What factors affect the bearing fit?

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

This kind of unfavorable sliding (called creep) will cause abnormal heating, wear of the mating surface (which will make the worn iron powder invade the bearing interior) and vibration, which will make the bearing unable to play its full role.

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

Dimensional tolerance of shaft and housing

The dimensional tolerance of shaft and housing hole of metric series has been standardized by GB / t275-93 "rolling bearings and shaft and housing fit". The fit of bearing and shaft or housing can be determined by selecting the dimensional tolerance.

Selection of bearing fit

The selection of bearing fit 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 rotate, the load borne by each ring can be divided into rotating load, static load or non directional load. The static fit (interference fit) should be adopted for the ferrule bearing rotating load and non directional load, and the transition fit or dynamic fit (clearance fit) with small clearance can be used for the ring bearing static load.

When bearing load is large or bearing vibration and impact load, its interference must be increased. When hollow shaft, Thin-walled bearing box or light alloy or plastic bearing box is used, the interference must also be increased.

When high rotation is required, high precision combined bearing must be used, and the dimensional accuracy of shaft and bearing box mounting hole shall be improved to avoid excessive interference. If the interference is too large, the geometry of the bearing ring may be affected by the geometric accuracy of the shaft or bearing box, thus damaging the rotation accuracy of the bearing.

If the inner and outer rings of non separable bearings (such as deep groove ball bearings) adopt static fit, it will be very inconvenient to install and disassemble the bearings. It is better to use dynamic fit on one side of the inner and outer rings.

1) Influence of load properties

The bearing load can be divided into inner ring rotating load, outer ring rotating load and non directional load according to its nature. The relationship between bearing load and fit can refer to bearing matching standard.

2) Influence of load size

Under the action of radial load, the radius direction of the inner ring is compressed and extended, and the circumference tends to increase slightly, so the initial interference will be reduced. The reduction of interference can be calculated by the following formula:

here:

⊿ DF: interference reduction of inner ring, mm

d: Bearing nominal inner diameter, mm

B: Nominal inner ring width, mm

Fr: radial load, n {KGF}

Co: basic rated static load, n {KGF}

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

In case of impact load, the fit must be tighter.

3) Influence of surface roughness

If the plastic deformation of the mating surface is considered, the effective interference is affected by the machining quality of the mating surface, which can be approximately expressed by the following formula:

[grinding shaft]

⊿deff=(d/(d+2))*⊿d......(3)

[turning shaft]

⊿deff=(d/(d+3))*⊿d......(4)

here:

⊿ deff: effective interference, mm

⊿ D: apparent interference, mm

d: Bearing nominal inner diameter, mm

4) Influence of bearing temperature

Generally speaking, the bearing temperature is higher than the surrounding temperature during dynamic rotation, and the inner ring temperature is higher than the shaft temperature when the bearing rotates with load, so the effective interference will be reduced by thermal expansion.

If the temperature difference between the inner bearing and the outer shell is ⊿ T, it can be assumed that the temperature difference between the inner ring and the shaft on the mating surface is approximately (0.01-0.15) ⊿ t. Therefore, the interference reduction ⊿ DT caused by temperature difference can be calculated by formula 5

⊿dt=(0.10 to 0.15)⊿t*α*d

≒0.0015⊿t*d*0.01......(5)

here:

⊿ DT: reduction of interference caused by temperature difference, mm

⊿ T: temperature difference between the inside of the bearing and the surrounding of the shell, ℃

α: The linear expansion coefficient of bearing steel is (12.5 × 10-6) 1 / ℃

d: Bearing nominal inner diameter, mm

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

In addition, due to the difference of temperature difference or coefficient of linear expansion between outer ring and outer shell, sometimes interference will increase. Therefore, attention should be paid to the use of sliding between the outer ring and the housing mating surface to avoid the thermal expansion of the shaft.

5) Maximum internal stress of bearing caused by fit

When the bearing is installed with interference fit, the ring will expand or shrink, thus producing stress.

When the stress is too large, sometimes the ring will break, which needs attention.

The maximum internal stress of bearing produced by matching can be calculated by the formula in Table 2. As a reference value, the maximum interference is not more than 1 / 1000 of the shaft diameter, or the maximum stress σ obtained from the calculation formula in Table 2 is not more than 120MPa {12kgf / mm2}.

Maximum internal stress of bearing caused by fit

here:

σ: Maximum stress, MPA {kgf / mm2}

d: Bearing nominal inner diameter (shaft diameter), mm

Di: inner ring raceway diameter, mm

Ball bearing Di=0.2(D+4d)

Roller bearing Di=0.25(D+3d)

⊿ deff: effective interference of inner ring, mm

Do: radius of hollow shaft, mm

De: outer raceway diameter, mm

Ball bearing De=0.2(4D+d)

Roller bearing De=0.25(3D+d)

D: Bearing nominal outer diameter (shell diameter), mm

⊿ deff: effective interference of outer ring, mm

DH: outer diameter of shell, mm

E: The elastic modulus is 2.08 × 105Mpa {21200kgf/


Post time: Dec-18-2020