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Abstract: Research on the life extension technology of gyro motor bearing, focusing on the improvement of cage material and size. The long-term stability of the cage material and the rationality of the dimensions are verified by the preliminary running-in and the user's installed life assessment test, which proves that the improvement plan is successful and feasible.
The working environment temperature of a certain type of high-precision and long-life gyro motor bearing is 80°C, and the actual internal working temperature is generally 80-100°C. At this temperature, the viscosity of the bearing lubricating oil will decrease. The working speed of the bearing is 36000r/min, and the service life is required to be more than 20,000 hours. The bearing is under very small load, so there is generally no surface fatigue failure, but the accuracy requirements are very high, and the failure is mostly caused by wear and tear. Due to high-speed operation, there is a lack of oil at the entrance of the elastohydrodynamic lubrication contact area, which reduces the thickness of the elastohydrodynamic oil film of the bearing by more than 30% [1]. Due to the inability to form a good elastohydrodynamic lubricating oil film, the lubrication conditions of the bearing are very poor, and any supplementary lubrication device cannot be added.
With the improvement of the understanding of the gyro motor bearing and the continuous improvement of the requirements for the life and reliability of the gyro, it is decided to improve this type of bearing to improve the life and reliability of the gyro motor.
1 Improvement measures
On the basis of the original bearing, focus on improving the cage material and size: (1) The cage adopts porous polyimide composite material with better strength and wear resistance, and the effective life of the bearing is extended through the transfer lubrication technology of lubricating oil, Reduce cage wear; (2) For the newly adopted cage material, find the appropriate cage pocket size and guide size through tests, and verify the rationality of the cage size through early running-in and user installed life assessment tests.
1.1 The basic principle of cage transfer lubrication
Due to the centrifugal force, the lubricating oil in the porous oil-containing cage tends to be transferred to the outer diameter surface. When the bearing is running, heat is generated due to friction and other factors, and the cage has a temperature rise. Since the volume expansion coefficient of the lubricating oil is larger than that of the cage skeleton material, the increase in the volume of the lubricating oil contained in the micropores of the cage material is larger than the increase in the volume of the micropores, which will cause a pressure that the lubricating oil leaks to the outer surface. internal pressure. As the temperature of the cage increases, the viscosity of the internal lubricating oil decreases and the fluidity increases, which tends to transfer to the surface of the steel ball. The steel ball keeps rotating, transferring the lubricating oil on its surface to the contacting rolling surfaces. Some of the lubricating oil is directly transferred to the outer guide surface of the bearing and lost, which is not only useless for bearing lubrication, but also causes the quality of the bearing to drift. Therefore, this part of the oil should be minimized during work. If the cage does not have the ability to absorb oil, the amount of lubricating oil inside the bearing channel will continue to increase and be lost to the surrounding. The porous oil-containing cage has the ability to recover oil, because the micropores in the porous cage material can be regarded as microcapillaries that penetrate each other, which generate capillary force for the lubricating oil. This force is inversely proportional to the pore diameter and proportional to the surface tension between the lubricating and porous materials. This force prevents the lubricating oil in the cage from losing too quickly, and at the same time sucks the excess lubricating oil in contact with it into the cage. When the external conditions such as speed, load, temperature and lubricating oil characteristics are constant, a dynamic balance will be established through operation. At this time, the oil output and oil absorption rates are equal, and the thickness of the oil film is stable. This is an ideal equilibrium state and a stable working period of the porous oil-containing cage. With the prolonged operation time, the lubricating oil will be lost or deteriorated, the lubricating performance will deteriorate, and sometimes there will be wear objects or foreign objects in the channel, these factors will lead to the destruction of the balance. If equilibrium cannot be established under the new conditions, the lubricating oil film cannot be established and stabilized, and the bearing begins to fail.
1.2 Improvement of cage material
The original bearing cage material is polyimide (PI) + polytetrafluoroethylene (PTFE) + tungsten disulfide (WS2) porous composite material, mainly using PI polymer material as the skeleton, filled with solids such as PTFE and WS2 transfer material. Since WS2 is an inorganic material and completely incompatible with other two organic polymers, WS2 is basically in a state of being covered by polymer materials, and may fall off when the cage is in contact, thereby affecting the effectiveness of the surface elastohydrodynamic oil film , and its dispersion strengthening effect can be completely guaranteed by adjusting the material pressing process parameters and improving the material strength, and its surface friction reduction effect can also be guaranteed by PTFE, so the improved bearing adopts PI+PTFE porous composite material, which has been used in It has been successfully used in a variety of high-speed gyro motors.
In addition to the cage material, the structural form, guiding method, surface roughness and quality of the cage also have a greater impact on the bearing friction torque and life. Since it is not possible to simulate the calculation through theory, the structural form and parameters of the cage need to be tested according to the type of cage material combined with the lubricating oil immersed in order to find a cage structure size with better high-speed stability.
2 Test verification
2.1 Bearing failure caused by cage
Under normal circumstances, the cage, the steel ball and the guide ribs are separated by a thin elastohydrodynamic oil film, but due to the existence of the guide gap, the cage keeps colliding under the drag force of the oil. During the collision, the cage continuously accelerates and decelerates, and also absorbs and consumes energy continuously. When the cage pocket gap and the guide gap are not matched properly, the energy absorbed by the cage is greater than the energy consumed. When the energy of the cage accumulates to a certain extent, it will rub violently with the steel ball and the guide ribs, accompanied by the whirling and whistling of the cage. This phenomenon is called the instability of the cage movement. Unstable operation of the cage can lead to sharp fluctuations in the dynamic performance of the bearing, as well as severe wear on the cage guide surfaces and pockets. On the one hand, these wear objects will block the micro-holes inside the cage, block the oil circulation path of the cage, and cause oil shortage on the working surface of the bearing; Early failure [2].
