Expert knowledge of bearings

I have done this before and have summarized it; I hope it will be useful to you: The market demands for various mechanical devices and instruments that use rolling bearings are becoming increasingly stringent, and the requirements and performance standards for these bearings are also becoming more diverse. To select the most suitable bearing from the numerous structures and sizes available, it is necessary to conduct studies from various perspectives. When selecting bearings, generally, factors such as the arrangement of bearings in the shafting system, the ease of installation and removal, the available space for the bearings, their dimensions, and their availability on the market are considered to roughly determine the bearing design. Secondly, the size of the bearing is determined by comparing the design life of various machines that use bearings with the different durability limits of those bearings. When selecting bearings, there is a tendency to consider only their fatigue life; however, the lifespan of the grease, wear, noise, and other issues arising from grease aging also need to be thoroughly studied. Furthermore, depending on the application, it is necessary to select bearings that are specially designed with specific requirements regarding precision, clearance, cage structure, lubricant, and so on. However, there is no fixed order or rule for selecting bearings; what should be given priority is the requirements and performance specifications for the bearings, as well as the most relevant factors, which is particularly practical. Generally, the steps for selecting a bearing can be summarized as follows: 1. Based on the operating conditions of the bearing (including the direction and type of load, speed, lubrication method, requirements regarding coaxiality, whether positioning is required or not, the installation and maintenance environment, ambient temperature, etc.), select the basic type of bearing, the tolerance grade, and the clearance ； 2. Based on the operating conditions, stress levels, and lifespan requirements of the bearing, the bearing model is determined through calculations; or the bearing model is selected according to the application requirements, after which its lifespan is verified ； 3. Verify the rated load and maximum speed of the selected bearing. The main factors considered when selecting a bearing are the maximum speed, the desired service life, and the load capacity. Other factors help determine the final specifications regarding the bearing type, design, dimensions, tolerance grades, and clearance. Various types of rolling bearings have different characteristics and are suitable for different applications in various machines. When selecting a bearing type, the following factors should generally be considered. Under normal circumstances: thrust bearings and angular contact bearings are used when bearing thrust loads, ball bearings are typically employed in high-speed applications, while roller bearings are chosen when dealing with heavy radial loads. In short, the appropriate type should be selected from a variety of bearing products manufactured by different manufacturers. 1) The space and position occupied by bearings in a machine: In mechanical design, the dimensions of the shaft are determined first, and then rolling bearings are selected based on those shaft dimensions. Generally, ball bearings are used for the small shafts, while roller bearings are used for the large shafts. However, when the bearing is constrained in the diameter direction of the machine, needle bearings or ball or roller bearings from the ultra-light and extra-ultra-light series are selected ； When the axial position of the bearing in the machine is restricted, narrow or extra-narrow series of ball or roller bearings can be used. 2) The magnitude, direction, and nature of the load on the bearing. The load is the most important factor in selecting a bearing. Roller bearings are used to bear heavier loads, while ball bearings are used for lighter or moderate loads. Bearings made of carburized steel or quenched in bainite can withstand impact and vibration loads. In terms of the direction of the load, when a pure radial load is applied, deep groove ball bearings, cylindrical roller bearings, or needle bearings can be used. When subjected to relatively small pure axial loads, thrust ball bearings can be used ； Thrust roller bearings can be used when subjected to large pure axial loads. When bearings are subjected to combined radial and axial loads, angular contact ball bearings or tapered roller bearings are generally selected. For cantilever support structures, tapered roller bearings or angular contact ball bearings are commonly used, and they are employed in pairs. 3) Self-aligning capability of bearings: When the center line of the shaft differs from that of the bearing housing, resulting in an angular error, or when the stiffness of the shaft is low due to a large distance between its two supports, making it prone to bending or tilting under stress, self-aligning ball or roller bearings, as well as outer ball bearings with good self-aligning capabilities, can be used. Such bearings can maintain normal operation even when the shaft is slightly tilted or bent. 4) Bearing stiffness The bearing stiffness refers to the magnitude of the force required to cause a unit deformation in the bearing. The elastic deformation of rolling bearings is very small, and it can be ignored in most machines. However, in some machines, such as machine tool spindles, bearing stiffness is an important factor; in such cases, cylindrical and tapered roller bearings should generally be used. This is because in these two types of bearings, the rolling elements are in point contact with the raceways when carrying loads, resulting in poor rigidity. Furthermore, various types of bearings can also achieve an increase in supporting stiffness through preloading. In bearings such as angular contact ball bearings and tapered roller bearings, in order to prevent shaft vibration and increase support stiffness, a certain axial force is often applied in advance during installation to press them together. It should be noted here that the pre-tightening amount must not be too large. If it is too large, it will increase bearing friction and raise temperature, affecting the bearing’s service life. 5) Rotational speed of the bearing: Each bearing model has its own maximum rotational speed, which is determined by physical characteristics such as size, type, and structure. The maximum rotational speed refers to the highest operating speed of the bearing (usually expressed in r/min); exceeding this limit can cause the bearing’s temperature to rise, the lubricant to dry out, and even lead to the bearing getting stuck. The speed range required by the application helps determine what type of bearing to use. D is