Page 264 - Machinery Component Maintenance

246 Machinery Component Maintenance and Repair

It is important to note that couple unbalance cannot be corrected by a
single mass in a single correction plane. At least two masses are re-
quired, each in a different transverse plane (perpendicular to the shaft
axis) and 180" opposite to each other. In other words, a couple unbalance
needs another couple to correct it. In the example in Figure 6-4B, for
instance, correction could be made by placing two masses at opposite an-
gular positions on the main body of the rotor. The axial location of the
correction couple does not matter as long as its value is equal in magni-
tude but opposite in direction to the unbalance couple.

Quasi-Static Unbalance
Quasi-static unbalance, Figure 6-5, is that condition of unbalance for
which the central principal axis of inertia intersects the shaft axis at a
point other than the center of gravity. It represents the specific combina-
tion of static and couple unbalance where the angular position of one cou-
ple component coincides with the angular position of the static unbal-
ance. This is a special case of dynamic unbalance.

Principal
Inertia Axis Shaft Axis

Figure 6-5. Quasi-static unbalance.

Couple
Unbalance Static Unbalance

Figure 6-5A. Couple plus static unbalance results in quasi-static unbalance provided one
couple mass has the same angular position as the static mass.