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9. FITS AND INTERNAL CLEARANCES
9.1 Fits
9.1.1 Importance of Proper Fits
In the case of a rolling bearing with the inner ring fitted
to the shaft with only slight interference, a harmful
circumferential slipping may occur between the inner
ring and shaft. This slipping of the inner ring, which is
called “creep”, results in a circumferential displacement
of the ring relative to the shaft if the interference
fit is not sufficiently tight. When creep occurs, the
fitted surfaces become abraded, causing wear and
considerable damage to the shaft. Abnormal heating
and vibration may also occur due to abrasive metallic
particles entering the interior of the bearing.
It is important to prevent creep by having sufficient
interference to firmly secure that ring which rotates
to either the shaft or housing. Creep cannot always
be eliminated using only axial tightening through
the bearing ring faces. Generally, it is not necessary,
however, to provide interference for rings subjected
only to stationary loads. Fits are sometimes made
without any interference for either the inner or outer
ring, to accommodate certain operating conditions, or
to facilitate mounting and dismounting. In this case,
to prevent damage to the fitting surfaces due to creep,
lubrication of other applicable methods should be
considered.
Load Application
Load
Stationary
Load Rotating
Load Stationary
Load
Rotating
Direction of load indeterminate due to
variation of direction or unbalanced load
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9.1.2 Selection of Fit Therefore, the effective interference &d should be roughness of the surfaces and may be estimated using
larger than the interference given by Equation (9.1). the following equations:
(1) Load Conditions and Fit However, in the case of heavy loads where the radial
The proper fit may be selected from Table 9.1 based on load exceeds 20% of the basic static load rating C 0r, For ground shafts & d= d & da . . . . . . . . . . . . (9.4)
the load and operating conditions. under the operating condition, interference often d+2
becomes shortage. Therefore, interference should be
(2) Magnitude of Load and Interference estimated using Equation (9.2): For machined shafts & d= d & da . . . . . . . . . . . . (9.5)
The interference of the inner ring is slightly reduced d+3
by the bearing load; therefore, the loss of interference
should be estimated using the following equations: & d≥0.02 Fr 10–3 . . . . . . . . . . (N)¹
B º . . . . . . . (9.2)
d 10 –3 . . . . . . . . (N)¹ where & d : Effective interference (mm)
& dF = 0.08 B Fr Fr 10–3 . . . . . . . . . {kgf}» & d a : Apparent interference (mm)
& dF = 0.25 B d : Bearing nominal bore diameter (mm)
º . . . (9.1) & d≥0.2
According to Equations (9.4) and (9.5), the effective
d Fr 10 –3 . . . . . {kgf}» interference of bearings with a bore diameter of 30 to
B 150 mm is about 95% of the apparent interference.
where & d : Effective interference (mm)
Fr : Radial load applied on bearing (5) Fitting Stress and Ring Expansion and
where & d F : Interference decrease of inner ring (N), {kgf} Contraction
(mm) B : Nominal inner ring width (mm)
When bearings are mounted with interference on
d : Bearing bore diameter (mm) (3) Interference Variation Caused by Temperature a shaft or in a housing, the rings either expand or
B : Nominal inner ring width (mm) Difference between Bearing and Shaft or contract and stress is produced. Excessive interference
Fr : Radial load applied on bearing Housing may damage the bearings; therefore, as a general
guide, the maximum interference should be kept under
(N), {kgf} The effective interference decreases due to the approximately 7/10 000 of the shaft diameter.
increasing bearing temperature during operation. If The pressure between fitted surfaces, expansion or
Table 9.1 Loading Conditions and Fits the temperature difference between the bearing and contraction of the rings, and circumferential stress
housing is & T (°C), then the temperature difference may be calculated using the equations in Section 15.2,
Bearing Operation Fitting between the fitted surfaces of the shaft and inner ring Fitting(1) (Pages A130 and A131).
is estimated to be about (0.1~0.15) & T in case that
the shaft is cooled. The decrease in the interference of 9.1.3 Recommended Fits
the inner ring due to this temperature difference & d T
may be calculated using Equation (9.3): As described previously, many factors, such as
the characteristics and magnitude of bearing load,
Inner Ring Outer Ring Load Inner Ring Outer Ring & d T = (0.10 to 0.15) × & T⋅α⋅d temperature differences, means of bearing mounting
Conditions H0.0015& T ⋅ d × 10–3 . . . . . . . . . . . . . . . . . . . (9.3) and dismounting, must be considered when selecting
the proper fit.
Rotating Stationary Rotating where & d T : Decrease in interference of inner ring If the housing is thin or the bearing is mounted on a
Stationary Rotating Inner Ring due to temperature difference (mm) hollow shaft, a tighter than usual fit is necessary. A
Tight Fit Loose Fit split housing often deforms the bearing into an oval
Load & T : Temperature difference between bearing shape; therefore, a split housing should be avoided
interior and surrounding parts (°C) when a tight fit with the outer ring is required.
Stationary The fits of both the inner and outer rings should be
Outer Ring α : Coefficient of linear expansion of tight in applications where the shaft is subjected to
bearing steel=12.5×10–6 (1/°C) considerable vibration.
Load The recommended fits for some common applications
d : Bearing nominal bore diameter (mm) are shown in Table 9.2 to 9.7. In the case of unusual
operating conditions, it is advisable to consult NSK.
Stationary Rotating Rotating In addition, depending on the temperature difference For the accuracy and surface finish of shafts and
Rotating Stationary Outer Ring between the outer ring and housing, or difference in housings, please refer to Section 11.1 (Page A100).
their coefficients of linear expansion, the interference
Load may increase.
Stationary Loose Fit Tight Fit (4) Effective Interference and Finish of Shaft and
Inner Ring Tight Fit Housing
Load Since the roughness of fitted surfaces is reduced
during fitting, the effective interference becomes
less than the apparent interference. The amount of
this interference decrease varies depending on the
Rotating or Rotating or Direction of Load Tight Fit
Stationary Stationary Indeterminate
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