Lubrication Of Electric Motor Bearings
Marilyn | March 1, 2008
Bearing failure or proper lubrication? The choice is yours.
Better lubrication practices could prevent the type of bearing damage that leads to costly premature motor failures in countless plants. How are you taking care of these crucial activities in your operations?
Proper lubrication of ball and roller bearings in electric motors is essential to their health. Grease reduces friction and protects the surface finish from rust during long idle periods and in unfavorable environmental conditions. It also transfers heat from the bearing and even helps protect the bearing from dirt and contaminants. Since bearing life—and, by extension, motor life—depends on proper lubrication, it’s important to use the right grease for the application and to re-lubricate bearings at the correct intervals.
Grease is a “dirt magnet,” so it’s surprising to many that packing it into the cavity around the bearing actually helps keep dirt and other contaminants from getting into this critical component.
On very old motors, lubrication was provided by oil-soaked felt that “wicked” oil to the bearings. Grease serves this function in today’s machines. Consisting of oil suspended in a base material like lithium, calcium or polyurea, it lubricates the bearing continuously while preventing the oil from leaching out. Depending on its composition, different greases may be better suited for one application than another. For example, one may be superior at high or low temperatures, another impervious to water, while still others retain oil better under extreme pressures.
The lesson here is to select the right grease for the application. An electric motor in an Arizona open-pit copper mine where the ambient temperature is 130 F requires different grease than an identical motor in the Arctic Circle.
Of course, it’s sometimes necessary to meet one stringent requirement at the expense of others. In the food process industry, for instance, the most important property of lubricants is that they won’t poison you if they somehow get into the can of beans you’re going to eat for supper.
An old professor of Texas history used to say, “Never mix gunpowder and alcohol, ’cause you can’t shoot it, and it tastes terrible!” Although it’s usually okay to combine lithium- and calcium-based greases, mixing lithium- and polyurea-based greases causes the oil to leach out much more quickly than normal, potentially starving the bearing of lubrication. Be sure you know which types of grease your plant uses—and know which ones are compatible with one another.
Table I provides general guidelines for grease compatibility, based on the variances in compatibility of different greases tested by the National Lubricating Grease Institute (NLGI), April 1983. Grease manufacturers often can provide similar charts.
Although compatibility guidelines are helpful, there are enough exceptions to warrant care. Before mixing two greases, check with both manufacturers. If both say it is all right to mix those specific greases, it probably is safe to do so. If either of them says no, don’t risk it (see Fig. 1). Note that in some instances both manufacturers may say it is safe to mix specific greases that are incompatible according to the general guidelines in Table I.
Types of grease in motor bearings
Some motor manufacturers have used polyurea-based grease—which performs well at high temperatures (over 250 F) and high speeds (10,000 rpm or higher)—almost exclusively for more than 30 years. Recently, though, several of them have switched to a second-generation polyurea grease that reportedly has even better properties than the old standby. Because these manufacturers produce tens of thousands of motors weekly, their decision to change grease is significant. Such a move indicates a high confidence level in that grease.
Bearing manufacturers, on the other hand, use various greases, depending on application requirements. As a result, the replacement bearings you buy from your local bearing supplier might not contain grease that is compatible with what you use in your plant. So, be careful.
Ultrasonic listening equipment, vibration analysis and thermography all can help predict bearing failures. But according to some sources, an operator tends to grease a bearing only when it “gets noisy enough that he can hear it” over the ambient sound of surrounding equipment. By that time, the damage has been done. Pumping in a few ounces of grease may mask the noise for a while, but it is too late to save the bearing.
Assuming you have a good predictive maintenance program and want to improve on preventive maintenance, how often should you grease the bearings in an electric motor? If you read the manuals for a dozen different electric motors, you’ll likely find 12 different recommendations.
Some of the factors that determine how often a bearing should be greased are:
- Operating hours
- Operating temperature
- Bearing size
- Bearing type (ball or roller)
- Cleanliness of environment
- Vibration levels
- Criticality of operation
One of the best charts for determining lubrication intervals is based on the bearing bore diameter, rpm, yearly operating hours and type (ball, roller, thrust, etc.). Unfortunately, this chart is not very practical. That’s because the person responsible for greasing the bearings usually doesn’t know the bearing sizes of every motor, and some motors have a different bearing size on each end.
