- Buyer's Guide
Keeping a handle on lubrication seems easy enough. All you need to do is to make sure the right lubricant is used in the right amount and at the right time. Unfortunately, it’s not that simple.
It has been estimated that 60 to 90 percent of all bearing failures are lubrication related. Bearing failures most often lead to unplanned downtime, which can impact production as well as affect all related components around the bearing. Downtime is costly. While the cost varies by incident and by plant, it can add up.
As the most common cause of bearing failure, lubrication is serious business. For many years, this “serious business” has been conducted in a way that makes perfect sense on the surface but in fact borders on being haphazard.
Many technicians have relied on preventive, time-based lubrication alone. That is, every “X” number of months, the grease gun comes out, and the bearings are lubricated. After all, underlubrication can be lethal, causing equipment failure, costly repairs and replacements, significant unplanned downtime, and lost profits. But by relying solely on time-based lubrication, or even a combination of planned maintenance and temperature readings to serve as a proxy for lubrication status, you run the risk of something just as bad if not worse - overlubrication.
Relying on time-based, periodic lubrication assumes bearings need to be greased at defined time periods. Often this evolves into a well-intentioned guessing game at best. Adding more lubrication to a bearing that is already adequately greased is a real risk.
By using ultrasound technology (along with normal practices such as removing old grease and replacing it with new), technicians can combine standard time-based maintenance with condition-based predictive maintenance, gaining in the process both a clearer picture of what’s really going on in their machines and improved reliability.
Ultrasonic equipment detects airborne and structure-borne ultrasounds normally inaudible to the human ear and electronically “transposes” them into audible signals, which a technician can hear through headphones and view as decibel (dB) levels on a display panel. In some instruments, the received sound can also be viewed on a spectral analysis screen. With this information, a trained technician can interpret the bearing condition in order to determine what, if any, corrective action is needed.
Ultrasound technology has many advantages:
Airborne and structure-borne ultrasound instruments are an extension of the user’s sense of hearing. Just like vibration feels what you can’t feel and infrared cameras see what you can’t see, ultrasound hears what you can’t hear. Noise in a typical plant environment (machines running, production equipment running, etc.) can prevent you from being able to hear other sounds such as compressed air leaks or electrical discharges like corona, tracking or arcing.
Screen shots of a good bearing (left) and a failing bearing (right)
Ultrasound instruments sense or listen for high-frequency sounds that are not heard in the audible range by normal human hearing. These high-frequency sounds are detected by the instrument and translated into an audible sound that is heard in the headset by the inspector. The decibel level is then indicated on the instrument’s display.
Traditional inspection of electrical components has been performed using an infrared camera. Users of this technology rely on images showing temperature changes that may represent electrical anomalies such as tracking and arcing. For mechanical inspection, vibration analysis has been the conventional method for condition monitoring of rotating equipment. Vibration analysis produces a visual spectrum or time waveform that reveals any fault harmonics. If the goal is to have a truly world-class predictive maintenance (PdM) program, the use of multiple technologies is recommended for various inspections. Just as a physician uses multiple tools to diagnose aches, pains and abnormalities, maintenance professionals should take the same approach when it comes to the assets they are responsible for in their facilities.
This overlay shows recorded ultrasound files of four motor outboard points on identical motors. One has an obvious bearing defect.
In addition to infrared and vibration, ultrasound can be used to complement other PdM technologies. Airborne and structure-borne ultrasound can give the user an “image” to analyze in order to diagnose and confirm mechanical and electrical conditions. The concept of ultrasound imaging involves recording sounds heard via the ultrasound instrument and then playing back those recorded sounds in spectrum analysis software. This can provide the inspector with the audible sound heard in the field during the inspection and a visual “image” or spectrum of the recorded ultrasound. This method can help to reduce the subjectivity of only relying on changes in the decibel level and in the sound tone or quality heard by the inspector.
Ultrasonic technology helps the lube tech take a lot of the guesswork out of lubrication. Ultrasound is a localized signal, which means that when a sensing probe is applied to a bearing, it will not be affected by “crosstalk” and will allow the technician to hear and monitor the condition of each bearing. Ultrasound looks at each bearing individually, much the same way medical ultrasound can detect exactly which artery is clogged or which vein is leaking.
How does ultrasound work in regard to lubrication? The first step is to establish both a baseline decibel level and a sound sample. This is ideally done when moving through a route for the first time by comparing dB levels and sound qualities of similar bearings. Anomalies will be easily identified. Once baselines are established, each bearing can be trended over time for any changes in either amplitude or sound quality.
Generally speaking, when the amplitude of a bearing exceeds 8 dB and there is no difference in the sound quality established at the baseline, the bearing needs to be lubricated. To prevent overlubrication, the technician should apply the proper lubricant a little at a time until the dB level drops.
Many organizations set up their condition-based lubrication programs by incorporating a two-stage approach. The reliability inspector uses a relatively sophisticated ultrasound instrument to monitor and trend bearings. A report of bearings in need of lubrication is produced. The lube tech then uses a specialized ultrasound instrument that alerts the tech when to stop adding grease.
