Bearing code
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Is there any relationship between the diameter of a rolling bearing, its width series code, and its actual physical dimensions?
Rolling Bearing Table of Contents Basic characteristics of rolling bearings Classification of rolling bearings Selection of rolling bearing types Coding of rolling bearings Common questions and answers regarding the installation of rolling bearings A precision mechanical component that converts the sliding friction between a rotating shaft and its housing into rolling friction, thereby reducing friction losses, is called a rolling bearing. Rolling bearings generally consist of an outer ring, an inner ring, rolling elements, and a cage. The inner ring serves to cooperate with the shaft and rotate together with it. The outer ring, on the other hand, cooperates with the bearing housing to provide support. The rolling elements are evenly distributed between the inner and outer rings by means of a cage. Their shape, size, and quantity directly affect the performance and service life of the rolling bearing. The cage ensures an even distribution of the rolling elements, prevents them from falling out, guides their rotation, and also facilitates lubrication. Rolling bearings are easy to use and maintain, reliable in operation, and have good starting performance; they also possess high load-carrying capacity at medium speeds. Compared to sliding bearings, rolling bearings have larger radial dimensions, poorer vibration damping capabilities, a shorter lifespan at high speeds, and generate more noise. Radial bearings in rolling bearings (which primarily bear radial loads) generally consist of four components: an inner ring, an outer ring, rolling elements, and a cage for the rolling elements. The inner ring fits tightly on the journal and rotates together with the shaft, while the outer ring is mounted in the bearing housing bore. Raceways are provided on both the outer periphery of the inner ring and the inner periphery of the outer ring. When the inner and outer rings rotate relative to each other, the rolling elements roll along the raceways of the inner and outer rings. They are separated by a cage to prevent mutual friction. A thrust bearing consists of a fixed ring and a movable ring. The stationary ring is tightly fitted to the sleeve, while the rotating ring is supported on the bearing housing. The rings and rolling elements are usually made of rolling bearing steel with high strength and good wear resistance; after quenching, their surface hardness should reach HRC60–65. Cages are mostly made by stamping soft steel; they can also be manufactured from copper alloys, bakelite with fabric reinforcement, or plastics. Basic characteristics of rolling bearings Advantages: 1. Low frictional resistance, low power consumption, high mechanical efficiency, and easy starting. 2. Standardized dimensions ensure interchangeability, making installation and removal straightforward as well as maintenance easier. 3. Compact structure, light weight, with reduced axial dimensions. 4. High precision, high rotational speed, low wear, and long service life. 5. Some bearings have the ability to self-adjust. 6. Suitable for mass production, offering stable and reliable quality as well as high production efficiency. Rolling bearings are used in the front axle, rear axle, and middle axle of bicycles. Disadvantages: 1. High noise level. 2. The structure of the bearing housing is relatively complex. 3. High cost. Classification of rolling bearings: 1. Classification by structural type of rolling bearings. Bearings are classified according to the direction of the load they can withstand or their nominal contact angle: Radial bearings – These are rolling bearings primarily used to bear radial loads, with a nominal contact angle ranging from 0 to 45 degrees. Based on the nominal contact angle, they are further divided into: radial contact bearings – centrifugal bearings with a nominal contact angle of 0; angular contact bearings – centrifugal bearings with a nominal contact angle ranging from greater than 0 to 45 degrees. Thrust bearings – rolling bearings primarily used to bear axial loads, with a nominal contact angle of more than 45 to 90 degrees. Based on the nominal contact angle, they are further divided into: axial contact bearings – thrust bearings with a nominal contact angle of 90 degrees; angular contact thrust bearings – thrust bearings with a nominal contact angle greater than 45 but less than 90 degrees. Bearings are classified according to the type of their rolling elements as follows:
1) Ball bearings – the rolling elements are balls.
2) Roller bearings – the rolling elements are rollers. Roller bearings can be further classified according to the type of rollers as follows: Cylindrical roller bearings – these are bearings whose rolling elements are cylindrical rollers, and the ratio of the length to the diameter of these cylindrical rollers is less than or equal to 3 ； Needle roller bearings – bearings whose rolling elements are needle rollers, with a length-to-diameter ratio of more than 3, but a diameter of 5 mm or less; Tapered roller bearings – bearings whose rolling elements are tapered rollers; Self-aligning roller bearings – bearings whose rolling elements are spherical rollers. Bears are classified according to their ability to self-align during operation as follows: 1) Self-aligning bearings – bearings whose raceways are spherical in shape, allowing them to accommodate angular deviations and angular movements between the axes of the two raceways. 2) Non-aligning bearings (rigid bearings) ---- Bearings capable of resisting angular offsets between the axes of the raceways. Bearings are classified according to the number of rows of rolling elements:
1) Single-row bearings – bearings that have one row of rolling elements. 2) Double-row bearing ---- A bearing with two rows of rolling elements. 3) Multi-row bearings ---- Bearings with more than two rows of rolling elements, such as three-row and four-row bearings. Bearings can be classified, based on whether their components can be separated, into:
1) Separable bearings ---- bearings with separable components ； 2) Non-separable bearings ---- Bearings in which the rings cannot be freely separated from each other after final assembly. Bears can also be classified into various structural types based on their shape and structure – such as the presence or absence of filling grooves, the presence of inner and outer rings as well as the shape of these rings, the design of the flanges, and even whether a cage is present or not. 2. Classification by rolling bearing size Bearings are classified according to their outer diameter: (1) Micro bearings——Bearings with a nominal outer diameter of 26 mm or less. (2) Small bearings----Bearings with a nominal outer diameter range of 28-55mm. (3) Medium and small bearings——Bearings with a nominal outer diameter range of 60–115 mm. (4) Medium and large bearings——Bearings with a nominal outer diameter range of 120–190 mm. (5) Large bearings——Bearings with a nominal outer diameter range of 200–430 mm. (6) Extra-large bearings ---- Bearings with a nominal outer diameter of 440 mm or more. Selection of rolling bearing types There are a variety of rolling bearing types, and the following factors can be considered when making a choice. a. Magnitude, direction, and nature of the load Ball bearings are suitable for bearing light loads, while roller bearings are suitable for bearing heavy loads and impact loads. When a rolling bearing is subjected to pure axial loads, thrust bearings are generally used ； When roller bearings are subjected to pure radial loads, deep groove ball bearings or short cylindrical roller bearings are