Oil-lubricated ball bearings used in motor rotors are characterized by low load capacity, high precision, long lifespan, low frictional torque, and high stability. Under normal operating conditions, the most important factor affecting bearing lifespan, frictional torque, and reliability is bearing lubrication. Under specific operating conditions (such as a vacuum environment), bearings rely solely on the lubricating oil contained in the cage to maintain lubrication and ensure normal operation of the motor rotor; this is considered a one-time lubrication. The special operating environment (vacuum) and long lifespan requirements of motor rotors make it difficult to meet the demands even with oil-cage bearings. A suitable oil supply system must be provided for supplementary oil supply to ensure adequate bearing lubrication. When supplying oil to the bearing, the bearing lubrication status needs to be analyzed to determine the approximate timing of supplementary oil supply.
While numerous theoretical formulas and practical applications exist for bearing lubrication, they all possess significant limitations. The mathematical models and empirical coefficients in these formulas are generally established under ideal process quality conditions, often resulting in values that deviate considerably from actual values. The complexity and systematic nature of bearing lubrication necessitate that experiments and experimental analysis serve as crucial bases for verifying bearing lubrication designs.
The single-use lubrication life of a bearing refers to the time during which the bearing can maintain normal operation under conditions where no external supplemental lubrication is required, relying solely on the circulating lubrication established by the lubricating oil contained in the cage. Bearing failure is not primarily due to fatigue, but rather to insufficient lubrication leading to precision failure, which in turn causes wear failure. To investigate the impact of single-use lubrication on the life of motor bearings, a 1:1 simulation experiment was conducted under identical operating conditions.
1. Experimental Methods
Sixteen sets of 7004 ball bearings were divided into eight groups for a general paired bearing test (for motor rotors). During the test, the operating current, voltage, and speed of the motor rotor were monitored and recorded. When the operating current of the motor exceeded the current performance index and an abnormality occurred, the bearing was considered to have failed.
Before the test, the bearings were screened and evaluated, and only qualified bearings were selected for testing. The test methods and procedures are as follows:
(1) Clean and dry the bearings according to the cleaning specifications.
(2) The cage is treated with an oil immersion process.
(3) According to the running-in specifications, the bearings shall be run-in initially after assembly.
(4) After initial running-in, install the bearing into the motor rotor.
(5) Based on the actual working conditions, conduct a one-time lubrication life test on the bearing, and monitor and record the operating current, voltage and speed of the motor rotor during the test.
(6) After the life test is completed, a curve of the motor operating current changing with time is plotted based on the test data, and the lubrication life curve of the bearing is plotted based on the curve; and the life curve is analyzed.
2. Test equipment and conditions
The testing equipment is a bearing running-in tester. This equipment drives the bearing to run using a drive motor, and also has the function of acquiring and recording bearing running-in data; the bearing running motor is an ironless brushless DC motor. The testing equipment enables real-time monitoring and recording of the current, voltage, and speed of the test bearing motor, accurately plots the current change curve over time, and can promptly determine the fluctuations in current and the fluctuations in bearing friction torque.
Test conditions:
(1) Ambient temperature (20±5)℃, vacuum degree ≤10Pa.
(2) The axial load of the bearing is 30-40N (to meet the working requirements of the motor rotor).
(3) The bearing speed is based on the actual operating speed of the motor rotor, which is 5600 r/min.
3. Experimental Results and Analysis
The experiment focused on real-time monitoring and recording of the motor rotor's current, voltage, and speed. Based on the test data, curves of time and operating current were plotted. Due to the long duration of the experiment and the large amount of test data, this paper only plots the curve of the bearing operating current changing with time based on the test data, as shown in Figure 1.
Based on the eight sets of curves in Figure 1, after sorting, the corresponding lubrication life curve of the bearing during a single lubrication process can be obtained, as shown in Figure 2.
As shown in Figure 2, the bearing operated under a single lubrication condition for approximately 25 months. The entire test process was divided into four stages: sufficient lubrication stage, lubrication balance stage, insufficient lubrication stage, and lubrication failure stage.
When the lubrication of the bearing is completely destroyed, the temperature rise and operating current of the bearing increase sharply, accompanied by a distinct and harsh dry friction sound. The operating time in the lubrication failure zone is less than one month.
4. Conclusion
(1) Without replenishing the oil supply to the motor rotor bearing, the bearing's service life can only meet 22 months of normal continuous operation.
(2) The oil supply time for supplementing lubrication of motor rotor bearing should be selected before the bearing is insufficiently lubricated. Considering that the bearing may have microscopic dry friction and insufficient lubrication in the latter half of the lubrication balance zone (not manifested in electrical performance), in order to ensure the reliability of bearing lubrication, the oil supply time can be appropriately advanced to 15 to 18 months.
