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Matching selection scheme of motor bearing

by:JNSN     2022-03-13
The purpose of the fit is to firmly fix the inner or outer ring of the motor bearing with the shaft or the housing, so as to avoid unfavorable axial sliding on the mating surfaces. This unfavorable axial sliding (called creep) can cause various problems such as abnormal heat generation, wear of the mating surfaces (and thus intrusion of worn iron powder into the bearing interior), and vibration, making the bearing unable to function fully. Fig. 1 The relationship between the dimensional tolerance of the shaft and the housing hole and the fit (bearing with 0-grade accuracy) The bearing's accuracy grade 0-grade clearance fit transition fit Cooperate with Dmpdmp. The selection of cooperation is generally carried out according to the following principles. According to the direction and nature of the load acting on the bearing and which side of the inner and outer rings rotates, the load on each ferrule can be divided into rotating load, static load or non-directional load. The ferrule that bears the rotating load and the non-directional load should take the static fit (interference fit), and the ferrule that bears the static load can take the transition fit or the dynamic fit (clearance fit). When the bearing load is large or subjected to vibration and shock load, its interference must be increased. When using hollow shaft, thin-walled bearing housing or light alloy, plastic bearing housing, it is also necessary to increase the amount of interference. When it is required to maintain high rotation accuracy, high-precision bearings must be used, and the dimensional accuracy of the shaft and bearing housing must be improved to avoid excessive interference. If the interference is too large, the geometrical accuracy of the shaft or bearing housing may affect the geometrical shape of the bearing ring, thereby compromising the rotational accuracy of the bearing. Both the inner and outer rings of the non-separable bearing use static fit, so it is extremely inconvenient to install and disassemble the bearing. It is best to use a dynamic fit for one of the inner and outer rings. (1) Influence of load properties Bearing load can be divided into inner ring rotation load, outer ring rotation load and non-directional load according to its properties. The relationship about the fit is as follows: Bearing rotation conditions Inner ring: rotating outer ring: static load direction: fixed Inner ring: stationary Outer ring: rotating Load direction: rotating with outer ring Inner ring: stationary Outer ring: rotating Load direction: fixed Inner ring: rotating Outer ring: static Load direction: rotating with inner ring (Interference fit) Inner ring: Dynamic fit (clearance fit) Outer ring: Dynamic fit (clearance fit) Outer ring: Static fit (interference fit). The relationship between the nature of the load and the fit (2) Influence of the size of the load Under the radial load, the inner ring is compressed and stretched in the radial direction, and the circumference tends to increase slightly, so the initial interference will be reduced. The amount of interference reduction can be calculated by the following formula. [When Fr≤0.25 C0][When Fr>0.25 C0]u003dBdF .F r0.08 d -310 (37)BFrd F u003d0.02 10-3 (38): Inner ring interference reduction, mm: bearing nominal Inner diameter, mm: nominal width of inner ring, mm: radial load, N{kgf}: basic static load rating, N{kgf}△dFdBFrCor Therefore, when the radial load is a heavy load (more than 25% of the C0 value), The fit must be tighter than with light loads. In the case of shock loads, the fit must be tighter. (3) The influence of the roughness of the mating surface If the plastic deformation of the mating surface is considered, the effective interference after fitting is affected by the machining quality of the mating surface, which can be approximately expressed by the following formula. Influence of temperature Generally speaking, the bearing temperature during operation is higher than the surrounding temperature, and when the bearing rotates with load, the inner ring temperature is higher than the shaft temperature, so thermal expansion will reduce the effective interference. Now suppose that the temperature difference between the inside of the bearing and the periphery of the outer casing is Δt, it may be assumed that the temperature difference between the inner ring and the shaft at the mating surface is approximately (0.01 ~ 0.15) Δt. Therefore, the interference reduction Δdt caused by the temperature difference can be calculated by the following formula. dt u003d(0.10 to 0.15)0.0015 t.d10-3 (41)t . .d△dt△tad: Interference reduction due to temperature difference, mm: Temperature difference between the inside of the bearing and the periphery of the housing, ℃: Linear expansion of bearing steel Coefficient, (12.510-6) 1/℃: Nominal inner diameter of the bearing, mm (5) Maximum stress inside the bearing caused by the fit When the bearing is installed with an interference fit, the ferrule will expand or contract, resulting in stress. When the stress is too high, sometimes the ferrule will crack, which requires attention. The maximum internal stress of the NSK motor bearing produced by the fit can be calculated by the formula of . As a reference value, it is safe to take that the maximum interference does not exceed 1/1000 of the shaft diameter, or the maximum stress* obtained by the calculation formula is not more than 120MPa{12kgf/mm}. When other accuracy requirements are particularly high, the accuracy of the shaft and housing should be improved. Compared with the shaft, the general housing is difficult to process and has low precision, so it is advisable to loosen the cooperation between the outer ring and the housing. When using hollow shafts and thin-walled housings, the fit must be tighter than usual. When using a double-half housing, the fit with the outer ring should be loosened. For cast aluminum or light alloy housings, the fit must be tighter than usual.
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