generally selected ； When a rolling bearing is subjected to a pure radial load along with a moderate axial load, deep groove ball bearings, angular contact ball bearings, tapered roller bearings, and self-aligning ball or roller bearings can be used ； When the axial load is high, angular contact ball bearings and tapered roller bearings with a larger contact angle can be used, or a combination of radial bearings and thrust bearings can be employed, which is particularly suitable for extremely high axial loads or when high axial stiffness is required. b. Permissible rotational speed This varies greatly depending on the type of bearing. Generally, bearings with low friction and minimal heat generation are suitable for high speeds. During design, efforts should be made to ensure that the rolling bearing operates at speeds below its limit speed. c. Stiffness When a bearing is under load, elastic deformation occurs at the contact points between the bearing rings and the rolling elements; the amount of deformation is proportional to the load, and this ratio determines the stiffness of the bearing. Generally, the stiffness of bearings can be improved by pre-tightening them ； Furthermore, in bearing support design, considering the combination and arrangement of bearings can also improve the support stiffness of the bearings. d. Self-aligning capability and installation errors After the bearing is installed in its operating position, poor installation and positioning often occur due to manufacturing errors. At this time, factors such as shaft deflection and thermal expansion often cause the bearing to be subjected to excessive loads, leading to premature damage. Self-aligning bearings can overcome defects caused by installation errors on their own, making them suitable bearings for such applications. e. Installation and removal Bearing types such as tapered roller bearings, needle bearings, and tapered roller bearings are of the type where the inner and outer rings can be separated (i.e., so-called separable bearings), making their installation and removal convenient. f. Marketability – Even bearings listed in the product catalog may not be available for sale in the market ； On the contrary, some bearings that are not listed in the product catalog are produced in large quantities. Therefore, it should be clear whether the bearings to be used are readily available. Code for rolling bearings: The code for rolling bearings is a product identifier that uses letters and numbers to indicate characteristics such as the bearing’s structure, dimensions, tolerance grades, and technical performance. **Standard GB/T272-93 specifies that the code for bearings consists of three parts: a prefix code, a basic code, and a suffix code. The basic code is the foundation of the bearing code. Both prefix codes and suffix codes are supplementary to the bearing code; they are used only when there are special requirements regarding the bearing’s structure, shape, material, tolerance levels, or technical specifications. In most cases, they can be omitted partially or entirely. a. Basic code The basic code indicates the basic type, structure, and dimensions of the bearing. It consists of the bearing type code, size series code, and inner diameter code. a1. Bearing type code: Digits or letters are used to represent different types of bearings; please refer to the database for details. a2: The size series code consists of two digits. The first digit denotes the width series (radial bearings) or height series (thrust bearings), while the second digit denotes the diameter series. The size series indicates that bearings with the same inner diameter can have different outer diameters, and bearings with the same outer diameter can have different widths (or heights); this allows them to meet various load-carrying requirements. a3: The inner diameter code indicates the nominal inner diameter of the bearing, expressed as a number. Example: Bearing 2 32 24 2 – Type code; self-aligning roller bearing ； 32-Size series code ； 24 – Inner diameter code, d=120mm; Example: Bearing 6208-2Z/P6. 6 – Type code, deep groove ball bearing ； 2-Size series code ； 08-Inner diameter code, d=40mm ； 2Z - Bearing with dust shields on both ends ； P6 – The tolerance grade meets the standard specification for grade 6. Common questions and answers regarding the installation of rolling bearings: 1. Are there any requirements for the installation surfaces and the installation location? Yes. If foreign particles such as iron filings, burrs, or dust get into the bearing, they can cause noise and vibration during operation; in severe cases, they may even damage the raceways and rolling elements. Therefore, before installing the bearing, you must ensure that the installation surface and the installation environment are clean. II. Must bearings be cleaned before installation? The surface of the bearings is coated with rust preventive oil; you must clean them thoroughly using clean gasoline or kerosene, and then apply clean, high-quality lubricant suitable for high speeds and temperatures before installing them for use. Cleanliness has a significant impact on the lifespan of bearings as well as on vibration and noise levels. But we would like to specifically remind you that fully sealed bearings do not require cleaning or lubrication. III. How to choose a grease? Lubrication has a crucial impact on the operation and lifespan of bearings. Here, we briefly introduce the general principles for selecting a grease. Grease is made up of base oil, thickeners, and additives. Greases of different types, as well as different grades within the same type, exhibit significant differences in performance, with varying allowable rotation limits; this must be taken into account when making a selection. The performance of grease is primarily determined by the base oil; generally, low-viscosity base oils are suitable for low temperatures and high speeds, while high-viscosity ones are suitable for high temperatures and high loads. Thickeners also affect lubricating performance; the water resistance of thickeners determines the water resistance of the grease. In principle, greases of different brands cannot be mixed, and even greases with the same thickener can have adverse effects on each other due to differences in additives. Fourth, when lubricating bearings, is it true that the more grease applied, the better? The idea that more grease is better when lubricating bearings is a common misconception. Excessive grease in the bearings and bearing housings will cause excessive mixing of the grease, resulting in very high temperatures. The amount of lubricant filled in the bearing should be 1/2 to 1/3 of the internal space of the bearing; at high speeds, it should be reduced to 1/3. V. How to install and remove it? When installing, do not strike the bearing’s end faces or the surfaces that are not under stress directly; instead, use pressing blocks, sleeves, or other installation tools to apply even force to the bearing. Never transmit force through the rolling elements. If the mounting surface is lubricated, the installation will be smoother. If the interference is large, the bearing should be placed in mineral oil and heated to 80–90°C before installation; the oil temperature must be kept strictly below 100°C to prevent a decrease in hardness due to tempering effects and to avoid issues with dimensional recovery. When encountering difficulties during disassembly, it is recommended to use a disassembly tool to pull outward while carefully pouring hot oil on the inner ring; the heat will cause the inner ring of the bearing to expand, making it easier to remove. 