An effective condition-monitoring program provides many benefits, ranging from improved asset availability to reduced energy consumption. One primary focus of condition monitoring is the status of bearings, whose health (or lack of it) can mean the difference between meeting production goals and causing chaos. Should even a single bearing fail, the consequences can be damaged equipment and product, a production run stopped, parts that must be ordered for repairs, and considerable man-hours wasted.
A program for monitoring bearing condition need not be extremely complicated, nor does it necessarily require a large investment of capital to get it going. One technology that is both cost-effective and relatively easy to incorporate in such a program is air-borne/structure-borne ultrasound.
How It Works
Most ultrasound instruments are lightweight and portable. They translate high-frequency sounds produced by bearings down to the audible range so they can be listened to with headphones and viewed as intensity levels (usually decibels) on a display panel. Digital instruments record test data, while some instruments have onboard sound recording and data logging capability. The ability to view sound levels while simultaneously listening to sound quality enhances the inspection effectiveness (Figure 1, page 24). Inspectors can quickly identify changes in bearing condition that occur from increases in decibel levels or by changes in sound quality as they collect data along their routes.
Figure 1. Bearing condition can be analyzed by listening to increases in decibel level or changes in sound quality; a visual display provides additional information
Ultrasonic bearing condition monitoring can be used for quantitative and qualitative analysis. Data can be recorded and reviewed in data management software and sound samples can be analyzed through spectral analysis software. This two-pronged approach enables inspectors to determine whether, or what type of, corrective action should be taken.
Sensing high-frequency sounds has its advantages. Sound emissions are localized to the point of origin, making it easy to identify or monitor a problem with little to no crosstalk from other mechanical components. Because ultrasound waves are small compared to those in the audible range, subtle changes can be detected before a part reaches the critical failure stage. Friction is one of the major contributors to bearing failure, and ultrasound instruments detect friction. Trending friction levels can help determine failure conditions related to lubrication issues.
Lubrication: Too Much vs. Too Little
While many bearings can fail due to lack of lubrication, over-lubrication is considered one of the major causes of bearing failure. Many lubrication programs are based on preventive maintenance programs in which bearings are lubricated according to a time-based schedule with predetermined amounts of lubrication applied. While useful, if this practice is followed without any feedback regarding the condition of a bearing, it may, in fact, lead to an over-lubricated condition that will eventually lead to bearing failure. Many maintenance departments are therefore switching to a combination of preventive and condition-based lubrication (Figure 2, page 25).A condition-based lubrication program requires trending of bearing decibel levels. A baseline decibel level is set, along with (if possible) a baseline sound sample, and an inspection schedule is established for periodic testing. When a bearing sound level exceeds 8 dB with no change in the sound quality (usually a smooth, "rushing" sound), the bearing is considered in need of lubrication. A lubrication technician, while listening to the bearing, will then apply lubricant, a little at a time, until the baseline level is reached. Stopping at that point prevents over-lubrication.
Figure 2. To maintain correct levels of lubrication, many maintenance departments use a combination of preventive and condition-based lubrication
Other causes of bearing failure include improper installation, corrosive atmospheres, particle contamination, faulty alignment, and use of the wrong lubricant.
Setting Up a Program
An effective program requires strategies for the efficient use of manpower. This entails documentation and analysis of all test data. Inspection routes must be manageable and logical to eliminate wasted time. Scheduling of inspection will depend on criteria such as criticality, potential for failure, or safety.
Ultrasound inspection is both relatively fast and accurate—only one test point is needed. For consistency when setting a baseline, if the instrument has frequency tuning capability it is advisable to choose one frequency, such as 30 kHz, for testing bearings. Once a test point has been marked, all an inspector has to do is touch the test point and listen to the bearing quality while noting the decibel level on the display panel. If the sound quality changes from smooth to, for example, crackling or grinding, and if the change is >8 dB, the inspector can note it immediately. The data are logged and, along with a recorded sound sample, subsequently analyzed.
A maintenance manager for a food manufacturing plant reported at a recent ultrasound conference a savings of $220,000 in their bearing-monitoring program. The company also reported that since initiating ultrasound condition monitoring at another of their facilities more than three years ago, there have been no unannounced failures in the 340 bearings they monitor, and that their motor repair cost was reduced from $2400 to $600 per motor. In fact, all their criteria for rebuilding a motor are based on condition rather than preventive, time-based procedures. This switch has resulted in a three-fold savings, which translates into $90,000 per year.
The benefits of an ultrasonic condition-monitoring program go beyond bearings. The instruments are used to reduce energy costs by locating steam and compressed air leaks (Figure 3, page 26). Identifying steam and air leaks has prevented potential loss of equipment availability by locating arcing and tracking problems in electrical apparatus such as transformers, switchgear, and motor control centers. Electrical gear is typically quiet, so sound emissions generally indicate a problem. Recorded samples can be analyzed with spectral analysis software to determine what's going on.
Figure 3. Ultrasonic condition monitoring can detect compressed air leaks as well as bearing noise
As for energy loss, both air and steam are pricey utilities. As energy costs continue to escalate, ultrasound leak detection can provide substantial savings. One automobile manufacturing company had a compressed air leak survey performed in their plant and were amazed to realize a savings of $157,000 as a result of locating and repairing the leaks. Similar savings have been reported for steam system surveys—one large chemical plant realized a $1,000,000 savings from a steam survey.
The benefits of ultrasonic condition monitoring are quite substantial. The initial investment is relatively inexpensive and the return on it could be immense. To help push an ultrasound program along, it is a good idea to attend a training course. There are certification courses available that cover all the major applications and provide all the information necessary to implement a successful program.