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Abstract: With the rapid development of modern agriculture, the development of agricultural mechanization is particularly important. As the core component of agricultural machinery, sliding bearings have a wide range of applications, mainly used to support shafts and bearing components, which can effectively ensure the rotational accuracy and reduce the friction and wear rate of the shaft, and their performance will directly affect the service life of the entire agricultural machinery. This study reviews the common faults of sliding bearings in agricultural machinery, analyzes the development status of high-performance sliding bearing materials, and discusses the future development trends of sliding bearing materials in agricultural machinery equipment.
Keywords: sliding bearings; agricultural machinery; strength; friction; corrosion
Sliding bearings in agricultural machines need to work at high speeds and are one of the important equipment in agricultural machinery. The sliding bearings of agricultural machinery play an important role in the development of modern agriculture, and they are also the most commonly used supporting shaft parts, with a wide range of applications for different agricultural machinery. The damage of sliding bearings has become a common problem in agricultural machinery equipment. This is mainly due to the fact that the working environment of agricultural machinery is often harsh, especially in mountainous areas where the environment is generally humid and the terrain is uneven; the continuous working time is long; during use, it often comes into contact with stones, water, mud, pesticide residues, etc.; during use, it is difficult to add lubricating oil. These situations can easily cause sliding bearings to crack, corrode, burn out, etc. Once the sliding bearings have problems, their agricultural machinery equipment will be directly affected, which may cause unstable equipment operation and insufficient accuracy, and even directly shorten the service life of the agricultural machinery equipment. Therefore, it has certain research value to analyze the common faults of sliding bearings in agricultural machinery.
After a period of use, the sliding bearings of agricultural machinery are prone to cracks. When working, the sliding bearings are often hit by hard objects such as stones, and cracks appear due to the impact force; when the agricultural machinery load is too large, the sliding bearings will deform or even crack [3]. These situations will greatly shorten the service life of agricultural machinery, and with the increase of use time, the cracks will increase, eventually causing the bearings to break. Therefore, it is necessary to further develop high-performance bearing materials to compensate for the strength defects of agricultural machinery sliding bearings, meet the daily needs of agricultural machinery equipment use, reduce the occurrence of faults, and extend the service life.
First of all, when using agricultural machinery equipment, most users add lubricating oil to the contact surface of the sliding bearings of the agricultural machinery equipment and the shaft. However, if the lubricating oil is excessively consumed during use, the lubricating film cannot be formed, causing the bearing and shaft to rub against each other, causing excessive bearing wear and damage, and easily causing burning or excessive wear [4]. Secondly, during the working process, moisture will enter the interior of the agricultural machinery and come into contact with the sliding bearings. This phenomenon will destroy the oil layer structure of the lubricating oil and ultimately cause damage to the sliding bearings [5]. It is difficult to add lubricating oil without disassembling some parts of the agricultural machinery equipment. Finally, during use, if high-hardness mud and sand particles flow in, it will cause wear on the sliding bearings, forming localized deep scratches, which will enhance the function of abrasive particles and further aggravate the damage process, forming a vicious cycle. Therefore, it is necessary to improve the friction performance and self-lubricating ability of sliding bearings in agricultural machinery equipment, to ensure that agricultural machinery equipment works to the maximum extent during operation, and to achieve efficient and high-quality farming.
Agricultural machinery equipment will come into contact with soil during farming, and the soil contains media such as water, air, and pesticide residues, which will corrode the agricultural machinery equipment and internal parts [6]. Corrosion is also a common problem in the use of agricultural machinery equipment, which will not only damage the machine body, but also invade the interior of the machine and corrode the internal bearings. Because it is difficult to clean the bearings after use and the storage conditions are often humid, the damage to the bearings is intensified, resulting in a shortened service life of agricultural machinery equipment. According to the degree of corrosion, surface damage can cause slight corrosion to the bearings, and severe corrosion can directly cause the bearings to break. When the corrosion reaches a certain degree, the bearings need to be replaced, which will cause a lot of waste of metal materials and economic losses. Therefore, in order to adapt to the harsh working environment, sliding bearing materials for agricultural machinery equipment are required to have higher strength, self-lubricating properties, and corrosion resistance.
The Babbitt alloy is divided into tin-based, lead-based, and cadmium-based types. Babbitt alloys have strong embeddability and compliance, high strength, and good corrosion resistance. The soft matrix of the Babbitt alloy forms an internal concave structure, while the hard particles protrude outward, forming many small voids. These voids can store lubricating oil well, making Babbitt alloy bearings have good friction performance in agricultural use. The outward protrusion of hard particles not only brings the advantage of storing oil but also enhances its bearing capacity. Sometimes, a small amount of arsenic is added to the manufacturing of Babbitt alloy bearings to reduce component segregation. However, Babbitt alloy bearings need strict control of the use temperature because the hardness and fatigue strength of Babbitt alloy bearings will decrease by two-thirds when the temperature is higher than 150 ℃. Although tin-based Babbitt alloy has stronger strength and corrosion resistance than lead-based Babbitt alloy, the economic cost of tin is higher, so it is used less.
The advantages of Babbitt alloy are good embeddability and compliance, while the disadvantages are low bearing capacity, poor heat resistance, and low fatigue resistance.