6. Is it true that the smaller the radial play of a bearing, the better? Not all bearings require the minimum possible operating play; you must choose an appropriate level of play based on the specific conditions. According to the national standard 4604-93, the radial clearance of rolling bearings is divided into five groups: Group -2, Group 0, Group 3, Group 4, and Group 5. The clearance values increase in order from smallest to largest, with Group 0 representing the standard clearance. The basic radial clearance group is suitable for general operating conditions, normal temperatures, and common interference fits ； For bearings operating under special conditions such as high temperature, high speed, low noise, and low friction, a larger radial clearance is preferable ； For precision spindles and bearings used in machine tool spindles, a smaller radial clearance is advisable ； A small amount of operating clearance can be maintained in roller bearings. Additionally, for separable bearings, there is no such thing as a clearance ； Finally, the operating clearance of a bearing after it is installed is smaller than its original clearance before installation, as the bearing has to rotate under certain loads, and there is also elastic deformation resulting from the bearing’s fit and the applied loads. 7. Differences between rolling bearings and sliding bearings The difference between rolling bearings and sliding bearings is evident first in their structure: rolling bearings rely on the rotation of rolling elements to support the rotating shaft, so the contact point is a single point; the more rolling elements there are, the more contact points there will be ； Sleeve bearings rely on a smooth surface to support the rotating shaft; therefore, the contact area is a surface. Secondly, the modes of motion differ; the mode of motion for rolling bearings is rolling ； The mode of motion in sliding bearings is sliding; therefore, the friction characteristics are completely different. Sliding bearings – Table of contents. Classification of sliding bearings. Structure of bearing bushes and bearing materials for sliding bearings. Common failures of sliding bearings. Issues to be considered in the structural design of sliding bearings. Properties and selection of lubricants for sliding bearings. Technical requirements for scraping and lapping of sliding bearings. A sliding bearing is a bearing that operates under conditions of sliding friction. Sliding bearings operate smoothly, reliably, and silently. Under liquid lubrication conditions, the sliding surfaces are separated by the lubricating oil and do not come into direct contact, which also helps to **reduce frictional losses and surface wear; furthermore, the oil film possesses certain vibration-damping capabilities. However, the starting frictional resistance is relatively high. The portion of the shaft supported by the bearing is called the journal, and the component that fits with the journal is known as the bearing shell. The layer of anti-friction material cast on the inner surface of a bearing shell to improve the frictional properties of its surface is called the bearing lining. The materials used for bushings and bearing liners are collectively referred to as sliding bearing materials. Commonly used materials for sliding bearings include bearing alloys (also known as Babbitt metal or white metal), wear-resistant cast iron, copper- and aluminum-based alloys, powder metallurgy materials, plastics, rubber, hardwood, and carbon-graphite, as well as polytetrafluoroethylene (PTFE) and modified polyoxymethylene (POM), etc. Sleeve bearings are generally used in applications involving low-speed and heavy-load conditions, or in operating parts where maintenance and lubrication are difficult to perform. Classification of sliding bearings There are many types of sliding bearings. ①Based on the direction in which they can bear loads, they can be divided into two types: radial (thrust) sliding bearings and thrust (axial) sliding bearings. ②Based on the type of lubricant, they can be classified into seven categories: oil-lubricated bearings, grease-lubricated bearings, water-lubricated bearings, gas bearings, solid-lubricated bearings, magnetic fluid bearings, and electromagnetic bearings. ③Based on the thickness of the lubrication film, bearings can be divided into two categories: thin-film lubricated bearings and thick-film lubricated bearings. ④Based on the bearing bush material, they can be classified into bronze bearings, cast iron bearings, plastic bearings, jewel bearings, powder metallurgy bearings, self-lubricating bearings, oil-impregnated bearings, etc. ⑤Based on the structure of the bearing shells, they can be classified into circular bearings, elliptical bearings, three-lip bearings, stepped-face bearings, tilting-pad bearings, and foil bearings, among others. Structure of the bearing shells and bearing materials in sliding bearings Bearing shells are divided into split-type and integral-type structures. To improve the frictional properties of the bearing shell surface, one or two layers of anti-friction material are often cast on its inner diameter surface; this is commonly referred to as the bearing lining. Therefore, there are bimetallic and tri-metallic bearing shells. Bearing materials Bushings or bearings are important components of sliding bearings; the materials used for bushings and bearing liners are collectively referred to as bearing materials. Since the bearing shells or bushings are in direct contact with the shaft journals, the shaft journal parts are generally quite wear-resistant; therefore, the main mode of failure for bearing shells is wear. The wear of bearing shells is directly related to the material of the shaft journal, the material of the bearing shells themselves, the lubricant, and the lubrication conditions. When selecting the material for bearing shells, these factors must be taken into account to improve the service life and performance of sliding bearings. The materials for bearings include: 1) Metal materials, such as bearing alloys, bronze, aluminum-based alloys, zinc-based alloys, etc ； 2) Porous metal materials (powder metallurgy materials) ； 3) Non-metallic materials. Among them: Bearing alloys, also known as white alloys, are primarily alloys of tin, lead, antimony, or other metals. Due to their good wear resistance, high plasticity, excellent running-in properties, good thermal conductivity, resistance to sticking, and good adhesion to oil, they are suitable for use under heavy loads and at high speeds. However, bearing alloys have relatively low strength and are expensive; they must be poured onto bronze, steel strip, or cast iron bearing shells to form a thin coating. Porous metal materials: Porous metal is a powder material with a porous structure; when immersed in lubricating oil, its micropores are filled with the oil, turning it into an oil-lubricated bearing that possesses self-lubricating properties. Porous metal materials have low toughness and are only suitable for steady, impact-free loads as well as at medium and low speeds. Bearing plastics: Commonly used bearing plastics include phenolic plastics, nylon, polytetrafluoroethylene, etc. Plastic bearings possess high compressive strength and wear resistance; they can be lubricated with oil and water, and also have self-lubricating properties, though their thermal conductivity is poor. Main faults of sliding bearings During operation, sliding bearings experience friction due to the contact between the shaft journal and the bearing bush, which leads to surface heating, wear, and even seizure. Therefore, when designing such bearings, it is necessary to use materials with good anti-friction properties for the bearing bushes, select appropriate lubricants and proper supply methods, and improve the bearing’s structure to achieve thick-film lubrication. 