Page 297 - Intro Predictive Maintenance

288 An Introduction to Predictive Maintenance

The best way to confirm a critical-speed problem is to change the operating speed of
the machine-train. If the machine is operating at a critical speed, the amplitude of the
vibration components (1¥, 2¥, or 3¥) will immediately drop when the speed is
changed. If the amplitude remains relatively constant when the speed is changed, the
problem is not critical speed.

14.1.2 Imbalance
The term balance means that all forces generated by, or acting on, the rotating element
of a machine-train are in a state of equilibrium. Any change in this state of equilib-
rium creates an imbalance. In the global sense, imbalance is one of the most common
abnormal vibration profiles exhibited by all process machinery.

Theoretically, a perfectly balanced machine that has no friction in the bearings would
experience no vibration and would have a perfect vibration profile—a perfectly flat,
horizontal line—however, no perfectly balanced machines exist. All machine-trains
exhibit some level of imbalance, which has a dominant frequency component at the
fundamental running speed (1¥) of each shaft.

An imbalance profile can be excited as a result of the combined factors of mechani-
cal imbalance, lift/gravity differential effects, aerodynamic and hydraulic instabilities,
process loading, and, in fact, all failure modes.

Mechanical
It is incorrect to assume that mechanical imbalance must exist to create an imbalance
condition within the machine. Mechanical imbalance, however, is the only form of
imbalance that is corrected by balancing the rotating element. When all failures are
considered, the number of machine problems that are the result of actual mechanical
rotor imbalance is relatively small.

Single-Plane. Single-plane mechanical imbalance excites the fundamental (1¥) fre-
quency component, which is typically the dominant amplitude in a signature. Because
there is only one point of imbalance, only one high spot occurs as the rotor completes
each revolution. The vibration signature may also contain lower-level frequencies
reflecting bearing defects and passing frequencies. Figure 14–1 illustrates single-plane
imbalance.

Because mechanical imbalance is multidirectional, it appears in both the vertical and
horizontal directions at the machine’s bearing pedestals. The actual amplitude of the
1¥ component generally is not identical in the vertical and horizontal directions and
both generally contain elevated vibration levels at 1¥.

The difference between the vertical and horizontal values is a function of the bearing-
pedestal stiffness. In most cases, the horizontal plane has a greater freedom of move-
ment and, therefore, contains higher amplitudes at 1¥ than the vertical plane.