Aluminum-based alloy bearings are relatively light in weight, high in strength, and have good bearing capacity. Especially at high temperatures, their bearing capacity is more prominent. Aluminum-based alloy bearings are mainly divided into Al-Sn, Al-Zn, and Al-Pb. Among them, the amount of tin in Al-Sn alloy bearings plays a crucial role in the alloy’s performance. According to the different tin contents, it can be divided into low-tin alloys, medium-tin alloys, and high-tin alloys. In the case of low tin, it has high compression resistance and strong anti-corrosion ability. As the tin content increases, the hardness, strength, and bearing capacity of aluminum-tin alloys are inversely proportional. However, their bite resistance performance and embeddability are better at high temperatures. The zinc content in Al-Zn alloys also plays a crucial role. If the zinc content is too low, the soft phase is difficult to play a role. If the zinc content is too high, the hardness will be too great. Therefore, it is necessary to strictly control the zinc content. Al-Pb alloys have a good self-lubricating effect and can reduce friction and prevent seizure. However, due to the toxicity of lead, attention should be paid during use.
The advantages of aluminum-based alloys are good wear resistance and corrosion resistance, while the disadvantages are weak dry friction performance and weak self-lubricating ability.
TiAl-based alloys have excellent high-temperature performance and low density, and are currently one of the most popular high-temperature structural materials in the world. Because TiAl-based alloys possess both metallic and covalent bonds in their crystals, TiAl-based alloys not only possess the characteristics of metallic toughness but also have the high-temperature resistance of ceramics [8,10]. Between 500 ℃ and 1000 ℃, the TiAl-based alloys that are commonly used include Ti3Al (α2), TiAl (γ), and TiAl3. The usage environment for Ti3Al (α2) cannot exceed 650 ℃, as the structure of Ti3Al (α2) is unstable in poor environments. TiAl (γ) has good comprehensive performance, low relative mass, and has the characteristics of high-temperature resistance. The solid solubility range of TiAl3 is too narrow, and the ductility at room temperature is poor. The temperature requirements for its production environment are very demanding, and processing at room temperature is difficult [8,10].
The advantages of TiAl-based alloys are their lightweight, high strength, and excellent properties such as creep resistance and oxidation resistance at high temperatures. However, the disadvantage of TiAl-based bearing alloys is their brittleness and poor ductility at room temperature, making them difficult to mechanically process.
Iron-based alloy materials are a common type of alloy material in agricultural machinery equipment. They have strong comprehensive properties, relatively low prices, and are suitable for the production and manufacturing of agricultural machinery bearings. Iron-based alloys are divided into Fe-C series bearing materials, Fe-Cu-C series bearing materials, Fe-Mo-C series bearing materials, etc. [8].
Due to the high hardness of iron-based materials, they are easily damaged over a long period of friction. Graphite, on the other hand, has a non-reactive chemical property and also has anti-corrosion properties, making it an excellent lubricating material that can be added to iron-based alloys. When burned in air, graphite forms carbon dioxide, thus forming organic acids. It is not easily reactive with water, pesticide residues, and other substances, making it suitable for use in harsh agricultural environments.
By adding Cu elements to Fe-C alloys, the resulting alloy not only retains the excellent properties of Fe-C but also has the advantages of Cu-based materials. The tensile strength of Fe-Cu-C materials can reach up to 400 MPa, and Cu can effectively suppress the growth of grain size, disperse grain size, and improve wear resistance and self-lubricating function [8,11-12].
Mo elements have good strength, hardness, and corrosion resistance at high temperatures, making them ideal for use in Fe-Mo-based bearing materials, which have excellent friction properties [8].
The advantages of iron-based alloy materials are their strong compressive ability, high hardness, and low economic cost. However, they have the disadvantages of high noise, poor corrosion resistance, ineffective friction reduction, weak embeddability and adaptability, and inadaptability to low-speed environments.
Copper-based alloys are alloys composed of brass as the base material and other trace elements. Copper-based alloys have excellent fatigue strength and carrying capacity, and also have good conductivity and anti-corrosion properties, and a low coefficient of friction. Therefore, copper-based alloys have been widely used in the manufacturing process of sliding bearings in agricultural machinery equipment. The most widely used sliding bearing material in copper-based alloys is bronze, which is mainly divided into leaded bronze, tin bronze, aluminum bronze, etc. [8]. Due to the strong toxicity of Pb, leaded bronze sliding bearing materials are limited. Tin-copper alloys are often used to make vehicle and other industrial products in contact-friction parts due to its good thermal conductivity and wear resistance. However, China’s tin reserves are dwindling, with declining grades and high prices [13]. Aluminum bronze is widely used in shafts, bushings, gears, and other components due to its excellent properties such as high strength, wear resistance, heat resistance, and corrosion resistance. Aluminum bronze-based bearing materials prepared by powder metallurgy not only have high strength and good corrosion resistance but also have a certain amount of pores inside, which can be used to store lubricating oil and play a self-lubricating role during service. To reduce the friction and wear coefficient of copper-based sliding bearing materials, lubricants such as graphite and molybdenum disulfide are usually added to enhance its anti-friction and anti-wear capabilities, thus enhancing the working stability and wear resistance of sliding bearing materials, allowing bearings to continue to work properly without adding lubricating oil.
The development of agricultural machinery plays an important role in the implementation of the rural revitalization strategy. As the core component of agricultural machinery, sliding bearings directly affect the service life of agricultural machinery. Therefore, the performance of sliding bearings is essential for the development of agricultural mechanization. With the rapid development of agricultural machinery, the requirements for sliding bearing materials are increasingly high. Moreover, due to the complex working environment of agricultural machinery, especially when agricultural equipment is used for operations in mountainous areas, it often aggravates problems such as sliding bearing fracture, friction and wear, and corrosion. Therefore, sliding bearing materials need to have higher hardness, self-lubricating ability, and corrosion resistance. Due to their high fatigue strength and carrying capacity, excellent anti-friction and anti-corrosion properties, excellent thermal conductivity, low coefficient of friction, and good self-repairing properties, copper-based alloys have excellent application and development prospects in the field of agricultural machinery.