Mounting and dismounting

THE IMPORTANCE OF CORRECT FITTING

A bearing can only perform to its full capacity when it is correctly fitted on the shaft and in the housing. Insufficient interference on fitting surfaces could cause bearing rings to creep in a circumferential direction. Once this happens, considerable wear occurs on the fitting surface and both shaft and housing are damaged. Furthermore, abrasive particles may enter the bearing causing vibration, excessive heat and damage to raceways. It is therefore necessary to provide bearing rings under rotating load with an adequate interference fit to prevent creep. When using thi-type bearings under low load, the bearings should be fastened by a nut. Statically loaded bearings generally do not need to be fitted with an interference fit. Only when subject to a high degree of vibration do both inner and outer rings require fitting with an interference fit. 

FITTING OF BEARING AND SHAFT

CONDITION ( STEEL SHAFT ) SHAFT BORE DIAMETER SHAFT TOLERANCE CLASS
THIN TYPE OTHERS
INNER RING ROTATING LOAD OR INDETERMINATE LOAD DIRECTION LIGHT LOAD<=0.06Cr OR FLUCTUATING LOAD 10≤d≤18
18≤d≤30
30≤d≤50
h5
h5
h5
js5
js5
js5
STANDARD LOAD=0.06~0.12Cr 10≤d≤18
18≤d≤30
30≤d≤50
js5
js5
js5
j5
k5
k5
OUTER RING ROTATING LOAD NECESSARY FOR INNER RING TURNING EASILY AROUND SHAFT ALL BORE DIAMETERS g5 g6
UNNECESSARY FOR INNER RING TURNING EASILY AROUND SHAFT ALL BORE DIAMETERS h5 h6

FITTING OF BEARING AND HOUSING

CONDITION ( ONE-PIECE HOUSING ) AXIAL DIRECTIONAL MOVEMENT OF OUTER RING TOLERANCE CLASS OF SHAFT HOUSING SEATS
THIN TYPE OTHERS
INNER RING ROTATING LOAD VARYING LOADS EASY TO MOVE H6 H7
LIGHT OR STANDARD LOAD EASY TO MOVE H7 H8
HIGH TEMPERATURE OF INNER RING AND SHAFT EASY TO MOVE G6 G7
LIGHT OR STANDARD LOAD PRECISE ROTATION AS A RULE, IMPOSSIBLE TO MOVE K5 K6
POSSIBILE TO MOVE JS6 J6
QUIET OPERATION EASY TO MOVE H6 H6
INDETERMINATE LOAD DIRECTION LIGHT OR STANDARD LOAD IN GENERAL, POSSIBLE TO MOVE JS6 J7
STANDARD OR HEAVY LOAD AS A RULE, IMPOSSIBLE TO MOVE K5 K7
LARGE SHOCK LOAD IMPOSSIBLE TO MOVE M5 M7
LIGHT OR FLUCTUATING LOAD

IMPOSSIBLE TO MOVE

M5 M7
OUTER RING ROTATING LOAD STANDARD OR HEAVY LOAD IMPOSSIBLE TO MOVE N5 N7
THIN-TYPE HOUSING SEATS HEAVY LOAD OR LARGE SHOCK LOAD IMPOSSIBLE TO MOVE P6 P7

CHARACTERISTIC OF LOAD AND FITTING

ROTATING RING LOAD LOAD CONDITION FITTIN

INNER RING

STATIC

Inner ring rotating load

Outer ring static load

Interference fit for inner ring

Clearance fit for outer ring


OUTER RING

ROTATING

OUTER RING

STATIC

Outer ring rotating load

Inner ring static load

Clearance fit for inner ring

Interference fit for outer ring


INNER RING

ROTATING
IN THE CASE OF FLUCTUATING LOAD DIRECTION OR UNBALANCED LOAD ROTATING OR STATIC Indeterminate load direction Interference fit for inner and outer ring

CALCULATIONS OF FITS

1) FITTING PRESSURE AND DIMENSIONAL CHANGES OF INNER AND OUTER RING

The right fit for each application is established taking various conditions into consideration such as load, speed, temperature, mounting and dismounting of the bearing. The interference fit should be greater than normal in thin housings, housings of soft material or on hollow shafts.

2) LOAD OF INTERFERENCE

The interference fit of shaft and inner ring decreases under radial load. The decrease in fit of shaft and inner ring is calculated by the following formula:

∆dF=0.08×√d/B•Fr×10-3 (mm)
∆dF=0.02×Fr/B×10-3 (mm)
 
The higher value from the two formula shown below shoud be used.

3) INFLUENCE OF TEMPERATURE ON BEARINGS, SHAFT AND HOUSING

Each inner ring, outer ring or rolling element of a bearing rotating under load generates heat which will affect the interference fits of the shaft and the housing. Assuming a temperature difference within the bearing and the housing of ∆T(°C), that of the mating surface of the shaft and of the bearing is (0.10 ~ 0.15) ∆T.

Consequently, ∆dT, the decrease of the inner ring interference fit due to temperature change, in calculated from the following formula: 

∆dT=(0.10 ~ 0.15)×∆T•a•d=0.0015×∆T•d×10-3 (mm)

whereby:

  • ∆dT = decrease of interference due to temperature difference (mm)
  • ∆T = temperature difference between bearing and surrounding housing (°C)
  • a = coefficient of thermal expansion for bearing steel = 12.5×10-6 (l/°C)
  • a = coefficient of thermal expansion for stainless steel = 10.3×10-6 (l/°C)
  • d = nominal bore diameter of bearing (mm)

It should also be noted that fit can increase due to temperature changes.

4) EFFECTIVE INTERFERENCE, SURFACE ROUGHNESS AND ACCURACY

The surface roughness is smoothed during fitting and the effective interference becomes smaller than the theoretical interference. The surface roughness quality of a mating surface han an influence on how much this theoretical interference decreases. Effective interference can usually be calculated as follows:

Grounded shaft : ∆d=d/(d+2)•∆da(mm)
Turned shaft : ∆d=d/(d+3)•∆da(mm)

whereby:

  • ∆d = effective interference (mm)
  • ∆da = theoretical interference (mm)
  • d = nominal bore diameter of bearing (mm)

By combining these factors, the theoretical interference fit required for inner ring and shaft where the inner ring is subjected to rotating load is calculated as follows:

∆da≥(∆dF+ ∆dT) ((d+3)/d o (d+2)/d) (mm)

Normally, shaft and housing seats have to meet the accuracy and roughness requirements as given below.

ACCURACY AND ROUGHNESS OF SHAFT AND HOUSING SEATS

CARATTERISTICHE SHAFT HOUSING
ROUNDNESS BELOW 50% OF SHAFT DIAMETER TOLERANCE BELOW 50% OF HOUSING BORE DIAMETER TOLERANCE
CYLINDRICITY BELOW 50% OF SHAFT DIAMETER TOLERANCE WITHIN BEARING WIDTH BELOW 50% OF HOUSING BORE DIAMETER TOLERANCE WITHIN BEARING WIDTH
SQUARENESS                                                                                          ≤3/1000(0.17°)
ROUGHNESS OF MATING SURFACE Rmax 3.2 Rmax 6.3

Mounting bearings with extra tight or light interference fits can lead to early bearing failure. In order to ensure safe operating conditions the tolerance variations of shaft seats, housing bores and bearing bore and outside diameter need to be reduced. We recommend the tolerance zones are divided into two bands and selective assembly is applied. Bearings sorted into two tolerance bands for inner and outer rings are available on request.