Powder metallurgy is an industry that uses special materials to make various parts. It is an industry that uses reduced iron powder or electrolytic copper powder as raw materials to make various castings, forgings, gears, bearings, and standard parts. It is widely used in transportation machinery, industrial machinery, and electrical machinery.
Powder metallurgy bearings are one of the fields, also known as oil containing bearings or sintered bearings, mainly used for micro motors, such as electroacoustic devices, household appliances, electronic computers, and other motors. They are made from iron powder and copper powder through mold pressing, high-temperature calcination, and high-pressure polishing. The product has a certain degree of porosity, and the main difference from general bearings is that it can contain oil. There is no need to add oil during use, and the oil content of oil bearing is generally controlled at no less than 18% (volume fraction, up to 22% to 24%). The lubricating oil inside the bearing is called powder metallurgy bearing lubricating oil. According to the metallurgical powder material of the bearing, powder metallurgy bearings are mainly divided into copper based and iron based. Copper based bearings are generally used in motors with higher speeds (such as motors above 20000r/min), while iron based bearings are mostly used in motors with relatively lower speeds (500-2500r/min) (there are also a few motors with speeds above 10000r/min), So the corresponding powder metallurgy bearing lubricants include copper based bearing oil for high speed and iron based bearing oil for low speed.
1. Domestic and international development status
The powder metallurgy industry developed earlier in foreign countries, starting in the 1880s. At that time, the products were mainly mechanical parts, friction materials, collector materials, etc. Bearing products were products from the 1930s, with Japan developing rapidly. In 1956, the total output of bearing products was 494 tons (317 tons based on copper and 177 tons based on iron). In 1963, it increased to 1586 tons (978 tons based on copper and 608 tons based on iron), an increase of three times, The corresponding lubricating oil was also added with antioxidants from the general mineral oil at the beginning of Z, to the later powder metallurgy bearing specific oil. The performance of specialized oils has also evolved from general lubricating oils to lubricating oils with high viscosity index, including anti oil splashing properties, oxidation resistance, wear resistance, and rust prevention.
In China, it began in the mid to late 1950s, and in the 1980s, there was a strong demand for copper based bearings. Later, due to the low cost of iron-based bearings, large-scale development of iron-based bearings began, and most of the products produced were used for general motors in gears and machine tools. With the development of the electronic industry and the emergence of micro motors, there have been supporting micro bearing production. The lubricating oil used is mainly mechanical oil, followed by 68 and 100 ordinary hydraulic oils, which are generally used according to the requirements of motor manufacturers. Export products use internal combustion engine oil (Mobil 1 # is commonly used in export products in Hong Kong, Macao, and southern regions), so the use of powder metallurgy bearing lubricating oil in China is relatively scattered, There is no national standard or specialized oil production yet.
Characteristics of powder metallurgy bearings and lubricating oil
Bearings are generally divided into two categories: rolling bearings and sliding bearings. The influence of lubrication conditions on rolling bearings is not significant, but it has a direct impact on the performance of sliding bearings. Sliding bearings are divided into dry bearings and powder metallurgy bearings. Dry bearings are lubricants (such as graphite, MoS2, Pb, etc.) dispersed in a bronze matrix, with high friction coefficients and suitable for high-temperature working conditions. Powder metallurgy bearings are suitable for areas with low corresponding loads and low friction coefficients in boundary lubrication areas. It has several characteristics: because it contains lubricating oil itself, it does not need to be frequently replenished (for general pumps) or not replenished (for micro motors) during use; Easy to produce, can be directly pressed into various shapes of bearings; In the absence of oil replenishment, the service cycle is not less than 1000h, and micro motors are generally not less than 2000h, which is very helpful for equipment in the home appliance industry; In addition, it has a lower production cost than general metal bearings and is suitable for mass production.
Due to the special manufacturing process of powder metallurgy bearings, their lubrication structure and lubrication mechanism are different from general metal bearings. Generally, when the shaft starts to rotate, lubricating oil that accumulates in the upper space that does not bear the load adheres to the upper part of the shaft. As the shaft rotates, the oil is gradually sent into the narrow slot at the lower part of the shaft, separating the shaft from the bearing and providing lubrication. When the oil bearing is not rotating, the contact surface between the shaft at the bottom of Z and the bearing is isolated by a thin layer of lubricating oil connected between the oil bearing gaps. When the shaft rotates, friction heat is generated due to the presence of concave and convex surfaces on the shaft and bearing surfaces, causing the surface temperature of the bearing to increase. The viscosity of the lubricating oil contained in the bearing itself decreases, accelerating its leakage from the gaps of the oil bearing, and isolating the shaft and bearing for lubrication. The characteristic of oil bearing lubrication is that lubricating oil flows from the unloaded side to the loaded side through the gap between the shaft and the bearing, while the opposite is true inside the bearing. Lubricating oil flows from the loaded side to the unloaded side.
3. Requirements for lubricating oil for powder metallurgy bearings
3.1 Appropriate PV value and friction performance
For general bearings, their load, speed, and their product are the basic factors for bearing load Z. For powder metallurgy bearings, when selecting bearing materials, it is necessary to consider the load P and allowable speed. In Japan and the United States, there are regulations for the load, speed, and product P of iron-based and copper based oil bearings.
