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Vibration vs. Ultrasound - Two Techniques to Consider in CBM

by Jason Tranter, Allan Rienstra and Tom Murphy

If someone tells you there is only one RIGHT way to do something - you should be highly dubious! As a result of the positive reader response from “RCM vs. FMEA - There Is a Distinct Difference!” (Dec/Jan 2013), Uptime magazine has invited several subject matter experts to write short, related segments that allow our readers to consider alternatives or options when approaching maintenance reliability improvements. In this issue, we examine two popular condition-based maintenance practices and applications — Vibration and Ultrasound.

Vibration

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Vibration analysis is an effective way to detect premature bearing failure AND the conditions that can lead to excessive bearing loading and ultimately bearing fatigue failure. (photo courtesy of SPM Instrument)

Vibration analysis is a very powerful technology. It can be used to detect a wide range of fault conditions in rotating machinery, including defects in rolling element bearings. Vibration analysis can be used to detect the root causes of bearing failure and the symptoms of bearing failure. But it should be noted that the best strategy of every condition monitoring technician is to utilize as many techniques as possible in order to gain the greatest confidence in any fault diagnosis. While the remainder of this article will describe the virtues of vibration analysis, it is recognized that other technologies exist, including oil analysis, wear particle analysis and ultrasound monitoring, all of which can aid in the detection of rolling element bearing faults.

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36% of bearing failures can be attributed to over-lubrication. Excessive grease in a bearing causes overheating; liquefaction of the grease and subsequent damage to the bearing.
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34% of premature bearing failure is due to fatigue caused by excessive loading, typically caused by shaft misalignment and rotor imbalance. Other causes include improper handling/installation (16%) and lubricant contamination (14%).

 

WHAT CAUSES BEARINGS TO FAIL?

Before we discuss the detection of bearing defects, let’s quickly look at why bearings fail. If the machine is out of balance, misaligned, or operates in a resonant condition, the additional load will greatly reduce the life of the bearing. Spectrum analysis and phase analysis will help you detect these bearing destroying conditions.

If the bearings are not installed correctly, for example the bearing is cocked on the shaft or cocked in the housing, then again spectrum and phase analysis can be used to detect the situation so remedial action can be taken.

Poor lubrication of bearings will also reduce their life. Inadequate lubrication and design issues, such as skidding and sliding, also can be detected with vibration analysis.

DETECTING BEARING DEFECTS

Thanks to the geometry of rolling bearings, a bearing will generate tell-tale frequencies that identify whether there is damage to the inner race, outer race, the rolling elements and/or the cage. Due to the nature of typical bearing failure modes, the vibration generated can be readily distinguished from other sources of vibration, including non-bearing related fault conditions. As a result, a well-trained vibration analyst will be able to readily detect bearing defects and get a very good idea of the nature and severity of the defect without even knowing which bearing is installed in the machine. For example, by diagnosing that a spall exists on the outer race and that the bearing will continue to operate with a low risk of failure for many months, enables the maintenance and production departments to determine the best possible time to replace the bearing.

Utilizing a combination of high-frequency techniques, such as the Shock Pulse Method, PeakVue and enveloping, and conventional spectrum analysis (supported by time waveform analysis), it is possible to track the condition of the bearing from the initiation of the bearing defect right through to the point where failure is imminent. Relying on quantitative techniques, where spectral patterns and amplitude levels provide a direct indication of bearing condition, puts the vibration analyst (and therefore the maintenance and production staff) in a position where the financial consequences of bearing failure will be significantly reduced. And by detecting and correcting the conditions that cause bearings to fail, the reliability of the bearings can be vastly improved.

Ultrasound

Beyond the small and somewhat niche group that makes up reliability and condition- based monitoring (CBM) practitioners, few consider ultrasound as a technology for predicting bearing defects. Ask everyday people off the street about ultrasound and most will relate to its more famous medical roles for diagnostic imaging and soft tissue repair. Some may even be familiar with very high frequency ultrasound used to identify material cracks and welding flaws. Notwithstanding the lack of visibility low frequency ultrasound has among commoners, it more than makes up for it within the confines of maintenance and reliability.

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Collecting ultrasound data through the grease fitting

Here, ultrasound is considered the most versatile predictive technology in the toolbox. Some revere ultrasound as their frontline defense against premature machine failures. Ultrasound’s simplicity, economics and broad spectrum of applications has led one clever company to christen the technology “predictive maintenance for the masses.”

If you have been involved with maintenance for a decade or more, then you have likely observed the evolution of ultrasound from an energy management and troubleshooting technology to a trending and diagnostic standard. Today, ultrasound fulfills a necessary task identifying early fault conditions in rotating and non-rotating machinery. As a companion technology to vibration analysis, ultrasound lends a symbiotic and complementary ear.

HOW DOES ULTRASOUND WORK?

Ultrasonic data collectors detect sound pressure waves usually in the 35 kHz to 40 kHz frequency range. The waves act upon a resonant sensor to create a small electrical charge. The charge is amplified, measured and converted (heterodyned) to a corresponding audible frequency that is presented in a high-quality headset and can be recorded to the data collector’s memory. Ultrasonic data may provide an immediate alarm in the field or be analyzed later. There are many defect symptoms and defect conditions to detect on rotating assets that are identifiable first with ultrasound.

