Machinery's Handbook, 31st Edition
2658 Electric Motor Maintenance 2) Brushes. Check brushes in holders for fit and free play. Check the brush-spring pres sure. Tighten brush studs in holders to take up slack from drying out of washers, making sure that studs are not displaced, particularly on DC motors. Replace brushes that are worn down almost to the brush rivet, examine brush faces for chipped toes or heels, and for heat cracks. Damaged brushes should be replaced immediately. 3) Commutators. Examine commutator surface for high bars and high mica, or evidence of scratches or roughness. See that the risers are clean and have not been damaged. 4) Ball or roller bearings. On hard-driven, 24-hour service ball- or roller-bearing motors, purge out old grease through drain hole and apply new grease. Check to make sure grease or oil is not leaking out of the bearing housing. If any leakage is present, correct the condition before continuing to operate. 5) Sleeve bearings. Check sleeve bearings for wear, including end-play bearing surfaces. Clean out oil wells if there is evidence of dirt or sludge. Flush with lighter oil before refilling. 6) Enclosed gears. For motors with enclosed gears, open drain plug and check oil flow for presence of metal scale, sand, or water. If condition of oil is bad, drain, flush, and refill as directed. Rock rotor to see if slack or backlash is increasing. 7) Loads. Check loads for changed conditions, bad adjustment, poor handling, or control. 8) Couplings and other drive details. Note if belt-tightening adjustment is all used up. Shorten belt if this condition exists. See if belt runs steadily and close to inside (motor edge) of pulley. Chain should be checked for evidence of wear and stretch. Clean inside of chain housing. Check chain-lubricating system. Note inclination of slanting base to make sure it does not cause oil rings to rub on housing. Annual or Biannual Inspection.— 1) Windings. Check insulation resistance by using ei- ther a megohmmeter or a voltmeter having a resistance of about 100 ohms per volt. Check insulation surfaces for dry cracks and other evidence of need for coatings of insulating material. Clean surfaces and ventilating passages thoroughly if inspection shows accu mulation of dust. Check for mold or water standing in frame to determine if windings need to be dried out, varnished, and baked. 2) Air gap and bearings. Check air gap to make sure that average reading is within 10 percent, provided reading should be less than 0.020 inch (0.51 mm). All bearings, ball, roller, and sleeve should be thoroughly checked and defective ones replaced. Waste- packed and wick-oiled bearings should have waste or wicks renewed, if they have become glazed or filled with metal or dirt, making sure that new waste bears well against shaft. 3) Rotors (squirrel-cage). Check squirrel-cage rotors for broken or loose bars and evi dence of local heating. If fan blades are not cast in place, check for loose blades. Look for marks on rotor surface indicating foreign matter in air gap or a worn bearing. 4) Rotors (wound). Clean wound rotors thoroughly around collector rings, washers, and connections. Tighten connections if necessary. If rings are rough, spotted, or eccentric, refer to service shop for refinishing. See that all top sticks or wedges are tight. If any are loose, refer to service shop. 5) Armatures. Clean all armature air passages thoroughly if any are obstructed. Look for oil or grease creeping along shaft, checking back to bearing. Check commutator for surface condition, high bars, high mica, or eccentricity. If necessary, remachine the commutator to secure a smooth fresh surface. 6) Loads. Read load on motor with instruments at no load, full load, or through an entire cycle, as a check on the mechanical condition of the driven machine.
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