Methods For Monitoring Bearing Performance

A range of traditional and advanced options are available to help you monitor your bearings. Don’t disregard any of them. 

By Galen Burdeshaw, Baldor Electric Co.

The health of your bearings is critical to the health of your equipment and processes. With proper performance monitoring, imminent failures can be identified and corrected. Conversely, without a sound monitoring program in place and subsequent corrective actions not being taken when needed, a single bearing failure can result in full machine shutdown and countless hours of lost production. 

Bearing monitoring is guided by three main human senses: sight, sound and touch. Basic monitoring has typically been conducted through elemental observations. Fortunately, there are a number of highly sensitive tools that can amplify these observations—making them more noticeable and recordable. They also include basic logic to assist with warning identification. Keep these technologies in mind as you seek to augment your bearing sense(s):

#1. Visual monitoring: Looking
Monitoring bearings visually through classical methods includes observing lubricant condition, corrosion and deterioration. Mounted bearings that are lubricated properly will purge grease from their seals. The condition of the grease upon purging can indicate improper relubrication intervals and/or contamination. Dark, cakey or milky grease are visual signs that relubrication intervals and procedures may be improved.

Evidence of corrosion is a valuable monitoring tool as well. High levels of corrosion can degrade material strength and performance. Deterioration of the surface, seals or obvious physical dimensional characteristics should also warrant further investigation. These observations are often signals of wear, heat and other abnormal performance prior to total bearing failure.

Sight-gauge bearings and thermal imaging guns are among the readily available monitoring tools that leverage visual observations. Bearings that are lubricated by oil rather than grease are often fitted with sight gauges to indicate the presence and quantity of oil available to the bearing. These types of gauges aren’t just practical—they’re inexpensive.

#2. Audible monitoring: Listening
Traditionally, audible monitoring is one of the most common methods of machinery monitoring. That’s because odd noises are obvious indicators of improper operation, even to the untrained user. This type of monitoring is conducted quickly—through an operator’s daily routines. After all, if a machine bearing doesn’t sound well, it usually isn’t.

There can be two problems with a bystander’s audible observations: (1) Such observations usually identify the later stages of bearing failure, when planning downtime for bearing replacement is impractical; and (2) audible feedback of a single bearing can be masked by the overall noise of its environment. This is where instruments such as stethoscopes (with amplification) and decibel-level meters are advantageous. Both are available with a wide range of features, including quantified readings and recording capabilities that allow users to trend bearing performance. These tools are also more useful at identifying improper operation at a less-threatening stage of failure.

Bearings should run quietly and smoothly—anything different likely will reflect a flaw in or problem with the bearing itself. Noises such as grinding or banging should be investigated quickly. These noises may indicate complete bearing failure and continued use may lead to catastrophic failure and/or damage to neighboring equipment. Bearing noises like light clicking and squealing may indicate looseness, faults or skidding, and should be inspected for cause and remedy.

Audible evaluation is not as sensitive as other monitoring techniques. It’s really more of a method of identifying failure than identifying poor performance. One more thing to keep in mind: Audible monitoring in the early stages of failure is more noticeable at higher operating speeds than lower speeds.

Many bearing manufacturers offer various permanently mounted sensors pre-installed on bearings that provide online, real-time monitoring of temperature and speed.

#3. Physical monitoring: Touching
Monitoring bearings by touch—and then trending the observations against historical performance—is by far the most useful and accurate means for assessing bearing condition and predicting failure. The touch method can be used to monitor temperature, vibration and lubrication parameters.

Temperature. . .
Operating temperature is the most practical and beneficial monitoring method for bearings because expensive tools are not required. It’s also appropriate to all types of applications (i.e., slow to high speeds and light to heavy loads). For example, the average threshold of pain for humans is approximately 130 F. Thus, if it’s difficult to maintain hand-to-bearing contact for several seconds, the temperature probably exceeds 130 F. (A related method, wherein water droplets that are placed on a bearing housing quickly boil, will indicate that bearing temperature has exceeded 212 F.)

Monitoring bearing temperatures is crucial: As these components fail, they get hotter. Trending their temperatures over time will help identify the early stages of failure. The most common tools for doing this include thermocouples and resistance temperature detectors (RTDs)—both of which can be permanently mounted to locations on the bearing housing for continuous real-time monitoring. 

Temperature switches that can be utilized for warning and/or shutdown at dangerous operating temperatures are also available. Many bearing manufacturers offer permanently mounted sensors that are pre-installed in bearing housings (in areas that accurately reflect the true bearing temperature, not just the housing-skin temperature).

Portable thermal imaging tools offer a quick and efficient means of monitoring bearing performance. These devices use infrared (IR) thermography to visually identify variations in temperature—the most common being the infrared thermometer. Although portable thermal imaging tools typically can’t measure temperatures over a broad area, they’re inexpensive and easy to use.

Utilizing a portable temperature-measuring tool, like a thermal imaging gun, will help you accurately monitor bearing temperature. As a bearing fails, its temperature will continually increase. Trending temperature over time will help identify a bearing in the early stages of failure.

Vibration. . .
Vibration analysis is the most information-rich method available for bearing analysis—and touch is a good way to distinguish between smooth and rough operation. As safety permits, feel the bearing housing during operation. Rough operation, jostling or grinding may indicate a bearing problem.

You may also consider vibration-measurement instruments to not only identify stages of bearing failure, but to also identify overall machine performance and problems. Sensors mounted to the bearing may include permanently mounted or portable magnetic-base accelerometers, displacement probes or velocity pickups. Sensor selection is dependent upon the bearing speed, sensitivity requirements and the application. Although vibration feedback is highly desirable, proper training is important due to the complexity in data collection and interpretation.

Lubrication. . .
Simple tests can also be conducted on purged grease to detect hard-particle contaminants. After re-lubrication, the technician should rub some freshly purged grease between his/her fingertips. Gritty grease may indicate a need for more frequent lubrication—or wear from a failing bearing. MT

Galen Burdeshaw is Baldor’s Customer Order Engineering Manager for DODGE Bearings and PT Components.