Page 62 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

SOURCES OF UNBALANCE

Most electric motor manufacturers standardize on balancing
their rotating elements with the entire keyway filled with a half
key. That is the shaft is totally filled. Coupling manufacturers
practice the same procedures when balancing their couplings.
When assembling these two elements, an L-shaped or T-shaped
key must be used to maintain the original balance. Using the ex-
ample above and viewing Figure 4-8 will illustrate the problems
with not using the proper key during assembly.
As the person responsible for maintenance and repairs, it
should be your goal to assure all possible sources of unbalance are
inspected and corrected if necessary. The less vibration a machine
has, the less stress it will be subjected to, and the longer it will
survive. Equipment with a history of coupling, bearing, or seal
failures should be inspected for sources of unbalance.
Figure 4-9 shows some of the more common sources of un-
balance associated with coupling installation. Any of these condi-
tions can lead to premature failure of bearings, couplings, or seals.

Extra Washer
Missing Washer
Excess Key Material
Missing Key Material

Hub Loose on Shaft
Bolt in Backwards
Bolt too Long

Figure 4-9. Sources of Unbalance

As in all maintenance activities, cleanliness is of extreme im-
portance in maintaining the inherent balance of rotating machinery.
A small particle of dirt and subsequent component failure.