Condition Monitoring Expert Tip #7 by Mobius Institute
Spectrum analysis provides a great deal of information about the health of rotating machinery. But you should consider the spectrum as a summary of the vibration within the machine.
The Fast Fourier Transform takes the time waveform and computes how much of each frequency is present and displays that as a line in the spectrum (grossly summarized, but that is basically the case). Therefore, if the vibration from the machine is generated by smooth periodic motion, then the spectrum provides a very good representation of what is happening inside the machine. But as damaged gears mesh together, and rolling elements pass over damaged areas on the raceway of the bearing, and as the pump vanes push through the fluid causing turbulence or cavitation, the vibration generated is not smooth and periodic. And there are a lot of other fault conditions that likewise do not generate smooth and periodic vibration. Thus, the only way to really understand what is happening inside the machine is to study the time waveform.
The time waveform is a record of exactly what happened from moment to moment as the shaft turns, the gears mesh, the vanes pass through fluid, and the rolling elements roll around the bearing. Each minute change that results from impacts, rubs, scrapes, rattles, surges, and so much more is recorded in the time waveform and then summarized in the spectrum. Therefore, it is critical to record the time waveform correctly and analyze it when you have any suspicion that a fault condition exists.
Special thanks to Mobius Institute for letting us share this condition monitoring expert tip with you!
by Ana Maria Delgado, CRL
Guest post by Ray DeHerrera, Mechanical Engineer at Pioneer Engineering
Vibration analyst use multiple tools to predict a potential fault in a machine; from transducers to accelerometers, the toolbox for vibration analysts is continually expanding to allow for more comprehensive and accurate data collection and interpretation. One tool that is absolutely important to the data analysis process is knowing how your equipment processes data. Vibration analysts needs to know how results are being derived from multiple calculations within your equipment. This allows for the development an efficient collection history that will produce more accurate results.
The calculations attempt to translate data banks into a model that can then explain the events occurring inside of your equipment. Often times the computer processed model may develop imaginary information, thus leading to more questions than answers. With basic background and knowledge of variables that may affect your post processed data, your questions will start to be answered.
To introduce the initial creation of our mathematical model that is displayed upon our data collector or computer screen, (such as the time wave form or spectrum) we will explore commonly used hardware such as the transducer. In general the function of the transducer is convert one form of energy to another. A commonly used transducer for case mounted readings is an accelerometer. The accelerometer mimics mechanical vibrations to produce a usable signal. The usable signal is so small that typically an internal amplifier will be needed for your data collector to harness the information. This process is the initial creation of our mathematical model of data, which has been created from a response of a mechanical device (transducer) sitting upon a machine and is now being converted to a digital signal that has been amplified.
Now our signal must be stored for further analyzing. There are a number of vibration collector types and manufactures. The collector is very similar to a computer giving it the ability to quickly process the original signal into various mathematical models. One must take the time to do their research before purchasing a collector and the associated software. Many desired post processing and collection capabilities maybe limited such as sampling rates. With a good collector and setup your mathematical models will be accurate. The accuracy and consistency in your collections is key when managing your periodic collections.
With the basic knowledge of how your equipment generates your post processed model will make your time more efficient and your results accurate. The analyst will be able to identify data that is imaginary and pick out what is real. Take the time to understand your hardware and how your computer generates each model.
by Ana Maria Delgado, CRL
Cross channel phase is a process of using a two-channel instrument to determine the phase relationship between two measurement locations without using a fixed reference tachometer. The cross channel phase uses two time waveform signals: Using channel “A” as a reference it takes one cycle in the time waveform as one revolution of the shaft and plots the “B” channel time waveform against channel “A” to determine the relative phase. This process is useful for confirming angular or offset misalignment, unbalance, loose parts, and problems such as soft foot (machine frame distortion), etc.
Cross channel phase cannot be used for performing a dynamic balance on a rotating component as it refers to relative phase while dynamic balancing requires absolute phase data.
by Gary James CRL
I recently used an audible recording of a time domain waveform acquired with the VibXpert to help demonstrate the value of the actual “audible” sound of a bearing fault during a recent condition monitoring survey (baseline measurements) of a multistage circulation pump. As we know, the time domain waveform signal is truly the “raw” data of vibration measurements. In this particular case, the frequency domain (FFT plot) measurements showed very low vibration amplitude levels and the determining factor for the early detection of the bearing fault was primarily the use of time domain waveform signals. Frequency domain (spectra plots) along with time domain (waveform plots) was presented to the customer for their review. Also, an “audible” recording of the time domain waveform signal was presented to the customer for review with very favorable results. The customer conveyed that the “actual” bearing impacting sound represented by the time waveform recording was easier to comprehend then reviewing the time signal plot. The customer stated that he felt like he was getting a combination of “ultra sound” technology along with vibration analysis and was very satisfied with the results.The VibXpert data collectors allow you to listen to the raw data collected on equipment. The OMNITREND software allows time domain waveform data to be played back in the comfort of your office. In certain situations these capabilities can be a great analysis tool. I have successfully used these capabilities in the past to capture “random” impacting events of ultra-slow operating equipment such as gear reducers on conveyor drives and building structure movement in connection with sensitive types of hospital equipment. Bottom line, I am planning on utilizing these capabilities much more in the future for illustrating time domain waveform signals to my customers.
ADVANCED MAINTENANCE TECHNOLOGIES
by Ana Maria Delgado, CRL