Page 56 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

Rotating elements tend to rotate about their center of gravity,
unless they are constrained. In a machine element, the center of
rotation and the center of gravity should be the same. When they
are not, vibration due to unbalance will result.

TYPES OF UNBALANCE

Unbalance can be of four basic types, static, couple, quasi-
static and dynamic. Static unbalance is shown in Figure 4-1.

STATIC UNBALANCE
Center of Rotation
Unbalance
Weight

Centerline of the Shaft

Figure 4-1. Static Unbalance

Static unbalance is defined as an unbalance where the center
of rotation is displaced parallel to the geometric center of the ro-
tating element. If the element were placed on knife-edges, it
would rotate until the heavy spot was on the bottom.
With a statically unbalanced element rotating, the amplitude
and phase of the vibration at both ends of the shaft would be the
same. This static unbalance can be easily corrected by adding or
removing the proper amount of weight as long as it is done in the
proper plane. Figures 4-2 and 4-3 show the proper way to balance
a pure static unbalance.
In Figure 4-2 a single weight equal to the unbalance is placed
exactly in the same plane as the unbalance and exactly 180 degrees
away. This brings the rotational center and the center of the shaft
in-line.