There are many factors affecting the sealing performance of bearings, such as the rationality of sealing structure design and the machining accuracy of products and seals.
Sealed bearing has the advantage of no additional lubrication equipment and mechanism, which can greatly reduce the volume of the whole machine in mechanical design and reduce the cost of the whole machine. At present, it is more and more widely used in motors, household appliances, mechanical equipment, automobiles and other equipment.
Sealed bearings lubricated with grease are generally used in parts that are not easy to disassemble. Moreover, due to the lack of lubrication device, the general space is relatively narrow. In a sense, the performance of sealed bearings is more important than fatigue life. On some supporting hosts, the performance change of sealed bearings can cause rapid failure and damage of bearings. Compared with foreign imported products, there is still a big gap in the three main indicators of bearing sealing performance: grease leakage rate, temperature rise and dust-proof performance, resulting in most of the sealed bearing market being occupied by foreign bearing companies.
There are many factors affecting the sealing performance of bearings, such as the rationality of sealing structure design, the machining accuracy of products and seals, the performance of lubricating grease and the filling amount of lubricating grease. This paper expounds the main reasons affecting the sealing performance of bearing through the statistics of the test data of sealing bearing performance of our company for a long time.
1. Influence of sealing structure on sealing performance
Sealed bearings are generally divided into contact seal and non-contact seal. During the operation of the bearing, there will be a "pumping" phenomenon on both sides of the sealing ring due to different pressures, that is, during the thermal expansion in the bearing, the internal air can be discharged from the bearing, on the contrary, the external medium is not allowed to enter, so the contact part should be the outer slope. There is an important mechanical balance in the sealed medium (Fig. 4), namely:
△ P = (Ps – Pv) - (Pa + Ph –Pc)
PS - hydraulic pressure of the medium itself
Pv -- pump suction pressure (eddy current pressure)
Ph -- backflow pressure
Pa -- Atmospheric pressure
Pc -- capillary pressure
△ P -- pressure difference between two sides of sealing medium (≤0).
Sealing is possible only when the design fully ensures that AP is always less than or equal to zero (△ P≤ 0). Therefore, if the sealing structure design is unreasonable, the sealing performance will be greatly reduced.
2. Influence of machining accuracy on sealing performance
At present, the sealing structure is mostly designed with the outer ring sealing groove. Because the detection of the outer ring sealing groove is very difficult, most domestic enterprises now guarantee the structural size of the sealing groove by forming tools. However, from the detection of many sealing performance tests of our company, the phenomena of different depth, width, roundness and coaxiality of the sealing groove often appear in our products. Even lead to the seal cannot be fully positioned in the outer seal groove, and there is activity or even relative rotation when the bearing rotates at high speed, resulting in the seal failure of the bearing.
3. Influence of grease filling amount on sealing performance
The amount of grease injected into the bearing has a great influence on the grease leakage rate of the bearing. The following table is the data chart of sealing test of 6216-2Z bearing in our position:
It can be seen from the table that with the increase of grease injection, the grease leakage rate of bearing maintains an upward trend. A large number of tests have proved that when the grease injection amount is less than 30% of the volume of the inner cavity of the bearing, there is little grease leakage because the grease in the bearing does not reach the saturation state. When the grease injection exceeds 30%, the bearing will be partially saturated. When the grease injection reaches 50%, the lubricating grease in the bearing will be fully saturated and the excess grease will be “squeezed out”. Special attention is paid to that sometimes, although there is not much grease injection, the grease injection method is not standardized and scientific, resulting in uneven grease in the bearing cavity, local saturation in the bearing and rapid "extrusion" of grease in the early stage.
The dust-proof performance is also closely related to the grease injection amount. The excessive leakage of lubricating grease leads to a sharp increase in the dust adsorbed on the sealing lip of the bearing, an increase in the amount of dust introduced into the bearing, and too much grease injection is unfavorable to dust prevention.
The test shows that with the increase of grease injection, the temperature rise of the bearing will also rise. This is because the amount of grease in the bearing increases, the viscous resistance of the lubricating grease increases, and the stirring heat of the steel ball and cage in the bearing increases, resulting in the increase of the temperature rise of the bearing.
4. Effect of grease performance on sealing
Grease is one of the main factors affecting the performance and service life of sealed bearings. By analyzing the causes of bearing damage, about 43% of the bearings are fatigued due to poor lubrication. According to the noise life and precision life, it is evaluated that the failure of bearings due to lubrication problems is more than 50%. The poor sealing performance of the bearing will make it difficult to ensure the good lubrication state of the bearing, so that the service life and reliability of the bearing cannot reach the expected indicators. Domestic bearing grease is mainly mineral lithium grease, and its quality still lags behind that of imported products, mainly due to low vibration and noise reduction ability, poor cleanliness and quality stability. Therefore, imported grease is widely used at present. In terms of grease alone, from the perspective of improving the grease leakage performance and dust-proof performance of the seal, it is better to select the grease with high viscosity. If the viscosity of the grease is high, it is not easy to flow out, but it is unfavorable to the temperature rise performance. The viscosity of the grease is high, the viscous resistance of bearing operation increases, and the temperature rise increases. Therefore, comprehensive consideration should be given to ensure the performance, service life and reliability requirements of the bearing.
5. Conclusion
There are other influencing factors, such as the machining accuracy of the bearing sealing structure, and even the accuracy of the bearing itself, which have an important impact on the sealing performance. However, according to the data summary of the sealing test conducted by our company for many years, the factors affecting the sealing performance of the bearing are mainly the above factors, which generally do not appear independently. Therefore, in order to fundamentally improve the sealing performance of the sealed bearing, we should strictly control each process from the structural design of the bearing to the processing, assembly and grease filling of the bearing.
More about Marginal Plastic Sliding Bearings:
Marginal Plastic plain bearings provide excellent wear resistance and low friction in both dry and lubricated operating conditions over a wide range of applications. Engineered plastic bearings are made from thermoplastic bearing material processed by injection moulding. It uses high-performance plastics as the raw material, and filled high-strength fibers and special solid lubricants.
As a specialized manufacturer of self-lubricating bearings & bushings, Marginal Bearing, which was founded in the year 2010, is devoted to researching and producing new self-lubricating bearing materials. After years of effort, we have successfully developed various bearing materials with many different standard bearing sizes, including Three-layer Metal-polymer, self-lubricating bearings, Two-layer, metal-polymer, self-lubricating bearings, Bi-metal composite, self-lubricating bearings, Copper wrapped boundary lubricating bearings, High performance steel bearing, Plastic compound materials bearings and Filament wound composite materials bearings.
With more than 10 years of experience on the researching, producing and promotion of self-lubricating bearings, Marginal Bearing can deliver not only high-performance products but also high-quality services, such as material selection, simulation test, engineering design and installation. The self-lubricating bearing and technology is now performing its beneficiary roles in the traditional industry area and further more providing variable solutions in the industry areas related to our daily life such as the public transportation, entertainment, energy, civil construction, environment protection facilities and aerospace industry.
Marginal Bearing will continue to create value through high-performance self-lubricating products and protect the earth by using environmentally RoHS-compliant materials. We believe that our responsibility is to promote enterprise change to create an internal driving force for our society.
