Resulting Mounting Fit
EXAMPLE: ABEC 7
Resulting Mounting Fit
EXAMPLE: ABEC 9
.0000 tight .0006 loose
.0000 tight .0004 loose
Tables covering recommended shaft and housing seat dimensions for superprecision (ABEC-7) ball bearings are shown on pages E8 through E15.
The “average” mounting fit is usually considered the ideal mounting condition and should be obtained.
To accomplish this, it is important to follow the tabulated tolerances, except when deviations are recommended by the Engineering Department. It is equally important that all shaft and housing shoulders be square and properly relieved to assure accurate seating and positioning of the bearings in the mounting.
On high-speed applications where nearby heat input is along the shaft, it is extremely important that the floating bearings can move axially to compensate for thermal changes. Ball bearings cannot float longitudinally if they are restricted by tight housing bores or by the radial expansion of the bearing itself due to temperature differentials. Therefore, in such cases, the housing mounting fit for the floating bearings is slightly looser than the tabulated average fit. Likewise, in spring-loaded ball bearing applications the housing mounting fit must be free enough to permit axial movement of the bearings under the spring pressure, during all conditions of operation. The recommended housing dimensions to ensure proper “float” of the bearings under average conditions are tabulated on pages E13 and E15.
Shafts are preferably made from steel hardened and ground all over; and where not otherwise unsuitable, a hardness of 45-50 Rockwell C has been successful. When designing a spindle or shaft it is highly desirable to plan so that it can be ground all over in one setting as a final operation. This promotes true balance and running accuracy, which is critical in high-speed work.
Spacers are used to increase shaft rigidity, moment stiffness and decrease deflection. Spacers, mounted between units of a pair of bearings, are preferably made of alloy steel, hard- ened and ground and should be sturdy in cross-section and equal in length. Equal lengths can be produced by grinding the inner-ring spacer and outer-ring spacer together. It is important that the faces of the spacers be square and that their parallelism be the best possible. All corners should be rounded to remove sharp edges and burrs.
The inside diameter of the inner-ring spacers should clear the shaft but not be so loose as to make it possible to mount and run them eccentrically. For short spacers and high operating speeds add clearance of not more than .0010 inch (.025mm) over the maximum shaft diameter has been found
generally acceptable. For long spacers and low speeds, this clearance may be increased to prevent the shaft from disturbing the face parallelism of the spacer. The spacer outside diameter should not be less than the outside diameter of the inner ring of the bearing.
The outside diameter of the outer-ring spacers should be about .0010 inch (.025mm) smaller than the minimum bore of the housing. These should have lubricant holes and grooves where necessary and are usually centrally located. Spacer end parallelism should be the same as the parallelism tolerance for the adjacent bearing.
Under certain conditions it may be desirable to control fits more accurately without the added expense of using closer tolerance bearings and assembly parts. This can be accom- plished by selective assembly of the bearings, shafts and housings after they have been sized and sorted according to bores and outside diameters. At the customer’s request, we can provide bearings with bore’s and O.D.’s coded in inch or metric dimensions from nominal size. A nominal charge is required for this additional inspection. This improved fit-up at assembly provides a higher degree of precision in the spindle. Generally, however, it is quite satisfactory for production and field servicing to use closer shaft and housing tolerances with bearings having a higher degree of precision.
Housings are usually made of cast iron or steel and generally heat treated to lessen possible distortion. For the smaller high- speed applications, steel housings are preferable.
The bore of the housing should be ground or bored and checked at a number of points throughout its length and diameter to assure that it is round and does not taper.
It is preferable to mount the bearings in one casting; this permits machining the two housing bores in one setting and assures accurate alignment of the bearings.
In many cases of housing design, it is advantageous to employ a sub-housing or a steel sleeve between the outer ring of the bearing and the machine frame, thus allowing assembly of the bearings on the shaft and insertion of the entire unit in to the machine frame. This method also provides a surface of proper hardness where machine frames are made of a material that has a low Brinell value, such as aluminum and other soft metals.
Shaft shoulders and housing shoulders should be square and true, and should be of such diameters as to meet the recommendations given on page E17 through E21. The choice between fillets and undercut reliefs rests with the individual shaft design and conditions surrounding its normal use.
Where screws are used to fasten end caps into the main housing, adequate section should be left between the screw hole and the hosing bore. This is required to prevent distortion of the housing bore when the screws are tightened and the covers or others parts pulled tightly into place.
Prior to assembly, shafts and housings, as well as all lubricant holes and channels, should be cleaned thoroughly, in order to remove all chips and particles which may be carried by the lubricant into the bearings to cause bearing damage.