WHERE IS ULTRASOUND USEFUL?

A defect symptom may be as simple as an increase in friction from poor lubrication. Another symptom may be the rubbing and skidding of rolling elements against the bearing raceway, or impacting due to mechanical flaws or contaminated lubricant. A defect condition could be misalignment, imbalance, or worn shaft couplings. These conditions also present earliest in the ultrasonic range. By identifying both symptom and cause, ultrasound serves to answer two fundamental questions that every reliability practitioner should ask:

  • Is my machine okay (Symptom)
  • If not, what’s wrong with it? (Condition)

ULTRASOUND’S ROLE IN CBM

As a front line defense against symptoms of failure, ultrasound raises alarm flags very early on. It answers the most simple and fundamental question: “Is my machine okay?” And it answers that question by collecting data quickly, economically and with a minimum of analysis. Some technicians continue to perform in-depth vibration analysis on hundreds, or even thousands, of bearings that may or may not have a fault condition. Other technicians recognize their time is better spent by first identifying defective assets with ultrasonic condition-based monitoring. By using ultrasound to filter the good from the bad, they afford themselves more time to perform deeper analysis on only the machines designated as “not okay” by ultrasound.

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Collecting dynamic ultrasound measurements on an electric drive motor

Recent advancements extend the usefulness of ultrasound as a diagnostic and analysis technology. Serving once again as a symbiotic companion to vibration analysis, ultrasound performs exceptionally well in assessing defects on slow rotating assets, an area where vibration analysis can struggle. One such advancement includes the ability to measure, store and analyze dynamic data.

Ultrasound detectors measure data in one of two ways: static and dynamic. Static data relates to a single value measurement taken at a point in time and is usually expressed as decibels per microvolt (dBµV). Dynamic data is a single measurement taken over a longer period of time (5, 10, 30, or even 60 seconds). Think of it this way: static data is a photograph, whereas dynamic data is a video. Both tell a compelling story about the bearing’s current condition, however, dynamic data can be analyzed in the time and frequency domain, much like its companion, vibration analysis. Where static data helps answer the fundamental question, “Is it okay,” dynamic data analysis extends the ultrasound technician’s capacity to help discover, “What is wrong with it.”

ARE WE GETTING THE COMPLETE PICTURE ON ULTRASOUND AND CBM?

While the importance of ultrasound as a symbiotic companion to vibration analysis may be the emphasis of this article, what is often overlooked are the many other amazing ways this technology should be used. Too often, the term CBM is thought of in terms of bearings and rotating machines only. But condition-based monitoring embraces so much more. Consider the following examples:

  • Can a compressed air or vacuum leak stop production? (Yes it can!)
  • Will too many failed steam traps create inefficiency, energy waste and poor product quality? (Of course they will!)
  • What are the potential hazards if an electrical fault in metal-clad switchgear goes undetected? (Fire, explosion, injury and death!)
  • What can happen to a process when a closed valve is not really closed? (Contamination and product waste, to name a few.)
  • What consequences can arise from a blockage in a hydraulic system? (Production lag or shutdown.)

Any one of these faults can negatively impact uptime and all of them can be monitored with ultrasound. Do any of the above fault conditions rotate? No, but they still require CBM. Ultrasonic CBM reveals the condition of non-rotating assets that are just as essential to uptime as things that rotate. When compressed air leaks are discovered with ultrasound, it reveals the condition of the compressed air system; it leaks! When failed steam traps are identified, the condition of the steam system is determined; it’s inefficient!

CONCLUDING STATEMENTS

Reliability practitioners identify ultrasound as an important technology for determining both symptom and cause of mechanical defects. Ultrasound provides the earliest warning that a bearing is developing a defect and entering failure stage. As a symbiotic companion to vibration analysis, ultrasound delivers a more complete picture about the health of rotating assets. Understanding the full range of opportunities ultrasound presents ensures the technology is used for all its intended purposes and not just for things that turn. It is equally important that the term CBM be associated with more than just bearings.


Part2Jason Tranter is the founder of Mobius Institute and author of iLearnVibration and other training materials and products. Jason has been involved in vibration analysis in the USA and his native Australia since 1984. Before starting Mobius Institute, Jason was involved in vibration consulting and the development of vibration monitoring systems. www.mobiusinstitute.com


Part2Allan Rienstra is the General Manager of SDT Ultrasound Solutions, providing ultrasound solutions to maintenance professionals since 1991. As co-author of the bookHear More, A Guide to Using Ultrasound For Leak Detection And Condition Monitoring,” Allan is recognized as a subject matter expert in the field. www.sdthearmore.com


Part2Tom Murphy is a Chartered Engineer with a degree in Acoustics and 29 years postgraduate experience in predictive maintenance using vibration, infrared and ultrasound. He teaches ultrasound to ASNT Level 1 and Level 2 standards, implements programs and provides assistance to companies whose PdM results could be better. Tom is the co-author of “Hear More”.


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