by Doug Waetjen
UE Systems, Inc.
Today for a handful of proactive industrial facilities predictive maintanance (PdM) programs are merely a means to an end. These plants are employing state-of-the art technologies to conduct a complete modal analysis of their operations. The ultimate goal is to reform the manufacturing process to dramatically improve performance and competitiveness. Because the learning curve for ultrasonics is quick and its applications are many, the state-of-the-art technology has become the single most cost-effective method for the job.
What is Modal Analysis?
Modal analysis is the study of the structural characteristics of machinery with respect to vibration — the natural frequencies, or mode shapes, of a structure. The comprehensive analysis involves a close examination of mass, stiffness and damping, i.e. losses in attenuation due to molecular construction, grease, dirt, oil, isolators or rubber mounts, which reduce and/or minimize the vibration amplitude that contributes to a machine’s decay.
Machinery diagnostic consultants bring new techniques, such as machinery performance trending and analysis, and technologies, such as ultrasonics, vibration analysis, and infrared testing into play. They plug in to process control and machinery applying the same software techniques that are used on standard predictive maintenance vibration readings and apply them to non-standard signals. These include tension signals, (i.e. pulling); dynamic analysis from a cold rolling mill; running FFT’s of the tensionmeter rolls; conducting a torsional analysis on drives and multiple drives to help characterize a system; and determining how to get the next half-percent increase in performance.
The objective is always is to reduce quality defects in a facility’s system, to target its critical applications either for root cause failure analysis and/or engineering improvement d esigns, and to help the facility to integrate new technologies into its process. For example, consultants might look at the characteristics of a gear box and determine how it will potentially hold up if the facility increases the line speed by another hundred feet per minute. Then they would suggest alternative designs that the company could evaluate.
Ultrasonics Technology: Versatile, Fast and Easy-to-Use
Ultrasonics is the jack-of-technologies for most of these tasks. Because ultrasonic instruments such as UE Systems, Inc.’s Ultraprobe 9000 are sensitive to sounds beyond the limits of normal human hearing, the technology is ideal for use in acoustic array testing, such as sound intensity, sound-source localization, and acoustic holography.
Compared to standard vibration testing techniques, an ultrasonic instrument is fast and easy to use. For example, if diagnosticians are analyzing a machine train in distress, the ultrasonic instrument enables them to listen to all bearing and gear locations in a matter of several minutes. An applications specialist can hook up the ultrasonic instrument to a vibration analyzer or use spectrum analysis software to determine the frequency content which typically identifies the specific fault.
Spectrum software takes ultrasonic signals detected by an ultrasonic instrument and converts any computer into a powerful, real-time spectral analysis workstation capable of diagnosing audio data visually. The software enables inspectors to analyze, trend and track everything from worn bearings, leaking or plugged valves, and electrical disturbances in high voltage equipment to faulty compression valves.
An ultrasonic instrument’s pistol shaped housing demodulates ultrasound signals emitted from operating equipment. All an inspector needs to capture the signal is a common tape recorder, or the signals can be entered directly into a notebook computer where they may be visually and audibly analyzed.
Observing the noise level of grease-lubricated ball and roller bearings, for example, helps an inspector pinpoint worn or faulty bearings and also aids in determining the point at which the grease reaches and quiets the bearing. It is also possible to know when additional lubrication is needed and whether the appropriate lubrication was used. Once an inspector notes what and where the problem is, a company can schedule maintenance and repairs.
From Modal Analysis to Process Reform
Here is an example of how modal analysis can work. A steel mill engaged an applications s pecialist to make recommendations on how to improve its manufacturing process. Part of the analysis involved investigating the continuous-casting branch of the facility which had steel oscillators with very complex drive set-ups complete with numerous gear boxes and bearings. The consultant traced a “bump” with standard vibration readings, which did not produce enough of a difference in the amplitude to be able to find the source of the problem.
However, using an ultrasonic instrument with a contact probe attachment (wave guide), it took minutes to inspect structural positions, bearings and gear boxes to locate a fault, which was caused by a rubber support bushing that was not machined to the proper fit. He discovered that the bushing was sliding back and forth and producing a mechanical impact, or snap, as it shifted position under load.
Using standard vibration testing, it would have taken days to localize the fault and then characterize it. Once the inspector discovered the faulty bushing, it was decided that because it was having no negative impact on the process, the facility could wait for a conveniently scheduled downtime to replace the part.
When an inspector does not have the luxury of being able to make direct contact with the machinery, there is another advantage of using an ultrasonic instrument. Its scanning probe attachment is acoustical and highly directional. Simply aim the instrument at a machine train in distress and monitor the amplitude which localizes the source of the sound. The frequency identifies the problem. This is a much easier and faster method than using the microphone arrays.
Yet another bonus of using an ultrasonic instrument is that an inspector can tune it anywhere from 20 to100 kHz and pinpoint the exact spot that is picking up the specific resonance being excited. Excellent frequency data reveals the source of the problem, e.g. the inner or outer race (gear problems) or a lubrication breakdown.
In another situation, machinery diagnostic consultants were hired to evaluate spindles prone to frequent failure. Repairs can be costly and time-consuming and the facility felt that the spindles (from three different manufacturers) were not lasting as long as they were supposed to. A consultant began his analysis as a standard pdM exercise using vibration analysis, ultrasonics and infrared to characterize the nature of the spindles’ failure mode.
He plugged the ultrasonic instrument into an FFT equalizer, examined the frequency content, which enabled him to see/hear gear teeth count for the internal splines. Were typically occurs here. The consultant then proceeded to characterize the performance for the spindle. Once this was defined, the next step was to evaluate which spindles had the longest lives.
Data showed that one of the spindles was superior to that of another made by a different manufacturer. The recommendation was to replace the inferior spindles with the preferred ones which resulted in increased production. But the diagnostician did not stop there. He took the analysis one step further and eliminated spindles and gearing completely substituting universal joints for the same applications. Conventional wisdom dictated that this could not be done, that universal joints would not hold up.
As a test, the facility implemented a set of universal joints in one of its rolling mill. Almost immediately, the inspector noted a 50 percent drop in vibration on that stand and close to a 25 percent drop in the vibration of each adjacent stand, the one downstream and the one upstream. This phenomenal difference in performance meant that the mill was no longer speed limited.
As a result of modal analysis, the steel mill has replaced all its spindles with universal joints and is now the top running cold mill in the world.
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If there is a moral to the story, it is that companies wise enough to incorporate predictive maintenance programs into their manufacturing schedules should not stop there.
Indispensible technologies like vibration analysis, infrared testing and especially ultrasonics have much to offer and maintaining the status quo is only the tip of the iceberg.