Grease-lubricated Electric Motors: A New Perspective
Tags: electric motor lubrication, greases
Over the past few years, there has been a renewed interest in the industry regarding the proper lubrication methods for grease-lubricated electric motors.
This article reviews the results from laboratory testing of a grease-lubricated NEMA-frame, 3,600 rpm electric motor manufactured to IEEE 841 standards and equipped with open deep-groove ball bearings and grease relief valves on bearing housing outlets.
Results and conclusions from this work were confirmed in the field and will be the basis for modification of a DuPont lubrication engineering standard covering electric motor lubrication.
Figure 1. IEEE 841 Test Motor
DuPont lubrication standard "PL 43 - Lubrication of Electric Motors" provides detailed guidance on acceptable lubricants and lubrication methods for electric motors, both grease and oil-lubricated. This article focuses on grease- lubricated NEMA-frame motors between 10 and 200 horsepower typically used to drive process pumps, mixer gearboxes, cooling tower pumps and fans, and other nonprocess-critical general-purpose applications.
Figure 2a. Inner Cap on Bearing Housing Fitted with Gits Cup
Motors can be equipped with either open-, single- or double-shielded deep-groove ball bearings depending upon the manufacturer's preferences. According to our motor lubrication standard, on motors equipped with shielded bearings, the shields are to be oriented toward the grease cavity in the end bells. All bearing housings are to be fitted with grease relief valves to prevent overlubrication and damage to the windings, as well as possible damage to shielded bearings should the bearing housings become full of grease. Use of a high-quality, polyurea-thickened, mineral base oil grease is recommended for general-purpose applications up to 3,600 rpm and ambient temperatures up to 40°C.
While two methods of relubrication were permitted in the DuPont standard, Motor Stopped and Motor Running, the Motor Stopped method was clearly preferred assuming that this was the best method to prevent overlubrication and damage to the windings. However, in actual practice, the Motor Running method is the predominant method used at the plant level, because operational demands precluded shutting down a piece of equipment for routine relubrication, as this could upset the process and cause an unanticipated plant shutdown.
Figure 3. Grease on Labrynth Seal Outer Diameter and on Inner Cap
In the early 1990s, DuPont entered into a preferred supplier alliance with an internationally known manufacturer of high-quality electric motors to provide new and/or replacement motors manufactured to the IEEE 841 standard. These 841-compliant NEMA-frame horizontal motors are equipped with open ball bearings, grease relief valves and rotating labyrinth seals on both bearing housings. In addition, motors were equipped with a patented bearing housing design to provide positive lubrication of motor bearings for long, trouble-free lives.
Prior Testing
In August 2001, DuPont conducted tests at an authorized electric motor repair facility in the Mid-Atlantic area, which raised even more questions on the effectiveness of the Motor Running method in preventing/minimizing grease from entering the windings of an in-service motor.
In this impromptu test, an industry-leading polyurea grease was added to the bearing housing of a newly rebuilt electric motor. The motor, equipped with double-shielded deep-groove bearings was operating at normal speed and ambient shop temperature. Grease was added slowly from a hand-operated grease gun until grease exited the bearing housing 1/8-inch NPT relief port opening. Once grease was observed at the exit port, the motor was stopped, disassembled and inspected for the presence of excess grease.
Inspection showed a substantial amount of grease pushed through the outboard shield, the rotating bearing elements and the inboard shield, and entering the motor winding cavity. These findings were somewhat surprising as well as troubling, because this arrangement was thought to provide the best system to prevent overalubrication.
Now, with our movement to the IEEE 841 standard design with its open bearing configuration, we became even more concerned about the feasibility of continuing to support the Motor Running lubrication method within our motor lubrication standard.
Even more concerning was that this test was conducted without any obstructions in the bearing housing outlet port, such as extension pipes and/or grease relief valves, which were permitted and even encouraged in the standard.
Also, the grease was absolutely fresh, whereas, long-life in-service motors could potentially have hardened or dried grease, possibly adding more restriction in the outlet path. Based on these factors, we felt additional testing on the new 841 design motor was warranted to determine the effectiveness of this design in preventing overlubrication and motor winding contamination.
Recent Testing
In April 2002, with the assistance of our alliance motor vendor, and other DuPont engineering and mechanical specialists, a comprehensive testing program was carried out at the vendor's Baytown, Texas repair facility.
Six test cases, two static and four dynamic, were developed to evaluate the following factors:
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Motor Stopped vs. Motor Running lubrication method
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Effectiveness of grease relief valves, Gits cup vs. Alemite pin design
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Effectiveness of positive lubrication system in reducing/preventing overlubrication and motor contamination
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Performance of industry-leading polyurea grease vs. a competitive low-noise polyurea - effects of physical/chemical properties on lubrication performance
A new IEEE 841 compliant motor (Frame 326T, 3,600 rpm) was supplied by the DuPont LaPorte THF area for the test. The motor was equipped with rotating labyrinth seals, open deep-groove ball bearings and grease relief valves on both motor bearing housings. A thermocouple was installed in contact with the output shaft bearing outer race to monitor bearing temperatures during the test.
Relief Valve Static Tests
The first series of static tests were conducted to evaluate the effectiveness of the two approved grease relief valves in controlling/minimizing grease entry into the motor in the Motor Stopped condition. The following test protocol was used:
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The motor end bell was removed and the motor bearings and grease cavity were packed 100 percent full of the test grease for all test conditions to increase the severity of the test.
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The end bell was carefully reassembled and then a predetermined amount of grease, based on the bearing dimensions, was slowly added to the bearing housing.
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The motor end bell was removed and the grease distribution was evaluated both qualitatively and quantitatively.