Another drawback of this method is that each motor in a plant probably will have a different lubrication schedule—motors could be installed at different times, they could operate a different number of hours/year, their usage could vary with the seasons. It’s easy to see why something that sounds simple (e.g., “Grease the bearing every 4000 operating hours with 1.0 ounces of fresh grease”) may be hard to implement.
Various industries have tried to simplify the task by developing practical guidelines like those in Table II. Each represents a compromise, though, so none of them works for every situation.
One thing that bearings and motor windings have in common is the 10-degree rule. Every 10 C degree increase in temperature cuts their life expectancy in half. If a blanket of grease raises the winding temperature 20 C degrees, the winding will last only one-fourth as long as it should have. With an increase of 50 C degrees, a winding that should last 20 years would have a life expectancy of only about eight months. Unless you really enjoy changing motors in the middle of the night, try not to do anything that increases the motor temperature!
Now we come to the recommended procedure for greasing bearings. Under normal conditions, first remove the grease drain plug and wipe all the dirt and debris off of the grease fitting and the nozzle of the grease gun. With the motor running, pump fresh grease into the bearing while observing the old grease that is being forced out of the grease drain. When the purged grease looks fresh, stop pumping. Run the motor for at least 20 minutes to purge any excess grease and then replace the drain plug.
Caution: Remember that the shaft is rotating. The motor is coupled or belted to something, so there are lots of things to get hung up in. You probably need all your fingers, so work safely.
Some manuals say to “pump 0.8 ounces of grease into the bearing.” That sounds simple enough. Many operators know how many pumps it takes to deliver an ounce of grease, because they actually have checked. But, it is hard to determine if the passage between the grease fitting and the bearing is full of grease or empty. What if that precise 0.8 ounces of grease doesn’t even fill the grease passage?
Ultrasonic equipment affords a more reliable way to know when the grease reaches the bearing. While listening to the bearing, pump in fresh grease until the sound changes for the better. If you pump four tubes of grease into a 5 hp motor and still don’t see any grease coming out of the drain, please stop! Tell the boss what you did, and be prepared for him to yell a little.
If he’s fair, you’ll probably get the task of removing the motor, cleaning out all that excess grease (Fig. 2) and reinstalling the motor.
There are some good, low-tech ways that make it easier to do a good job. One way is to replace the drain plug with a low-pressure (0.5 to 1 psi) pressure relief fitting. That makes removing the drain plug or waiting for the grease to purge unnecessary.
For motors installed in out-of-the-way places, bearing suppliers sell another useful device—a small grease can powered by a watch battery that provides a regulated flow of fresh grease to the bearing (see Fig. 3). Simply screw it onto the pipe in place of the grease fitting. Be sure to write the date on it and replace it annually or semi-annually.
All the specialized equipment in use today around the world makes grease selection more complicated. Specialty applications like kilns or ovens may be good places for synthetic grease. Synthetic grease typically can handle 30 C-degree higher temperatures than conventional grease, but it’s not as suitable for high-speed operation. To avoid compatibility problems, be sure to identify all special cases.
Belted applications may require an extreme-pressure (EP) grease. It might be a good idea to identify these motors in some clear way—like painting the end bracket a different color from your other motors. The color won’t match the rest of the motor, but it will make it easier to identify a roller bearing that has a shorter relubrication interval and requires an EP grease. Be sure to tell your service center whether a motor is direct-coupled or belted when sending it out for repair.
Most premature motor failures result from bearing damage that may have been prevented with good lubrication practices. Choosing the right grease for the application and following the correct lubrication schedules and procedures will assure long, trouble-free motor life with a minimum of unscheduled downtime. It’s also important to avoid mixing incompatible greases and over-greasing. Finally, when sending a motor out for repair, make sure the service center motors knows what grease you use. MT
Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association (EASA), in St. Louis, MO. EASA is an international trade association of more than 2100 firms in 50 countries that sell and service electrical, electronic and mechanical apparatus. Telephone: (314) 993-2220; Web: www.easa.com