Basic knowledge of bearings

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Structure of sliding bearings Structure of sliding bearings Generally, sliding bearings are composed of bearing bushes and bearing seats. Sliding bearings can be divided into radial sliding bearings (mainly bearing radial loads) and thrust sliding bearings (mainly bearing axial loads) according to the direction of the load they bear. Commonly used radial sliding bearings have two structural forms: integral type and split type. 1. Integral sliding bearing Figure 5-11 is a common integral radial sliding bearing, which is connected to the frame with bolts. A bearing bush (or bushing) made of anti-friction material is pressed into the bearing seat hole. An oil cup is installed on the top of the bearing seat. There is an oil inlet hole on the bushing, and an axial oil groove is opened on the inner surface to distribute lubricating oil for lubrication. The biggest advantage of integral sliding bearings is their simple structure, but the bearing clearance cannot be adjusted when the bearing working surface is too worn. ； The journal can only be installed from the end, which is inconvenient or even impossible to install on thick and heavy shafts or shafts with intermediate journals. To overcome these two shortcomings, split sliding bearings can be used. 2. Split sliding bearings Split sliding bearings are shown in Figure 5-12 and consist of bearing seats, bearing caps, split bearing bushes (divided into upper and lower bushes) and connecting bolts. The split surface of the bearing should be nearly perpendicular to the load direction. Most bearing split surfaces are horizontal or inclined. The split surfaces of the bearing cap and the bearing seat are often made into a stepped shape to facilitate positioning and prevent misalignment during work. The bearing clearance after the bearing bush is worn can be adjusted by reducing the metal gasket at the split surface or scraping the bearing metal. Split sliding bearings are easy to assemble and disassemble, and the gap between the bearing bush and the shaft can be adjusted, so they are widely used. Figure 5-11 Figure 5-12 Bearing structure Bearing structure Bearing structure The bearing is an important part of the sliding bearing. Commonly used bearings are divided into two structures: integral type and split type. 1. Integral bearing bush (shaft sleeve) Integral bearing bush generally has oil holes and oil grooves on the bushing for lubrication, as shown in Figure 5-13b. Bushings made of powder metallurgy generally do not have oil grooves, as shown in Figure 5-13a. Figure 5-13 2. Split bearing bush The split bearing bush is composed of upper and lower half bushes. The upper bearing bush has oil holes and oil grooves. The cast split thick-walled bearing bush is shown in Figure 5-14. In order to improve the friction properties of the bearing surface, a layer of anti-friction material (such as bearing alloy) can be cast on the inner surface, called a bearing lining. The oil hole on the bearing bush is used to supply lubricating oil, and the function of the oil groove is to distribute the lubricating oil evenly. The shape of the common oil groove is shown in Figure 5-15, and it should be opened in the non-load-bearing area. Figure 5-14 Rolling bearings Characteristics and basic structure of rolling bearings 1. Characteristics of rolling bearings Rolling bearings are made using the principle that rolling elements roll between the shaft diameter and the support race. It uses rolling friction instead of sliding friction. Compared with sliding friction bearings, the characteristics of rolling bearings are as follows: (1) Advantages ① Low friction factor under normal use conditions, small friction torque during operation, sensitive starting, and high efficiency ； ②　Preloading method can be used to improve support stiffness and rotation accuracy ； ③　For journals of the same size, the width of the rolling bearing is small, which can make the axial size of the machine compact. ； ④　The lubrication method is simple and the damaged bearings are easy to replace. (2) Disadvantages ① Poor ability to withstand impact loads ； ②　Loud noise during high-speed operation ； ③　Larger radial size than sliding bearings ； ④　Compared with sliding bearings, the service life is lower. Rolling bearings can work within a wider range of load, speed and accuracy, and are more convenient to install and maintain. Rolling bearings are standardized and serialized parts, which can be organized into specialized mass production and are cheap. Therefore, it has gradually replaced sliding bearings in many occasions and has been widely used. 2. Basic structure of rolling bearings A common rolling bearing is shown in Figure 5-16, which consists of an inner ring, an outer ring, rolling elements and a cage. The inner ring is installed on the journal, and the outer ring is installed in the frame hole. There are raceways on the surfaces of the inner and outer rings that are in contact with the rolling elements, and the rolling elements roll along the raceways. The function of the cage is to separate the rolling elements and distribute them evenly on the circumference of the seat ring to prevent adjacent rolling elements from contacting and causing friction during movement. Figure 5-16 Common rolling elements are shown in Figure 5-17, including balls, cylinders, cones, etc. Rolling elements are indispensable parts for forming rolling friction. The code of rolling bearings The code of rolling bearings is stipulated in the national standard GB/T 272-1993 that the rolling bearing code consists of a basic code, a prefix code and a suffix code. The arrangement is as follows: Prefix code + basic code + postcode (1) Basic code (except needle roller bearings) The basic code is the basis of the bearing code, which consists of the type code, the size series code (including the width (height) series code and the diameter series code) and the inner diameter code, see Table 5-5. Table 5-5 Basic code type code Dimension series code Inner diameter code is represented by one digit, or one to two letters, see Table 5-7 Width series code Diameter series code is usually represented by two digits. Please check the manual for the inner diameter codes of d＞500, d＜10 and d=22, 28 and 32. The inner diameter code of 10≤ｄd≤500 is as follows: Indicates series with the same inner diameter and outer diameter, but different widths (height for thrust bearings). Use one digit to express a series that represents the same inner diameter but different outer diameters. Use one digit to represent the inner diameter code 00 01 02 03 04 ~ 99. The size series codes are used together. For most bearings, the width series code is 0 and can be omitted. The width series code 0 of tapered roller bearings and spherical bearings should be marked with the inner diameter/mm 10 12 