Page 36 - needle bearings

Bearing Fits
NTN

7. Bearing Fits

6.1 Interference ¡Crack and earlier separation of bearing ring, and
For rolling bearings, the inner ring and outer ring are displacement of bearing ring
fixed on the shaft or in the housing so that relative ¡Wear of bearing ring, shaft and housing caused by
movement does not occur between the fitted surfaces of creep and fretting corrosion
the bearing ring and the shaft or housing in radial, axial ¡Seizure (sticking) caused by less internal clearance
and rotational directions when a load acts on the bearing. ¡Insufficient running accuracy and abnormal noise
Such a relative movement, if occurred, would result in caused by deformed raceway surface
wear, fretting corrosion, friction crack, etc. on the fitted
surfaces, which would then cause damage of the bearing
6.3 Fit selection
and the shaft or the housing. Furthermore, wear powder
invades into the bearing, then causing imperfect rotation, Fit selection is generally done in accordance with the
abnormal overheat, vibration, etc. rule specified hereunder.
The most effective way to fix a bearing is to assign an The loads acting on each bearing ring are divided into
proper interference to the fitted surfaces between the running load, stationary load and directionally unstable
bearing ring and the shaft or the housing and to thereby load according to the direction and characteristic of loads
apply "tight fit" to the bearing. Furthermore, as its acting on the bearing.
advantage this tight fit method supports the thin-walled "Tight fit" can be selected for a bearing ring subjected
bearing ring with uniform load throughout its entire to running load and directionally unstable load and
circumference without any loss of load carrying capacity. "stationary fit" or "loose fit" be selected for a bearing ring
The needle roller bearing is a bearing type enabling to subjected to stationary load. (See Table 6.1)
separate the inner ring and the outer ring from one Where load of high magnitude or vibration and shock
another and, therefore, it can be installed on a shaft or in loads act on a bearing or a light alloy/plastic housing is
a housing with an interference applied to both of its inner used, it is necessary to secure a large interference. In
ring and outer ring. In the case of "tight fit", the easiness that case, however, the housing rigidity must be
of bearing installation and removal is lost and, therefore, considered carefully so as not allow occurrence of
the bearing ring subjected to stationary load can be split damage, etc.
"loose-fitted". For an application subjected to high running accuracy,
bearings of high accuracy must be used with a shaft and
a housing of higher dimensional accuracy so as not to
6.2 Necessity of proper fit apply a large interference thereto. Applying a large
Improper fit could lead to damage and shorter life of interference would cause the shaft or housing profile to
bearing. Therefore, advance careful analysis is needed be transferred to the bearing track, which could then
for selection of proper fit. Representative examples of interfere with the bearing running accuracy. The said
bearing defects caused by improper fit are as described bearings of high accuracy are used to prevent such
below. possible phenomena.
Table 6.1 Radial load and bearing fit
Bearing fit
Load
Bearing running conditions Sketch
characteristic Inner ring Outer ring
Inner ring : Rotation Rotating inner
Outer ring: static ring load
Load direction: constant
Loose fit
Tight fit
Inner ring: static acceptable, too
Outer ring: rotation Static outer
Load direction: rotating ring load
with outer ring

Inner ring: static
Rotating inner
Outer ring: rotation
ring load
Load direction: constant
Loose fit
Tight fit
acceptable, too
Inner ring: rotation
Outer ring: static Static outer
Load direction : rotating ring load
with inner ring
Inner ring: rotation or static Load direction is
Outer ring: rotation or static non-constant due to Directionally Tight fit Tight fit
Load direction: The direction directional fluctuation, unstable load
can not be fixed. unbalanced load, etc.

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