Chemical Processing - August 2007 - (Page 30) >> Hybrid design Figure 2. Use of ceramic balls or rollers effectively insulates the bearing from the inside. Type of lubrication. Under normal speed and temperature conditions, the bearings in electric motors usually are lubricated with grease. Grease allows for simpler morecost-effective housing and sealing designs, offers better adhesion of lubricant to critical surfaces, and provides more reliable protection against contaminants than oil. The life expectancy of grease depends upon several factors, including the type of bearing, the type of grease, the orientation and speed of the motor, and the operating temperature of the bearings. Small ball bearings in standard electric motors usually are tted with seals or shields and lubricated for life — they aren’t intended to be relubricated but replaced at normal motor repair intervals. Severe-duty motors, regardless of size, often are supplied with open bearings and provisions for regreasing. (If the life of the grease is shorter than the expected life of the bearing, the bearings obviously will need to be relubricated while the grease is still performing as intended.) Sometimes, rotational speeds or operating temperatures make it impractical or impossible to use grease because the grease life or relubrication times are too short. These cases demand oil lubrication. In general, only large electric motors are oil-lubricated, in part due to the need for more sophisticated seals and the potential risk of leakage from the systems. Tips for longer life When motors fail, bearings may be the culprit but possible non-bearing causes also abound. These encompass windings, wiring, grease or seal failures, which, in turn, may result in bearing failures (although bearings are not the root cause). Improper motor use and inadequate maintenance can add to potential problems and premature bearing failure. Here are some potential problems along with pointers on how to avoid or address them: 30 • August 2007 Electric arcing. In variable-speed motors, stray currents from arcing may cause bearing damage. Although arcing typically tends to be isolated and localized, the effect on a bearing is almost like a series of small lightning strikes that melt and retemper internal bearing surfaces. The result is that some surface material akes away and spalls out to create noise in the bearing and potentially shortened service life. Users can be sure that bearing damage from electric arcing has occurred when they notice characteristic “ uting” patterns (Figure 1). Fluting is caused by the dynamic effect of the rolling elements continually moving over the microcraters and etching a rhythmic pattern into the running surfaces of a bearing’s races. Noise and vibration from the bearing increases and, eventually, the deterioration will lead to complete bearing failure. Even if a bearing isn’t affected by these discharges, its lubrication could be. Grease composition can degrade rapidly due to the high localized temperatures generated by current discharges. One way to head off arcing problems is to insulate the bearings from the shaft currents. Specialized ceramic coatings can be applied on the outside or inside diameter of the bearing to prevent currents from owing through the bearings. Hybrid bearing designs, which substitute ceramic balls or rollers for the metal rolling elements within a bearing, offer another solution (Figure 2). They effectively insulate bearings “from the inside.” Moisture. This can’t always be controlled but it can be managed. When motors are running, humidity usually isn’t harmful. However, when they’re turned off and cool, condensation builds up. Condensation can’t be stopped but using grease forti ed with rust inhibitors in bearing as- >> Proper lubrication Figure 3. Bearings require the right amount of grease as well as appropriate regreasing intervals. www.chemicalprocessing.com http://www.chemicalprocessing.com
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