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.
Vibration Characteristics of Rolling Element Bearings
The vibrations produced by machines with rolling element bearings can be separated into three frequency regions. (See example on page 8 for calculation of monitor filter ranges for rolling element bearing applications).
1. Rotor Vibration Region
Rotor-related vibrations normally occur in the range of 1/4 to 3 times shaft rotative speed (1/4X – 3X) and are best measured in terms of velocity or displacement. Many rolling element bearing failures are the direct result of a rotor-related malfunction (e.g., unbalance, misalignment, or rotor instability). Rotor-related malfunctions must be corrected to eliminate bearing overloading and subsequent failure. Most general purpose equipment with speeds from 1200 to 3600 rpm generate rotor-related vibration signals between 10 and 500 Hz (600 cpm to 30 kcpm). It is, therefore, imperative for diagnostics to monitor this frequency range in order to determine when/if a bearing failure is caused by a rotor-related malfunction. Without this data, the rotor-related malfunction would remain undetected, and bearings will continue to fail and need periodic replacement.
It should be mentioned here that the Rotor Vibration region is not exclusive to rotor-related vibration components. Bearing-related frequencies can also occur in this region. A damaged cage can produce vibrational components below 1/2 shaft rotative speed. In addition, studies have shown that with the REBAM® monitoring system, spalling on the inner race can produce signal components in the Rotor Vibration region.
2. Prime Spike Region
The second vibration frequency region to monitor for machines with rolling element bearings is the Prime Spike (element passage) region. As previously mentioned, a rolling element bearing generates characteristic frequencies based on its geometry and speed. Prime Spike is a term used by Bently Nevada to describe a vibration frequency range which includes those bearing frequencies that are generated by the rolling elements traversing either an inner or outer race flaw. This frequency range is normally 1 to 7 times the Outer Race Element Passage frequency (1-7 EPx). Vibrations in this range can be measured effectively in terms of acceleration, velocity or displacement. Field studies indicate that approximately 90% of all bearing failures are related to either an inner or outer race flaw. The other 10% are related to either a rolling element flaw, which produces vibrations in the Prime Spike region in acceleration and velocity signals, and the Rotor region in REBAM® signals, or a cage flaw which produces vibration in the Rotor region. By establishing a frequency band around the predominant bearing failure frequencies and filtering out the rotor-related vibration frequencies, it is possible to gain improved monitoring of bearing condition.
3. High Frequency (Spike Energy) Region
The third region is the high frequency (Spike Energy) region. This region covers frequencies from 5 kHz to approximately 25 kHz and is measured in terms of acceleration. If high frequency region measurements are used for bearing failure detection, they should be used as a supplement to measurements made in the Rotor-Vibration and Prime Spike regions. High frequency measurements have two primary uses:
1. High frequency signals occasionally provide an early indication of a bearing problem at the prefailure stage. Care must be exercised because “self-peening” of bearing flaws results in decreasing readings in this frequency region as a bearing failure progresses.
2. High frequency signals are useful to help detect certain other machine malfunctions such as cavitation, rubs, steam or gas leaks, valve problems, blade passage or gear mesh problems. High frequency vibration energy attenuates very rapidly with distance from the source. This can be both bad and good. Bad, in that one needs to be very close to the source to obtain data; good, in that the localized nature of the vibration can be used to isolate the source of a problem.
Based on observation of many rolling element bearings in the field by Bently Nevada Corporation and many of our customers, most of the information on the performance of rolling element bearings and warning of their failure occurs in the Prime Spike region (1-7 EPx). Information about rotor behavior generally occurs in the region between 1/4 X and 3X times rotative speed. Information at very high frequencies (8 EPx and higher to the megahertz region) may contain very early warning information, as well as other data concerning machinery condition (e.g., rubs, gear noise, cavitation, valve noise, etc.).
Causes of Failure in Rolling Element Bearings
A rolling element bearing has a finite life and will fail due to fatigue, even if operated under ideal design conditions. Rolling element bearing manufacturers realize this fact and have developed design life limits (L10/B10) to let users know how long a bearing should last when installed and operated within design limits. L10/B10 is defined as the rating life of a group of apparently identical rolling element bearings operating under identical loads and speeds with a 90% reliability before the first evidence of fatigue develops. Unfortunately, most “real world” installations are not under ideal conditions and the bearings prematurely fail well before reaching their design life. Most premature bearing failures can be attributed to one or more of the following causes:
1. Excessive loading
- a. Steady-state (e.g., misalignment or static
- b. Dynamic (e.g., unbalance or rotor system instability)
2. Improper lubrication (insufficient or excessive)
3. External contamination
4. Improper installation
5. Incorrect sizing (e.g., wrong design)
6. Exposure to vibration while not rotating (false brinelling)
7. Passage of electric current through the bearing
When analyzing premature rolling element bearing failures, it is important to not only determine that the bearing is failing, but also to determine the underlying cause of that failure. The above list includes the major causes of premature bearing failure and can be used as an initial guide to determine the reason for a bearing failure. To ensure success, elimination of premature bearing failures must be a major goal of any predictive maintenance program.
A rolling element bearing progresses through three failure stages:
1. Prefailure
2. Failure
3. Near Catastrophic/Catastrophic
Note: Each of these different failure stages exhibits specific vibration characteristics which require specific diagnostic/ monitoring techniques.
Prefailure Stage — During the prefailure stage, the bearing is in the earliest stages of failure. It develops hairline cracks or microscopic spalls that are not normally visible to the human eye, since most of the early damage occurs below the surface of the race. During this stage there may be an increase in the high frequency (7 EPx) vibration produced by the bearing. If temperature or Prime Spike vibration measure-ments are taken during this stage, the levels will be normal. At this stage, the bearing usually has a significant amount of safe operating life left, and it is not economical to replace it at this time.
Failure Stage — During the failure stage, the bearing develops flaws that are visible to the human eye. At this stage, the bearing usually produces audible sound, and the temperature of the bearing will rise. Vibration amplitudes in the “bearing-related” range (Prime Spike) reach easily detectable levels. Once the failure stage is reached, it is necessary to either change the bearing or increase the frequency of monitoring to estimate how long the bearing will safely operate before causing a catastrophic machine failure. This stage is considered the economical time at which to replace the bearing. If the bearing is not removed during the failure stage, it will eventually enter the final progression of failure, the near catastrophic/catastrophic stage.
Near Catastrophic/Catastrophic Stage— When the bearing enters this stage, rapid failure of the bearing is imminent. Audible noise produced by the bearing significantly increases, and the bearing temperature increases until the bearing overheats. Rapid wear causes the bearing clearance to increase, which then allows significant shaft motion relative to the bearing. Since a rolling element bearing is designed to restrict shaft motion, it can be very dangerous to allow the bearing to reach this stage due to the probability of creating a rub within the machine. Bearing-related (Prime Spike) vibration amplitude levels will show significant increases in this stage. High frequency vibration data may be unreliable in this stage and caution should be used in its interpretation. Due to “self-peening” of the bearing flaws, high frequency amplitude levels often decrease during this stage, and it can appear that the bearing is in an earlier stage of failure. The occurrence of this “self-peening” phenomena is especially true for low speed machines.