Predictive Maintenance through the Monitoring and Diagnostics of Rolling Element Bearings
Introduction
The predictive maintenance philosophy of using vibration information to lower operating costs and increase machinery availability is gaining acceptance throughout industry. Since most of the machinery in a predictive maintenance program contains rolling element bearings, it is imperative to understand how to monitor and diagnose problems associated with them. Bently Nevada has adopted a two-part philosophy with regard to rolling element bearing monitoring and diagnostics: (1) the monitor system will provide adequate warning to avert catastrophic machine failures and (2) diagnostic data will be available so that when warning is given, the bearings will have visible damage. This philosophy should be kept in mind during the following discussion.
Rolling Element Bearing Characteristics
Any discussion of monitoring and diagnostics for rolling element bearings would not be complete without a comparison with the techniques used for fluid film bearings. The construction of a fluid film bearing is such that the shaft is supported by a fluid film during operation. By design, the shaft can experience motion relative to the bearing. Because of this freedom of motion, the industry-accepted vibration measurement for a fluid film bearing machine is a shaft relative measurement, i.e., proximity probe.
A rolling element bearing, by design, has extremely small clearances which do not allow a significant amount of shaft motion relative to the bearing (Figure 1).
Forces from the shaft are transferred through the rolling elements to the bearing outer race and then ultimately to the bearing housing. Because of this transmission, a casing (bearing housing) measurement is normally acceptable for monitoring machines with rolling element bearings. However, as explained later in this discussion, a method called REBAM® is available from Bently Nevada Corporation that allows vibration measurements directly at the bearing outer ring, which contains the outer race. This direct measurement greatly enhances bearing vibration data, and in some cases, this is the only measurement that can provide adequate vibration information.(Reference Hansen, J. Seven and Harker, Roger G., ”A New Method for Rolling Element Bearing Monitoring in the Petrochemical Industry,” Presented at the Vibration Institute Seminar, New Orleans, Lousiana, June 1984.) Shaft relative vibration measurements (i.e., proximity probe) are also useful when clearances increase during failure and for observation of rotor problems that are not related to bearings. A classical characteristic of rolling element bearings is the generation of specific vibration frequencies based on the bearing geometry, number of rolling elements and the speed at which the bearing is rotating Click to lik to read terms of frequencies in vibration. (Reference Foiles, Bill, “Rolling Element Bearing Frequencies,: Edited by Bently Nevada Corporation.)
The most prominent of these characteristic bearing frequencies are the Outer Race Element Pass frequency, Inner Race Element Pass frequency, Element Spin frequency, and the Fundamental Train frequency or Cage frequency. These vibrational components are generated even in a new bearing, but the amplitudes are small. Bently Nevada defines the frequency range from the Outer Race Element Pass frequency (1EPx) to seven times this value (7EPx) as the Prime Spike frequency region. This range contains the Inner and Outer Race Element Pass and Element Spin frequencies, and is therefore a valuable region to monitor bearing condition. The Cage frequency lies below 1/2 rotor speed (for a stationary outer ring) and cage damage would therefore show up in the Rotor frequency region, defined below. Distinguishing between these two regions enhances the ability to determine if a vibration increase is caused by a failing bearing or a rotor-related malfunction (imbalance, misalignment, fluid induced instability, etc.). It should be kept in mind that, from a plant manager’s point of view, it is much more important to determine when a bearing needs to be replaced to avert a machine failure and unnecessary downtime, than it is to determine what components within the bearing are being damaged. The primary goal of a rolling element bearing monitoring system is to satisfy this need. The secondary goal is to provide data that is appropriate for diagnosing the failure of the bearing with the purpose of determining the root cause (improper mounting, lubrication, loading, etc.) so that similar failures can be avoided in the future.