1. Tile surface corrosion: Spectral analysis revealed abnormal concentrations of non-ferrous metal elements ； Sub-micron wear particles of many non-ferrous metal components appeared in the ferrography ； Excessive moisture and acid value in the lubricating oil. 2. Corrosion on the journal surface: Spectral analysis revealed abnormal iron concentration; the ferrograph showed many sub-micron particles of iron, and the moisture content or acid value of the lubricating oil was above the specified limits. 3. Scratches on the journal surface: Iron-based cutting particles or black oxide particles are present in the ferrography, and a tempered color is visible on the metal surface. 4. Slight moving wear on the roof: Spectral analysis revealed an abnormal iron concentration; the ferrography showed numerous sub-micron wear particles containing iron. Additionally, the moisture content and acid value of the lubricating oil were also abnormal. 5. Scratches on the bearing surface: Cutting abrasives were found in the ferrography, and these abrasives were composed of non-ferrous metals. 6. Tile flaking: The ferrography revealed many large-sized fatigue-flaked alloy wear particles as well as layered abrasive grains. 7. Bearing wear: The ferrography shows a large number of large-sized alloy abrasive particles and black metal oxides. Issues to consider in the structural design of sliding bearings Sliding bearings involve surface contact; therefore, a certain oil film must be maintained between the contacting surfaces. Hence, the following points should be taken into account during design: 1. The oil film must be able to enter the friction surfaces smoothly. 2. Oil should enter the bearing from the non-load-bearing area. 3. Do not arrange the full-ring oil groove in the middle of the bearing. 4. Like oil bearings, oil grooves are provided at the joints. 5. To ensure sufficient and reliable oil supply by the oil ring. 6. The fuel filler hole must not be blocked. 7. Do not create areas where oil cannot flow. 8. Prevent the occurrence of sharp edges and corners that could break the oil film. Performance and Selection of Sliding Bearing Grease Sliding bearings can also be lubricated with grease, and the following factors should be considered when selecting grease: (1) When the bearing is subjected to high loads and operates at low speeds, grease with a low penetration value should be chosen; otherwise, grease with a high penetration value is appropriate. For high-speed bearings, lubricants with a lower cone penetration and good mechanical stability should be used. It is particularly important that the viscosity of the base oil in the grease be low. (2) The drip point of the selected grease is generally 20–30°C higher than the operating temperature; under conditions of continuous operation at high temperatures, care should be taken to ensure that it does not exceed the allowable operating temperature range for the grease. (3) When sliding bearings operate in a wet or humid environment, calcium-based, aluminum-based, or lithium-based greases with good water resistance should be selected. (4) Choose a grease with good adhesion. 2. Selection of lubricating grease for sliding bearings: When the load is <1 MPa, the circumferential speed of the shaft journal is below 1 m/s, and the maximum operating temperature is 75°C, calcium-based grease No. 3 should be used ； Load: 1–6.5 MPa; journal peripheral speed: 0.5–5 m/s; maximum operating temperature: 55°C. Use Type 2 calcium-based grease ； For loads >6.5 MPa, with a journal peripheral speed of 0.5 m/s or less, and a maximum operating temperature of 75°C, use Type 3 calcium-based grease ； Load < 6.5 MPa, journal circumferential speed 0.5–5 m/s, maximum operating temperature 120°C; use lithium-based grease No. 2 ； For loads >6.5 MPa, with a journal peripheral speed of 0.5 m/s or less and a maximum operating temperature of 110°C, use Type 2 calcium-sodium based grease ； Load: 1–6.5 MPa; circumferential speed of the shaft journal: below 1 m/s; maximum operating temperature: 50–100°C; use lithium-based grease No. 2 ； Load > 5 MPa, circumferential speed of the shaft journal: 0.5 m/s, maximum operating temperature: 60°C; use grade 2 rolling machine grease ； In humid environments at temperatures of 75–120°C, calcium-sodium-based grease should be considered for use. In humid environments, where the operating temperature is below 75°C, aluminum-based greases can be used in place of calcium-based grease No. 3. At operating temperatures of 110–120°C, lithium-based or barium-based grease can be used. When centralized lubrication is used, the consistency should be lower. 3. Lubrication cycle for grease used in sliding bearings: For occasional use on non-critical components, when the shaft speed is <200 r/min, the lubrication cycle is once every 5 days ； Shaft speed > 200 r/min; lubrication cycle is once every 3 days. Intermittent operation: Shaft speed < 200 r/min, lubrication every 2 days ； Shaft speed > 200 r/min; lubrication should be performed once a day. Continuous operation, with operating temperature below 40°C: shaft speed < 200 r/min, lubrication interval of once per day ； Shaft speed > 200 r/min; lubrication is required once per shift. Continuous operation, operating temperature 40–100°C: shaft speed < 200 r/min, lubrication once per shift ； Shaft speed > 200 r/min; lubrication is required twice per shift. Technical requirements for lapping of sliding bearings – Sliding bearings: Basic requirements. It is necessary to ensure a uniform and tight contact between the shaft journal and the sliding bearing, while also maintaining a certain amount of clearance. The contact angle refers to the central angle subtended by the contact surface between the journal and the plain bearing. The contact angle should not be too large nor too small. An excessively small contact angle can increase the pressure on plain bearings; in severe cases, it may cause significant deformation of the bearings, accelerate wear, and shorten their service life ； A too large contact angle can affect the formation of an oil film, preventing proper liquid lubrication. Experimental studies have shown that the limit of the contact angle for sliding bearings is 120°. When the sliding bearing wears down to this contact angle, liquid lubrication is compromised. Therefore, without affecting the pressure conditions on the sliding bearing, the smaller the contact angle, the better. Based on the theoretical analysis of frictional torque, the frictional torque is minimum when the contact angle is 60°. Therefore, it is recommended that for sliding bearings with a rotation speed above 500 r/min, a contact angle of 60° should be used; whereas for sliding bearings with a rotation speed below 500 r/min, a contact angle of 90° can be used, or 60° as well. Contact points: The actual condition of contact between the journal and the surface of the sliding bearing can be expressed by the number of actual contact points per unit area. The more, finer, and more uniform the contact points are, the better the lapping of the sliding bearing; conversely, this indicates poor lapping of the sliding bearing. Generally speaking, the finer and more numerous the contact points, the greater the difficulty of scraping and lapping. In production, the number of contact points should be determined based on the performance and operating conditions of the sliding bearing. The information listed in the table below can serve as a reference:
Sliding bearing speed (r/min) – Number of contact points (per 25×25 mm area):
Below 100: 3–5
100–500: 10–15
500–1000: 15–20
1000–2000: 20–25
Above 2000: Over 25
Machinery with precision grades I and II can use the values listed in the table above; for machinery with precision grade III, these values can be halved.
