Rolling-element bearings used in electric motors are at risk for various modes of failure if an incorrect maintenance or lubrication strategy is implemented. These include incorrect lubricant selection, contamination, loss of lubricant and overgreasing. This article discusses several effective strategies to minimize the likelihood of these failure modes.
Most electric motors are designed with grease-lubricated, antifriction, rolling-element bearings. Grease is the lifeblood of these bearings because it provides an oil film that prevents the harsh metal-to-metal contact between the rotating element and races. Bearing troubles account for 50 to 65 percent of all electric motor failures, and poor lubrication practices account for most of these bearing troubles. Proper maintenance procedures, planning and the use of the correct lubricant can increase productivity by reducing these bearing troubles and motor failures.
Get to know the failures. By knowing the failure modes, focus can be placed on reducing or even eliminating them.
Incorrect Lubricant - It is important to use the correct grease for specific applications. Regreasing with the wrong grease can lead to premature bearing failure. Most oil suppliers have grease that is specifically designed for electric motors, which is different from their multipurpose extreme purpose (EP) grease.
Grease Incompatibility - Greases are made with different thickeners, such as lithium, calcium or polyurea. Unfortunately, not all greases are compatible with each other, even those with the same thickener type. Therefore it is important to use the same grease or compatible substitute throughout the life of the bearing.
Motor Casing Full of Grease - If the grease cavity is overfilled, and high pressure from the grease gun is applied, the excess grease can find its way between the shaft and the inner bearing cap and press into the inside of the motor. This allows the grease to cover the end windings of the insulation system and can cause both winding insulation and bearing failures.
Lubricant Starvation - There are several possible causes of lubrication starvation. The first is insufficient grease being added during installation. The second is inappropriate, elongated relubrication intervals. The third involves the possibility that the oil has separated from the thickener base due to excessive heat.
Overpressurization of the Bearing Housing - Anytime there is an overpressurization of the bearing housing, stresses are placed on parts that were not designed to handle the pressure. Keep in mind that the standard manual grease gun can produce pressures up to 15,000 psi.
Overheating Due to Excess Grease -Too much volume will cause the rotating bearing elements to churn the grease, trying to push it out of the way. This results in parasitic energy losses and high operating temperatures, and increases the risk of oil separation and bearing failure.
To begin, a plan must first be in place. The following suggestions are the bare minimum that need to be discussed and implemented to get the program started.
Make an equipment list that includes all the assets needed in the program.
Verify the type of bearings and their seals that are installed in both the inboard and outboard ends of motors. This will determine if the bearings are regreasable. A policy should also be determined for the regreasing of shielded bearings, commonly found in motors. (Some experts recommend not greasing double-shielded bearings.)
Choose a grease type that will be adequate for the program. Remember that once a grease type and manufacturer are chosen, it's best to not deviate from this choice. If this grease is different from a grease previously used in the bearings, the previous grease will need to be cleaned out or flushed from the bearings and housings.
Make all necessary modifications to the electric motors. This includes adding fittings and making them accessible.
Establish a set of procedures for maintaining the motors.
There are many choices to make when deciding on a preventive maintenance (PM) system. In some plants it may be beneficial to use only a spreadsheet, while others have the need for complete dedicated systems. The end goal is the same. Each motor needs to be tracked as an asset, accomplished by noting the attention each motor receives. Some factors to include in the PM system are: date of installation, horsepower, frame size, rpm, bearing type and environmental conditions. Setting up a system like this may take some time, but once completed it will be a great tool.
When searching for a lube type and manufacturer or supplier, there are several things to consider. The following is a list of qualities of a good electric motor grease:
Good channeling characteristics
NLGI Grades 2 to 3
Base oil viscosity of an ISO VG 100 to 150 or more specifically 90 to 120 cSt at 40°C
High dropping point, 400°F minimum
Low oil bleed characteristics, per D1742 or D6184
Excellent resistance to high-temperature oxidation
Good low-temperature torque characteristics
Good antiwear performance (but not EP)
Polyurea grease is popular with many bearing and motor manufacturers. A significant proportion of equipment manufacturers also specify some type of polyurea grease in their electric-powered machinery. A polyurea-based grease is an excellent grease for electric motors; however, this thickener is incompatible with most other thickeners. Some manufacturers don't recommend mixing one brand of polyurea with another. Instruct the motor rebuild shop on what grease to use, and make sure the grease type is specified on new motor purchase orders.
There are several methods for determining a regrease time cycle. It is important to realize that no single method will provide a magical answer to a plant's problems. The multiple calculators, tables and charts available can provide a good starting point. They can be used to determine how to set the cycles. The real fine tuning, however, must be done by trial and error. The factors that most calculators have in common are load, operation time, bearing type, temperature, environment and speed. This is where the database that was built will be beneficial.
Grease volume control has been a long-standing problem for industry, and simply following OEM recommendations may not be enough to solve this problem. There is a simple equation that takes a logical approach to determining the volume of grease to be added. The formula is:
G = 0.114 x D x B
Where G = the amount of grease in ounces, D = the bearing outside diameter in inches and B = the bearing width in inches.
Once the volume is found, it must be converted into shots, or pumps of the grease gun. There is one way to obtain the value used to convert the number; for this the user will need the grease gun to be used and a postal scale. After finding the output per full stroke of the handle, label the gun so that it is now "calibrated". The average value is approximately 18 shots per ounce for most manual guns but grease gun output can vary by a factor of 10, so be sure to calibrate each gun.
Feedback from the lubrication points is needed to verify that the proper frequency and volume has been chosen. Ultrasonic instrumentation might be the best tool available to dial in the optimum relubrication frequency. A conservative approach is to use a frequency generation method as a starting point, and continuously refine that value based on feedback from the ultrasonic equipment. Likewise with volume, ultrasonics can be used to hone in on the correct amount of grease by using the hybrid method. Ultrasonics will be fully discussed in a later issue of ML because it is somewhat of an art form and warrants a separate article on this subject.
The intent of a good maintenance program is to extend the service life of a motor. In most cases, improper electric motor lubrication procedures can have a negative impact on the program. A basic set of procedures should include some variation of the following:
Ensure the grease gun contains the appropriate lubricant.
Clean the areas around the relief and fill fittings.
Remove the grease relief valve or drain plug.
Grease the bearing with a calculated amount of grease. Slowly add grease to minimize excessive pressure buildup in the grease cavity.
Watch for grease coming out the relief port. If excessive amounts of grease are pumped into the motor and the old, used grease is not being purged, stop and check for hardened grease blocking the relief passage.
If regreasing is performed with the motor out of service, run the motor until bearing temperature increases to operating temperature to allow for thermal expansion of the grease. Ensure the relief valve or drain plug is left out during this process.
Allow the motor to run at this temperature for a short time to expel any excess grease before installing the bottom grease relief valves.
After excessive grease has been purged, reinstall the drain plug and clean excessive grease from the relief port area.
This article was written to inform the reader of some of the thought processes that go into the creation of a electric motor lubrication program. Remember to take your time and do it right the first time. The rewards are worth the effort.