Fully consider the suitability of bearings in the design process
For motor designers, the selection of bearings is crucial. One of the main tasks of motor structure design is to analyze and calculate the design life and fatigue life of bearings, and determine the bearing size.
The selection of bearings should not only consider the grease life, wear, and noise caused by grease aging, but also comprehensively evaluate the accuracy, fit, clearance, cage, grease, sealing structure, loading and unloading, and other special requirements according to the different uses of the motor. Today, Ms Participate in a discussion with everyone about some aspects related to bearings in the process of motor design and manufacturing.
Control elements of motor bearings in different stages
Mechanical usage and design lifespan
When selecting bearings, increasing the fatigue life coefficient means choosing larger bearings, but it is necessary to consider the strength, rigidity, installation size, etc. of the shaft, and fatigue life is not necessarily the only limiting condition. The bearings used in various machinery have a benchmark design life based on their usage conditions, expressed as an empirical fatigue life coefficient.
Bearing installation and coordination
When installing bearings, the fit between the inner diameter of the bearing and the shaft, as well as the outer diameter and the housing, is very important.
When the fit is too loose, the mating surface will experience relative sliding, rapidly wearing down the mating surface like an avalanche, damaging the shaft or housing; At the same time, wear powder will invade the interior of the bearing, further exacerbating wear, causing heating, vibration, and damage.
If the fit is too tight or the interference is too large, on the one hand, it will cause the outer diameter of the outer ring to decrease or the inner diameter of the inner ring to increase, reducing the internal clearance of the bearing; On the other hand, due to uneven deformation of the outer or inner ring, the bearing noise increases. In addition, the geometric accuracy of shaft and housing processing can also affect the original accuracy of bearing rings, thereby affecting the performance of bearings.
Principles for Selection of Rolling Bearing Coordination
The condition of the bearing ring relative to the load
For rings that rotate or swing relative to the load direction, interference fit or transition fit should be selected. The ring with a fixed load direction should choose a clearance fit. When using non separable bearings as flow supports, a fixed ring relative to the load direction should be used as the moving ring, and clearance fit or transition fit should be selected.
● Type and size of load
When subjected to impact loads or heavy loads, a tighter fit should generally be chosen compared to normal or light loads. The size of the load on the radial bearing is divided by the ratio of the radial equivalent dynamic load to the radial rated dynamic load, and the larger the load, the greater the fit interference.
● Bearing clearance
The use of interference fit for bearing clearance can lead to a reduction in bearing clearance. It is necessary to check whether the clearance of the installed bearing meets the usage requirements in order to correctly select the fit and bearing clearance.
● Other factors
The material, strength, and thermal conductivity of the shaft and bearing seat; External factors and the heat transfer pathways and heat generated in bearings, as well as the performance of support installation and adjustment, can all affect the choice of fit.
Factors affecting bearing clearance
The so-called internal clearance of a bearing refers to the amount of movement that occurs when the inner or outer ring of the bearing is fixed before being installed on the shaft or bearing housing, and then the unsecured side is moved radially, axially, or diagonally. According to the direction of movement, it can be divided into radial clearance, axial clearance, and angular clearance.
Due to the fit of the bearing and the temperature difference between the inner and outer rings during operation, the clearance of the bearing is generally smaller than the initial clearance. In theory, if the bearing has a slightly negative operating clearance during operation, its lifespan is maximized. But maintaining this optimal clearance is very difficult. As the usage conditions change, the negative clearance of the bearing will correspondingly increase, resulting in a significant decrease in bearing life or heating. Therefore, the initial clearance of the bearing is generally set to be slightly greater than zero.
(1) The reduction in clearance caused by interference fit
When the bearing is installed on the shaft or bearing housing with a static fit, the inner ring expands and the outer ring contracts, resulting in a decrease in internal clearance of the bearing. The expansion or contraction of the inner or outer ring varies depending on the bearing form, shaft and bearing housing shape, size and material, and is approximately 70% to 90% of the interference fit.
(2) The reduction in clearance caused by the temperature difference between the inner and outer rings
When the bearing is running, the outer ring temperature is generally 5-10 ℃ lower than the inner ring or rolling body temperature. If the bearing box generates a large amount of heat or the shaft is connected to a heat source, or there is thermal fluid flow inside the hollow shaft, the temperature difference between the inner and outer rings will be greater, and the difference in thermal expansion caused by this temperature difference between the inner and outer rings will become the reduction in clearance.
