The function of sliding bearings is to support loads and slide against each other. Sliding bearings can be divided into two types: increased friction and reduced friction. Internal combustion engine crankshaft sliding bearings are anti friction, and their bearing alloy layer must be a low friction coefficient material, which also reduces the friction and wear of the bearings. Alloy bearing bushing are important friction pairs in engines. The wear-resistant alloy used to manufacture sliding bearings and their lining is called bearing alloy, also known as sliding bearing alloy or bearing alloy.
1. Performance characteristics and microstructure of alloy bearing bushing for internal combustion engines
The function of sliding bearings is not only to support rotating parts but also to withstand the inertial force of gas pressure moving parts, as well as to withstand the thermal load generated by friction. On the bearing, the part of the bearing that directly contacts the shaft bears the load and has relative motion with the shaft. In order to reduce friction and wear, various requirements are put forward for the material of the bearing. In addition to requiring low friction coefficient and wear resistance between the friction pairs, the following points should also be met: it should have sufficient compressive strength, fatigue resistance, and the ability to withstand impacts.
The part of the shaft supported by bearings is called the journal, the parts that match the journal are called the bearing bushing, the cylindrical bearing bushing are called the sleeve, and the part where the bearing bushing are installed is collectively called the shell. The upper half is called the bearing cover, and the lower half is called the bearing seat. The cover and seat are connected by bolts, and the joint surface of the two is located by a stop or a pin. Different thicknesses of washers can be placed to adjust the bearing clearance.
The bearing shell of an internal combustion engine is a sliding component in the sliding bearing of the internal combustion engine, with a semi-circular cylindrical surface in the shape of a tile, called the bearing shell. Due to the heavy weight of the internal combustion engine shaft, rolling bearings cannot be used, only sliding bearings can be used. The joint surface of most bearings is horizontal, and there are also inclined ones to adapt to the requirement of the load direction being close to perpendicular to the joint surface. For the convenience of lubricating oil entering between the friction surfaces, there are oil injection holes on the bearing cover and axial oil grooves on the bearing bushing for distributing lubricating oil. The bearing covers and seats are mostly made of cast iron, and those that bear heavy loads are made of cast steel or welded steel plates.
There are several types of radial sliding bearings, including split type, integral type, flange type, and self positioning type. The bearing cover of the split type bearing can be opened for easy installation into the shaft neck, and it is easy to adjust the bearing clearance after the bearing shell is worn. The integral bearing structure is simple, but the journal must be installed from one end, and the clearance cannot be adjusted after wear. The installation surface of the flange bearing is perpendicular to the centerline of the bearing. Self positioning bearings can automatically adjust the axis to adapt to the bending deformation of the shaft. The inner diameter of a radial sliding bearing is called the bearing diameter. The axial dimension of the bearing shell is called the bearing width. The ratio of bearing width to bearing diameter is called the aspect ratio, which is generally taken as 0.4-1.5. To reduce axial dimensions (such as in internal combustion engines), it is also taken as 0.25.
In sliding bearings, the bearing shell and lining are directly used in conjunction with the shaft neck, causing friction between them and also bearing alternating and impact loads. Due to the complex manufacturing process, high cost, and difficult replacement of the shaft, which is an important component on the machine, to ensure minimal wear on the shaft, the hardness of the bearing shell should be much lower than that of the journal. If necessary, the worn bearing shell can be replaced and the shaft can continue to be used.
Sliding bearings should have sufficient compressive strength and fatigue resistance; Good anti friction performance (low friction coefficient); Good reserve of lubricating oil function; Good running in performance; Good thermal conductivity and corrosion resistance; Good process performance; Make it easy to manufacture and affordable. If one material cannot meet the above performance requirements simultaneously, sliding bearing alloys can be cast onto the bearing bushing of 08 steel by casting method to produce bimetallic bearings.
2. Commonly used alloy materials for internal combustion engine bearing bushing
Internal combustion engine sliding bearing materials are materials used to manufacture parts such as bearing bushing and sleeves. These parts are friction couples that directly fit with the shaft or journal. Sliding bearing materials must have good friction reduction, wear resistance, embedding, compliance, corrosion resistance, and sufficient load-bearing capacity to reduce friction and wear and extend the service life of bearings. The bearing bushing are made of sliding bearing materials with good anti friction properties, including metal materials (such as copper alloy, aluminum alloy, and babbitt alloy) and non-metallic materials (such as plastic, graphite, rubber, and wood). Some metal materials with good anti friction properties have low strength (such as Babbitt alloy), and only a layer is cast on the surface of the bearing with high material strength, called bearing liner. The part that supports the bearing liner is called the bearing backing and is commonly made of soft steel, cast iron, or bronze. To further improve the surface performance of the bearing liner, a thin layer of metal (such as indium) with better anti friction properties can be plated on its surface. A bearing made of multiple layers of different metal materials is called a multi-layer metal bearing. A thick walled bearing is one in which the shape of the inner hole is not significantly affected by the shape of the bearing seat hole; The shape of the inner hole mainly depends on the shape of the bearing seat hole, which is called a thin-walled bearing. Thin walled bearing bushing are widely used in mass-produced machines such as internal combustion engines and air compressors.
