Page 35 - needle bearings

Bearing Internal Clearance
NTN

5.2.2 Calculation of running clearance
Ўt ʹЋɾ∆TɾDo ʜʜʜʜʜʜʜʜʜʜʜ(5.3)
The running clearance in bearing can be determined from
initial radial clearance, reduced internal clearance where,
incurred by effective interference, and temperature Ўt : Reduced clearance value incurred by
difference between inner ring and outer ring, using temperature difference mm
formula (5.1). Ћ : Linear expansion coefficient of bearing steel
-6
Ўeff ʹЎo ʵʢЎf ʴЎt ʣ ʜʜʜʜʜʜʜ(5.1) 12.5 ʷ 10 /˚C
∆T : Inner ring – outer ring temperature difference ˆ
where, Do : Outer ring raceway diameter mm
Ўeff : Running clearance mm d : Bearing bore diameter mm
Ўo : Initial radial clearance mm D : Bearing outer diameter mm
Ўf : Reduced internal clearance incurred by
effective interference mm When a shaft or a housing is used as a direct raceway,
Ўt : Reduced internal clearance incurred by temperature difference between the shaft and the
temperature difference between inner ring housing is applied as temperature difference (∆T).
and outer ring mm
5.3 Fits and bearing radial clearance
(1) Reduced internal clearance incurred by effective Where the allowable tolerances for the shaft and the
interference housing hole are already decided, the simple nomogram
When a bearing is installed on a shaft or in a housing as shown in Fig. 5.1 is available as a guideline to
with some effective interference, the inner ring expands decision of the initial radial clearance of bearing so as to
and the outer ring shrinks and, as the result, the radial enable to get an optimal clearance after the bearing was
clearance in the bearing reduces correspondingly. installed on the shaft/in the housing. The nomogram in
The expansion or shrinkage of inner ring or outer ring Fig. 5.1 is used as the guideline as stated above. For the
is approximately equivalent to 85% of the effective detail feel free to contact NTN.
interference though depending on bearing type, shaft or For example, where the fit condition for needle roller
housing profile, dimension and material. For the detail bearing with inner ring is already given as J7m6, Fig.5.1
refer to Table 6.4 on page A-35. shows that clearance C3 must be secured to get the
Ўf ʹ 0.85ɾ∆deff ʜʜʜʜʜʜʜʜʜʜʜ(5.2) standard running clearance after installation.
where,
Ўf : Reduced internal clearance incurred by
effective interference mm
∆deff : Effective interference mm Tolerance range class
for housing hole
H6/7 J6 J7 K6 K7 M6 M7 N6 N7
(2) Reduced internal clearance incurred by
temperature difference between inner ring and
outer ring
When a bearing is in running, the temperature of its
outer ring is lower by 5 to 10˚C than that of its inner ring Radial clearance
or rolling elements. Also, temperature difference between Ordinary C3 C4
the inner ring and the outer ring gets larger when heat is
radiated more from the housing or when the bearing shaft
is communicated with a heat source and a heated fluid is
Tolerance range class
flowing through a hollowed shaft. In such a case, the for shaft
internal clearance reduces corresponding to thermal
h5/6 j5 j6 k5 k6 m5 m6 n5 n6
expansion difference between the inner ring and the
outer ring which is incurred by this temperature
difference. Fig. 5.1 Relationship between bearing fits and radial clearance

A-31