To improve efficiencies, it is good practice for the technician to note when the equipment was last greased and how much grease was applied in order to calculate roughly how much lubricant is used per week. By utilizing ultrasound to lubricate each and every time, the technician produces historical data that can be employed as a guide to help determine whether the lubrication schedule can be modified to save man-hours and if the manufacturer’s suggested lubricant amount is accurate. If less is needed, there’s cost-saving potential.
While most of this article has focused on the dangers of underlubrication and overlubrication, ultrasound is just as reliable in picking up other potential bearing failure conditions. A technician using ultrasound can hear telltale “grinding” sounds and other anomalies, which are often accompanied by an amplitude increase. In regard to lubrication, the advantage of ultrasound is that it is able to isolate bearings and determine their individual needs, thus reducing the possibility that some bearings are too “dry” and preventing others from overlubrication.
It is always a daunting proposition to make a new investment in technology. Will it pay off? Will your staff actually have an easy time using it? Is it a flash in the pan or a truly reliable modality that will stand the test of time?
Although more plants are utilizing ultrasound and adopting a predictive or proactive approach rather than a reactive mindset, there are still many that are figuratively using crystal balls and outdated methodologies. The end result is poor reliability, unnecessary man-hours, downtime, and lost productivity and profit. While ultrasound can’t cure all reliability ills, it has proven itself in a wide variety of settings to be a valuable and powerful diagnostic tool that technicians should add to their toolkits.
Using an ultrasound instrument while greasing allows technicians to know when they have added enough grease or when too much grease has been applied.
When it comes to something as important to reliability as lubrication, the real question becomes, “Can you afford not to use ultrasound technology?” Consider the plant that went from almost 30 bearing failures a year before using ultrasound to having no bearing failures for three years once ultrasound was added to its maintenance arsenal. It’s no coincidence. Ultrasound works.
When it comes to lubrication practices in plants, there are three scenarios that could be considered as good, better and best. A good scenario would be to follow the manufacturer’s recommendations as to the frequency, type of lubricant and amount of lubricant to be applied to a certain piece of equipment. A better scenario would be to still make use of the timed interval, but instead of lubricating with just a grease gun, utilize an ultrasound instrument while greasing. This will at least allow the lube techs to know when they have added enough grease or when too much grease has been applied. Another benefit is that the individual lubricating the equipment can listen to the bearing while greasing it. This enables the inspector to hear if other bearing defects are present that lubrication may not help. In a sense, the lubricators become fault finders. If a bearing does not sound normal or has an increased decibel level, that bearing can be documented and complementary vibration or ultrasound data can be collected to determine what the issue/defect is.
Finally, the best scenario is to use an ultrasound instrument with data storage and data management software to create routes. Data such as decibel levels and sound files can be recorded periodically. How often the readings are taken should be based on an asset criticality assessment. Once a baseline has been established, a low-level alarm can be set for when a lack of lubrication condition has been detected for a bearing. A high-level alarm is also set to indicate when a bearing has reached the point of initial failure.
If lubrication is not the solution to the problem, a more detailed test would be required to identify the specific defect, such as an inner or outer race bearing fault. When a point along a route has been determined to be lacking lubrication, an inspector can go to that point and apply lubricant until the decibel level drops back down to the baseline level.
Ultrasound imaging or recording is a new concept that involves using a special instrument to analyze ultrasounds, which are then viewed in spectrum analysis software. Although this is a fairly new technique, the use of ultrasound for both mechanical and electrical inspections is growing. The spectrum analysis of recorded ultrasounds can enhance diagnostic accuracy and reduce the subjectivity of only comparing decibel levels or simply what is heard by the inspector in the headset.
Adding ultrasound to mechanical inspections can also allow for better use of other tools such as vibration analysis. If there are too many assets to monitor with vibration, ultrasound can be included to complement the vibration analysis program. Critical assets can benefit from having both vibration and ultrasound data collected. For noncritical assets, which may not necessitate the time to collect vibration data but still need to be monitored, ultrasound data can quickly and easily be used.
Another reason to complement a vibration program with ultrasound is if the vibration analyst’s time is limited. A vibration route can be very time-consuming, but ultrasound can help reduce the time it takes to collect vibration data. Ultrasound can be used first, and once the decibel level has risen to trigger an alarm, vibration can further diagnose the problem and the reason for the increase in noise level.
The beauty of adding ultrasound to an existing predictive maintenance and reliability program is that it can be used for many different applications. Typically, those who are starting out on a reliability journey or are just beginning to utilize the technology will employ ultrasound for compressed air and gas leak detection. Tremendous energy savings can be realized through the use of ultrasound for both steam trap and compressed air/gas inspections. Based on these savings, the maintenance and reliability department can gain buy-in from both management and floor personnel. The savings associated with the energy-conservation efforts can then be reinvested in the program by means of additional tools, training, certification courses or even manpower.
The same method for energy-conservation applications can also be used for electrical and mechanical applications. This is what makes ultrasound a versatile and easy-to-use tool for both the well-established maintenance and reliability programs, and those that are just beginning their journey.