Controlling maintenance and repair costs in a facility has always been a challenge for maintenance and reliability professionals. In today’s environment, controlling costs has become critical in helping to remain competitive in your respective industry and operating an efficient facility. Improving condition monitoring and predictive maintenance techniques and procedures can go a long way in increasing asset availability and reducing unplanned downtime, thus greatly reducing costs associated with operating in a reactive maintenance mode.
Lubrication related bearing failures seem to always be one of the major challenges in any plant and facility. The costs associated when machines fail and production ceases can be damaging to the bottom line of even the most profitable company. Identifying potential failures before they become catastrophic has become the new standard for maintenance and reliability departments worldwide. Facilities that continue to operate reactively, and without utilizing predictive maintenance tools often find themselves behind their competition and always in “firefighting” mode.
One technology in particular can easily be implemented into an existing maintenance and reliability program. Airborne and structure-borne ultrasound is an easy to use technology and can be applied to many different energy conservation and equipment reliability applications. This article discusses the benefits of utilizing airborne and structure-borne ultrasound assisted lubrication to reduce bearing failures due to over and under lubrication conditions.
Introduction to Ultrasound Assisted Lubrication
When you consider that as many as 60 to 80 percent of all bearing failures are lubrication related, this represents an area where many opportunities lie where these types of bearing failures can be avoided by utilizing predictive technologies such as ultrasound. Lubrication related failures includes both over and under lubrication. Also, the use of the improper lubricant can greatly decrease the life expectancy of the bearing. Consulting your lubricant provider for recommendations for the correct lubricant for your application is critical.
Typical lubrication programs rely solely on a timed based interval. This interval is set up is usually based on either the bearing/equipment manufacturer’s recommended amounts, or the recommendations from the lubricator based on how much grease they have been applying at each PM. The tendency for time based lubrication routes is to over lubricate. Additionally, what if the bearing already has enough grease? If the bearing already has enough lubricant, more grease will be applied during a scheduled PM. Another question that should be asked is what if the bearing needs more grease than what is being applied during the scheduled PM?
Adding ultrasound assisted lubrication to standard lubrication best practices that are already in place can help to reduce the number of failures due to over and under lubrication of bearings. The advantages of condition based lubrication rather than a time based lubrication approach are fewer bearing failures, extended motor & bearing life, and a decrease in the amount of lubricant used. All of these add up to potential savings in maintenance costs, man-hours spent lubricating, and improved asset availability.
Ultrasound Technology and Bearing Lubrication
Hand-held airborne and structure-borne ultrasound instruments sense and receive high frequency sound waves that are produced from rotating equipment, electrical disturbances, and compressed air & gas leaks, just to name a few. These high frequency sounds are above the range of normal human hearing, and therefore cannot be heard in the audible range. The instrument receives the high frequency sound, and through a process called heterodyning, translates the high frequency sound into an audible sound heard through the headset by the inspector. The sound is then measured as a decibel (dB) on the display panel of the instrument.
There are several sources of ultrasound that are detected with an ultrasonic instrument. For instance, turbulence is created when a gas under pressure escapes from a small crack or orifice in a piping system, creating a compressed gas leak. For condition based lubrication with ultrasound, friction is the primary source of the ultrasound that is produced from bearings and rotating equipment. When bearings do not have sufficient lubrication, there is an increase in friction. When friction increases, so does the decibel level (dB) indicated on board the ultrasonic instrument.
An increase in the decibel level is also noted when a bearing is in an over lubricated condition. As lubricant begins to build up inside the bearing housing, there is an increase in both pressure and friction. Thus, as friction and pressure increase, so does the decibel level.
Additionally, as friction increases, so does the temperature. Increases in temperature can also have a damaging effect on the bearing, such as lubricant degradation, expansion, and oxidation. All of which can have a damaging effect on both the bearing and eventually the piece of equipment that the bearing is operating on.
A bearing that is properly lubricated will have a much lower decibel level than one that is under lubricated. A properly lubricated bearing will have a steady uniform sound like that of rushing air. When listening to bearings while lubricating, if the bearing needs grease, one will note a decrease in the decibel level as grease enters the housing. Once a sufficient amount of grease has surrounded the bearings, the decibel level drops, and stays at a constant/steady dB level, and lubrication will need to stop being applied. If more grease continues to be applied, the dB level will gradually begin to increase. At this point, the bearing has reached an initial over lubrication condition. Lubrication would need to be stopped if the dB begins to increase. Also, if grease is applied and there is no change in the dB level, further investigation would need to be conducted to determine if the bearing is in a progressive stage of failure that lubrication will not silence or dampen.
The image below shows a recorded sound file of a bearing taken while the bearing was being lubricated. Notice the dramatic difference between the noise level before lubrication, and then after lubrication.
The image below shows a recorded sound file of a bearing taken while the bearing was being lubricated. This example shows the effects of over lubrication when grease continues to be applied after a drop in the dB level when the proper amount of grease has been applied.
When monitoring the dB level of a bearing, a recommended standard of an 8dB increase above a preset baseline, or above the previous reading, represents a lack of lubrication. As an example, if a baseline of 50dB has been established for a certain location on a piece of equipment such as a motor out board, and the current reading taken is 58dB, this location would be in a lack of lubrication condition. Lubrication would then be applied until the dB level has decreased back to the baseline or normal level of 50dB. Once the dB has reached the baseline level, lubrication would stop being applied.
If a digital ultrasound instrument is being used for ultrasound assisted lubrication, routes can be created with the included software, and then loaded up to the instrument. Data and sound files are recorded, and then downloaded back into the route in the software. A best practice is to record both data and sound files so that the baseline sound files can then be compared to the current reading. Once data has been collected, baselines can be established. Based off of the baselines, alarm levels can be set. When establishing alarm levels, one alarm level is set for a lack of lubrication, and a high alarm level is set for a more significant increase in the dB level which would indicate a more severe bearing failure other than a lack of lubrication. When data is downloaded into the software, a report is generated showing the locations that are currently in an alarm level. Lubrication would be applied only to the points that are currently in the lubrication alarm level. This method eliminates lubricating points that are still below the established low level alarm or lubrication alarm, thus decreasing the amount of time spent lubricating, the amount of lubricant used, and a reduction in lubrication related equipment failures.
Other Applications for Ultrasound
Bearing lubrication is just one application for which you can use ultrasound technology. Other conditions in rotating equipment that can be detected with ultrasound include inner race, outer race, and cage defects, fluting (electrical arcing in bearings), misalignment, soft foot, rotor bar issues, and pump cavitation just to name a few. Other applications include compressed air & gas leak detection, vacuum leak detection, electrical inspection (corona, tracking, arcing), steam trap inspection, and vessel integrity checks around seals and fittings.
Being able to identify failures before they become more severe is critical. Airborne & structure-borne ultrasound is a technology that can be used for many different applications. When considering ultrasound for equipment reliability and ultrasound assisted lubrication, the benefits are quite substantial. Cost justification is simple when compared to the savings that can be realized from decreases in lubricant used, less man hours spent greasing equipment, less downtime due to undetected failures, and reductions in the amount of failures due to lubrication issues. Not to mention potentially being able to avoid downtime due to unexpected bearing failures. Ultrasound is a versatile tool with a short learning curve, thus making it an ideal technology to begin a predictive maintenance program around, or complement an existing technology that is being used such as vibration analysis or infrared thermography.
This article was previously published in the Reliable Plant 2013 Conference Proceedings.
By Adrian Messer, UE Systems, Inc.
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