Rolling Bearing Shaft & Housing Fits
     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.
     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.
     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.
     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"
    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
    2. "Stationary Inner Ring Load"
    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.
    3. "Rotating Outer Ring Load"
    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.
    4. "Stationary Outer Ring Load"
         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.

     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.
 
 
     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.
     The tolerances for geometric accuracy are shown below:
    1. Out of round and taper tolerance-one-half the recommended shaft O.D. tolerance.
    2. Runout of shaft shoulder-recommended shaft O.D. tolerance.
    3. Concentricity of one bearing seat to the other - recommended shaft O.D. tolerance.
    4. Conformity of bearing seat to a straight edge - 80%.
     Out-of-roundness of the shaft can affect the dynamic accuracy of bearing rotation and affect the vibration of a machine.
     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.
     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.
     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.
     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.
     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?
    2. The part that rotates gets the press fit.
    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.
     The fillet radius on the shaft (Figure 4) and in the housing (Figure 5) should be made according to bearing manufacturers' specification.
Figure 4
Figure 5
 
     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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