Fitting of Bearings

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 thin-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
10d18
18
d30
30
d50
h5
h5
h5
js5
js5
js5
STANDARD LOAD=0.060.12Cr
10d18
18
d30
30
d50
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
POSSIBLE 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 IMPOSSIBLTE TO MOVE N5 N7
THIN-TYPE HOUSING SEATS HEAVY LOAD OR LARGE SHOCK LOAD IMPOSSIBLE TO MOVE P6 P7

CHARACTERISTICS OF LOAD AND FITTING

ROTATING RING LOAD LOAD CONDITION FITTING

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 condetions into consideration such as load, speed, temperature, mounting 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:


(3) INFLUENCE OF TEMPERATURE ON BEARINGS, SHAFTS AND HOUSINGS
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(), that of the mating surface of the shaft and of the bearing is (0.100.15) T.
Consequently, dr, the decrease of the inner ring interference fit due to temperature change, is calculated from the following formula:

dr=(0.100.15)Tad0.0015Td10-3(mm)
dr : DECREASE OF INTERFERENCE DUE TO TEMPERATURE DIFFERENCE(mm)
T : TEMPERATURE DIFFERENCE BETWEEN BEARING AND SURPOUNDING HOUSING()
a : CONEFFICIENT OF THERMAL EXPANSION FOR BEARING STEEL12.510.6-6(l/)
a : CONEFFICIENT OF THERMAL EXPANSION FOR STAINLESS STEEL10.310.6-6(l/)
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 is smoothed during fitting and the effective interference becomes smaller than the theoretical interference. The surface quality of a mating surface has an influence on how much this theoretical interference decreases. Effective interference can usually be calculated as follows:

Ground Shaft : d=d/(d+2)da(mm)
Turned Shaft : d=d/(d+3)da(mm)
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 or (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

. 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

Mountings bearing 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. These bearings are marked as follows:

SELECTIVE CLASSIFICATION OF OUTER AND BORE DIAMETER TOLERANCES AND INDICATION MARK



NOTE: 1.THIS IS APPLIED TO BOTH BEARINGS OF ABEC 5P AND P5.
2.UPON YOUR REQUEST, PLEASE SPECIFY THE MARK LISTED BELOW.
ZC1.... 2 SELECTIVE CLASSIFICATIONS FOR BORE DIAMETER TOLERANCE (0-d/2,-d/2-d)
1 SELECTIVE CLASSIFICATIONS FOR OUTER DIAMETER TOLERANCE (0-D)
ZC2... 1 SELECTIVE CASSIFICATION FOR BORE DIAMETER TOLERANCE (0-d)
2 SELECTIVE CASSIFICATION FOR OUTER DIAMETER TOLERANCE (0-D/2,-D/2-D)
ZC3... 4 SELECTIVE CASSIFICATION FOR BOTH BORE DIAMETER TOLERANCE (0-d/2,-d/2-d,0-D/2,-D/2-D)
D... MINIMUM VALUE OF OUTER DIAMETER TOLERANCE
d... MINIMUM VALUE OF BORE DIAMETER TOLERANCE