Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/339575
EQUIPMENT MANAGEMENT 13 AGGREGATES MANAGER July 2014 It's also important to select the right grease when relubricating the bearings. e motor nameplate or manual will specify which type of grease should be used. Diff erent greases contain diff erent soap bases and additives, and are designed for specifi c applica- tions. ey also have diff erent lubricating characteristics. When diff erent greases are mixed, a reaction can take place between incompatible ingredients, altering the characteristics of the grease and interfering with its ability to lubricate properly. is can o en result in a catastrophic bearing failure in only a few hours. In an at- tempt to provide a guide for mixing greases, see Table 2. If greases must be mixed, the grease manufacturers should be contacted to determine whether the greases are compatible. While failures may occur due to lack of lubrication, bearings may also fail due to grease that is contaminated by water or other materials. In severe or extreme applications, bearing seals should be considered. e amount of water or dirt present o en will determine if a sha slinger, contact seal, or non-contact seal is required. Installation of a seal on the motor bearings may dramati- cally extend the life of the motor. Routine vibration measurements of the entire equipment train should be taken at regular intervals so that problems can be found well in advance of a bearing failure. e frequency at which these measurements are taken will depend on the importance of the equipment being monitored. e foundation and equipment base should be checked regularly for movement or looseness. Stator winding Another common cause of motor failures is stator winding failures. To ensure long motor life, it is important the motor operate within the tempera- ture class of its insulation system. e motor should be kept clean and free of particle build up on the frame surface, air inlet, and fans. Totally enclosed fan-cooled (TEFC) motors depend on heat transfer from the frame to the ambient air in order to conduct heat out of the stator winding. A build up of dirt and particles on the frame surface will increase the motor's temperature rise. Exces- sive motor temperatures may lead to premature winding failures. ere are several simple tests that can be performed to detect and prevent premature failure of your motor winding. First, motor current can be measured to determine if a motor is overloaded. Measuring current is performed with a portable clamp-on cur- rent transformer with an appropriate voltage insulation level. If the equipment has a panel-mounted amp meter, that instrument may be used. Current levels should be less than, or equal to, the nameplate values. Current levels in excess of the nameplate rating should be reviewed. Measurement of voltage imbalance is the second test. Voltage imbalance between phases may increase mo- tor temperature and cause the motor to exceed rated temperature. For most industrial installations, voltage measurements at the motor starter are close enough to the motor so that measurements are satisfactory. e phase-to-phase voltage of all three phases should be measured. Each measured value should be within 10 percent of the motor's nameplate voltage, and all measured values should be within 1 percent of each other. If voltages are not within 1 percent of each other, the voltage motor horsepower should be derated per Graph 1. Acceptable Fair Rough, Needs Attention Velocity (in/sec) Velocity (in/sec) Velocity (in/sec) <0.15 .015 to 0.22 >0.22 Bracket vibration limits Aluminum Complex Barium Calcium Calcium 12-hydroxy Calcium Complex Clay Lithium Lithium 12-hydroxy Lithium Complex Polyurea Aluminum Complex X C C Barium X C Calcium X C C C B C Calcium 12-hydroxy C C C X B C C C C Calcium Complex B X C C Clay C C X Lithium C C X C C Lithium 12-hydroxy B C C X X Lithium Complex C C C C C C X Polyurea C X Grease mixture compatibility C = Compatible, B = Borderline Table 3 Table 2