Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/388231
46 September - October 2014 | www.machinerylubrication.com characteristics of the Babbitt material make it susceptible to damage during installation. This also means the Babbitt can be easily destroyed if subjected to significant dynamic loading for long periods of time. The Babbitt will eventually crack and break off, leaving voids, as shown in Figure 2. Another common cause of bearing fail- ures is the machine operating at slow speeds either during startup or shutdown. For a bearing to work properly, the shaft surface must be moving at a sufficient speed to draw in the cool lubricant, pressurize it to form a hydrodynamic layer and expel it with any debris formed during the process. Figure 1 illustrates how hard particles can lead to bearing damage. This can occur with small particles at slow roll when the shaft and bearing surfaces are close together or with larger particles during normal operation. Figure 3 shows a journal bearing that was damaged during shutdown as the speed slowed to less than 200 rpm. For Babbitt to work properly, a small portion of the tin is expected to melt and be washed away by the lubricant. This creates channels around the harder parti- cles of antimony and copper, which actually support the shaft and carry the load. These channels allow oil to pass through them, cooling the surfaces and washing away any debris that was formed during normal operation. Case Study #1: Steam-turbine-driven Boiler Feed Pump A large pump experienced a bearing failure during normal operation. The shaft journal was 5.75 inches, and the typical bearing clearance was 7 to 9 mils. A post- mortem test was conducted to determine the possible cause of the failure. Figure 4 shows the bearing when it was removed from the pump. The bearing suffered from significant loss of overlay material, as seen in Figure 5. Melting along the edges of the overlay material can be seen in Figure 6. This likely was due to significant heating, although there was no thermal data indicating an event had occurred. This picture also shows the redeposition of melted material on the bearing surface. A highly magnified image of the bearing surface (Figure 7) from a scanning electron microscope (SEM) shows the surface microstructure of the Babbitt alloy. It includes copper needles and antimony cuboids embedded in the tin alloy. The shaft surface actually rides on the harder needles and cuboids, while the lubrication flows around them in channels within the tin. This allows for cooling of the bearing surface and the removal of any debris that might enter the bearing. Plant personnel believed electrical arcing was the root cause of failure. The study results found no evidence of electrical arcing but pointed toward overheating (likely from a loss of lubrication) as the primary cause. Since the bearing was severely damaged, an accurate diagnosis was difficult. This allowed for the possi- bility of a simple failure in the bond between the base metal and the Babbitt material as the root cause. Case Study #2: Fan Bearing Failure A large centrifugal fan (shown in Figure 8) was operating normally when the bearing metal temperature spiked to 212 degrees F. This was captured by the instrumentation and monitoring systems, initiating a critical alarm condition. Checks of the fan indicated that the bearing metal temperature spike occurred BEARING LuBRICATIOn FIGuRE 3. Bearing damage incurred during low-speed operation FIGuRE 4. Failed pump bearing FIGuRE 5. Loss of overlay (Babbitt) material down to the base metal FIGuRE 6. Melted Babbitt and material redeposition FIGuRE 7. A highly magnified view of the Babbitt surface Fatigue Cracks Cracks Propagate Cracks to Base Metal Material Loss Babbitt Base Material FIGuRE 2. Fatigue cracking caused by excessive hydrodynamic forces