the bearing size, which generally refers to the pitch diameter of the bearing. When selecting a bearing, the average of the inner and outer diameters of the bearing is used, with the unit being mm. The pitch diameter D multiplied by the shaft’s rotational speed (in r/min) yields a critical speed factor (DN), which is very important when selecting the type and size of bearings. The product catalogs of most bearing manufacturers provide the maximum rotational speed values for their products, and practice has shown that it is better to operate at speeds that are 90% below these maximum values. The maximum speed of lip-lubricated bearings is lower than that of oil-lubricated bearings, and the method of supplying oil to the bearings affects the maximum speed that can be achieved. It should be noted that for lip-lubricated bearings, their maximum speed is generally only 80% of the maximum speed achievable when such bearings are equipped with a high-quality recirculating oil system; whereas for oil-mist lubrication systems, the maximum speed is usually 50% higher than that of similar basic lubrication systems. The design and structure of the cage also affect the bearing’s maximum speed, as there is a sliding contact between the rolling elements and the cage surface. Using a costly, well-designed cage made from high-quality, low-friction materials not only helps to separate the rolling elements but also contributes to maintaining the lubrication film in the sliding contact area. However, inexpensive retainers such as stamping retainers can usually only keep the rolling elements separated. Therefore, they suffer from accident-prone and troublesome sliding contact, resulting in a lower limit speed. Generally, in applications operating at high speeds, deep groove ball bearings, angular contact bearings, and cylindrical roller bearings are preferred ； In applications operating at lower speeds, tapered roller bearings can be selected. The maximum speed limit for tapered roller bearings is generally about 65% of that for deep groove ball bearings, 70% for cylindrical roller bearings, and 60% for angular contact ball bearings. Thrust ball bearings have a low maximum speed and can only be used in applications with lower speeds. For bearings of the same type, the smaller the size, the higher the allowable speed. When selecting bearings, care should be taken to ensure that the actual rotational speed is below the limit speed. 6) Bearing play and axial displacement: Normally, a shaft is supported by two bearings at a certain distance apart. To accommodate the varying degrees of thermal expansion affecting the shaft and the housing, one bearing should be fixed axially during installation, while the other should allow it to move along the shaft (i.e., it should serve as a floating support), in order to prevent jamming caused by the expansion or contraction of the shaft. Swing bearings typically use cylindrical roller bearings without ribs on the inner or outer ring (formerly series 2000 and 32000) as well as needle bearings, mainly because the internal structure of these bearings allows for appropriate axial movement between the shaft and the housing. At this time, a tight fit can be used between the inner ring and the shaft, as well as between the outer ring and the housing hole. When non-separable bearings are used as floating supports, such as deep groove ball bearings or self-aligning roller bearings, it is necessary during installation to allow a loose fit between the outer ring and the housing bore, or between the inner ring and the shaft, so that free axial movement is possible. Tapered roller bearings, self-aligning roller bearings, and deep groove ball bearings are essentially of the positioning type; when used in a non-positioning capacity, they are installed with a loose fit. All thrust roller bearings are positioning bearings. 7) Facilitates the installation and removal of bearings. When selecting a bearing type, it is also necessary to take into account the ease of installing and removing it, especially this is crucial for large and extra-large bearings. Commonly used angular contact ball bearings, tapered roller bearings, cylindrical roller bearings, and needle roller bearings with separable outer rings are easy to install and remove, as their inner and outer rings can be mounted on the shaft or in the holes of the housing separately. In addition, self-aligning roller bearings, double-row cylindrical roller bearings, and self-aligning ball bearings with a conical bore in the inner diameter and set screws are also relatively easy to install and remove. 8) Other requirements: In addition to the factors mentioned above, consideration should also be given to the operating temperature of the bearing, the bearing sealing, as well as any special requirements regarding friction torque, vibration, noise, etc. Clearance selection: Clearance is an important factor for the proper operation of rolling bearings, and it is divided into axial clearance and radial clearance. Choosing the appropriate clearance allows the load to be distributed evenly among the bearing rollers ； It can restrict the axial and radial displacement of the shaft (or housing), ensuring the rotational accuracy of the shaft ； Enables the bearing to operate properly at specified temperatures ； Reducing vibration and noise helps to increase the lifespan of bearings. Therefore. When selecting bearings, it is necessary to choose the appropriate bearing clearance. When selecting bearing clearance, the following factors should be considered: 1. The operating conditions of the bearing, such as load, temperature, speed, etc ； 2. Requirements for the performance of bearings (rotational accuracy, friction torque, vibration, noise) ； 3. An interference fit between the bearing and the shaft as well as the housing holes results in a reduction in the bearing clearance. Rolling bearings are precision components, and their use must also be carried out with due caution. No matter how high-performance the bearings are, if they are not used properly, the expected high performance will not be achieved. The precautions for using bearings are as follows. (1) Keep the bearing and its surrounding area clean. Even tiny dust particles that are invisible to the eye can have a negative impact on bearings. Therefore, it is necessary to keep the surroundings clean to prevent dust from entering the bearings. (2) Use with caution. Applying strong impacts to the bearing during use can cause scratches and indentations, which may lead to accidents. In severe cases, cracks and breaks can occur, so caution is necessary. (3) Use appropriate operating tools. Avoid replacing with existing tools; the appropriate tools must be used. (4) Be careful of bearing rust. When handling bearings, sweat on the hands can cause rusting. Make sure to use clean hands; it’s best to wear gloves if possible.