Page 59 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Vibration Due to Unbalance

Unlike static unbalance, couple unbalance cannot be detected
by placing the element on knife-edges. Thus a couple unbalance
can only be detected with the element rotating. The element is
essentially statically balanced. A couple is basically two equal
forces acting in opposite directions, and through two different
planes. Again, unlike static unbalance, couple unbalance cannot be
corrected in a single plane, but rather it requires corrections to be
made in two or more planes. In only a few cases will either pure
static or pure couple unbalance be detected in machinery. Most
often it is a combination of both. These combinations are classified
as dynamic and quasi-static unbalance. Figure 4-6 shows quasi-
static unbalance. Note that the quasi-static unbalance is made up
of a static unbalance and a Couple unbalance.

QUASI-STATIC UNBALANCE
Center of Rotation

Center of the Shaft

Figure 4-6. Quasi-Static Unbalance

Quasi-static unbalance is the condition of unbalance where
the center of rotation intersects the element’s geometric centerline,
but not at its center of gravity. Quasi-static unbalance can be de-
tected by the amplitudes of the vibration being very different at
each end of the shaft, and being out of phase by approximately
180 degrees. Once again, this type of unbalance must be corrected
in two or more planes.
The fourth type of unbalance is dynamic unbalance and is the
most common type encountered in machinery. Dynamic unbal-
ance is defined as unbalance where the axis of rotation does not
coincide or touch the element’s geometric centerline. Dynamic
unbalance is depicted in Figure 4-7.