Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 1, Basic Information
Section 3, Using Phase to Determine How One Part of a Machine is Shaking Compared to Another
So far we have been considering vertical vibration measured only at one point of the machine, such as at a specific bearing housing. However, for analysis we need to compare the vibration at one point with the vibration at another point.
Now, assume that the source of the vibration was pure static unbalance. (Unbalance at each plane in-phase with each other.) The vibration generated at each end would have the same phase. (In non-technical terms, in-phase vibration means vibrations that are moving in the same direction at the same instant in time.)
If the first reading is taken at the rotor's left end (pickup vertical), then the phase mark will show up at 7:00 o'clock (210°). With the pickup moved to the right end of the rotor (pickup vertical), the vibration waveform generated from the pure static unbalance should follow the same timing as the first waveform. (The phase response will occur after the rotor rotated 270°.) Therefore, the phase mark will be indicated in exactly the same position; also at 7:00 o'clock (210°).
Now assume that instead of pure static unbalance, there is pure couple unbalance (equal unbalances in two planes, 180° opposite each other). When the unbalance force at one end is "pushing" straight down, the unbalance force at the other end will be "pushing" straight up.
In Fig. 7, the two vibration waves (and resulting phases) are 180° out-of-phase relative to each other. Another way of looking at it is that a phase response is due to the left end vibration, but the phase response due to the right end vibration doesn't occur until the rotor has rotates an additional half turn (or 180° after the left end causes the strobelight to flash).
If instead of either pure static or pure couple unbalance, some other combination of dynamic unbalance occurs (such as unbalances at approximately 90° to each other), then the phase relationships are something like those in Fig. 8.
This textbook contains only part of the information in our Practical Vibration Analysis seminar.