For motor engineers, selecting bearings for motors requires consideration from various perspectives. First, the design life of the motor and bearing should be compared with the bearing's fatigue life to determine the bearing size. At the same time, attention must be paid to grease aging, which affects grease life, wear, and noise. Furthermore, depending on the motor's specific application, selection must consider precision, fit, clearance, cage, grease, sealing structure, assembly/disassembly, and other special requirements.
When installing bearings, the fit between the bearing's inner diameter and the shaft, and between its outer diameter and the housing, is crucial. If the fit is too loose, relative sliding will occur on the mating surfaces, a phenomenon known as creep. Creep can wear down the mating surfaces, damage the shaft or housing, and allow wear particles to penetrate the bearing, causing heat, vibration, and damage. Excessive interference fit can lead to a smaller outer ring diameter or a larger inner ring diameter, reducing the bearing's internal clearance. Furthermore, the geometric precision of the shaft and housing machining can affect the original precision of the bearing rings, thus impacting the bearing's performance.
The selection of motor bearings should be based on a comprehensive evaluation and decision-making process considering various factors such as bearing size, precision, fit, clearance, cage, seal, lubrication, special requirements, installation, and use.
Factors to consider when selecting motor bearings
1. Selection of motor bearing size
The selection of bearings is based on the machine being used and the design life. Intentionally increasing the fatigue life coefficient when selecting bearings not only requires choosing larger bearings, which is uneconomical, but also means considering factors such as shaft strength, rigidity, and installation dimensions, which are often not solely based on fatigue life. Bearings used in various types of machinery have a benchmark design life, expressed as an empirical fatigue life coefficient, depending on the operating conditions. Bearing manufacturers will recommend specific selection methods.
2. Selection of motor bearing precision
In general industrial motors, P6 or even P0 precision levels are sufficient to meet their usage requirements. Bearing precision mainly refers to dimensional accuracy and rotational accuracy. National standards, referencing international standards, specify different precision levels and tolerances and values for major dimensions. Bearing precision is divided into five levels: P0, P6, P5, P4, and P2, increasing sequentially.
The precision of rolling bearings is determined by dimensional tolerances and rotational accuracy. Dimensional tolerances include the tolerances for the bearing's inner diameter, outer diameter, width, and assembly width; the tolerance limits for chamfer dimensions; and the tolerance values for different widths.
Rotational accuracy factors include the tolerance values for radial runout of the inner and outer rings; the tolerance values for axial runout of the inner and outer rings; the tolerance values for lateral runout of the inner ring; and the tolerance values for the inclination of the outer diameter surface of the outer ring.
3. Selection principles for rolling bearing fits
● The condition of the bearing races relative to the load. For races that rotate or oscillate relative to the load direction, an interference fit or transition fit should be selected. For races that are fixed relative to the load direction, a clearance fit should be selected. When a non-separable bearing is used as a floating support, the races that are fixed relative to the load direction should be used as floating races, and a clearance fit or transition fit should be selected.
● Load type and magnitude. When subjected to impact loads or heavy loads, a tighter fit should generally be selected than for normal or light loads. The magnitude of the load on a radial bearing is determined by the ratio of the radial equivalent dynamic load Pr to the radial rated dynamic load Cr; the larger the load, the greater the interference fit.
Under light load, Pr/Cr is less than or equal to 0.06; under normal load, Pr/Cr is greater than 0.06 and less than 0.13.
Under heavy load, Pr/Cr is greater than 0.13.
● As the bearing size increases, the larger the interference fit, the larger the clearance fit.
● The bearing clearance after installation should be checked to ensure it meets the usage requirements, so as to correctly select the fit and bearing clearance.
●Other factors affecting the selection of fit include the material, strength, and thermal conductivity of the shaft and bearing housing; the heat conduction path and heat generated externally and within the bearing; and the support installation and adjustment performance.
4 Selection of motor bearing clearance
The internal clearance of a bearing refers to the amount of movement that occurs when one of the inner or outer rings of the bearing is fixed in place before it is installed on a shaft or in a bearing housing, and the unfixed ring moves radially, axially, or angularly. Based on the direction of movement, it can be classified as radial clearance, axial clearance, and angular clearance.
During bearing operation, due to bearing fit and temperature differences between the inner and outer rings, the initial clearance is generally smaller than the initial clearance. Theoretically, a slightly negative operating clearance during bearing operation maximizes bearing life. However, maintaining this optimal clearance is very difficult. As operating conditions change, the negative clearance of the bearing will increase accordingly, leading to a significant decrease in bearing life or the generation of heat. Therefore, the initial clearance of a bearing is generally set slightly greater than zero. The operating clearance can be calculated from the initial clearance, the reduction in clearance due to interference fit, and the change in clearance caused by external temperature differences.
● Reduction in clearance due to interference fit. When a bearing is statically fitted onto a shaft or bearing housing, the inner ring expands while the outer ring contracts, resulting in a reduction in the bearing's internal clearance. The amount of expansion or contraction of the inner or outer ring varies depending on the bearing type, and the shape, size, and material of the shaft and bearing housing, but is generally approximately 70% to 90% of the interference fit.
● The reduction in clearance caused by the temperature difference between the inner and outer rings. During bearing operation, the outer ring temperature is generally 5-10°C lower than the inner ring or rolling elements. If the bearing housing dissipates a large amount of heat, the shaft is connected to a heat source, or there is hot fluid flowing inside the hollow shaft, the temperature difference between the inner and outer rings will be even greater. The difference in thermal expansion between the inner and outer rings caused by this temperature difference becomes the reduction in clearance.
5. Selection of materials and structure for motor bearing retainers
In deep groove ball bearings, the cage serves to isolate the rolling elements at equal intervals, prevent them from falling out, and guide and drive the rolling elements to rotate. Different cages correspond to different bearing usage requirements.
● Stamped riveted cages are generally used in deep groove ball bearings with large loads, resistance to impact loads, medium speeds, and an outer diameter greater than 26mm.
● Stamped wave-shaped cages are generally used in deep groove ball bearings with large loads, medium speeds, low frictional torque, and an outer diameter greater than or equal to 13 and less than 26 mm.
● Stamped crown-shaped cages are generally used for deep groove ball bearings with small loads, low speeds, low frictional torques, low noise requirements, and outer diameters less than 13mm.
● Solid nylon cages are generally used for deep groove ball bearings with smaller loads, higher speeds, lower frictional torque, and higher noise requirements, and are suitable for bearings with an outer diameter of 22mm or less.
