Keeping Motors Running
Reliable motor performance contributes to effective power management and energy efficiency
"For want of a nail, the shoe was lost,” goes the adage by Benjamin Franklin, which goes on to offer a warning about the way failing to address small but essential tasks can undermine larger goals.
The warning can apply to the efforts of many institutional and commercial organizations to streamline their power-management efforts and improve energy efficiency. Specifically, organizations might want to pay closer attention to motor maintenance to give them a better chance of achieving their larger efforts.
By developing and fine-tuning preventive maintenance programs for their facilities’ motors, managers can ensure equipment failures don’t get in the way of energy efficiency.
Repairs to the housing generally address mechanical damage, whether from internal misalignment of the rotor due to bearing failure, external misalignment due to abrupt shock from a gearbox or other connected load failure, or direct external damage to the housing. While not the most common occurrence, such damage does happen.
Most housings have three parts: the two end bells containing the bearings the shaft turns in, and the cylindrical cover between the two end bells. Axially mounted bolts hold the three parts together. Before disassembling them, the technician should scribe the cylinder and each bell with a mark across the separation line to be able to reassemble them in exactly the same relationship. This precaution ensures a good fit and easier assembly.
If the damage is minor, the technician can reshape the steel housings with a ball-peen hammer. If the damage is major or the housing is cast iron, the only solution is to replace the parts.
Several symptoms occur when bearings or bearing seals are bad. Grease or oil from inside the bearing appears outside the housing around the shaft if the outer seal is broken. Noise and vibration, sometimes accompanied by localized heat at the bearing location, occurs if the bearing is over-lubricated or if the races or rollers inside the bearing are beginning to deteriorate.
If over-greasing, dirt, or moisture is a problem, disassembly, cleaning, and repacking with new grease and seal replacement usually is sufficient. If, while the bearing is out the housing, visual inspection shows cracking or fine metal particles present, a broken retainer ring or severe rust, the technician should replace the bearing.
When no visual indications appear, another method to detect a bad bearing is to rotate the inner race very slowly with one hand while holding the outer race in the other hand. A sudden resistance or hitch in the movement indicates a particle of dirt or possibly a metal cutting has lodged in the clearance between the rollers and the races.
The technician should reclean and recheck the bearing. If the problem is metal cuttings, which will be picked up by a small magnet, the bearing should be replaced. At normal motor speeds, it will not last much longer.
Technicians also should replace seals any time they remove them because the seals are opened slightly when sliding along the shaft. Any shaft roughness will cause tiny tears in the sealing material.
The motor-base portion of the housing occasionally might develop a cracked foot where the anchor-bolt hole is located, so it needs to be welded. Some frames have a built-in screw adjustment for moving the frame to correct the center distance from the driven shaft on belt-drive loads.
These screws have lock nuts that can loosen from severe vibration. Checking and tightening should always take place when working on other motor problems.
Couplings, Pulleys or Sheaves
Shaft couplings connect the motor shaft directly to the driven shaft. Both shafts should turn on the same center line if they are properly aligned. Pulleys and sheaves are used for belt-driven loads using flat and V-belts, respectively.
Depending on the type of coupling — chain, bolted and bushed, gear, flexible steel, or rubber insert — the technician will need to replace or periodically lubricate the wearing element. The frequency depends on the severity of service.
Before assembling the coupling with the new element, the technician should check the alignment with a precision rule or dial indicator, depending on the required precision.
Pulley and sheave alignment are important to motor life, pulley and sheave life, and belt life. Misalignment can cause vibration, which is harmful to motor life and causes rubbing or lateral pressure on the belt. This situation will result in slipping, rapid sheave or pulley wear, and rapid belt wear.
A simple alignment check on pairs of pulleys involves stretching a line taut across the face of the driven pulley so it touches the rim on both sides of the shaft. If the motor pulley is aligned correct, the line also will touch its rim.
This method can detect misalignment, either because the shafts are not parallel or because one pulley is in a different parallel plane than the other. It also will indicate the amount and type of pulley adjustment to realign the two pulleys.
Coils and Windings
Depending on the motor type, electromagnetic coils and windings located in the stator or rotor can be open, totally enclosed, or encapsulated in plastic. Dirt, moisture and heat cause most electrical failures in motors, so the preventive maintenance program should begin with regular cleaning, inspection, and testing for good reliability.
Cleaning stator or rotor windings consists of blowing out open motors with compressed air to remove dirt, dust, and water vapor, cleaning them with a solvent, and painting or varnishing them. In addition to lengthening insulation and wire life, this practice gives technicians an opportunity to visually inspect the motor’s internal circuit.
In doing so, the technician can identify and correct any major problems, such as loose wires, as well as physical damage, such as cracked insulation, heat-darkened wire, or loose coils. But some flaws and impending problems will escape visual inspection. So motors also require periodic testing to uncover these hidden problems. It is much more effective to do this testing before a shutdown has occurred because of undetected repair needs.