Performance characteristics, comparative test of plastic cage and result Analysis.

Abstract: This paper introduces the current use of plastic cages, analyzes the composition of plastic cage materials, and describes the processing technology and characteristics of plastic cages. Through the high-speed and high-temperature tests of two kinds of plastic cages made of different materials, the high-speed and high-temperature performance of the two kinds of plastic cages and the differences between them are explained.

Key words: rolling bearing; plastic cage; performance; high temperature test; high speed test

Cage is one of the important parts of rolling bearing. Its basic function is to keep the rolling elements separate. During the operation of the bearing, the rolling elements are evenly distributed along the circumference of the bearing, and guide the rolling elements to roll on the correct rolling track. In general applications, traditional metal stamping cages are mainly used. In the application fields of high temperature, high speed, low noise and long life, the solid cage is mainly used.

According to the different working conditions, there are metal cages, phenolic resin cages and plastic cages in the solid cages. The processing and assembly of metal solid cage and phenolic resin cage are relatively complex. Due to the high production cost, it is suitable for single piece and small batch production. The processing and assembly of plastic cages are relatively simple. It adopts mold injection molding for one-time molding, mass production, low cost, and is suitable for mass production. With the development of plastic material technology, the performance of plastic composites has been greatly improved, and plastic cages are more widely used. In particular, some foreign bearing enterprises, such as SKF Company, have selected plastic cages of various bearings as standard cages within a certain size range. NSK and fag companies' bearings with plastic cages account for about 30% of their output.

1. Performance characteristics of plastic cage

1.1 Cage material

The main material composition of plastic cage is polyamide, also known as nylon, abbreviated as PA, which has good comprehensive properties, including mechanical properties, heat resistance, wear resistance, chemical resistance and self-lubrication, low friction coefficient, certain flame retardancy and easy processing. There are many kinds of nylon, such as PA6, PA46, PA66 and pal010. With the increase of temperature, its performance will change, such as strength reduction, thermal expansion and deformation, and easy aging after long-term use. These shortcomings limit the application of conventional plastic cages. It is a hot spot in this field to select appropriate modifiers to modify plastics to develop towards high impact strength, high performance and prolonged aging time. Glass fiber (GF) is the most common modifier, which is widely used in the reinforcement and modification of all kinds of general plastics. Adding a certain amount of glass fiber and anti-aging agent to these materials can enhance the modification of materials, improve the strength and elasticity of materials, and expand the application range of materials. Nylon reinforced by modified glass fiber maintains some of the original advantages of plastic, and at the same time, its wear resistance, mechanical properties and heat resistance are also significantly improved. At present, glass fiber reinforced nylon 66 and nylon 46 are two relatively successful materials used in bearing cages. Glass fiber reinforced nylon 66 is a cage material used more in low-noise and high-speed applications. Glass fiber reinforced nylon 46 is relatively less used because of its high price.

1.2 Characteristics of plastic cage

Cages made of nylon have many features. First, the light weight makes the bearing more flexible. If it is used in electrical appliances or power consumption products, it plays a great role in energy saving. Second, low noise is the first choice for silent bearings, especially for household appliances with strict noise requirements. Its application can significantly improve the low-noise performance of household appliances. Third, it can effectively extend the service life of the bearing and reduce the friction between the cage and the rolling element; Fourth, it has good properties such as wear resistance, anti-magnetism and low friction.

2. Comparative test of plastic cage

In order to further test the performance of glass fiber reinforced nylon cage, two plastic cage bearings made of different materials (PA66-GF25 and PA46-GF30) were used for comparative tests. Taking two groups of double row angular contact ball bearings 3207DDU as an example, the overall dimension of the bearing is d × D × B=35mm × 72mm × 27mm, basic rated dynamic load Cr=33.5kN, contact sealing structure on both sides of the bearing. According to JB / T50013-2000 code for life and reliability test of rolling bearings and the structural parameters of the bearing, the test conditions are formulated. According to the characteristics that the cage will be subject to great impact and wear when the bearing operates at high speed, and it is easy to age when it operates at high temperature for a long time, the high-speed test and high-temperature test are carried out on the bearing to test the wear resistance and high-temperature aging resistance of the cage.

(1) High speed test: test speed n=5000r / min, radial load Fr=3.35kN, equivalent dynamic load p=0.1Cr, test time 200h.

(2) High temperature test: test speed n=3700r / min, radial load Fr=8.38kN, equivalent dynamic load p=0.25Cr, test time 200h, bearing outer ring temperature ≥ 120 ℃.

2.1 installation of test bearing

B30-60 bearing testing machine was used in the test. The installed capacity is 4 sets / set at a time, and the installation of bearings is shown in Fig.1.

Fig. 1 installation diagram of test bearing

In the figure, four sets of test bearings are installed at stations a, B, C and D respectively. The material of bearing cages at stations A and B is PA46-GF30, and the material of bearing cages at stations C and D is PA66-GF25. Apply a certain radial load to the bearing through the test device, and drive one end of the spindle to test the bearing. In the test device, the spindle tolerance is m6, which is an interference fit with the inner ring of the bearing. The shell tolerance is H7, which is a clearance fit with the outer ring of the bearing. Heating rods are respectively implanted in the shells at both ends to heat the bearings to the required temperature. Monitor the temperature and noise of the outer ring of the bearing during the test, and record the bearing temperature every 2h.