1) Ball bearings – the rolling elements are balls.
2) Roller bearings – the rolling elements are rollers. Roller bearings can be further classified according to the type of rollers as follows: Cylindrical roller bearings – these are bearings whose rolling elements are cylindrical rollers, and the ratio of the length to the diameter of these cylindrical rollers is less than or equal to 3 ； Needle roller bearings – bearings whose rolling elements are needle rollers, with a length-to-diameter ratio of more than 3, but a diameter of 5 mm or less; Tapered roller bearings – bearings whose rolling elements are tapered rollers; Self-aligning roller bearings – bearings whose rolling elements are spherical rollers. Bears are classified according to their ability to self-align during operation as follows: 1) Self-aligning bearings – bearings whose raceways are spherical in shape, allowing them to accommodate angular deviations and angular movements between the axes of the two raceways. 2) Non-aligning bearings (rigid bearings) ---- Bearings capable of resisting angular offsets between the axes of the raceways. Bearings are classified according to the number of rows of rolling elements:
1) Single-row bearings – bearings that have one row of rolling elements. 2) Double-row bearing ---- A bearing with two rows of rolling elements. 3) Multi-row bearings ---- Bearings with more than two rows of rolling elements, such as three-row and four-row bearings. Bearings can be classified, based on whether their components can be separated, into:
1) Separable bearings ---- bearings with separable components ； 2) Non-separable bearings ---- Bearings in which the rings cannot be freely separated from each other after final assembly. Bears can also be classified into various structural types based on their shape and structure – such as the presence or absence of filling grooves, the presence of inner and outer rings as well as the shape of these rings, the design of the flanges, and even whether a cage is present or not. 2. Classification by rolling bearing size Bearings are classified according to their outer diameter: (1) Micro bearings——Bearings with a nominal outer diameter of 26 mm or less. (2) Small bearings----Bearings with a nominal outer diameter range of 28-55mm. (3) Medium and small bearings——Bearings with a nominal outer diameter range of 60–115 mm. (4) Medium and large bearings——Bearings with a nominal outer diameter range of 120–190 mm. (5) Large bearings——Bearings with a nominal outer diameter range of 200–430 mm. (6) Extra-large bearings ---- Bearings with a nominal outer diameter of 440 mm or more. Selection of rolling bearing types There are a variety of rolling bearing types, and the following factors can be considered when making a choice. a. Magnitude, direction, and nature of the load Ball bearings are suitable for bearing light loads, while roller bearings are suitable for bearing heavy loads and impact loads. When a rolling bearing is subjected to pure axial loads, thrust bearings are generally used ； When roller bearings are subjected to pure radial loads, deep groove ball bearings or short cylindrical roller bearings are generally selected ； When a rolling bearing is subjected to a pure radial load along with a moderate axial load, deep groove ball bearings, angular contact ball bearings, tapered roller bearings, and self-aligning ball or roller bearings can be used ； When the axial load is high, angular contact ball bearings and tapered roller bearings with a larger contact angle can be used, or a combination of radial bearings and thrust bearings can be employed, which is particularly suitable for extremely high axial loads or when high axial stiffness is required. b. Permissible rotational speed This varies greatly depending on the type of bearing. Generally, bearings with low friction and minimal heat generation are suitable for high speeds. During design, efforts should be made to ensure that the rolling bearing operates at speeds below its limit speed. c. Stiffness When a bearing is under load, elastic deformation occurs at the contact points between the bearing rings and the rolling elements; the amount of deformation is proportional to the load, and this ratio determines the stiffness of the bearing. Generally, the stiffness of bearings can be improved by pre-tightening them ； Furthermore, in bearing support design, considering the combination and arrangement of bearings can also improve the support stiffness of the bearings. d. Self-aligning capability and installation errors After the bearing is installed in its operating position, poor installation and positioning often occur due to manufacturing errors. At this time, factors such as shaft deflection and thermal expansion often cause the bearing to be subjected to excessive loads, leading to premature damage. Self-aligning bearings can overcome defects caused by installation errors on their own, making them suitable bearings for such applications. e. Installation and removal Bearing types such as tapered roller bearings, needle bearings, and tapered roller bearings are of the type where the inner and outer rings can be separated (i.e., so-called separable bearings), making their installation and removal convenient. f. Marketability – Even bearings listed in the product catalog may not be available for sale in the market ； On the contrary, some bearings that are not listed in the product catalog are produced in large quantities. Therefore, it should be clear whether the bearings to be used are readily available. Code for rolling bearings: The code for rolling bearings is a product identifier that uses letters and numbers to indicate characteristics such as the bearing’s structure, dimensions, tolerance grades, and technical performance. **Standard GB/T272-93 specifies that the code for bearings consists of three parts: a prefix code, a basic code, and a suffix code. The basic code is the foundation of the bearing code. Both prefix codes and suffix codes are supplementary to the bearing code; they are used only when there are special requirements regarding the bearing’s structure, shape, material, tolerance levels, or technical specifications. In most cases, they can be omitted partially or entirely. a. Basic code The basic code indicates the basic type, structure, and dimensions of the bearing. It consists of the bearing type code, size series code, and inner diameter code. a1. Bearing type code: Digits or letters are used to represent different types of bearings; please refer to the database for details. a2: The size series code consists of two digits. The first digit denotes the width series (radial bearings) or height series (thrust bearings), while the second digit denotes the diameter series. The size series indicates that bearings with the same inner diameter can have different outer diameters, and bearings with the same outer diameter can have different widths (or heights); this allows them to meet various load-carrying requirements. a3: The inner diameter code indicates the nominal inner diameter of the bearing, expressed as a number. Example: Bearing 2 32 24 2 – Type code; self-aligning roller bearing ； 32-Size series code ； 24 – Inner diameter code, d=120mm; Example: Bearing 6208-2Z/P6. 