In a previous article for Machinery Lubrication magazine on the subject of electric motor lubrication, all of the static and dynamic test results were run on what was considered the industry-leading polyurea long-life electric motor grease.
The motor-stopped and motor-running test conditions were repeated with a new low-noise polyurea grease from another major U.S. lubricants supplier to determine if its formulation provided any performance advantages over the previous test grease.
Table 1 summarizes some of the key properties of both test greases as provided in the public domain literature from both suppliers.
Table 1. Properties of Test Greases
Test Condition 5: Motor stopped; cold start; low-noise polyurea
The industry-leading polyurea grease was removed from the output shaft bearing and end bell cavity and inner cap assembly. All components were refilled to 100 percent capacity with a competitive mineral oil-based, low-noise polyurea grease.
The standard test conditions detailed for Run No. 3 were duplicated with this new grease, and the drive-end bearing temperature was recorded at one-minute intervals until the temperature plateaued and started to drop. This indicated that the bearing housing had purged all of the excess grease and reached equilibrium.
The motor was stopped and quickly opened and all internal parts were evaluated for grease migration. The photographs document the findings.
Figure 1. Excessive Grease Migration Past Internal Bearing Cap and onto Windings
All of the excess grease in the bearing housing again migrated down the narrow opening between the motor shaft and inner bearing cap (Figure 1). Some grease either dripped or flung off of the shaft into the motor windings and also in the drive end bell (Figure 2).
Figure 2. Excess Grease in Motor End Bell
However, most of the excess removed from inside the motor was still clinging to the backside of the inner bearing cap.
As in the previous motor-stopped lubrication test, no grease exited the grease relief valve during the entire test.
Also, no grease was observed purging though the labyrinth seal, as with the previous test grease. This was considered a significant improvement in performance from the previous test run.
The most significant difference seen in this test, however, was in the heat-generation measurements taken on the drive end bearing (shown in Figure 3).
Figure 3. Heat-generation Measurements
Using the new low-noise polyurea grease under the same operating conditions resulted in a reduction in bearing temperature of almost 30 degrees Fahrenheit when compared to the industry-leading polyurea grease.
Conclusions from Test Condition 5
1. Tests of different greases confirmed the findings in Test Condition 3 that utilizing the motor-stopped relubrication method under or at near-full housing conditions results in excess lubricant being forced into the motor internals.
2. Again, the grease relief valve did not function at all because of temperature gradients in the bearing housing.
3. Grease composition/physical characteristics can have a significant impact on performance, resulting in increased component life as well as lubricant life.
Test Condition 6: Motor running normal; low-noise polyurea
After cleaning the interior surfaces of the motor from the previous test, the motor drive end bell was reinstalled. The motor was restarted and the bearing temperature monitored until the temperature stabilized.
Fifteen shots (three-fourths of an ounce) of low-noise polyurea grease were slowly added to the bearing housing using a standard hand-operated grease gun.
The outer race temperature was monitored until a steady-state operating condition was obtained. The motor was stopped, the drive end bell was carefully removed and the motor components were inspected for the presence of excess grease.
Grease migrated through the bearing and down the opening between the inner cap and the motor shaft (Figure 4).
Figure 4. Minimal Grease Migration Past Internal Bearing Cap
Figure 5. No Grease Present in Motor End Bell
Figure 6. No Grease Migration into Motor Windings
No grease was expelled from the labyrinth seal, indicating a more stable soap structure.
No grease dripped or splattered into the motor-end bell housing or windings.
As in the previous motor-running test (Condition 4), the grease relief valve functioned as designed and grease exited through the relief valve. Approximately one-third of the excess grease exited the bearing housing through the relief valve and two-thirds migrated inside the motor.
As shown in the dynamic tests, the heat-generation measurements showed a distinct advantage with the low-noise polyurea grease over the industry-leading polyurea grease (Figure 7).
Figure 7. Heat-generation Measurements
While the temperature reduction was not as significant as the motor-stopped test results, the steady-state operating temperature of the drive end bearing using the low-noise polyurea grease was almost 20°F cooler than the industry-leading polyurea grease.
Since the completion of this work in 2002, these findings have been confirmed in the field from inspection of in-service electric motors, providing support for the following conclusions and recommendations:
Conclusions and Recommendations
1. Neither the positive lubrication system with open bearings nor other motors equipped with shielded bearings can prevent the ingression of grease into the motor windings.
2. Grease relief valves are most effective in preventing gross overlubrication in a motor-stopped ambient temperature condition, but may not function during an ambient temperature restart, resulting in even more grease entering the motor internals.
3. Grease relief valves can reduce the amount of grease that enters the motor internals when relubrication is performed while the motor is running and at steady-state operating temperature conditions.
4. Grease relief valves should be thought of as excess lubricant indicators. If grease is observed exiting the relief valve port, bearing housings are full. Reduce the amount of lubricant or increase the lubrication interval.
5. The motor-running method should be the preferred lubrication procedure for grease-lubricated electric motors requiring relubrication.
6. Adding a measured amount of grease at regular intervals should minimize the amount of grease getting into the windings without adverse effects on motor life.
7. All polyurea greases are not the same.
8. Use of shear, stable, low-noise polyurea formulations can reduce operating temperatures, lengthen lubrication intervals and extend bearing lives.
About the Author
John Underwood is a corporate lubricants consultant for DuET Rotating Machinery. DuPont Engineering Technology, known as DuET, develops marketable research ideas into full-scale products and technologies. With world-class skills, personnel, resources and equipment, DuET ably provides the company – and its global customers – with product and process optimization support. For more information on DuPont, visit www.dupont.com.
Underwood, John. “Grease-lubricated Electric Motors – A New Perspective.” Machinery Lubrication magazine, January 2008. Available online at www.machinerylubrication.com.