Matching of bearings

The purpose of coordination is to make the inner ring or outer ring of the bearing firmly fixed with the shaft or shell, so as to avoid unfavorable axial sliding on the matching surface.

This unfavorable axial slide (called creep) will cause abnormal fever, composite Surface Wear (and then make the worn iron powder intrude into the bearing) and vibration problems, so that the bearing cannot play a full role.

Therefore, for bearings, due to bearing load rotation, it is generally necessary to let the ring carry surplus so that it is firmly fixed with the shaft or shell.

Dimensional Tolerances for shafts and casings

The dimensional tolerances for the axes and housing holes of the metric series have been standardized by the GB/T275-93 in combination with the shafts and housing, the matching between the bearing and the shaft or housing can be determined by selecting the dimensional tolerances.

Selection

The selection of cooperation is generally based on the following principles.

Based on the bearing load direction, nature and the side of the internal and external ring rotation, the load of each ring can be divided into rotary load, static load or non-oriented load. Ring Bearing rotation load and non-oriented load should be static (interference fit), bearing static load, can transition or dynamic fit (GAP fit ).

When the bearing load is large or is subject to vibration or impact load, the excessive profit must increase. When hollow shaft, thin-walled Bearing Box or light alloy or plastic bearing box is used, the excess profit must be increased.

When high rotation is required, high precision bearings must be used, and the dimensional precision of the shaft and bearing box should be improved to avoid excessive interference. If the interference is too large, the geometric shape precision of the shaft or bearing box may affect the geometric shape of the Bearing Ring, thus compromising the bearing rotation accuracy.

If both the inner and inner circles of non-separated Bearings (such as deep groove ball bearings) Adopt static cooperation, it is inconvenient to install and disassemble the bearings. It is best to use dynamic cooperation for one side of the inner and inner circles.

1) Impact of load nature
Bearing load can be divided into inner ring rotation load, outer ring rotation load and non-oriented load according to its nature. The relationship between bearing load and matching is referred to the bearing matching standard.

2) Impact of load size

When the inner ring is under radial load, the radius is compressed and stretched for years, and the circumference tends to be slightly increased. Therefore, the initial interference will be reduced. A small amount of excessive profit reduction can be calculated in the following formula:

Here:
When DF: the inner ring is reduced by a small amount of interference, mm
D: nominal bearing diameter, mm
B: nominal width of the inner ring, mm
FR: radial load, n {kgf}
CO: Basic rated static load, n {kgf}

Therefore, when the radial load is heavy (more than 25% of the CO value), the combination must be less than the light load.

If it is an impact load, the combination must be tighter.

3) Influence of composite Surface Roughness

If the plastic deformation of the matching surface is considered, the effective interference after the combination is affected by the processing quality of the matching surface, which can be expressed as follows:

[GRINDING axis]
Includeff = (D/(D + 2) * includ ...... (3)
[Turning axis]
Includeff = (D/(D + 3) * includ ...... (4)

Here:
Includeff: Valid interference, mm
Direction D: view interference, mm
D: nominal bearing diameter, mm

4) Temperature Effect

In general, the bearing temperature is higher than the surrounding temperature during the dynamic rotation, and the inner ring temperature is higher than the shaft temperature when the bearing is rotated with load, so the thermal expansion will reduce the effective interference.

If the temperature difference between the bearing and the outer casing is t, it is assumed that the temperature difference between the inner ring and the shaft is approximately (0.01-0.15) T. Therefore, a small amount of excess profit loss resulting from the temperature difference can be calculated by formula 5:

When dt = (0.10 to 0.15) Then T * α * d
Limit 0.0015 tb t * D * 0. 01 ...... (5)

Here:
⊿ DT: reduce the amount of excess profit produced by the temperature difference, mm
⊿ T: the temperature difference between the bearing and the surrounding housing.
α: linear expansion coefficient of Bearing Steel, (12.5 × 10-6) 1/℃
D: nominal bearing diameter, mm

Therefore, when the bearing temperature is higher than the shaft temperature, the combination must be tight.

In addition, between the outer ring and the shell, due to the difference in temperature difference or linear expansion coefficient, in turn, sometimes the profit increases. Therefore, when considering the use of Sliding between the outer ring and the matching surface of the shell to avoid the thermal expansion of the shaft, pay attention to it.

5) The maximum internal stress of the bearing generated in combination

When the bearing is installed with interference, the ring will expand or contract, resulting in stress.

When the stress is too high, sometimes the ring will break, you need to pay attention.

The maximum internal stress of the bearing can be calculated by the formula in table 2. As a reference value, taking the maximum interference not greater than 1/1000 of the shaft diameter, or the maximum stress σ not greater than 120Mpa {12kgf/mm2} calculated in table 2 is safe.
Table 2 maximum internal stress of the Bearing

Here:
σ: maximum stress, MPA {kgf/mm2}
D: nominal bearing diameter (shaft diameter), mm
DI: inner ring diameter, mm
Ball bearings ...... DI = 0.2 (D + 4d)
Roller bearings ...... DI = 0.25 (D + 3d)
Includeff: Valid interference of the inner ring, mm
Do: hollow axis radius, mm
DE: outer ring diameter, mm
Ball bearings ...... De = 0.2 (4d + D)
Roller bearings ...... De = 0.25 (3D + D)
D: bearing nominal outer diameter (shell aperture), mm
Includeff: Valid interference of outer ring, mm
DH: outer casing diameter, mm
E: elastic modulus, 2.08 × 105 MPa {21 200kgf/mm2}

6) Others

When the accuracy requirements are particularly high, the accuracy of the shaft and shell should be improved. Compared with the shaft, the general shell is difficult to process, low precision, so relax the outer ring and shell with appropriate.

When using hollow shaft and thin-walled shell, the combination must be tight than the normal.

When using a double half shell, the outer ring should be relaxed. For cast aluminum or light alloy casings, the fit must be tighter than usual