Keeping Your Motor Running

A vital piece of equipment in today’s modern manufacturing processes, the electric motor is found in practically every type of application. Its simple and reliable operation makes it rather easy to overlook during everyday maintenance practices. Unfortunately, it’s often too late to employ minor repairs when an electric motor begins to show signs of neglect.

However, there are several standard maintenance tools and practices that are easy to implement to help keep electric motor rebuilds and replacements at a minimum.

First, it’s important to ensure new and rebuilt motors are in prime condition. Here are some critical steps.

Perform vibration checks. Make sure to perform vibration checks for defects. The recommended limit is no more than 0.05 in.-per-second overall (0-120,000 cpm).

When purchasing, request mechanically balanced motors. Always ask for mechanically balanced rotors, especially on applications using high-efficiency/high speed models. Acceptable limits can be found in the International Standards Organization (ISO) specifications. Specifications are provided in G-6.3 for lower horsepower (under 100 hp) or less severe motors, and G-2.5 for highhorsepower, vertical or critical motors.

Store motors properly. The best way to prevent moisture from entering the bearing cavity and causing corrosion before installation is to store motors with the bearings fully packed with grease. If your motors are lubricated with oil, make sure you discuss proper storage with your motor supplier.

Rotate stored motor shafts. Make sure new and rebuilt motors are in prime condition by rotating the rotor by hand every month. This will prevent fretting corrosion, a wear process that produces false brinelling — evenly spaced depressions in the bearing races that may appear to have been formed by indentations due to shock loading.

When you need to repair a motor that has bearing problems, it’s important to handle replacement bearings with care.

Store bearings in their original packaging. The best way to store bearings is horizontally in a dry place and in their original, unopened packages. Never place bearings on a dirty surface.

Rotate bearings while in storage. For greased bearings, periodically turn over sealed and shielded bearings to prevent the grease from settling on one side.

Replace with identical bearings. Use identical bearings for replacements. If needed, contact your authorized bearing distributor or manufacturer for interchange information.

Never wash new bearings. Never wash new bearings before installation. Bearing manufacturers take great care to package and ship bearings that are dirt-free and ready for lubrication. There’s really no need to wash them or remove the protective slushing compound.

If you need to replace electric motor bearings, ensure you use proper installation and repair practices.

Be sure to handle bearings with care. Never pound directly on a bearing or ring. If a bearing is dropped, it is best not to install it. If using a vise to assist mounting, protect the shaft with a sheet of soft metal.

Inspect bearings carefully. Inspect the shafts and housings for size and damage. Remove any nicks and burrs with emery paper. If shafts or housings show obvious signs of wear or damage, then replace or repair them.

Avoid overheating. Never bring a flame in direct contact with a bearing and never heat one beyond 446ºC (230ºF). During heat-mounted operations, place the bearing against the shaft shoulder until it cools and locks in place to ensure the bearing does not creep away from the proper position.

Pay attention to the bearing press fit. Ensure the bearing’s proper fit by applying even pressure to the outer race with a press. A bearing that has an outer diameter of less than 100 mm (4 in.) should be installed in this way. Uneven pressure on the ring without a bearing press will damage the raceways.

Use the right tools for the job. It is really important to use the right tools for the job. Using the right tools can lower the possibility of damaging the bearings and will certainly help speed the installation process. Talk with your bearing supplier for help in selecting the proper tools.

Finally, it’s important to adopt the best practices for both lubrication and condition monitoring of electric motors and their bearings.

Proper lubrication is critical. For grease-lubricated bearings, the lubricant does more than prevent wear; it protects rolling elements against rusting and helps seal out dust, dirt, waste and other atmospheric contaminants. Be sure to talk with a lubrication engineer about the recommendation that’s right for your operation, especially for those motors that require oil instead of grease.

Use the correct lubrication practices. Over 90% of electric motor bearing failures are due to overgreasing. Overgreasing also causes seals to burst, thus allowing grease to bypass the bearing and go straight into the motor windings.

Adopt stringent alignment limits. Another cause of electric motor failure is misalignment. There are many methods to use to align the driver and the component it is driving, but what is most important is to align to tight tolerances. Remember, the bigger and faster the motor, the tighter the tolerances need to be.

Follow good housekeeping practices. Don’t let dirt or process materials build up on the motor, as it can act as insulation and generate unnecessary heat that has a negative impact on bearing life.

Lastly, watch for danger signs and determine the causes of failures. Keep alert for the three sure signs of improper bearing lubrication: excessive noise, vibration and temperature. Assuring proper vigilance for these signs will go a long way in maintaining uptime. Bearings are built to last, so you need to question the cause of motor bearing failures. Frequent failures may point to an installation or lubrication problem. Ask your bearing supplier for help from its experts and include your lubrication engineer in identifying the root cause of problems and for preventing future failures.