The main items (indicators) for evaluating sliding bearing materials include: compressive strength and fatigue resistance, wear resistance, running in, friction compatibility to prevent adhesion with the journal, compliance to compensate for initial poor fit of the sliding surface, embedding of hard particles to reduce journal scratches, thermal conductivity, corrosion resistance, processing technology, price, etc.
The selection of sliding bearing materials should be based on factors such as load, speed, temperature, lubrication conditions, and service life. Common sliding bearing materials include babbitt alloy, copper based and aluminum based alloys, wear-resistant cast iron, plastic, rubber, wood, and carbon graphite. With the development of modern internal combustion engines towards high speed and large-scale, higher requirements have been put forward for the bearing capacity and reliability of sliding bearings. The bearing capacity of traditional Babbitt alloy bearing materials, especially at high temperatures, is relatively low. As the temperature increases, serious adhesive wear may occur, leading to tile burning accidents; Copper based bearing alloys have high load-bearing capacity and fatigue strength, but their surface compliance, embedment, and running in properties are poor, which can easily damage the shaft. Therefore, their surface? A layer of lead tin binary alloy or ternary alloy material is required.
3. Sliding bearings made of tin based bearing alloys
A low melting point alloy mainly composed of tin, antimony, and copper, also known as white alloy or bearing alloy. In 1839, American Babbitt obtained a US patent for this alloy. The matrix of the alloy is a solid solution soft tissue containing copper and antimony dissolved in tin, in which hard particles composed of tin copper antimony compounds are distributed. Soft tissues have good friction compatibility, compliance, and embeddedness, while hard particles have a certain ability to support loads. Babbitt alloy has low strength and can only be used as a bearing liner for soft steel, cast iron, or bronze bearings. Later, bearing alloys mainly composed of lead, antimony, and copper emerged. The former is called tin based babbitt alloy, while the latter is called lead based babbitt alloy. Tin based Babbitt alloy has a high price and is mainly used for important bearings and large bearings for high-speed and heavy-duty applications.
The performance of lead based Babbitt alloy is not as good as that of tin based alloy, but it is inexpensive, widely used, suitable for bearings with medium speed, medium load, and relatively stable load. Adding trace elements such as chromium and beryllium to manufacture high-strength babbitt alloys is a new development direction. The representation method of tin based bearing alloys is the same as that of other cast non-ferrous metals, for example, ZSnSb4Cu4 represents tin based bearing alloys with an average mass fraction of 4% for antimony and 4% for copper. Babbitt alloy is relatively expensive and has low mechanical properties. It is usually cast into the bearing shell of steel (08 steel) to form bimetallic bearings.
The disadvantage of tin based bearing alloy is its low fatigue strength and low allowable temperature (not higher than 150 ℃). Once the lubrication conditions are abnormal, the bearing is easily burned out. At the same time, its compressive strength is not high enough, making it easy to be damaged when subjected to high speed and heavy loads. Due to the high cost of tin, if conditions permit, lead based bearings can be used instead of tin based bearing alloys.
4. Sliding bearings made of lead based bearing alloys
Lead based bearing alloy is also a type of bearing alloy with soft matrix and hard points, which is composed of alloy elements such as antimony, tin, and copper. The strength, hardness, thermal conductivity, and corrosion resistance of lead based bearing alloys are lower than those of tin based bearing alloys, and the friction coefficient is larger, but the price is cheaper. Suitable for manufacturing bearing bushing with medium and low loads, such as automotive, tractor crankshaft bearings, railway vehicle bearings, etc. The soft matrix in commonly used lead based bearing alloys is a solid solution formed by antimony dissolving into lead (i.e α Solid solution and lead containing solid solution based on compound SnSb (i.e β The eutectic composed of solid solution (i.e α＋β Eutectic). Hard dots are compounds SnSb and Cu3Sn. The hardness, strength, and toughness of lead based bearing alloys are lower than those of tin based bearing alloys, but the friction coefficient is larger, the price is cheaper, and the casting performance is good. Lead antimony based lead based bearing alloys are the most widely used and are commonly used in manufacturing bearing alloys that can withstand low speeds and loads. Such as the crankshaft, connecting rod bearings, and electric motor bearings of automobiles and tractors, but their working temperature cannot exceed 120 ℃. The strength of lead based and tin based babbitt alloys is relatively low, and they need to be cast into steel bearing bushing (usually stamped with 08 steel) to form a thin and uniform inner lining in order to function effectively. This process is called hanging lining.
