Failure Analysis of Bearings
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Bearing Failure Analysis
Bearing Failure Analysis
Bearing Failure Analysis
Bearing Failure Analysis
This section will give the attendee a broad overview of failure analysis. The best way to gain expertise in this subject is to examine as many damaged bearings as possible. Although each failure is unique, evidence will emerge that often allows the determination of root cause, which will lead to the application of the proper corrective action to reduce or eliminate future failures.
In designing the bearing mounting, the first step is to decide which type and size of bearings to use. This choice is usually based on a certain desired life for the bearing. The next step is to design the application, with allowance for prevailing service conditions. Unfortunately, too many of the ball and roller bearings installed never attain their calculated life expectancy because of something done, or left undone, in handling, installation, and maintenance.
The calculated life expectancy of any bearing is based on four assumptions:
1. Good lubrication in proper quantity will always be available to the bearing.
2. The bearing will be mounted without damage.
3. Dimensions of parts related to the bearing will be correct.
4. There are no defects inherent in the bearing.
However, even when properly applied and maintained, the bearing may be exposed to one further cause of failure; fatigue of the bearing material. Fatigue is the result of shear stresses cyclically applied immediately below the load carrying surfaces, and is observed as spalling away of surface metal. Although spalling can be readily observed, it is necessary to discern between spalling produced at the normal end of a bearings useful life and that which is triggered by causes found in the three major classifications of premature spalling: lubrication, mechanical damage, and material defects. Most bearing failures are attributed to one or more of the following causes:
Defective bearing seats on shafts and in housings
Faulty mounting practice
Incorrect shaft and housing fits
Vibration while the bearing is not rotating
Passage of electric current through the bearing
Transportation, storage, and handling
The life of a rolling bearing is defined as the number of revolutions (or the number of operating hours at a given constant speed) which the bearing is capable of enduring before the first sign of fatigue occurs on one of its rings or rolling elements (flaking, spalling).It is, however, evident from both laboratory tests and practical experience that seemingly identical bearings operating under identical conditions have different lives. A clearer definition of the term life is therefore essential for the calculation of bearing size. All information presented by SKF on dynamic load ratings is based on the life that 90 percent of a sufficiently large group of apparently identical bearings can be expected to attain or exceed. This is called basic rating life, and agrees with the ISO definition. The median life is approximately five times the calculated basic rating life.There are several other bearing lives. One of these is the service life, which is the actual life achieved by a specific bearing before it fails. Failure is not generally by fatigue in the first instance, but by wear, corrosion, seal failure, mishandling, etc. Another is specification life. This is the life specified by an authority, based on hypothetical load and speed data supplied by the same authority. It is generally a requisite L10 (basic rating life), and is assumed that the authority has related the specification to experience gained with similar machinery, to obtain adequate service life.
Load-Path Patterns and their Meanings
There are many ways bearings can be damaged before and during mounting, and in service. The pattern or load zone produced by the action of the applied load and the rolling elements on the internal surfaces of the bearing is a clue to the cause of failure.
To benefit from a study of load zones, you must be able to differentiate between normal and abnormal patterns. The figure illustrates how an applied load of constant direction is distributed among the rolling elements of a bearing. The large arrow indicates the applied load. The series of small arrows show the share of this load supported by each ball or roller in the bearing.
The rotating ring will have a continuous 360 degrees zone, while the stationary ring will show a pattern of approximately 150 degrees. The figure illustrates the load zone found inside a ball bearing when the inner ring rotates and the load has a constant direction.
The figure illustrates the load zone resulting if the outer ring rotates relative to a load of constant direction, or where the inner ring rotates and the load also rotates in phase with the shaft.
Combined thrust and radial load will produce a pattern shown in the figure above. With combined load, the loaded area of the inner ring is slightly off-center, and the length in the outer ring is greater than that produced by radial load, but not necessarily 360 degrees. In a double-row bearing, a combined load will produce load zones of unequal length. The thrust-carrying row will have a longer stationary load zone. If the thrust is of sufficient magnitude, one row of rolling elements can be completely unloaded.
The load path shows uniform wear on both the inner and outer ring. Pure thrust (axial) load is rare. If axial load is present, it is usually accompanied by radial load.
Certain types of bearings can tolerate only very limited amounts of misalignment. A deep groove ball bearing, when misaligned, will produce load zones not parallel to the ball groove on one or both rings, depending on which ring is misaligned. The figure illustrates the load zone when the outer ring is misaligned relative to the shaft.
Here, the inner ring is misaligned with respect to the outer ring. Cylindrical roller bearings and angular contact ball bearings are also sensitive to misalignment, but it is more difficult to detect this condition from the load zones.
Misalignment is a common source of premature spalling, occurring when a shoulder is not square with the journal, or where a housing shoulder is out-of-square with the housing bore. Misalignment arises when two housings are not on the same centerline. A bearing ring can be misaligned even though it is mounted on a tight fit, yet not pressed against its shoulder causing it to be left cocked on its seat. Bearing outer rings in slip-fitted housings that are cocked across their opposite corners can also result in misalignment.
Using self-aligning bearings does not cure some of the foregoing misalignment faults. When the inner ring of a self-aligning bearing is not square with its shaft seat, the inner ring is required to wobble as it rotates. This results in smearing and early fatigue. Where an outer ring is cocked in its housing across corners, a normally floating outer ring can become axially held and can be radially pinched in its housing.
Roll the bearing rings on a flat surface and note the position of the wear patterns. Misaligned patterns will slalom back and forth across the raceway surface. Thrust loads will simply move the wear path to one side.
Distorted or out-of-round housing bores can radially pinch an outer ring. The figure above illustrates the load zone found in a bearing where the housing bore was initially out-of-round or became out-of-round by bolting the housing to a concave or convex surface. In this case, the outer ring will show two or more load zones depending on the type of distortion.
The figure is a picture of a bearing that had been mounted in an out-of-round housing that pinched the stationary outer ring. This is a mirror view and shows both sides of the outer ring raceway.
If the fit is too tight, the bearing can be internally preloaded by compressing the rolling elements between the two rings. In this case, the load zones observed in the bearing indicate that this is not a normal life failure. Both rings are loaded through 360 degrees, but the pattern will usually be wider in the stationary ring out-of-round where the applied load is superimposed most on the internal preload.
Some applications, such as shaker screens, polishing machines, and other vibratory sorters, employ a weight attached to the shaft to produce eccentric motion in the machine. Since the load rotates in phase with inner ring raceway, a stationary load zone results.
In unbalanced applications, the load does not rotate in phase with either ring, producing a load zone on both rings. This condition often produces creep if the outer ring is loosely fit. Fan applications are a common source of this load pattern.
Failure Mode Classification
1. Causes of Failures Have Identifiable Characteristics2. Failure Mechanisms Have Identifiable Failure Modes
3. Observed Damage Can Identify Failure Causes The primary cause of failure analysis is to identify the true cause of failure. Corrective actions and verification of success are impossible without this first step. This classification system is in the development stage, and may change significantly prior to ISO submission. The proposed system arose from a