Bearings are critical components of machines and with proper performance monitoring, imminent failures can be identified and corrected. However, without a monitoring program in place, and subsequent corrective actions taken, a single bearing failure can result in full machine shutdown and countless hours of lost production.
Bearing monitoring is guided by three main senses: sight, sound and touch. Basic monitoring is conducted through elemental observations. However, many highly sensitive tools are available that amplify these observations so they are more noticeable, recordable, and include basic logic to assist with warning identification.
Monitoring bearings visually through classical methods include 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.
Several monitoring tools commonly available to leverage visual observations include site gauges for oil lubricated bearings, and thermal imaging guns. Bearings that are lubricated by oil rather than grease are often fitted with site gauges, which will give an indication of the presence of oil and the quantity of oil available to the bearing. These gauges are practical and inexpensive.
Traditionally, audible monitoring is one of the most common methods of monitoring machinery because odd noises are obvious indicators of improper operation, even to the untrained user. It is conducted quickly through an operator’s daily routines. After all, if a bearing within the machine doesn’t sound well it usually isn’t well.
The main problems with bystander audible observations is that (1) it usually identifies the later stages of bearing failure, when planning downtime for bearing replacement is impractical and (2) when audible feedback of a single bearing is masked by the overall noise of its environment. That’s when instruments such as stethoscopes (with amplification) and decibel level meters are advantageous. Both tools are available with a wide range of features that include quantified readings and recording features so bearing performance can be trended. These tools are also more useful at identifying improper operation at a less threatening stage of failure.
Bearings should run quiet and smooth; anything different will likely reflect a flaw or a 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 such as 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 is primarily a method of identifying a failure more so than identifying poor performance. Additionally, audible monitoring in the early stages of failure is more noticeable at higher operating speeds than lower speeds.
Physical (Touch) Monitoring
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 bearing failure. The touch method can be used to monitor temperature, vibration, and lubrication.
Operating temperature is the most practical and beneficial monitoring method for bearings because expensive tools are not required and is appropriate to all types of applications; slow to high speeds, light to heavy loads. For example, the average threshold of pain for humans is approximately 130°F. If it is difficult to maintain hand-to-bearing contact for several seconds then the temperature is likely above 130°F. Furthermore, water droplets placed on a bearing housing that quickly boil will indicate that the bearing temperature will have easily exceeded 212°F.
There are also many useful tools available to measure and monitor bearing temperatures. The most common 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 are also available that can be utilized for warning and/or shutdown at dangerous operating temperatures. Many bearing manufacturers offer various permanently mounted sensors pre-installed in bearing housings in areas that will most accurately reflect the true bearing temperature, rather than the housing skin temperature.
Portable thermal imaging tools are also a quick and efficient means to monitor bearing performance. These tools use infrared thermography to visually identify variations in temperature over a broad area. However, the most common portable temperature measurement tool is the infrared thermometer. Although it does not measure temperatures over a broad area, they are inexpensive and easy to use.
Monitoring and trending bearing temperature is important because as a bearing fails, the temperature will continually increase. Trending temperature over time will help identify a failing bearing in the early stages of failure.
Vibration analysis is the most information-rich method available for bearing analysis, and touch can help identify smooth versus rough operation. As safety permits, feel the 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 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 beneficial, proper training is important due to the complexity in data collection and interpretation.
Simple tests can also be conducted on purged grease to detect hard particle contaminants. Upon relubrication, rub some of the freshly purged grease between fingertips. Gritty grease may indicate a need to lubricate more often or wear from a failing bearing.
Many traditional and advanced options are available to monitor and evaluate bearing performance. Leveraging instrumentation to support traditional observations is a valuable practice in support of a predictive maintenance program.
Galen Burdeshaw is Baldor’s customer order engineering manager for DODGE bearings and power transmission components. For more information, visit www.baldor.com.