15 17 Code × 5 (2) Front and rear codes. The front and rear codes are supplementary codes added to the left and right of the basic code when the structural shape, size, tolerance, technical requirements, etc. of the bearing change. Their arrangement is as shown in Table 5-6. Table 5-6 Arrangement of front and rear codes Bearing code Front code Basic code Post code (group) sequence 1 2 3 4 5 6 7 8 Complete set of bearing components Internal structure Seal and dust-proof ferrule Variation Cage and its material Bearing material tolerance grade Clearance configuration Other bearing code examples (3) Needle roller bearing basic code The basic code of needle roller bearings consists of the bearing type code and the dimensions that characterize the bearing's matching installation characteristics. Needle roller bearing code example: Table 5-7 (see page 78 of the book) lists the commonly used rolling bearing types, size series codes, basic codes and bearing characteristics. Selection of rolling bearing types Selection of rolling bearing types According to the characteristics of various types of rolling bearings, the magnitude and direction of the load, speed, support stiffness and installation accuracy should be considered when selecting bearings. You can refer to the following principles when choosing: (1) Bearing load When the load is large, line contact roller bearings should be selected. Ball bearings are point contact and are suitable for light and medium loads. When there is impact load, spiral roller or ordinary roller bearings are often used. For purely axial loads, thrust bearings are used. Radial bearings are often used for pure radial loads. Where there are both radial loads and axial loads at the same time, if the axial load is relatively small, radial angular contact bearings or deep groove ball bearings should be used. When the axial load is very large, a combination structure of radial ball bearings and thrust bearings can be used. (2) When the bearing speed is high, point contact ball bearings should be used. Generally, ball bearings have a higher limit speed. If higher speed requirements are required, ultra-light and extra-light series bearings should be selected to reduce the impact of the centrifugal force of the rolling elements. (3) Rigidity and alignment performance requirements When the support stiffness requirements are large, a paired radial thrust bearing combination structure or a preloaded bearing method can be used ； When the support span is large, the bending deformation of the shaft is large, the stiffness is low, or there is an error in the center position of the two bearing housing holes, the deflection angle between the axis of the inner and outer rings of the bearing should be considered, and a self-aligning bearing needs to be selected. Spherical ball bearings or spherical roller bearings can be used. This type of bearing allows a larger deflection angle. (4) Assembly and disassembly require the use of bearings with internal tapered holes, which can adjust the radial clearance of the bearing, improve the rotation accuracy of the bearing, and facilitate installation on long shafts. ； Bearings with separable inner and outer rings are used for easy assembly and disassembly. In addition, attention should also be paid to economy to reduce product prices. Generally, single row radial ball bearings are the lowest priced, roller bearings are more expensive than ball bearings, and the higher the bearing precision, the higher the price. The fixing method of rolling bearings The circumferential fixing of the inner and outer rings of rolling bearings is ensured by the cooperation between the inner ring and the shaft and between the outer ring and the base hole. For its axial positioning, different positioning methods are selected according to different situations, see Table 5-8 and Table 5-9. Table 5-8 Simplified diagram of positioning components of the inner ring Application description Simplified diagram of positioning components Application description The nut is used when the bearing rotates at a high speed and bears a large axial load. The end surface of the nut in contact with the bearing ring must be perpendicular to the rotation center line of the shaft. In order to prevent the nut from loosening during rotation, a nut and a stop washer can be used to fasten the shaft with a retaining ring. When the axial load is not large, the bearing speed is not high, and it is difficult to thread the journal, a resilient retaining ring with a rectangular cross-section is used for axial positioning. This method is easy to install and disassemble, takes up little space, and is simple to manufacture. End-face thrust washers are used when the axial load is large, the rotation speed is relatively high, and it is difficult to thread the journal. Two screws are used for positioning at the shaft end, and an iron wire anti-loosening adapter sleeve is used. For the spherical bearings with low rotational speed and stable radial load and moderate axial load, a tapered adapter sleeve is installed on the journal. The adapter sleeve is positioned with a nut and a stop washer. Table 5-9 Positioning of the outer ring. The simplified diagram of positioning components should be used. Description. The simplified diagram of positioning components should be used. Description. The end cover is suitable for various radial bearings with high rotation speed and large axial load. The end cover uses screws to press the bearing outer ring, and the end cover can be made into a sealing device stop ring. When the stop rib cannot be processed in the bearing shell hole due to limited conditions, and the outline size must be reduced, a deep groove ball bearing with a stop groove on the bearing outer ring can be used. When assembling small bearings, a hand hammer and a simple auxiliary sleeve can be used, as shown in Figure 5-18. For medium and small bearings, a hydraulic press can be used to apply pressure on the inner ring during installation to press the bearing onto the journal. ； For larger medium and large bearings, the temperature difference method is often used for assembly.: After the bearing is heated in hot oil, the bearing is inserted into the journal. The heating temperature is generally 80 ~ 100 ℃, and is not allowed to exceed 120 ℃. Pay attention to prevent local overburning of the bearing. For small bearings with loose fit, you can use a hand hammer and a copper rod to gently tap out the bearing from the back along the inner ring of the bearing, as shown in Figure 5-19. The pressure method is used to disassemble the bearing. The most commonly used puller is the puller (commonly known as the puller), as shown in Figure 5-20. It relies on three claws to hook the inner ring of the bearing to remove the bearing. For this purpose, sufficient height should be left on the inner ring shoulder. Figure 5-18 Figure 5-19