Everyone needs phase analysis. A phase study should be performed on problem machines when the source of the vibration is not clear, or when it is necessary to confirm suspected sources of vibration. A phase study might include points measured only on the machine bearings or it may include points over the entire machine from the foundation up to the bearings. The following are examples of how phase can help analyze vibration.


The term soft foot is used to describe machine frame distortion. It can be caused by a condition where the foot of a motor, pump, or other component is not flat, square, and tight to its mounting, or many other things, such as pipe stress, machining errors, bent or twisted feet, and non-flat mounting surfaces. Soft foot increases vibration and puts undue stress on bearings, seals, and couplings. Soft foot on a motor distorts the stator housing creating a non-uniform rotor to stator air gap resulting in vibration at two times line frequency. A good laser shaft alignment system should be used to verify soft foot by loosening the machine feet one at a time. Phase analysis can be used to identify soft foot while the machine is in operation. Measure the vertical phase between the foot and its mounting surface. If the joint is tight, the phase angle is the same between surfaces. If the phase angle is different by more than 20 degrees, the foot is loose or the machine frame is cracked or flimsy. Download our Soft Foot Find-and-Fix infographic which outlines types of Soft Foot including causes and corrections.

Phase Measurement-Soft Foot


Phase is used to detect cocked bearings and bent shafts. Measure phase at four axial locations around the bearing housing. If the bearing is cocked or the shaft is bent through the bearing, the phase will be different at each location. If the shaft is straight and the bearing is not twisting, the phase will be the same at each location.


A once-per-revolution radial vibration usually means rotor unbalance. Use phase to prove imbalance is the problem. To confirm imbalance, measure the horizontal and vertical phase on a shaft or bearing housing. If the difference between the phase values is approximately 90 degrees, the problem is rotor unbalance. If the phase difference is closer to zero or 180 degrees, the vibration is caused by a reaction force. An eccentric pulley and shaft misalignment are examples of reaction forces.

Phase Measurement-Unbalance


Phase is used to detect loose joints on structures and bending or twisting due to weakness or resonance. To check for looseness, measure the vertical phase at each mechanical joint. When joints are loose, there will be a phase shift of approximately 180 degrees. The phase angle will not change across a tight joint.


Shaft misalignment is easily verified with phase. Measure each bearing in the horizontal, vertical, and axial directions. Record the values in a table or bubble diagram. Compare the horizontal phase from bearing to bearing on each component and across the coupling. Repeat the comparison using vertical then axial data. A good alignment will show no substantial phase shift between bearings or across the coupling.


Instead of comparing the phase and magnitude numbers from a table or bubble diagram, operational deflection shape software (ODS) can be used to animate a machine drawing. An ODS is a measurement technique used to analyze the motion of rotating equipment and structures during normal operation. An ODS is an extension of phase analysis where a computer-generated model of the machine is animated with phase and magnitude data or simultaneously measured time waveforms. The animation is visually analyzed to diagnose problems. ODS testing is able to identify a wide variety of mechanical faults and resonance issues such as looseness, soft foot, broken welds, misalignment, unbalance, bending or twisting from resonance, structural weakness, and foundation problems.

Phase and magnitude were measured from permanently mounted X and Y displacement probes on a turbine generator. The values listed in the table were used in ODS software to animate a stick figure drawing of the high- and low-pressure turbine shafts and the generator shaft. The picture to the right of the table is a capture from the ODS animation showing the vibration pattern of each shaft and the relative motion between shafts at 3,600 cycles per minute (turning speed). Many machines vibrate due to deteriorated foundations, looseness, the resonance of the support structure, and other problems that occur below the machine bearings. A phase study might include hundreds of test points measured all over the machine and foundation. Good ODS software can make it easier to analyze phase and magnitude data from a large number of test points. Analysis of an ODS involves observation and interpretation of the machine in motion.

Why Do I Need to Use Phase Analysis?

Filed under:
by Gary James CRL