2.2 Cage test plan and purpose
For the improved cage material, select 2 cage sizes. First, a short-term running-in test is carried out, and a better cage size is screened out by the low-speed dynamic (LSD) torque meter according to the inertia time of the gyro motor and the milliwatt meter, and then confirmed by the host user and a life assessment test is carried out to verify the cage. Long-term stability of the material.
2.3 Cage test research
The drag at the contact between the steel ball and the channel and the friction between the cage and the steel ball play a key role in determining the movement of the cage [3]. Long-term experience has shown that the main factors for the matching between the cage of the high-speed gyro motor and the ferrule are the clearance of the cage (pocket clearance and guide clearance) and the drag force curve of the lubricating oil. Limited to the conditions, the following only studies the optimal clearance of the cage.
2.3.1 Test procedure
(1) The cage is ultrasonically cleaned, extracted, dried, vacuum immersed in oil, oiled, and packaged in groups.
(2) Clean the bearing parts and gyro motor components for the test.
(3) Assembly, trial operation, LSD torque meter test, and preload adjustment.
(4) Record the change curve of mW meter.
(5) Stop the gyro motor and measure the inertia time.
(6) Repeat the test three times to confirm the test results.
2.3.2 Test parameters
LSD torque meter, milliwatt meter and DC permanent magnet brushless gyro motor are used as analysis and test methods. The output voltage is 40V, the improved bearing speed is 36000r/min, the cage material is porous material (PI+PTFE), the test oil is 4129 high and low temperature instrument oil, the axial load is 7N, the radial load is 0.7N, and the cage contains oil. The amount of 4 ~ 8mg, etc. remains unchanged from the original bearing.
2.3.3 Test results
2.3.3.1 Scheme 1
The outer diameter of the cage is 6.58+0.020mm, the inner diameter is 5.07+0.02-0.02mm, and the diameter of the pocket is 1.75+0.030mm. After running in for 100h, disassemble the bearing, as shown in Figure 1.
It can be seen from Figure 1 that the cage pockets are slightly worn, the steel ball running-in belt is normal, and the oil film on the steel ball and the channel can be clearly seen. If the gyro motor bearing has a good distribution of the oil film in the channel after the early running-in, the bearing can run for tens of thousands of hours without failure [4].
2.3.3.2 Scheme 2
The outer diameter of the cage is 6.7+0.01-0.01mm, the inner diameter is 5.18+0.050mm, and the pocket diameter is 1.75+0.030mm. After 30h of running-in, since the inertia time of comparative measurement is continuously shortened, the bearing is disassembled, as shown in Figure 2.
It can be seen from Figure 2 that the cage pockets are severely worn, the steel balls are black, the running-in mark on the outer channel is very obvious and the running-in belt is wide, and no clear oil film can be seen on the inner and outer rings. If the distribution of the oil film in the channel is not good after the early running-in, it indicates that the chemical properties of the bearing surface are not good. If the bearing continues to run, the service life will not be very long [4].
2.4 Host Verification
In order to verify the long-term stability of cage material and the rationality of size in scheme 1, 5 gyro motors were assembled, and LSD torque test, inertia time and milliwatt meter test were carried out after early running-in.
2.4.1LSD torque test
The LSD torque meter can qualitatively identify common bearing faults under the condition of simulating the actual bearing preload and low speed. At low speeds, the lubricating oil film of the bearing is very thin, causing the steel balls to come into direct contact with the raceway. In this case, there are two forces that affect the low-speed torque: one is the sliding friction force generated by the contact area between the steel ball and the channel; the other is the geometric profile force caused by the rough surface of the steel ball and the channel. The sliding friction torque shows the loading of the bearing, and the LSD torque meter can be used to adjust the bearing preload and identify the coaxiality of the matched bearing. The geometric profile moment can show the surface roughness, damage and particle contamination of the steel ball or channel [4].
The LSD torque curve of 3# gyro motor is shown in Figure 3, the average value is 430mV, and the curve amplitude is small. The LSD torque is required in the process to be 300-700mV, which fully meets the process requirements from the fluctuation range of the curve.
2.4.2 Stop time
The inertia time of the gyro motor is shown in Figure 4. It can be seen from Figure 4 that the inertia time of the five gyro motors is increasing to varying degrees. Although the inertia time of the 2# gyro motor shows a downward trend in the 3rd to 5th days, it is longer than the inertia time on the first day. In the process, the inertia time is required to be 120s≤t≤180s. It can be seen from the figure that the inertia time of the five gyro motors is within the qualified range after 8 days.
2.4.3 Milliwattmeter test
The milliwatt meter is an instrument for measuring the small change of the input power of the hysteresis motor in the gyro, which can detect the potential performance change of the gyro motor, especially for the detection of over-lubrication when the gyro motor works at high speed. Howling caused by normal operation [4].
The milliwatt meter curves of 1#～5# gyro motors are shown in Figure 5. As can be seen from Figure 5, except for the millipower of the 2# gyro motor, which is about 50mW, the millipower of the other four gyro motors is basically about 30mW, which meets the requirement of less than 100mW in the assembly process.
3 Conclusions
(1) The cage size has a great influence on the operation of the cage. According to the newly adopted PI+PTFE material formula, the optimal matching size is selected, that is, the outer diameter of the cage is 6.58+0.020mm, and the inner diameter is 5.07+0.02-0.02mm. The diameter of the pocket is 1.75+0.030mm.
(2) The installation test of the main engine unit proves that the improvement plan is successful and feasible. The long-term stability of the cage material has been verified by a life assessment test of nearly 20,000 hours.
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