The test was repeated using the same test protocol, but the Gits cup was replaced by an Alemite 314700 grease relief valve, rated at 1 psig opening pressure, and the same predetermined grease volume was again added to the bearing housing. The following results were obtained:
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There was little difference in the performance of either relief valve in the cold static test: Gits cup (Figure 2a), Alemite fitting (Figure 2b). One valve discharged 16 grams of grease, the other 14 grams.
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A negligible amount of grease migrated past the inner cap down the motor shaft as can be seen in Figures 2a and 2b.
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Both valves are effective in preventing gross overlubrication in a Motor Stopped cold condition.
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The Gits cup was judged slightly better than the Alemite fitting due to a larger bore opening in the fitting. Even after the grease gun was removed from the supply zerk fitting, grease continued to purge from the Alemite relief fitting for several seconds, which indicated residual pressure buildup in the bearing housing.
Based on the results of these static tests, the remainder of the dynamic tests were conducted with the Gits cup directly connected to the bearing housing outlet.
Dynamic Tests
Test Condition 3 - Motor Stopped Cold Start - Industry-leading Polyurea
In this test condition, the motor, which was at ambient conditions and bearing housing 100 percent grease full, was started on the test bench and allowed to run at its no-load speed of 3,600 rpm. The drive end bearing temperature was recorded at one-minute intervals until the temperature plateaued and started to drop, indicating that the bearing housing had purged all the excess grease and reached an equilibrium condition.
The motor was stopped and quickly opened, and all internal parts evaluated for grease migration.
A considerable amount of grease migrated past the inner cap and flung off the rotating shaft onto the motor windings. A considerable amount of grease was also found in the drive end bell. No grease exited the grease relief valve during the test.
Another interesting observation during this test was the fact that grease pushed through the drive end labyrinth seal and small bits of grease were thrown against the inner cap retainer bolt heads and onto the working surfaces around the motor (Figure 3).
From a practical perspective, this test condition simulated relubrication in the Motor Stopped condition and the grease relief valve did not function at all in this condition.
The drive end bearing operating temperature results are plotted in Figure 4.
The bearing temperature increased rapidly from a 70°F ambient to 120°F within the first minute of operation, then climbed steadily, reaching a maximum temperature of 176°F approximately 90 minutes into the test. The temperature then dropped slowly to a steady-state operating temperature of 152°F, approximately two and a half hours into the test.
The following interim conclusions were drawn from Test Condition 3:
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The long temperature stabilization time indicted that the nominal 30-minute wait time in the lubrication standard may be inadequate for this relubrication method.
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Due to temperature differences across the bearing housing and grease shearing in the ball path zone, undisturbed grease in the end bell at the exit port acts as a plug, preventing excess grease from exiting the bearing housing, whether or not a grease relief valve is present.
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Operating at or near full housing conditions will force grease into the windings after a Motor Stopped lubrication event.
Test Condition 4 - Motor Running Normal - Industry-leading Polyurea
In the next test, 15 shots (3/4 ounce) of grease were added slowly to the stabilized, normal-operating motor using a standard hand-operated grease gun.
Again the outer race temperature was monitored and the motor components were inspected for excess grease after the test.
As observed in the previous test, grease migrated through the gap between the inner cap and the motor shaft and either dripped or was flung off the shaft into the motor winding and end bell.
However there was much less grease inside the motor, because unlike the previous test, the grease relief valve functioned as designed. However, after collecting and weighing all the excess grease from inside the motor and that which was expelled from the Gits cup, approximately one-third of the grease exited the relief valve and two-thirds leaked inside the motor.
Grease also continued to be purged out of the rotating labyrinth seal.
The consistency of the grease was found to be soft and drippy, suggesting loss of structural stability of the soap structure. It was not determined if this loss of stability was temporary or permanent.
The drive end bearing temperature is shown in Figure 5.
Again, the temperature increased rapidly over 20°F in the first minute of operation and then slowly increased over the next 40 minutes to approximately 175°F as seen in the previous Test Condition 3. However, the equalization time for this lubrication method is much closer to the generally accepted 30 minutes purge time contained in the standard and generally accepted within the industry.
The most important finding from this test was the fact that the grease relief valve system did function as designed and partially relieved the pressure in the bearing housing. This reduced the level of contamination in the motor cavity.
A plausible explanation for this result is that in the normal running condition, the grease within the bearing housing is much softer and more pliable, thus offering less resistance to flow of excess grease within the bearing system. It now appears that the Motor Running lubrication method reduces/minimizes the amount of grease that purges into the motor internals, especially under full housing conditions and should be the preferred method of lubrication for grease-lubricated motors.
Recommendations
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Neither the Reliance positive lubrication system (PLS) with open bearings nor potentially other motors equipped with shielded bearings can prevent the ingression of grease into the motor windings.
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Grease relief valves are most effective in preventing gross overlubrication in the Motor Stopped ambient temperature condition, but may not function during an ambient temperature restart, resulting in even more grease entering the motor internals.
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Grease relief valves can reduce the amount of grease that enters the motor internals when relubrication is performed while the motor is running at steady-state operating temperature conditions.
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Grease relief valves should be thought of as "excess lubricant indicators". If grease is observed exiting the port, bearing housings are full. Reduce the lubricant amount or increase the lubrication interval.
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The Motor Running method should be the preferred lubrication procedure for grease-lubricated electric motors requiring relubrication.
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Adding a measured amount of grease at regular intervals should minimize the amount of grease getting into the windings without adverse affects on motor life.
The increase in temperature illustrated here may vary with different greases depending upon their chemical structure and physical properties as well as base oil viscosity.
Read more on electric motor lubrication:
Improving Electric Motor Lubrication Policies