6 – Type code, deep groove ball bearing ； 2-Size series code ； 08-Inner diameter code, d=40mm ； 2Z - Bearing with dust shields on both ends ； P6 – The tolerance grade meets the standard specification for grade 6. Common questions and answers regarding the installation of rolling bearings: 1. Are there any requirements for the installation surfaces and the installation location? Yes. If foreign particles such as iron filings, burrs, or dust get into the bearing, they can cause noise and vibration during operation; in severe cases, they may even damage the raceways and rolling elements. Therefore, before installing the bearing, you must ensure that the installation surface and the installation environment are clean. II. Must bearings be cleaned before installation? The surface of the bearings is coated with rust preventive oil; you must clean them thoroughly using clean gasoline or kerosene, and then apply clean, high-quality lubricant suitable for high speeds and temperatures before installing them for use. Cleanliness has a significant impact on the lifespan of bearings as well as on vibration and noise levels. But we would like to specifically remind you that fully sealed bearings do not require cleaning or lubrication. III. How to choose a grease? Lubrication has a crucial impact on the operation and lifespan of bearings. Here, we briefly introduce the general principles for selecting a grease. Grease is made up of base oil, thickeners, and additives. Greases of different types, as well as different grades within the same type, exhibit significant differences in performance, with varying allowable rotation limits; this must be taken into account when making a selection. The performance of grease is primarily determined by the base oil; generally, low-viscosity base oils are suitable for low temperatures and high speeds, while high-viscosity ones are suitable for high temperatures and high loads. Thickeners also affect lubricating performance; the water resistance of thickeners determines the water resistance of the grease. In principle, greases of different brands cannot be mixed, and even greases with the same thickener can have adverse effects on each other due to differences in additives. Fourth, when lubricating bearings, is it true that the more grease applied, the better? The idea that more grease is better when lubricating bearings is a common misconception. Excessive grease in the bearings and bearing housings will cause excessive mixing of the grease, resulting in very high temperatures. The amount of lubricant filled in the bearing should be 1/2 to 1/3 of the internal space of the bearing; at high speeds, it should be reduced to 1/3. V. How to install and remove it? When installing, do not strike the bearing’s end faces or the surfaces that are not under stress directly; instead, use pressing blocks, sleeves, or other installation tools to apply even force to the bearing. Never transmit force through the rolling elements. If the mounting surface is lubricated, the installation will be smoother. If the interference is large, the bearing should be placed in mineral oil and heated to 80–90°C before installation; the oil temperature must be kept strictly below 100°C to prevent a decrease in hardness due to tempering effects and to avoid issues with dimensional recovery. When encountering difficulties during disassembly, it is recommended to use a disassembly tool to pull outward while carefully pouring hot oil on the inner ring; the heat will cause the inner ring of the bearing to expand, making it easier to remove. 6. Is it true that the smaller the radial play of a bearing, the better? Not all bearings require the minimum possible operating play; you must choose an appropriate level of play based on the specific conditions. According to the national standard 4604-93, the radial clearance of rolling bearings is divided into five groups: Group -2, Group 0, Group 3, Group 4, and Group 5. The clearance values increase in order from smallest to largest, with Group 0 representing the standard clearance. The basic radial clearance group is suitable for general operating conditions, normal temperatures, and common interference fits ； For bearings operating under special conditions such as high temperature, high speed, low noise, and low friction, a larger radial clearance is preferable ； For precision spindles and bearings used in machine tool spindles, a smaller radial clearance is advisable ； A small amount of operating clearance can be maintained in roller bearings. Additionally, for separable bearings, there is no such thing as a clearance ； Finally, the operating clearance of a bearing after it is installed is smaller than its original clearance before installation, as the bearing has to rotate under certain loads, and there is also elastic deformation resulting from the bearing’s fit and the applied loads. 7. Differences between rolling bearings and sliding bearings The difference between rolling bearings and sliding bearings is evident first in their structure: rolling bearings rely on the rotation of rolling elements to support the rotating shaft, so the contact point is a single point; the more rolling elements there are, the more contact points there will be ； Sleeve bearings rely on a smooth surface to support the rotating shaft; therefore, the contact area is a surface. Secondly, the modes of motion differ; the mode of motion for rolling bearings is rolling ； The mode of motion in sliding bearings is sliding; therefore, the friction characteristics are completely different. Sliding bearings – Table of contents. Classification of sliding bearings. Structure of bearing bushes and bearing materials for sliding bearings. Common failures of sliding bearings. Issues to be considered in the structural design of sliding bearings. Properties and selection of lubricants for sliding bearings. Technical requirements for scraping and lapping of sliding bearings. A sliding bearing is a bearing that operates under conditions of sliding friction. Sliding bearings operate smoothly, reliably, and silently. Under liquid lubrication conditions, the sliding surfaces are separated by the lubricating oil and do not come into direct contact, which also helps to **reduce frictional losses and surface wear; furthermore, the oil film possesses certain vibration-damping capabilities. However, the starting frictional resistance is relatively high. The portion of the shaft supported by the bearing is called the journal, and the component that fits with the journal is known as the bearing shell. The layer of anti-friction