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This information reprinted courtesy of the Rolling Bearing Institute
and SKF, Inc. It is representative of the type of information
you'll find on the Rolling
Bearings Troubleshooter's Guide on CD-ROM as
featured in the Product
Showcase. |
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In order for a ball or roller bearing to perform satisfactorily, the fit
between the inner ring and the shaft, and the fit between the outer ring
and the housing must be suitable for the application. For example, too
loose a fit could result in a corroded or scored bearing bore and shaft,
while too tight a fit could result in unnecessarily large mounting and
dismounting forces and too great a reduction in internal bearing clearance. |
All rolling bearing manufacturers make bearings to standardized tolerances
set forth by the Anti Friction Bearing Manufacturers Association (AFBMA)
and the International Standards Organization (ISO). The proper fits can
only be obtained by selecting the proper tolerances for the shaft. Each
tolerance is designated by a letter and a numeral. The small letter is
for shaft fits, and the capital letter is for housing bores, and they locate
the tolerance zone in relation to the nominal dimensions and the numeral
gives the magnitude of the tolerance zone. In Figure 1, X illustrates
the bearing bore tolerance, and Y the bearing outside diameter tolerance.
The sectional rectangles indicate the location and magnitude of the various
shaft and housing tolerance zones which are used for ball and roller bearing
and they should be followed. |
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Bearing manufacturers' catalogs show the specific size for each tolerance
zone. The selection of fit is dependent on the character of the load, the
bearing dimensions, the bearing operating temperature, the heat expansion
of the shaft and other parts, the design and the required running accuracy.
The choice of tolerances for bearing housings is influenced by the material
and housing wall thickness. Also, consideration must be given to the fact
that the shaft deforms differently when it is solid than when it is hollow. |
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In determining suitable fits for the inner ring and the outer ring in any
given application, the direction of the load in relation to the respective
bearing ring must be known. Various load conditions are discussed below: |
1. "Rotating Inner Ring
Load"
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The shaft rotates in relation
to the direction of the load. All points on the inner ring raceway come
under load during one revolution.
Example: Bearings
in a motor that belt drives a fan
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2. "Stationary Inner
Ring Load"
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The shaft remains stationary
in relation to the direction of the load so that the load is always directed
towards the same portion of the inner ring raceway.
Example: Car front
wheel.
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3. "Rotating Outer Ring
Load"
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The bearing housing rotates
in relation to the direction of the load. All points on the outer ring
raceway are exposed to the load every revolution.
Example: Car front
wheel hub.
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4. "Stationary Outer
Ring Load"
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The bearing housing remains stationary in relation to the direction of
the load so that the load is always directed towards the same portion of
the outer ring raceway. Example: Bearings in a motor that belt drives
a fan.
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To facilitate bearing assembly, most bearings are fitted loosely to either
the shaft or housing depending on what part rotates. The part that rotates
must have a press fit in order to eliminate wear from differential rolling
or "creep.' Creep occurs when the loose-fitted ring is rotating with respect
to the load direction. Figure 2 shows a bearing loosely fitted to
the shaft with the inner ring rotating and the load direction fixed. The
inner ring is held between the shaft and the rolling elements. The velocity
of a point on the shaft outside diameter is equal to the velocity of the
inner ring bore, if the motion is pure rolling. Since the shaft circumference
is less than the inner ring bore circumference, the inner ring revolves
slightly less than one revolution for each full revolution of the shaft.
The relative linear movement per shaft revolution is pi (p) times the amount
of the loose fit. For example, a bearing fitted .002" (.0508 mm) loose
on a shaft rolls a distance of (.002") (p) inches (.0508 mm) per revolution.
With an 1800 RPM shaft speed, the inner ring can creep around the shaft
a distance of 1.07 miles (1.7220 km) in 100 hours of operation. The longer
the inner ring creeps, the more wear occurs and, consequently, the creeping
increases. An equivalent condition occurs with a loose-fitted outer ring
if there is relative motion between the load direction and the outer ring.
