Q&A Webinar: Detecting Misalignment using Vibration Analysis

September 22, 2011

Thank you for joining us for our Webinar Detecting Misalignment through Vibration Analysis. We hope you found the presentation to be valuable and very informative. If you missed our Webinar, you can view the recorded version at any time. Watch now!

Here are the answers to your questions:

Q: How do you account for thermal growth when installing new or repaired equipment?

A: If you already know that the machines will move as you run them, you must misalignment them ‘cold’ to compensate, so they grow into alignment as you run them. The trick, of course, is to know exactly how much! There are various methods to ascertain this precisely, the best being to perform a live monitoring job with PERMALIGN or ROTALIGN ULTRA Live Trend. To fully answer your question, I’d suggest that you take a peek at our Webinar “Thermal Growth and Machinery Alignment

Q: If using dual channel phase without tacho, do you recommend orientating accels axially for angular misalignment detection and orientating accels radially for parallel misalignment? Is this relative to using 2 channel cross channel phase to determine angular or parallel phase?

A: Both types of phase measurements are easy to take. Relative phase is the most convenient way to measure phase on a machine because the machine does not need to be stopped to install reflective tape on the shaft. Phase can be measured at any frequency. Most single-channel vibration analyzers can measure absolute phase. Multi-channel vibration analyzers like the VIBXPERT have standard functions for measuring both absolute and relative phase. See below section “When to use Phase Analysis”

Q: Can’t you only find imbalance from phase?

A: Phase data can be used to verify a lot of vibration issues including imbalance, but it is not only for imbalance itself. I have included a few examples of what issues can be detected below by using phase. Both types of phase measurements are easy to take. Relative phase is the most convenient way to measure phase on a machine because the machine does not need to be stopped to install reflective tape on the shaft. Phase can be measured at any frequency. Most single-channel vibration analyzers can measure absolute phase. Multi-channel vibration analyzers like the VIBXPERT have standard functions for measuring both absolute and relative phase. See below section “When to use Phase Analysis”

When to use Phase Analysis
Everyone needs phase analysis. A phase study should be made 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 can include points over the entire machine from the foundation up to the bearings. The following are examples of how phase can help analyze vibration.

  • Soft Foot
    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 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. I’d suggest that you take a peek at our Webinar “Soft Foot
    Phase can be used to identify soft foot while the machine is in operation. Measure 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.
  • Cocked Bearings and Bent Shafts
    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.
  • Confirm Imbalance
    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.
  • Looseness, Bending or Twisting
    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
    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. Compare the horizontal phase from bearing to bearing on each component and across the coupling. Repeat the comparison using vertical then axial data. Good alignment will show no substantial phase shift between bearings or across the coupling.
  • Operational Deflection Shapes
    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, 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.

Q: Will coupled drive and belt drive systems show up the same on the spectra?

A: Yes, vibration data on a belt drive system will look different from a coupled drive but not when looking at the common defects of a machine. For example the vibration data on a belt drive motor will show additional vibration below turning speed due to the belts. The common defects such as imbalance, looseness, and misalignment (to name a few) will show up the same on a belt drive system or a coupled drive system.

Q: In the below spectrum we see a looseness, do we need to correct first the looseness before the misalignment?

Spectrum: Looseness and Misalignment

A: When looking at vibration data there is always more than one issue showing within the vibration data. I suggest that correcting the highest amplitude vibration issue first and the secondary issue will usually decrease in amplitude. For example on this vibration spectra I would suggest correcting the misalignment issue first. The misalignment is the root cause affect of the looseness. Once the misalignment issue is corrected the looseness issue will not be seen on the spectra data.

Q: Will this show up in both horizontal and vertical or is it predominately just one or the other?

A: The answer is that an alignment issue will show up both the horizontal and vertical orientation when collecting vibration data. Of course depending on the type of misalignment the horizontal or vertical orientation could be higher. Once a single orientation has a high amplitude level of misalignment it will cross over into the other orientation as well. Of course if the alignment issue has a small amplitude level it might only be noticed in a certain orientation and again that depending on the type of misalignment that is present.

Q: How can vibration analysis instrumentation be integrated into a building’s BAS computer system?

A: There are generally two different vibration programs that exist in a plant. One would be walk around data collection. This is where a person goes out and places a sensor on a machine to collect the vibration data and then transfers that vibration data in a database. The second would be continuous monitoring where sensors are permanently mounted to a machine. Vibration data is collected every minute or less depending on the need. The walk around vibration database does have a SAP export feature that would allow you to export certain data from the database over into your system.

Most cases the continuous monitoring is best to feed data into another system. We have customers that use our continuous monitoring devices (online systems) to feed into their SCADA systems. This allows for an overall value of 4-20 MA signal to be sent across into another system. Now the other system, like the SCADA, can now alert if certain levels have been reached.

Q: What vibration is typically associated with a damaged impeller or damaged fan blade?

A: The vibration associated with a damaged impeller or fan blades is called vane pass frequency. When looking at the spectra vibration data you would be interested in the number of vanes or impellers that are on the machine. For example if you had six vanes the vibration data would show a peak in the spectra data at 6 times turning speed, 12 times turning speed, 18 times turning speed, 24 times turning speed, 30 times turning speed. Depending on the frequency maximum for your spectra would determine how far out the vane pass frequency would occur or be seen.

Q: When we laser align our mechanics say they can’t keep their coupling gap correct. What is a good method to laser align and maintain coupling gap at the same time?

A: There are two kinds of coupling gap we have to keep in mind when aligning a machine. The first is the simple gap difference between the coupling faces arising out of any angular misalignment between the shafts, and the second is the axial installation gap specification and tolerance that is demanded by manufacturer of the coupling.

Typically, you rough align the machines and then set the installation gap of the coupling before you completely tighten it down. If the hubs are shrunk fit, then you guide yourself by the position of the hub on the shaft ends and hope your holes in the base are drilled in the right place when your machines are set down. The correction of any misalignment (angular and offset) typically will never affect the installation gap by enough to make any difference, since we are talking about changes to angularity in thousandths of an inch, whereas the installation gap may have a tolerance of as much as a quarter inch in or out. So I’m not exactly sure what your mechanics are concerned about. With a good laser alignment system, you perform both angular and offset corrections simultaneously, and the axial gap between the couplings is not a concern.

I hope these answers were beneficial to all of you. If you have any additional questions, please feel free to contact me.

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