Pumps and Systems, February 2007
When evaluating bearing distress, the babbitted shoe surface is commonly the only area that is examined. Although a great deal of information can be extracted from the babbitt appearance, additional information exists elsewhere.
These "secondary sources" of diagnostic information often prove to be very valuable, since the babbitted surfaces are usually destroyed in a catastrophic bearing failure. Even a bearing wipe, which is the most common appearance of distress, may hide valuable information.
The "textbook" cases of distress modes are especially useful in diagnosing problems prior to the damage that occurs when a bearing can no longer support an oil film. Through the prudent use of temperature and vibration monitoring equipment, routine oil analyses, lubrication system evaluations and machine operational performance reviews, bearing distress may be identified and evaluated before catastrophic failure occurs.
Bearing health is commonly monitored through the use of temperature measurements. Be aware that temperature sensors may be mounted in a wide variety of locations, with a corresponding variation in temperature. The specific location and type of sensor must be known in order for the measured temperature data to have any real value.
Beginning An Evaluation
To start, the bearing assembly should be completely disassembled so that all of the bearing components may be evaluated. Do not clean the bearing, since valuable information may be lost.
Figure 1. Thrust and journal bearing part schematic .
Base Ring
Examine the base ring. During routine operation, the lower leveling plates may form indentations in the base ring, on either side of the dowels that locate them. The indentations should be identical and barely noticeable. Deep, wide indentations are an indication of a high load. The rocking strip on the bottom of the lower leveling plates contacts the base ring, and its condition presents another indication of bearing load.
The cleanliness of the bearing and oil can also be determined, since deposits are often trapped in the base ring. Evidence of water contamination, particularly in vertical machines, may go unnoticed unless the base ring is examined.
Leveling Plates
The spherical pivot in the rear of each thrust shoe rests in the center of a flat area on the hardened upper leveling plate. This flattened area is susceptible to indentation due to the point contact of the pivot. The indentation is easily identified by a bright contact area. This area indicates where the shoe operates on the upper leveling plate, and its depth gives an indication of load. Close examination of the upper leveling plate near the contact area may also produce evidence of electrical pitting.
The upper leveling plates interact with the lower leveling plates on radiused "wings." The upper leveling plates are typically hardened; the lowers are not. When new, the leveling plates have line contact. There is little friction between the wings, and the bearing can react quickly to load changes. Depending on the nature and magnitude of the thrust load, the wing contact areas will increase in time. The contact region of the wings, again noted by bright areas, will normally appear larger on the lower leveling plates. If the rotating collar is not perpendicular to the shaft axis, the leveling plates will continuously equalize, causing rapid wear.
Shoe Support
The shoe support is the hardened spherical plug in the rear face of each thrust shoe. Based on the magnitude and nature of the thrust loads, the spherical surface will flatten where it contacts the upper leveling plate. The contact area will appear as a bright spot on the plug. If evidence of hard contact exists (a large contact spot), rest the shoes (pivot down) on a flat surface. If the shoes do not rock freely in all directions they should be replaced.
The pivot can also appear to have random contact areas, indicating excessive end play, or it may be discolored, indicating lack of lubrication.
Shoe Body
The shoe body should be periodically examined for displaced metal or pitting. Indentations routinely occur where the shoe contacts the base ring shoe pocket in the direction of rotation. Displaced metal exhibiting a coarse grain may indicate erosion damage; bright or peened spots may indicate unwanted contact. Depending upon the shape of the individual pits, pitting may indicate corrosion or undesirable stray shaft currents.
Shoe Surface
When evaluating the shoe surface, the first step should be to determine the direction of rotation. This may be accomplished by evaluating:
- Abrasion scratches
- Discoloration (75-75 location)
- Babbitt flow
- Babbitt overlay
- Thrust shoe/base ring contact
Use caution when evaluating babbitt overlay (babbitt "rolled over" the edges of the shoes), since it may appear on both the leading and trailing shoe edges.
Normal
A healthy shoe will exhibit a smooth finish, with no babbitt voids or overlays. The dull grey finish of a brand new shoe may remain unchanged after many hours of operation, or it may appear glossy in spots or in its entirety. Routine thermal cycling of the bearing may cause the emergence of a mild "starburst" or mottled pattern in the babbitt. This is harmless, providing the shoe is flat and cracks do not exist.
Scratches
Abrasion
A bearing surface exhibiting circumferential scratches is the result of abrasion damage (see Figure 2). Abrasion is caused by hard debris, which is larger than the film thickness, passing through the oil film. The debris may embed itself in the soft babbitt, exhibiting a short arc on the shoe surface, ending at the point the debris becomes embedded. Depending on the debris size, the scratch may continue across the entire shoe surface.
Figure 2. Thrust shoe surface abrasion.
Abrasion damage becomes worse as time progresses. Surface scratches allow an escape for lubricating oil in the oil wedge, decreasing the film thickness. This will eventually lead to a bearing wipe.
Another source of abrasion damage is a rough journal, collar or runner surface. Roughness may be due to previous abrasion damage. It may also be from rust formed after extended periods of down time. New bearings should not be