Outer ring creep in a housing is often encountered in applications where
there are unbalanced loads. In this instance, press fits are required. |
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If a shaft has been correctly designed, the aspects of shaft quality that
affect bearing performance are geometric and dimensional accuracy, surface
finish, deflections, material and hardness. |
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The tolerances for geometric accuracy are shown below: |
1. Out of round and taper
tolerance-one-half the recommended shaft O.D. tolerance.
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2. Runout of shaft shoulder-recommended
shaft O.D. tolerance.
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3. Concentricity of one
bearing seat to the other - recommended shaft O.D. tolerance.
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4. Conformity of bearing
seat to a straight edge - 80%.
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Out-of-roundness of the shaft can affect the dynamic accuracy of bearing
rotation and affect the vibration of a machine. |
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Dimensional accuracy requirements are necessary for both the shaft diameter
and the axial locations of the shoulders. Out of tolerance shoulder locations
can result in excessive axial loading of the bearings. Shoulders
out of squareness can result in misalignment. Oversize bearing seats can
cause overheating or preloading of the bearing. Undersize shafting can
cause creeping as mentioned previously. |
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A rough surface finish on the shaft will cause a loss of press fit and
excessive wear of the bearing seat. A maximum surface finish of 63 microinch
AA is required for bearing seats. When a seal contacts the shaft surface,
a finish of 16 microinch AA for both the bearing seat and the seal surface
is required. A plunge ground and not a centerless ground shaft (that develops
a helical pattern) should be used for the seal surface, otherwise there
may be seal leakage. |
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For installations on needle roller bearings and cylindrical roller bearings,
the shaft surface is sometimes substituted for the inner ring. The shaft
hardness must be Rockwell C59 minimum and a maximum roughness of 15 microinch
AA. |
Bearings with tapered bores generally require a tighter fit on the shaft
than bearings with a cylindrical bore. The inner ring of the bearing is
secured by forcing it up a tapered shaft or a tapered adapter, or by driving
a tapered sleeve between it and the shaft. An inner ring installed in this
manner expands and the bearing internal clearance is reduced which is a
measure of the amount of interference fit. Shaft sizes for adapter mounted
bearing and tapered sleeves are shown in Table 1. |
| Important: |
| The bearing seat on the
shaft should be checked for diameter, roundness, taper, conformity to a
straight edge and squareness of the shoulders according to a print or specifications. |
To check for shaft conformity to a straight edge, obtain a piece of 1/8"
(3.175 mm) gauge stock that has a length 1/2" (12.75 mm) longer than the
bearing Beat. Apply a thin coating of Prussian blue to the 1/8" (3.175
mm) surface. Then move the gauge stock axially back and forth 1/4" (2.35
mm) on the shaft and look for a transfer of the blueing to the shaft (Figure
3). There should be at least 80% transfer of blueing. If there is less
than 80%, the shaft should be reworked. Any nicks should be removed with
a file or fine emery and then the shaft should be wiped with a lint-free
cloth. Seal contact surfaces on the shaft should be free of any wear. |
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If a shaft has to be repaired it should be done by a competent person using
metal spray buildup after a properly prepared surface. The shaft should
then be finished to the specified size. |
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It should be remembered, there is no substitution for having the correct
shaft and housing fit if you expect to obtain the maximum life out of a
bearing. |
| In summary, the following
questions should be asked before determining the shaft size or housing
size: |
1. What part rotates ---
inner ring or outer ring?
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2. The part that rotates
gets the press fit.
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3. What bearing will be
the fixed bearing and what one will be the floating bearing? A floating
bearing is necessary to prevent parasitic thrust loads from thermal expansion
of the shaft. The fixed bearing locates the assembly.
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The fillet radius on the shaft (Figure 4) and in the housing (Figure
5) should be made according to bearing manufacturers' specification. |
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Figure 4
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Figure 5
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A typical shaft installation is shown in Figure 6. The fixed bearing
is shown at "B" and this locates the shaft already. The free or floating
bearing is at "A" and allows the shaft to either expand or contract depending
on the temperature. |
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