A recent two-year study of failed bearings from a variety of applications - bearings returned for performance analysis - revealed that more than 86% of them failed because of end-user errors - storage and handling, installation and operational errors, among others. By far, operational errors account for most bearing failures and ineffective lubrication and contamination head the list of causes of failures due to operational errors.
Research also shows that regardless of how well one follows predictive- or preventive-maintenance procedures (discussed later), bearing failures invariably occur on rotating equipment. Bearing failures mean downtime and lost productivity. For that reason, it is often necessary to subject a failed bearing to failure analysis to find out why it failed. Such analyses serve as a basis for improving both operations and maintenance and increasing uptime.
THE FAILURE-ANALYSIS PROCESS
Often, performing rolling-element bearing-failure analysis is a relatively straightforward process. First, the analyst observes the overall appearance of the tracking-pattern "load zones" on the surfaces of a bearing's internal components. These internal load zones provide two clues: 1) they indicate how the bearing was loaded, and 2) they give an indication of any abnormal conditions that were present in the application. In other words, the pattern reveals the operating conditions experienced by the bearing and allows a skilled analyst, who can distinguish between normal and abnormal patterns, to determine whether abnormal operating conditions occurred. Bearings fail with a certain sequence of events that can be read by a trained analyst.
In more difficult cases, the analyst may need detailed information about the failed bearing. For example, it often is very difficult to determine the mode or modes of failure when a bearing is severely damaged. In such situations, a history of the bearing in the form of condition-monitoring data from sensors installed on the equipment is valuable and, perhaps, essential in order to determine the cause of failure. In any case, the person performing the failure analysis must have a complete understanding of the function of the various internal bearing components as they relate to the actual operating conditions applied to the bearing. Then, using a systematic approach based on the observable characteristics exhibited on the failed bearing, the analyst can explore and eliminate various factors that affect bearing performance. When the cause of failure is determined, real solutions can be implemented to increase the life of the replacement bearing.
In most cases, a bearing subjected to adverse operating conditions manifests its failure as a lubrication failure. The reason is that an abnormal operating condition typically produces excessive heat within the bearing. The heat buildup reduces the viscosity of the lubricant separating the internal rolling and sliding surfaces. On a microscopic level, the lower lubricant viscosity leads to a reduction in the fluid-film thickness separating the component surfaces and results in metal-to-metal contact. The friction and surface damage resulting from metal-to-metal contact subsequently increases the bearing's operating temperature and further reduces the lubricant's viscosity and fluid-film thickness. Continued operation under these conditions seriously jeopardizes the function of the bearing and ultimately results in premature bearing failure. In such situations, it is easy to incorrectly diagnose the problem as lubricant-related.
|Figure 1. A bearing subjected to adverse operating conditions manifests its failure as a lubrication failure. For example, this inner ring shows heat discoloration - straw/brown indicating temperatures of 300°F to 350°F and blue indicating temperatures of 400°F to 450°F. While the root cause of the overheating could have been ineffective lubrication, incorrect shaft fits or excessive loading is also a candidate. (Photo courtesy of SKF Reliability Systems.)|
There are many "predictive" technologies that can spot the degeneration of a bearing before it fails completely. In the hands of experienced bearing-failure analysts, the data from such predictive methods are invaluable. Following are descriptions of three of those technologies.
If the technician responsible for analyzing damaged, rolling-element bearings has the experience and the necessary tools, there is a way for him or her to detect an abnormal operating condition prior to catastrophic failure. Vibration-monitoring equipment installed on rotating equipment can detect and analyze the condition of various components, including rolling-element bearings. By analyzing the vibration signatures produced by the inner ring, outer ring, rolling elements and cage, a vibration analyst can pinpoint bearing damage caused during operation. Any unusual pattern generated at one of these suspected frequencies is cause for immediate concern. The level of vibration measured at the location may even dictate an equipment shut down and component change.
|Figure 2. "Spalling" on one raceway of this inner ring of a spherical roller bearing indicates the bearing experienced edge loading. Spalling is the last stage of failure before a catastrophic failure. (Photo courtesy of SKF Reliability Systems.)|
|Figure 3. If a failing bearing is not removed from service soon enough, the resulting damage can hide important clues needed to correctly identify the root cause of failure. However, in this example of a catastrophic failure, experts determined that the spherical roller bearing, whose inner ring is shown here, failed because of excessive speeds and loads. The white material in the photo, fire extinguisher residue, indicates how severe the failure was. (Photo courtesy of SKF Reliability Systems.)|
Pinpointing specific problems using condition-monitoring equipment requires training and experience. In practice, vibration signatures of newly installed equipment should be obtained immediately after installation. These readings provide a benchmark, and benchmarking can greatly reduce the time required for bearing-failure analysis. Since the database already includes the equipment's vibration signature, any extraneous vibration reading not related to the bearings can be eliminated. Once a benchmark is established, a comparison of later vibration readings with the benchmark aids in assessing the condition of the bearings, allowing their removal from service before catastrophic failure.
Another popular method for determining the condition of rolling-element bearings in rotating equipment is lube-o