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Bearing Interference Fit

When a ball bearing is used, it is not used alone. It is likely to be used on a shaft or in a housing bore by an Interference Fit. The Interference Fit is described as a value of tightness between the shaft and bearing bore or the housing bore and bearing outside diameter. The three main types of interference fits are distinguished as Loose Fit, Light Interference Fit and Interference Fit.

As subject of the Fit, the purpose is to prevent creeping. Once creeping occurs, the temperature will be abnormally generated by the slipping abrasion. Creeping is associated with the slipping between fitted surfaces. It will be a cause of premature life failure or that the abrasion particles will enter the bearing and cause it to become noisy and eventually burn up.

Typically a large interference fit prevents creeping, but it is necessary to select the interference fit by giving attention to the reduction of radial clearance (negative clearance), the temperature distribution during the bearing operation (the change of the interference fit by temperature) and the influence to the raceways.

More details about what interference fits are, how creeping works, and how interference fits and creeping are related are provided below.

The following formula sets up a standard to calculate how much radial clearance is reduced when a bearing is pressed onto a shaft or into a housing.



Interference fit of the inner ring to the shaft

The sketches drawn in solid lines and dotted lines are the bearing prior to a fit, and the bearing after a fit, respectively. When press fitted with an interference " i ", the inner ring groove diameter d2 increases by an amount δ. This δ is also equal to the decrease in radial clearance.

Press fits of the inner ring to the shaft :
Bearing Interference Fit - Shaft
Figure10-1
d:nominal bore diameter,
nominal outside diameter of the shaft
d1: Shaft bore
(In the case of solid shafts   (d/d1)2+ 1 = 1 )
(d/d1)2- 1
d2:Inner ring groove diameter
 i:Interference (i/2 in radial direction)
Eb:Young's modulus of inner ring (Outer Ring)
Es:Young's modulus of shaft
mb:Poisson's ratio of inner ring (outer ring)
ms:Poisson's ratio of shaft
δ= 2i (d2/d)
{ (d2/d)2 - 1 } (d2/d)2 + 1 + 1 + Eb (d/d1)2 + 1 - 1
(d2/d)2 - 1 mb Es (d/d1)2 - 1 ms
 

Interference fit of the outer ring to the housing

The sketches drawn in solid lines and dotted lines are the bearing prior to a fit, and the bearing after a fit, respectively. When it is press fitted with an interference " I ", the outer ring groove diameter D1 decreases by an amount Δ. This amount Δ is also equal to the decrease in radial clearance.

Press fits of the outer ring to the housing :
HousingBearing Interference Fit - Housing
Figure10-2
D:nominal outside diameter of outer ring, nominal housing bore
D1:Outer ring groove diameter
D2:Housing outer diameter
I:Interference (I/2 in radial direction)
Eh:Young's modulus of housing
mh:Poisson's ratio of housing
Δ= 2I (D/D1)
{ (D/D1)2 - 1 } (D/D1)2 + 1 - 1 + Eb (D2/D)2 + 1 + 1
(D/D1)2 - 1 mb Eh (D2/D)2 - 1 mh
 

Securing with glue

When the bearing is fitted to the shaft and housing by glue without interference, it is necessary to select the proper clearance to enhance the effectiveness of the glue. It is recommended to consult with the glue manufacturer because the proper clearance depends on the type of glue. Please be aware that the roundness of the ring raceways could worsen because of the curing stress of the glue.

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Referred from JIS B 0401-1

Deviation of holes for common fits

(unit : μm)
Dimensional division
(mm)		 G H JS K M N P
Over Incl. G7 H5H6H7 JS5JS6JS7 K5K6K7 M5M6M7 N6N7 P7
- 3 +12
+2		 +4
0		 +6
0		 +10
0		 ±2 ±3 ±5 0
-4		 0
-6		 0
-10		 -2
-6		 -2
-8		 -2
-12		 -4
-10		 -4
-14		 -6
-16
3 6 +16
+4		 +5
0		 +8
0		 +12
0		 ±2.5 ±4 ±6 0
-5		 +2
-6		 +3
-9		 -3
-8		 -1
-9		 0
-12		 -5
-13		 -4
-16		 -8
-20
6 10 +20
+5		 +6
0		 +9
0		 +15
0		 ±3 ±4.5 ±7.5 +1
-5		 +2
-7		 +5
-10		 -4
-10		 -3
-12		 0
-15		 -7
-16		 -4
-19		 -9
-24
10 18 +24
+6		 +8
0		 +11
0		 +18
0		 ±4 ±5.5 ±9 +2
-6		 +2
-9		 +6
-12		 -4
-12		 -4
-15		 0
-18		 -9
-20		 -5
-23		 -11
-29
18 30 +28
+7		 +9
0		 +13
0		 +21
0		 ±4.5 ±6.5 ±10.5 +1
-8		 +2
-11		 +6
-15		 -5
-14		 -4
-17		 0
-21		 -11
-24		 -7
-28		 -14
-35