material cast on the inner surface of a bearing shell to improve the frictional properties of its surface is called the bearing lining. The materials used for bushings and bearing liners are collectively referred to as sliding bearing materials. Commonly used materials for sliding bearings include bearing alloys (also known as Babbitt metal or white metal), wear-resistant cast iron, copper- and aluminum-based alloys, powder metallurgy materials, plastics, rubber, hardwood, and carbon-graphite, as well as polytetrafluoroethylene (PTFE) and modified polyoxymethylene (POM), etc. Sleeve bearings are generally used in applications involving low-speed and heavy-load conditions, or in operating parts where maintenance and lubrication are difficult to perform. Classification of sliding bearings There are many types of sliding bearings. ①Based on the direction in which they can bear loads, they can be divided into two types: radial (thrust) sliding bearings and thrust (axial) sliding bearings. ②Based on the type of lubricant, they can be classified into seven categories: oil-lubricated bearings, grease-lubricated bearings, water-lubricated bearings, gas bearings, solid-lubricated bearings, magnetic fluid bearings, and electromagnetic bearings. ③Based on the thickness of the lubrication film, bearings can be divided into two categories: thin-film lubricated bearings and thick-film lubricated bearings. ④Based on the bearing bush material, they can be classified into bronze bearings, cast iron bearings, plastic bearings, jewel bearings, powder metallurgy bearings, self-lubricating bearings, oil-impregnated bearings, etc. ⑤Based on the structure of the bearing shells, they can be classified into circular bearings, elliptical bearings, three-lip bearings, stepped-face bearings, tilting-pad bearings, and foil bearings, among others. Structure of the bearing shells and bearing materials in sliding bearings Bearing shells are divided into split-type and integral-type structures. To improve the frictional properties of the bearing shell surface, one or two layers of anti-friction material are often cast on its inner diameter surface; this is commonly referred to as the bearing lining. Therefore, there are bimetallic and tri-metallic bearing shells. Bearing materials Bushings or bearings are important components of sliding bearings; the materials used for bushings and bearing liners are collectively referred to as bearing materials. Since the bearing shells or bushings are in direct contact with the shaft journals, the shaft journal parts are generally quite wear-resistant; therefore, the main mode of failure for bearing shells is wear. The wear of bearing shells is directly related to the material of the shaft journal, the material of the bearing shells themselves, the lubricant, and the lubrication conditions. When selecting the material for bearing shells, these factors must be taken into account to improve the service life and performance of sliding bearings. The materials for bearings include: 1) Metal materials, such as bearing alloys, bronze, aluminum-based alloys, zinc-based alloys, etc ； 2) Porous metal materials (powder metallurgy materials) ； 3) Non-metallic materials. Among them: Bearing alloys, also known as white alloys, are primarily alloys of tin, lead, antimony, or other metals. Due to their good wear resistance, high plasticity, excellent running-in properties, good thermal conductivity, resistance to sticking, and good adhesion to oil, they are suitable for use under heavy loads and at high speeds. However, bearing alloys have relatively low strength and are expensive; they must be poured onto bronze, steel strip, or cast iron bearing shells to form a thin coating. Porous metal materials: Porous metal is a powder material with a porous structure; when immersed in lubricating oil, its micropores are filled with the oil, turning it into an oil-lubricated bearing that possesses self-lubricating properties. Porous metal materials have low toughness and are only suitable for steady, impact-free loads as well as at medium and low speeds. Bearing plastics: Commonly used bearing plastics include phenolic plastics, nylon, polytetrafluoroethylene, etc. Plastic bearings possess high compressive strength and wear resistance; they can be lubricated with oil and water, and also have self-lubricating properties, though their thermal conductivity is poor. Main faults of sliding bearings During operation, sliding bearings experience friction due to the contact between the shaft journal and the bearing bush, which leads to surface heating, wear, and even seizure. Therefore, when designing such bearings, it is necessary to use materials with good anti-friction properties for the bearing bushes, select appropriate lubricants and proper supply methods, and improve the bearing’s structure to achieve thick-film lubrication. 1. Tile surface corrosion: Spectral analysis revealed abnormal concentrations of non-ferrous metal elements ； Sub-micron wear particles of many non-ferrous metal components appeared in the ferrography ； Excessive moisture and acid value in the lubricating oil. 2. Corrosion on the journal surface: Spectral analysis revealed abnormal iron concentration; the ferrograph showed many sub-micron particles of iron, and the moisture content or acid value of the lubricating oil was above the specified limits. 3. Scratches on the journal surface: Iron-based cutting particles or black oxide particles are present in the ferrography, and a tempered color is visible on the metal surface. 4. Slight moving wear on the roof: Spectral analysis revealed an abnormal iron concentration; the ferrography showed numerous sub-micron wear particles containing iron. Additionally, the moisture content and acid value of the lubricating oil were also abnormal. 5. Scratches on the bearing surface: Cutting abrasives were found in the ferrography, and these abrasives were composed of non-ferrous metals. 6. Tile flaking: The ferrography revealed many large-sized fatigue-flaked alloy wear particles as well as layered abrasive grains. 7. Bearing wear: The ferrography shows a large number of large-sized alloy abrasive particles and black metal oxides. Issues to consider in the structural design of sliding bearings Sliding bearings involve surface contact; therefore, a certain oil film must be maintained between the contacting surfaces. Hence, the following points should be taken into account during design: 1. The oil film must be able to enter the friction surfaces smoothly. 2. Oil should enter the bearing from the non-load-bearing area. 3. Do not arrange the full-ring oil groove in the middle of the bearing. 4. Like oil bearings, oil grooves are provided at the joints. 5. To ensure sufficient and reliable oil supply by the oil ring. 6. The fuel filler hole must not be blocked. 