2022 November the Second Week KYOCM Technical Knowledge: Improvement on Technology of Lower Noise Deep Groove Ball Bearing

Abstract: From the aspects such as increasing process accuracy of inner ring and outer ring ,improving superfinishing method of raceway and increasing assemble quality of the bearing ,the technology method of 6204 bearing has been improved ,and the feasibility and validity are verified by theory analysis and practical measure.

Key words: deep groove ball bearing; lower noise bearing; vibration; technology; improvement

The vibration and noise of rolling bearings is one of the basic properties of bearings. Precision machinery and high-end household appliances require rolling bearings to have less vibration, lower noise and a certain vibration and noise life. Aiming at the problems existing in the bearing production process of a company, based on theoretical analysis and production practice, this paper puts forward the method of controlling bearing vibration and noise, improves its 6204E process method, and conducts 2000 sets of trial production.

1. Improve the positioning accuracy of the end face of the ring

The machining error of face grinding has an impact on all subsequent processes. The high-precision face provides a good process benchmark for groove grinding and superfinishing processes, and can ensure the symmetry of the ring groove. The trial production uses a double end grinder, which avoids the overlap of positioning error and processing error, and there is no processing error caused by the uneven magnetic table and the deformation of the magnetic attraction workpiece. Compared with the processing of ordinary bearings, the process of grinding the end face is added, which further improves the accuracy of the end face. During the trial production of 6204E, 40 outer rings and 40 inner rings were randomly selected to measure the parallelism of two end faces. The larger value was taken as the effective value. See Table 1 for the measurement results.

Table 1 Measurement Results of Parallelism of 6204E Low Noise Deep Groove Ball Bearing End Surface

It can be seen from Table 1 that the parallelism of 6204E inner ring is better than that of outer ring. This is because the inner ring is thick and the area of the stressed end face is large, so it is not easy to deform.

2.Improve the accuracy of the outer diameter of the ferrule

The outer ring surface of rolling bearing is the installation and use benchmark of bearing products. During processing, the outer ring surface is used as the positioning benchmark, and its surface error will be transferred to the subsequent process, affecting the roundness of the groove. When grinding the outer diameter with a cylindrical centerless grinding, four processes are used: rough grinding and fine grinding. If the geometric layout is improper in centerless grinding, it will often produce a very serious surface error - circular error (roundness), which can be guaranteed by trimming the guide wheel and adjusting the geometric parameters of the machine tool to ensure the stable movement of key surfaces. After grinding the outer diameter, the grinding process is added to further improve its accuracy.

3. Method of improving channel superfinishing

The process of superfinishing is a process in which the cutting effect of the oilstone on the machined surface is gradually transformed into the finishing effect. Superfinishing can effectively reduce the circular deviation (mainly the waviness), improve the geometric shape error of the channel cross-section, remove the grinding metamorphic layer, reduce the roughness value, and make the surface have residual compressive stress. The process is improved as follows:

(1) Mechanical compression type centerless clamp is used for ultra precision. Since the low noise bearing has high requirements on the shape accuracy and position accuracy of the ring groove, it is not suitable to use double rollers to support the super precision groove. Therefore, the machine tool with a mechanically compressed centerless clamp is used. Its clamping force is provided by two symmetrically placed rollers (small bearings) acting on the end face of the ring, which avoids a series of adverse effects caused by the magnetic force of the electromagnetic centerless clamp during the super precision. Its super precision quality is better than that of the double roller support. However, since the outer diameter is used for positioning when superfinishing the channel, the influence of geometric shape error will be reflected to the channel, so the process of grinding the outer diameter is added. At the same time, because the groove has a high requirement on the original parallelism of the reference end surface, the end grinding process is added in this trial production process to improve the positioning accuracy of the end surface.

(2) A two-step process is adopted. In the trial production process, one machine tool is used to realize two stations and can be automatically switched. The first station uses white corundum abrasive oilstone for coarse super finishing, and the second station uses green silicon carbide abrasive for fine super finishing, which can not only improve the cutting efficiency, but also reduce the roughness value and the oilstone wear speed. The processing quality can be improved by adjusting the pressure, vibration frequency, amplitude, workpiece speed, superfinishing time and other parameters of the oilstone. In this trial production process, the vibration frequency of the refined timeout oilstone is reduced, and the channel surface is formed with fine texture stripes. When the bearing rotates at high speed, it is easy to form oil film lubrication.

4. Holder

In this trial production process, the corrugated wide edge cage with tight nails is used, which can better control the radial movement of the steel ball, and polish the cage before assembly, so as to remove the burrs on the cage surface and reduce the factors that generate noise. On the premise of ensuring the flexible rotation of the finished product, shrink the cage as tight as possible.