2.2 test

In the first stage of 200h high-speed test, the temperature of the bearing outer ring of station A and D is about 90 ℃, and the temperature of the bearing outer ring of station B and C is about 150 ℃. Due to the tightness of the testing machine body, the temperature of the outer ring of the middle two sets of bearings is much higher than that of both ends under the high-speed state. There is no abnormal sound in the bearing during high-speed operation, which indicates that the bearing is not damaged or has inflexible rotation. The cages of these two materials can meet the requirements of smooth operation of the bearing at high speed.

During the 200h high temperature test in the second stage, heat the bearings at both ends, and the temperature of the bearing outer rings of stations a and D is maintained at about 120℃, and the temperature of the bearing outer rings of stations B and C is maintained at 150 ~ 160℃. There is no abnormal sound in the high temperature test of the bearing, indicating that the bearing cage is not damaged. The cages of these two materials have certain high temperature resistance.

After the high-speed and high-temperature tests, the bearing operates normally, indicating that the cages made of two different materials have maintained high performance. In order to further assess the durability and high temperature resistance of the cages made of these two materials, after the completion of the first two stages of the test, increase the ambient temperature of the bearing operation and continue the test. The temperature of the outer ring of the bearing in station a, B, C and D4 is controlled at 180 ~ 190 ℃. The bearing has been running continuously for more than 400H, and abnormal sound is detected in the bearings of station C and D. this may be due to the aging or fracture of the cage, which changes the performance and affects the normal operation of the bearing. The test was continued under the same high temperature conditions, and each set of bearings was tested for 1185h. The running noise did not increase significantly, and then the test was stopped.

2.3 test results

After the test, remove the bearing for inspection. The bearings of station A and B are in good appearance without failure. C. D the cage of the two station bearing has many fracture marks and has completely failed.

Disassemble the bearings and check the condition of all parts. The rings and steel balls of the four sets of bearings have not been fatigue spalling. That is to say, the abnormal sound produced during the test is not caused by the failure of ferrules or steel balls. Remove the cage from the bearing, and the cages of station a and station B are intact in appearance and maintain a certain toughness (Fig. 2 and Fig. 3); C. D. brittle fracture occurred in the two station cage, forming several broken blocks, and the material has been significantly aged (Fig. 4, Fig. 5).

Fig. 2 appearance of station A cage          Fig. 3 appearance of station B cage

（PA46 - GF30）                         （PA46 - GF30）

Fig. 4 appearance of station C cage           Fig. 5 appearance of station D cage

（PA66 - GF25）                         （PA66 - GF25）

Clean the cage, measure the size of the outer edge of the pocket, check the performance of the plastic cage and test the wear of the cage. See Table 1 for the measurement results. PA66-GF25 cage is more worn than PA46-GF30 cage.

Table 1 dimension of cage pocket edge mm

3. Result Analysis

According to the temperature of the outer ring of the bearing monitored during the test, the working conditions of the bearing cages at station a and D are the same, and the working conditions of the bearing cages at station D and C are the same. B. C during the high-speed test of the two station bearing cage, the temperature of the outer ring of the bearing is 150 ℃.

In the high temperature test stage, the outer ring temperature is 150-160 ℃. In the additional high temperature test stage, the outer ring temperature is 180-190 ℃, and the cage has been in a high temperature state. At the end of the test, the cage made of PA46-GF30 was not broken, and the cage made of PA66-GF25 was completely broken, and the fracture of station C was more serious than that of station D. This is due to the different high-temperature anti-aging properties of the two raw materials. Compared with PA66-GF25, PA46-GF30 has a higher anti-aging temperature and longer time.

During the high-speed operation of the bearing, the stress on the cage mainly comes from friction, inertia force and impact force. In the high temperature environment, it is also eroded by chemicals such as lubricants and their aging products. These corrosive substances adhere between the cage and the steel ball, which intensifies friction, wear and heating, resulting in serious wear of the cage and affecting the normal operation of the bearing. After the test, the average size of the pocket edge of the cage PA66-GF25 is much smaller than that of PA46-GF30. On the one hand, it is caused by the wear of the cage, indicating that the wear resistance of the material PA66-GF25 is not as good as that of PA46-GF30; On the other hand, the shrinkage of the model PA66-GF25 is larger than that of PA46-GF30 at high temperature, and the cage has a certain aging shrinkage deformation.

4. Conclusion

It can be seen from the test that no abnormality occurred in the four sets of bearings in the first stage of high-speed test. In the second stage of high temperature (150 ℃) test, there was no abnormality in the bearing. It shows that under certain high-speed and high-temperature conditions, the plastic cages of two different materials maintain good performance, so that the bearings can operate normally and flexibly. In the third stage, when the ambient temperature was further increased (the temperature of the outer ring of the bearing reached 180 ~ 190 ℃), the cages (made of PA66-GF25) of the bearings of two stations C and d had obvious wear and aging after long-time operation; A. The cage of B two station bearing (made of PA46-GF30) is relatively good, indicating that PA46-GF30 has better high temperature resistance and aging resistance.

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