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FITS
 
8.1 Bearing Fits
The inner and outer rings of bearings support a load that rotates, and are therefore mounted on the shaft and housing. In this case, fitting of the inner ring with the shaft, and outer ring with the housing differs according to nature of the load, assembly of the bearing and ambient environment, depending upon whether the fit is provided with clearance or interference. The three basic types of fitting are as follows:
 
(1) Clearance Fit
Mounted with clearance in the fit.

(2) Transition Fit
Mounted with both clearance and interference in the fit.

(3) Interference Fit
Mounted in if fixed position with interference in the fit.
 
Fig. 8.1 Bearing Creep
The most effective method of mounting a bearing to support a load is to provide interference by fastening with an interference fit. There are also advantages in providing clearance, such as mounting, dismounting and absorption of expansion and contraction of the shaft and housing due to change in temperature. If you do not provide interference that matches the load, creep may be produced by rotation. As shown in Fig. 8.1, if there is creep in the clearance difference of the fit that turns while receiving the load, slipping may be produced by the difference in the inner ring bore and circumference length, resulting in abnormal heat, abrasion and powder which negatively affect the bearing. Even if there is no clearance, creep may occur if the load is large. You should therefore decide the proper fit using Table 8.2 as a guideline.
 
Table 8.1 Nature and Fit of Radial Loads
  Diagram   Rotation division   Nature of load   Fit
Inner ring turning load

Outer ring static load
Inner ring:
Interference fit

Outer ring:
Clearance fit
   
Inner ring static load

Outer ring turning load
Inner ring:
Clearance fit

Outer ring:
Interference fit
   
 
Interference or clearance range on the other hand is decided by dimension tolerance of the bearing, shaft and housing. Fit therefore requires sufficient consideration.
 
 
8.2 Fit Selection
Proper fit selection is dependent upon thorough analysis of bearing operating conditions:
 
» Shaft and housing material, wall thickness, rigidity and finished surface precision

» Machinery operating conditions (nature and magnitude of load, rotating speed, temperature, etc.)
 
The basic philosophy for fit concerns whether it is the inner or outer ring that turns. Fit is decided by which of the bearing rings the load moves along, and is as given in

Table 8.1.
The relationship of dimension tolerance for the housing and shaft on which the bearing is to be mounted is as shown in Fig. 8.2. Some of the general fitting criteria for various types of bearings under various operating conditions is given in Figs. 8.2 through 8.4.
 
Table 8.2 Tolerance Class of Shaft Used for Radial Bearings (Class 0, 6X, 6)                     Click for Table
 
Table 8.3 Tolerance Class of Housing Bore Used for Radial Bearings (Class 0, 6X, 6)        Click for Table
 
Table 8.4 Tolerance Class of Shaft Used for Thrust Bearings (Class 0, 6X, 6)                    Click for Table
 
8.3 Fit Calculation
As was previously stated, standards for bearing fits have already been set, but problems such as creep, bearing ring cracking and premature flaking may occur depending on conditions such as actual assembly, load and temperature. The following items must be checked if interference is necessary.
 
1. Load and interference
  When a radial load is placed on a bearing, interference between the inner ring and shaft is reduced. Thus, interference varies according to the size of the load. The required interference is calculated by the following equation. (The equation supposes that a solid steel shaft is used.)
 
2. Temperature and interference
  The temperature of the shaft and housing generally rises while the bearing is operating. As a result, interference between the inner ring and shaft is reduced. In this case, interference is calculated by the following equation.
 
Δdr=0.0015・d・ΔT (8.3)
Where:
Δdr : Required effective interference
according to temperature difference
(μm)
ΔT : Difference between bearing
temperature and ambient temperature
(°C)
d : Bearing bore (mm)
 
3. Interference and surface roughness of fit surface
  Fit surface roughness of the shaft and housing is crushed to a certain extent, reducing interference by that amount. The amount that interference is reduced differs according to roughness of the fit surface, but this is generally compensated somewhat when calculating inner ring expansion and outer ring contraction factors.
 
4. Maximum interference
  Tensile stress is produced in the bearing ring mounted on the shaft when interference is provided. If excessive interference is applied, the bearing ring could be cracked or life reduced. The upper limit value for interference is generally 1/1000 of the shaft diameter or less.

In the case of heavy or shock loads, calculate fit stress with detailed analysis. It is generally safe as long as 13 kgf/mm2 is not exceeded for bearing steel, or 18 kgf/mm2 for carburizing steel.
» For more information kindly check NTN catalog or Contact Us
 
 
     
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1. Bearing History
2. Types of Bearings
3. Types of loads
4. FITS
5. Clearance
6. Bearing Tolerance
7. Lubrication
8. Bearing Storage
9. Mounting and Dismounting
10. Bearing Failures
11. Prefix and Suffix Interchange
11. Allowable Speed
   
 
 
 
 
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