Deviation of shafts for common fits

(unit : μm)
Dimension
division
(mm)		 f g h js k m n p r
Over Incl. f6 g5g6 h4h5h6 js4js5js6 k4k5k6 m5m6 n6 p6 r6
- 3 -6
-12		 -2
-6		 -2
-8		 0
-3		 0
-4		 0
-6		 ±1.5 ±2 ±3 +3
0		 +4
0		 +6
0		 +6
+2		 +8
+2		 +10
+4		 +12
+6		 +16
+10
3 6 -10
-18		 -4
-9		 -4
-12		 0
-4		 0
-5		 0
-8		 ±2 ±2.5 ±4 +5
+1		 +6
+1		 +9
+1		 +9
+4		 +12
+4		 +16
+8		 +20
+12		 +23
+15
6 10 -13
-22		 -5
-11		 -5
-14		 0
-4		 0
-6		 0
-9		 ±2 ±3 ±4.5 +5
+1		 +7
+1		 +10
+1		 +12
+6		 +15
+6		 +19
+10		 +24
+15		 +28
+19

Referred from JIS B 1566

Fits of inner ring in radial bearings*1

Bearing grade Rotating inner ring load or indeterminate direction load Stationary inner ring load
Tolerance zone class of shaft*2
0, 6 grades r6 p6 n6 m6
m5		 k6
k5		 js6
js5		 h5 h6
h5		 g6
g5		 f6
5 grades - - - m5 k4 js4 h4 h5 - -
fits interference fit intermediate fit clearance fit

Fits of outer ring in radial bearings*3

Bearing grade Stationary outer ring load Indeterminate direction load or rotating outer ring load
Tolerance zone class of hole*2
0, 6 grades G7 H7
H6		 JS7
JS6		 - JS7
JS6		 K7
K6		 M7
M6		 N7
N6		 P7
5 grades - H5 JS5 K5 - K5 M5 - -
fits interference fit intermediate fit clearance fit
*1 tolerance of bearing bore is based on JIS B 1514-1.
*2 symbol of tolerance zone class is based on JIS B0401.
*3 Tolerance of outer diameter of bearings is based on JIS B1514-1.

More Information About Interference Fits and Creeping

What Is Interference Fit?

If you're not completely familiar with the idea of fit as it relates to a bearing, here is a brief primer to clarify matters. Essentially, the fit in a ball bearing assembly refers to how much "play" or "give" there is between the parts of the bearing. Specifically, this is the fit between the shaft and bearing bore or bearing and housing bore. This space between the two parts is known as clearance, and a fitting where there is clearance between the parts is known as a clearance fit. In the case of an interference fit, sometimes known as a press fit, the two parts are pressed together so that there is a minimum of give between the two parts of the bearing assembly; in fact, the two parts are overlapping and "interfering" with one another. This occurs typically when one part is slightly larger than the part it is supposed to fit into. Once the two parts are joined, they deform to accommodate the lack of space and are effectively fused together as one.

How Is the Fit in an Interference Fit Created?

The fit can be created in one of two ways. The first is force, where a hydraulic press or similar pressure-applying device simply forces the oversized part into the other, compelling the parts to conform under the pressure. A less violent method is through thermal expansion. The part to be inserted is cooled, causing it to contract, then it is inserted into the other part. When the metal heats up and expands, the proper fit is created. The level of interference between the two parts determines whether or not the bearing has a loose fit, a light interference fit, or an interference fit. The interference fit is the tightest fit. NMB engineers can calculate the allowance which will produce these different kinds of fits for different sizes and construction types of ball bearing assemblies. The allowance is a planned deviation from the nominal, expected size of the part and the actual size of the part.

What Is Creeping?

Creeping is the tendency of metals to move or permanently deform as a response to the constant stresses placed upon them. Ball bearing assemblies are particularly vulnerable to creep as they are often consistently subject to pressures and high temperatures for extended periods of time. As you may imagine, creep can destroy the integrity of the ball bearing assembly, which can result in a failure of the application that is making use of the ball bearings. Interference fit can defend against creeping, and this is a large part of what makes this kind of fit desirable.

Definitions

Rotating inner ring load :
the line of action of the load is rotating in relation to the inner ring of the bearing
Stationary inner ring load :
the line of action of the load does not rotate in relation to the inner ring of the bearing
Stationary outer ring load :
the line of action of the load does not rotate in relation to the outer ring of the bearing
Rotating outer ring load :
the line of action of the load is rotating in relation to the outer ring of the bearing
Indeterminate direction load :
the direction of the load cannot be determined.
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