Page 63 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Vibration Due to Unbalance

When performing maintenance activities, care should be
taken to clean all buildup of dirt and debris from rotating compo-
nents, such as fan blades. Uneven buildup will result in an unbal-
ance that will lead to early failure of bearings and seals.

Example 4.3
A 6-foot diameter nodular iron flywheel weighs 8,634
pounds, and is assembled onto an engine’s crankshaft with bolts
in a 12-inch circle. During assembly, a piece of debris .001-inch
thick wedges between the flywheel and the crankshaft. The unit
operates at 300 rpm. How much force is generated as a result of
this misalignment of the flywheel?
A cross-sectional view of the flywheel is shown in Figure 4-10.

QUASI-STATIC UNBALANCE
12”
12”

36”

C L

Figure 4-10. Cross-sectional View of the Flywheel

Step 1. The weight of the flywheel is displaced .001" from the
center, thus the unbalance in inch-ounces is U = 8634 × 16 × .001
= 138 inch-ounces. The unbalance force is F = 1.774 × (300/1000) 2
× 138 = 139.8 pounds.

The above example illustrates that even low rpm machinery
can have excessive forces due to unbalance. However, the higher
the rpm the less the unbalance that can be tolerated.
Vibration due to unbalance occurs at a frequency equal to the