5. Sliding bearings made of aluminum based bearing alloys
With the development of automotive engines towards high speed, heavy load, and turbocharging enhancement, the working conditions of bearings have deteriorated, therefore, higher requirements have been put forward for bearing materials. Aluminum based bearing alloys basically possess the advantages of the above-mentioned bearing alloys and make up for their shortcomings, therefore, their application in automotive engines is increasing day by day. Aluminum based alloys include aluminum antimony magnesium alloy, aluminum tin alloy, and aluminum silicon alloy. They have high compressive strength and fatigue resistance, good thermal conductivity and corrosion resistance, and are inexpensive. However, their friction compatibility, embeddedness, and compliance are poor, and they are widely used in internal combustion engines and compressor bearings. Aluminum based bearing alloy is an alloy composed of antimony, tin and other alloying elements added to aluminum as the matrix, with low density, good thermal conductivity and corrosion resistance, and high fatigue strength. The advantages of this alloy are high thermal conductivity, corrosion resistance, fatigue strength, and high-temperature strength, as well as its low price. The disadvantage is that the coefficient of expansion is large and the anti bite ability is poor. Currently, high tin aluminum based bearing alloys are the most widely used. Suitable for manufacturing high-speed (13m/s) and heavy-duty (3200MPa) engine bearings. The commonly used brand is ZAlSn6Cu1Ni1.
Aluminum based bearing alloy is a new type of anti friction material with abundant raw materials and low price. It is widely used in high-speed, heavy-duty working automotive, tractor, and diesel engine bearings. But its coefficient of linear expansion is large, making it easy to bite and wear the shaft during operation. This can be solved by increasing the hardness of the shaft neck, increasing the bearing clearance, and reducing the surface roughness value of the bearing and shaft neck.
6. Copper based alloys used as sliding bearings
The copper based alloys used in sliding bearing materials mainly include brass, which is mainly composed of copper and antimony; Bronze primarily composed of copper and tin; And copper lead alloy (also known as lead bronze). Copper based alloys have high strength, good thermal conductivity and wear resistance, and allow for higher working temperatures than Babbitt alloys. However, their compliance, embeddedness, and friction compatibility are not as good as Babbitt alloys, and can be used as bearing pads or lining materials. Commonly used tin and phosphorus containing tin bronze, suitable for medium speed heavy-duty or impact loaded bearings; Tin bronze containing tin, zinc, and lead, suitable for bearings with medium speed and medium load; Lead bronze has a high load-bearing capacity and fatigue strength, making it suitable for bearings with high speeds and subjected to impact loads; Aluminum bronze has high strength and good corrosion resistance, making it suitable for low-speed and heavy-duty bearings. Brass generally has lower performance than bronze, but it is inexpensive and mainly used in low-speed bearings. However, brass containing manganese and silicon has better performance than tin bronze. Some bronze can also be used to manufacture bearings, so they are called copper based bearing alloys, such as tin bronze, aluminum bronze, lead bronze, and antimony bronze. Compared with Babbitt alloy, it has high fatigue strength and load-bearing capacity, excellent wear resistance, thermal conductivity, and low friction coefficient. Therefore, it can be used as a bearing to withstand high loads, high speeds, and high temperatures.
7. Sliding bearings made of multi-layer bearing alloys
Multilayer bearing alloy is a composite anti-wear material. For example, rolling one of tin antimony alloys, lead antimony alloys, copper lead alloys, aluminum based alloys, etc. together with low-carbon steel strips to form bimetallic composites. In order to further improve compliance, inlay and corrosion resistance, a soft and thin coating can be applied to the surface of the double-layer anti-wear alloy, which constitutes a three-layer anti-wear material with better anti-wear and abrasion resistance. The characteristics of this multi-layer alloy are to increase the steel back and reduce the thickness of the anti friction alloy layer to improve fatigue strength, and to use coatings to improve surface properties.
8. Conclusion
In summary, the structural structure of bearing alloys is characterized by the uniform distribution of hard phase particles on a soft phase matrix, or the uniform distribution of soft phase particles on a hard phase matrix. The bearing alloy should have good wear resistance and wear reduction performance; Has a certain compressive strength and hardness, and sufficient fatigue strength and bearing capacity; Good plasticity and impact toughness; Has good anti bite properties; Good compliance; Good inlay performance; It should have good thermal conductivity, corrosion resistance, and a small coefficient of thermal expansion. The bearing material requires a low friction coefficient with the shaft surface, minimal wear on the shaft neck, and the ability to withstand sufficient specific pressure, low expansion coefficient, and good corrosion resistance. For general performance engines, bearing bushing made of high tin aluminum alloy have a high cost performance ratio. Therefore, it has become one of the main materials for the bearing bushing of internal combustion engines. With the continuous development of science and technology and the automotive industry, the design of modern engines has developed towards the direction of "high speed, high pressure, high power, and low energy consumption". Researchers have applied micro nano technology in the field of special anti friction alloy materials, and have developed microcrystalline alloy materials that have special needs to meet certain single performance requirements. Special microcrystalline bearing materials not only fill the gap in anti friction materials in China, but also maintain synchronous development with world microcrystalline alloy technology in terms of material single performance.
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