Bearing failure causes a high percentage of electric motor breakdowns.
Good bearing maintenance is essential for maximum bearing life. The
Troubleshooting
maintenance schedule, the right lubricant, and the amount of lubricant should fit the needs of the motor and its running conditions. (If there is frequent bearing failure, adjusting one or more of these three factors may be all that’s necessary.)
Bearings should be replaced if a motor is disassembled for any reason (if it has been in service for a year or more). The cost of new bearings is minimal when compared to the cost of downtime or the cost of replacing or rewinding the motor.
Bearings are manufactured in a sterile, dust- and lint-free, airconditioned, low-humidity environment. Cleanliness is very important when handling ball bearings. (A grain of sand or a few iron fillings can destroy a bearing prematurely.) It should be wrapped in a grease-compatible plastic as soon as it is removed from the motor’s end bracket.
All motor components, tools, and work surfaces should be clean before a new bearing is unwrapped. Immediate motor assembly and bearing installation is recommended. If this is not possible, the bearing should be rewrapped.
A new bearing should never be washed. If the bearing is submerged in oil for heating/installation, the oil must be clean or (preferably) new.
Motor Storage Concerns
Motors should be stored in a dry, clean, vibration-free environment. Humid weather can cause condensation inside a motor. Condensation can be minimized by maintaining the storage room temperature 15 to 20 degrees above the outside temperature.
DC motors develop commutator corrosion in a high-humidity environment. In addition, brush boxes rust, resulting in stuck brushes. Remove the brushes from their boxes and relax the spring tension (for longterm storage).
‘TUrn the motor’s shaft a few revolutions at least once a month—daily if there is a vibrating problem. This redistributes the grease and recoats the contact surface between the ball and race.
Ball bearings eventually force their way through grease and make metal-to-metal contact with the races. (If there is vibration, the metal-tometal contact accelerates, and destructive wear is certain.) The balls wear the races, creating wear spots equal to the ball spacing. This is called false brinelling. Bearings with this problem are noisy and will fail prematurely, even if the motor is new.
Motors in service can also have false brinelling if they sit idle for long periods in a vibrating environment. TUrning the shaft or running the motor every day will minimize this problem.
Bearing Failure Causes
Mechanical causes of bearing failure include vibration, excessive belt tension, misalignment, housing or shaft distortion, wrong internal clearance, and preloading from axial thermal growth of the shaft.
Vibration
Many early bearing failures are caused by out-of-balance motor and/or load components. Motors should be balanced to accepted specifications.
A vibration check should be made on the mounting base of a motor. The base shouldn’t vibrate more than one-third of the acceptable balance specified for the motor itself. Keeping a history of all vibration tests is recommended.
Excessive Belt Tension
Belts should be tightened only enough to keep them from slipping. Formulas for this recommend about 1/64 inch of up-and-down belt movement per inch between shaft centers. Worn grooves in the drive pulley require excessive belt tension.
Misalignment
Misalignment is a frequent cause of vibration and preloading-related bearing failure. Although couplings are built to flex and accommodate some misalignment, it should be kept to a minimum. Laser alignment is a very precise method of alignment.
Housing or Shaft Distortion
When the shaft or housing is distorted, the affected bearing race will distort accordingly. The bearing will run hot and will fail prematurely. Check both the shaft and the housing if there is frequent bearing failure.
Wrong Internal Clearance
Bearings have internal clearance that allows for two factors: shaft expansion (from heat) and interference fit (or press fit, which keeps the bearing race from slipping). The extent of shaft (and housing) expansion is related to the
motor’s enclosure, size, and cooling method. The amount of interference fit varies from motor to motor.
Heat develops in the squirrel cage rotor and travels to the shaft, causing it to expand. The end bracket (end bell) dissipates this heat through the bearing’s outer race. (The motor’s cooling fan gives the end bracket a different expansion rate than that of the shaft.)
The expansion of the shaft and the pressed fit of the end bracket must be absorbed by the bearing’s internal clearance. Motor manufacturers select bearings that fit these requirements. Replacement bearings must have the same specifications as the original ones.
Bearings are manufactured with clearance ratings that allow for expansion (for example, CA). If there isn’t enough clearance, the bearing will run hot, causing the grease to fail.
Smaller motors (fractional to 10 horsepower) often have excessive clearance in either the shaft fit or the housing fit. The bearing, which is much harder than either the shaft or the housing, will slip or spin while the motor runs and will wear away material from these components. Over time, a very loose fit develops. This allows the rotor to drag on the stator.
To prevent this problem, an epoxy developed for this purpose can be used (Fig. 7.18). The epoxy remains resilient and allows for expansion, but keeps the bearing race from moving.
FIGURE 7.18 An epoxy designed to keep a bearing race from rotating. Locktite Corp.
Axial Shaft Expansion
Axial expansion occurs as a motor comes up to its running temperature. If the expansion isn’t allowed for, preloading becomes a problem. Preloading causes the bearing to overheat and fail prematurely.
The amount of axial expansion is related to the length of the shaft. Thrust washers (made of spring steel) are used to absorb the axial expansion in some motors.
Other motors have hubs that hold the shaft-end bearing captive. The opposite bearing must have room for the shaft’s axial expansion. (This can be checked after the motor is completely assembled.) Loosen the hub opposite the shaft. If the hub cover moves outward when the bolts are loosened, the bearing is preloaded. Shims can be used to respace the hub cover. If spring steel thrust washers are used, they shouldn’t be completely compressed. Some motors have a smaller bearing on the end opposite the shaft. Manufacturers do this to cut costs.