5. Improve the assembly quality of bearings

(1) Improve demagnetization quality of ferrule. The electromagnetic centerless fixture is used in the grinding process of bearings, and demagnetization is required after such processes are completed. Measure the value with the residual magnetism measuring instrument to ensure that the residual magnetism is within the allowable range. If the demagnetization of parts is not clean, dirt such as iron filings will be absorbed, which will seriously affect the vibration value and noise of bearings.

(2) Strictly control the surface defects of the working surface of bearing parts caused in and between processes, such as scratches left by bumps and bruises. Such scratches generate periodic vibration and noise, which is characterized by wide spectrum and large energy. Therefore, from the first process of grinding to the completion of assembly, it is always required to handle the products with care to avoid bearing bruises.

(3) Improve bearing cleanliness. During the grinding process of bearings, the cleaning quality should be improved. The grinding fluid and cleaning fluid used for treatment shall be purified in real time. During cleaning, it is strictly forbidden to bump or scratch the trench. Carry out effective and thorough ultrasonic cleaning for spare parts and finished products after assembly.

(4) When assembling the bearing, use CM group clearance. In the process of trial production, the vibration and noise of bearings with different clearance groups were compared and analyzed. From the test results, small clearance is conducive to reducing the vibration and noise of bearings. This is because when the clearance is small, the number of steel balls bearing the load increases, the contact stress decreases, and the operation is stable. At the same time, the axial and radial displacement values of the bearing during operation are reduced, so as to avoid the formation of excessive impact values, and to a certain extent, the sources of vibration and abnormal sound are reduced.

(5) High precision steel balls with good waviness, roughness and spherical deviation and surface strengthening treatment are adopted.

(6) Low noise has high requirements on the radial circular runout of the inner and outer rings of the complete set of bearings and the runout of the inner and outer ring ends of the complete set of bearings on the raceways. Therefore, this process measures the parallelism of the end faces and the symmetry of the raceways to ensure high ultra precision quality.

6. Vibration test results

The lubricating grease with high cleanliness and low noise is used. After assembly, according to JB Π T7047-1999 Technical Conditions for Vibration (Acceleration) of Deep Groove Ball Bearings of Rolling Bearings. The 50 sets of 6204E randomly selected were tested without and after greasing with an acceleration vibration tester S0910. See Table 2 for the test results.

Table 2 Vibration Test Results of 6204E Finished Bearing

In order to better evaluate the noise level of the bearing, in the actual measurement, measure the vibration value of three evenly distributed points on the front and back of each set of bearings, and take the larger test value as the effective vibration value of the bearing.

(1) It can be seen from Table 2 that low-noise lubricating grease can greatly reduce the vibration and noise of bearings. After grease injection, 100% of them meet the requirements of Group Z2 (40 dB), and 98% meet the requirements of Group Z3 (38 dB). The products have a certain reserve accuracy. Affected by the stability of the machine tool, the unevenness of the quality of the grinding wheel and the dressing quality of the grinding wheel, the unevenness of the replacement and dressing quality of the oilstone and the oilstone in the channel superfinishing process, and other factors that are difficult to avoid in the system, the entire process system has a certain volatility when meeting the requirements of the processing accuracy, so the vibration value data in Table 2 has a certain degree of dispersion.

(2) During the test, it was found that the vibration value of five sets of bearings tested on one side was relatively high, and the grease was washed away. After disassembly, it was found that one side of the channel had poor superfinishing quality, leaving grinding wheel marks. This shows that there is a certain deviation between the swing center of the oilstone and the groove curvature center after groove grinding, which affects the surface processing quality of the groove. According to the amplified sound signal of the loudspeaker, it is judged that there are 5 sets with obvious abnormal sound, and foreign matters are found to invade the lubricating grease inside, and the ultra precision quality of the channel is not high.

7. Conclusion

(1) Through the trial production of 2000 sets of 6204E with a new process, the final vibration value test results verify the feasibility and effectiveness of this process, which has accumulated desirable production experience for the production of low-noise bearings.

(3) Improve the positioning accuracy of the end face and outer diameter, provide a good process reference for the grinding and superfinishing of the bearing raceway, and help to process a good raceway surface quality. The vibration and noise level of the bearing can be effectively reduced by adjusting the process parameters of channel superfinishing to form uniform and fine stripe lines on its surface.

(4) It is also an effective measure to reduce bearing vibration and noise to improve the assembly quality of bearings, strictly implement the process specifications, and eliminate the bumps on the working surface of bearings.

More about KYOCM Cylindrical Roller Bearing：

The bearings can meet the challenges of applications faced with heavy radial loads and high speeds. Accommodating axial displacement (except for bearings with flanges on both the inner and outer rings), they offer high stiffness, low friction and long service life.

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