradually—first one, then
the other, and back again—so that the rotor is aligned evenly. On flange-mounted
rotors such as flywheels, it is important to clean the mating surfaces and the bolt holes.
Clean bolt holes are important because high couple imbalance can result from the
assembly bolt pushing a small amount of dirt between the surfaces, cocking the rotor.
Burrs on bolt holes can also produce the same problem.

Other. Other assembly errors can cause vibration. Variances in bolt weights when one
bolt is replaced by one of a different length or material can cause vibration. For
setscrews that are 90 degrees apart, the tightening sequence may not be the same at
final assembly as during balancing. To prevent this, the balancer operator should mark
which setscrew was tightened first.

Key length. With a keyed-shaft rotor, the balancing process can introduce machine
vibration if the assumed key length is different from the length of the one used during
operation. Such an imbalance usually results in a mediocre or “good” running machine
as opposed to a very smooth running machine.

For example, a “good” vibration level that can be obtained without following the
precautions described in this section is amplitude of 0.12in./sec. (3.0mm/sec.). By
following the precautions, the orbit can be reduced to about 0.04in./sec. (1mm/sec.).
This smaller orbit results in longer bearing or seal life, which is worth the effort to
ensure that the proper key length is used.

When balancing a keyed-shaft rotor, one half of the key’s weight is assumed to be
part of the shaft’s male portion. The other half is considered part of the female portion
that is coupled to it. When the two rotor parts are sent to a balancing shop for rebal-
ancing, however, the actual key is rarely included. As a result, the balance operator
usually guesses at the key’s length, makes up a half key, and then balances the part.
(Note: A “half key” is of full-key length but only half-key depth.)

In order to prevent an imbalance from occurring, do not allow the balance operator
to guess the key length. It is strongly suggested that the actual key length be recorded
on a tag that is attached to the rotor to be balanced. The tag should be attached so that
another device (such as a coupling half, pulley, fan, etc.) cannot be attached until the
balance operator removes the tag.

Theory of Imbalance. Imbalance is the condition when more weight is exerted on one
side of a centerline than the other. This condition results in unnecessary vibration,