The lubrication of powder metallurgy bearings belongs to boundary lubrication. At the beginning, due to the generation of metal friction, the friction coefficient is relatively high, about 0.15-0.30; But as it operates, the oil between the gaps of the oil bearing slowly forms an oil film, reducing metal friction and friction coefficient, which is about 0.05-0.10, equivalent to fluid lubrication. The friction coefficient varies with different PV values. Generally, the friction coefficient gradually decreases with the increase of P value. When the Ijg value is constant (i.e. a metallurgical material is selected), the friction coefficient is also related to the viscosity of the lubricating oil used. If the viscosity is too high, the oil film is thick, and the oil is not easily absorbed into the bearing; If the viscosity is too low and the oil film is thin, oil can easily overflow from the bearings and seep out of the motor during operation. Therefore, if the viscosity of the oil is too high or too low, it may increase the friction of the bearings. Therefore, appropriate viscosity and higher viscosity index are important factors for oil bearing.
3.2 Appropriate viscosity and higher viscosity index
Electric motors equipped with powder metallurgy bearings generally do not require oil replenishment during use. Insufficient or no lubricating oil in the bearings can cause shaft and bearing wear, resulting in friction noise and bearing burnout. Therefore, in addition to the general lubricating oil performance, powder metallurgy bearing oil requires that the lubricating oil in the bearing does not leak out of the motor under high-speed operation conditions (i.e., the anti splash oil characteristics of the oil), that is, it requires a high viscosity index and good adhesion ability. When the loss of leaked oil reaches 50%, it affects the service life of the motor, and the oil content of powder metallurgy bearings (test method GB5165) is closely related to the viscosity of the oil. Low viscosity oil has a high oil content, while high viscosity oil has a low oil content. When the motor is running at high speed, the oil temperature rises, which can easily cause a decrease in oil viscosity, evaporation of light components, and a decrease in oil content in the bearings, So it is required that the light components of the oil should be small, that is, the viscosity of the oil should not be too low, and it must have appropriate viscosity and a high viscosity index to maintain the oil content in the bearings at around 18%.
3.3 Anti oil spill characteristics
There are a large number of pores in the oil bearing body that are connected to the outside world. Therefore, during the use of high-speed motors, friction heat is generated due to the relative sliding between the bearing and the shaft, causing the lubricating oil in the oil bearing to seep out or be thrown out, resulting in oil outside the motor. At the same time, due to the loss of lubricating oil in the bearing, friction increases, noise increases, and even the bearing bites and burns the motor, affecting the service life of the motor. Therefore, additives for anti oil spill components must be considered in powder metallurgy bearings.
3.4 Other performance
In addition to the above performance, powder metallurgy bearing oil must also have good oxygen resistance. The acidic substances and oil sludge generated by oil oxidation can clog the bearing gaps, causing oil free contact between the shaft and the bearing, resulting in wear and motor burnout. At the same time, for iron-based bearings, they are prone to rust when in contact with water in the air. It is required that powder metallurgy bearing oil must have excellent rust prevention performance, in addition to certain wear resistance and non-toxic (food grade) properties used in food crusher motors.
4. Market situation of lubricating oil for powder metallurgy bearings
According to incomplete statistics, there are currently no less than 600 domestic units producing powder metallurgy bearings, and some well-known manufacturers such as Shenzhen Factory. Their production capacity is generally between 100-500t/a, and these manufacturers, Zhende Warehouse Electric Motor Co., Ltd. and Yangzhou Baolaide Powder Metallurgical Bearings, are mainly concentrated in the economically developing Guangdong, Shenzhen, and Southeast Along Co., Ltd., as well as in the social welfare powder metallurgical product areas of Xinyi City, Guangdong Province.
From the perspective of oil usage in the market, most bearing manufacturers use mechanical oil, 68 # or 100 # ordinary hydraulic oil, and some also use Mobil 1 #, Mobil 1130, or Japanese brand 68 # hydraulic oil according to the requirements of motor manufacturers. However, the common problem is that there is oil leakage during the effective use period of the motor, And Yangzhou Baolaide Powder Metallurgical Bearing Co., Ltd. uses specialized oil designated by Japanese bearing manufacturers to prevent oil leakage.
From the market and current oil usage of powder metallurgy bearings, it can be seen that the market for lubricating oil for powder metallurgy bearings is very large. It is expected that several bearing manufacturers with an annual output of over 100 tons will all use the matching powder metallurgy bearing special oil, and the amount of this oil will be greater than 100 tons.
5. Conclusion and recommendations
(1) Powder metallurgy bearing lubricating oil is a lubricating oil that needs to be added in the production of powder metallurgy bearings. Due to the special structure of powder metallurgy bearings, powder metallurgy bearing lubricating oil is required to have properties that match its performance, such as appropriate viscosity, viscosity temperature characteristics, anti oil splashing characteristics, etc.
At present, powder metallurgy bearings account for an increasing share of the domestic bearing market, while the lubricating oil used is still only in the mechanical oil stage, and there is a common phenomenon of motor oil leakage. The development of copper based specialized oils for use at high speeds and iron based lubricants with rust resistance at general speeds has certain market potential and technological content.
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