7. Do not create areas where oil cannot flow. 8. Prevent the occurrence of sharp edges and corners that could break the oil film. Performance and Selection of Sliding Bearing Grease Sliding bearings can also be lubricated with grease, and the following factors should be considered when selecting grease: (1) When the bearing is subjected to high loads and operates at low speeds, grease with a low penetration value should be chosen; otherwise, grease with a high penetration value is appropriate. For high-speed bearings, lubricants with a lower cone penetration and good mechanical stability should be used. It is particularly important that the viscosity of the base oil in the grease be low. (2) The drip point of the selected grease is generally 20–30°C higher than the operating temperature; under conditions of continuous operation at high temperatures, care should be taken to ensure that it does not exceed the allowable operating temperature range for the grease. (3) When sliding bearings operate in a wet or humid environment, calcium-based, aluminum-based, or lithium-based greases with good water resistance should be selected. (4) Choose a grease with good adhesion. 2. Selection of lubricating grease for sliding bearings: When the load is <1 MPa, the circumferential speed of the shaft journal is below 1 m/s, and the maximum operating temperature is 75°C, calcium-based grease No. 3 should be used ； Load: 1–6.5 MPa; journal peripheral speed: 0.5–5 m/s; maximum operating temperature: 55°C. Use Type 2 calcium-based grease ； For loads >6.5 MPa, with a journal peripheral speed of 0.5 m/s or less, and a maximum operating temperature of 75°C, use Type 3 calcium-based grease ； Load < 6.5 MPa, journal circumferential speed 0.5–5 m/s, maximum operating temperature 120°C; use lithium-based grease No. 2 ； For loads >6.5 MPa, with a journal peripheral speed of 0.5 m/s or less and a maximum operating temperature of 110°C, use Type 2 calcium-sodium based grease ； Load: 1–6.5 MPa; circumferential speed of the shaft journal: below 1 m/s; maximum operating temperature: 50–100°C; use lithium-based grease No. 2 ； Load > 5 MPa, circumferential speed of the shaft journal: 0.5 m/s, maximum operating temperature: 60°C; use grade 2 rolling machine grease ； In humid environments at temperatures of 75–120°C, calcium-sodium-based grease should be considered for use. In humid environments, where the operating temperature is below 75°C, aluminum-based greases can be used in place of calcium-based grease No. 3. At operating temperatures of 110–120°C, lithium-based or barium-based grease can be used. When centralized lubrication is used, the consistency should be lower. 3. Lubrication cycle for grease used in sliding bearings: For occasional use on non-critical components, when the shaft speed is <200 r/min, the lubrication cycle is once every 5 days ； Shaft speed > 200 r/min; lubrication cycle is once every 3 days. Intermittent operation: Shaft speed < 200 r/min, lubrication every 2 days ； Shaft speed > 200 r/min; lubrication should be performed once a day. Continuous operation, with operating temperature below 40°C: shaft speed < 200 r/min, lubrication interval of once per day ； Shaft speed > 200 r/min; lubrication is required once per shift. Continuous operation, operating temperature 40–100°C: shaft speed < 200 r/min, lubrication once per shift ； Shaft speed > 200 r/min; lubrication is required twice per shift. Technical requirements for lapping of sliding bearings – Sliding bearings: Basic requirements. It is necessary to ensure a uniform and tight contact between the shaft journal and the sliding bearing, while also maintaining a certain amount of clearance. The contact angle refers to the central angle subtended by the contact surface between the journal and the plain bearing. The contact angle should not be too large nor too small. An excessively small contact angle can increase the pressure on plain bearings; in severe cases, it may cause significant deformation of the bearings, accelerate wear, and shorten their service life ； A too large contact angle can affect the formation of an oil film, preventing proper liquid lubrication. Experimental studies have shown that the limit of the contact angle for sliding bearings is 120°. When the sliding bearing wears down to this contact angle, liquid lubrication is compromised. Therefore, without affecting the pressure conditions on the sliding bearing, the smaller the contact angle, the better. Based on the theoretical analysis of frictional torque, the frictional torque is minimum when the contact angle is 60°. Therefore, it is recommended that for sliding bearings with a rotation speed above 500 r/min, a contact angle of 60° should be used; whereas for sliding bearings with a rotation speed below 500 r/min, a contact angle of 90° can be used, or 60° as well. Contact points: The actual condition of contact between the journal and the surface of the sliding bearing can be expressed by the number of actual contact points per unit area. The more, finer, and more uniform the contact points are, the better the lapping of the sliding bearing; conversely, this indicates poor lapping of the sliding bearing. Generally speaking, the finer and more numerous the contact points, the greater the difficulty of scraping and lapping. In production, the number of contact points should be determined based on the performance and operating conditions of the sliding bearing. The information listed in the table below can serve as a reference:
Sliding bearing speed (r/min) – Number of contact points (per 25×25 mm area):
Below 100: 3–5
100–500: 10–15
500–1000: 15–20
1000–2000: 20–25
Above 2000: Over 25
Machinery with precision grades I and II can use the values listed in the table above; for machinery with precision grade III, these values can be halved.
This post was last edited by rdmy on 2011-2-25 00:11. Generally, the inner diameter of a bearing is related to its code; the last two digits of the code, multiplied by 5, give the inner diameter: 6205 has an inner diameter of 25, 6207 has an inner diameter of 35, 6222 has an inner diameter of 110, and 6310 has an inner diameter of 50. NJ311 has an inner diameter of 55, while NJ312 has an inner diameter of 60
Reply to 3#: What kind of bearing is NJ311? What are its application areas and advantages and disadvantages?
What kind of bearing is the uc3513?
The UC3513 model is incorrect. The only available bearing sample is the 3513; its new designation is 22213. It is a spherical roller bearing with an inner diameter of 65, an outer diameter of 120, and a thickness of 31. The same rules apply to the inner diameter size; multiply the last two digits of the bearing code by 5 to obtain the inner diameter size.
Reply to 4# Blue Sky NJ311 is a cylindrical roller bearing. NJ denotes a cylindrical roller bearing with a single rib on the inner ring. It is mainly used in motors, machine tools, the petroleum industry, rolling mill handling machinery, and various industrial machines.