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All About That Vibration: Understanding Vibration Phase

Vibration is any oscillatory motion caused by a restorative force. Free vibration occurs when a bell is rung or a guitar string is plucked. Forced Vibration occurs when a repetitive force is applied to a body. Industrial machinery, particularly rotating equipment, sees forced vibration as a consequence of a spinning shaft. In this scenario the rotation of the shaft applies a restorative force to to the surrounding system. The most common vibration frequency is directly caused by this spinning shaft is dubbed fundamental shaft speed or running speed and is denoted as "1X vibration."

Vibration Phase Diagram

This vibration can be caused by many conditions, but among the most common is unbalance. Unbalance occurs when the rotor has a heavy spot. To the left is an example of an overhung rotor. The black spot indicates a heavy spot on the static plane of the rotor, thus indicating a static unbalance for this configuration. Heavy spots can be caused by rotor bow, runout, debris buildup, deterioration, distortion, or even fans throwing balance weights from previous attempts at field balancing.

Vibration Phase

Understanding Vibration Signatures

Vibration readings taken at different points around the bearings will give different signatures. For example, it cannot be expected to have the exact same signature if one sensor is horizontally mounted and one sensor is vertically mounted and data collected simultaneously. We would expect there to be delay from the time the heavy spot moves past the horizontal sensor to reach the vertical sensor. This delay is known as Phase Lag. In the image that shows the red Channel A arrow (right), the Phase shift from the the horizontal to vertical location is 90° as this is tracked in the direction of rotation. Phase can be thought of as another word for angle, or angle offset. 

If the sensor is moved from the red Channel A location to the green Channel B location, we would expect a corresponding shift in phase. The next image (below) shows a phase shift of 270° with the green indicator arrow for Channel B on the bottom of the bearing assembly.

Phase lag 1
Phase lag 2

Why is Vibration Phase Important?

Vibration phase becomes particularly important in not only dynamic balancing, but also in helping to understand relative motion between machine train components and resolving if dominant forces are rotating or stationary. This analysis process is commonly called dual, or multi-channel phase analysis. Another common term is Operating Deflection Shape or ODS. ODS analysis it typically performed by collecting referenced phase data for large numbers of points across a structure or support assembly. Phase analysis can confirm such maladies as soft foot, unbalance, misalignment, looseness, resonance, bent shafts, cocked bearings, and more.

The final animation illustrates what simultaneous collection of both horizontal and vertical locations would look like if plotted on the same graph. We see two sine waves that are offset, or shifted from one another. The phase shift between the horizontal and vertical planes is 90° for this orientation.

Vibration is much more than just a magnitude, or overall amplitude. While amplitude may be a general indicator of severity, we need the other components (frequency and phase) to fully understand the problem. When high amplitudes at running speed are present (above 0.1 ips rms), phase data helps the analyst understand the forces present and how destructive the vibration truly is. If a motor is moving in phase axially with a pump, we would not expect a lot of energy absorbed at the coupling. However, if a motor is moving 180° out of phase form, or completely opposite the pump it is coupled to then we can assume that a significant amount of energy is being absorbed by the coupling.

Phase Lag

Understanding phase is a big step in becoming a better analyst. This article serves as an introduction to what phase is, but fully understanding how to interpret and use this information is a much longer process. Email Red Wolf Reliability or call 970-266-9005 for more information on training, mentoring, or about how to apply phase analysis in your plant.