Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
MOUNTING DESIGNS
Obtaining good spindle accuracy depends not only on selecting the
proper precision bearings but also on the following factors:
• Good design and machining of the components that
support the bearing (roundness and alignment of the seats,
squareness of backing shoulders of both the spindle and the
housing, and surface finish).
• Correct use of information given on bearings.
• Correct fitting practices.
• Appropriate bearing setting.
Selection of the most appropriate mounting design is largely dictated
by optimizing the stiffness, speedability and ease of assembly.
DESIGN AND ACCURACY OF MOUNTING SURFACES
The total runout of a spindle-bearing-housing system is a
combination of the runout of each component. A precision bearing
will assume the shape of the spindle and perpetuate whatever
runout is present. If the runout is caused by a defective housing, the
spindle and bearing will simply transmit the error to the workpiece.
Therefore, particular attention needs to be paid to the design and
accuracy of the mounting surfaces.
The primary function of the inner or outer ring seat and abutment
is to positively establish the location and alignment of the bearing
under all loading and operating conditions. To achieve optimum
bearing performance, it is essential to design housing seats and
abutments that are round and square in alignment with the spindle
axis. Shoulders must be of sufficient section and design to resist
axial deflection under load. The shoulder diameters should be
respected to help obtain optimum bearing performance.
HOUSING DESIGN
Housings are usually made of cast iron or steel and are generally
heat-treated to lessen possible distortion. For smaller high-speed
applications, steel housings are preferred.
The bore of the housing should be ground or bored and checked
at a number of points throughout its length and diameter to ensure
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, as well as accurate
alignment of the bearings.
In many cases of machine design, it is advantageous to employ a
subhousing 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 into 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 suggestions
shown with the part numbers given. The choice between fillets
and undercut reliefs rests with the individual shaft design and
conditions surrounding its normal use. Suggested housing geometry
requirements are discussed on pages 89 and 91.
Where screws are used to fasten end caps into the main housing,
adequate section should be left between the screw hole and the
housing bore. This is required to prevent distortion of the housing
bore when the screws are tightened and the covers or others parts
are pulled tightly into place.
Prior to assembly, shafts and housings, as well as all lubricant holes
and channels, should be cleaned thoroughly to remove all chips and
particles that may be carried by the lubricant into the bearings and
cause bearing damage.
HOUSING SEALS
A labyrinth combination of slinger and end cover provides a highly
effective seal against the intrusion of foreign matter. This seal is
suggested for use over a wide range of speeds. For slower-speed
applications, a combination of slinger and a commercial contacttype
seal is usually employed.
Slingers should be machined all over to assure true-running. Their
diameters should be concentric with the bore. The outside diameter
of the slinger is often tapered to throw off cutting compounds,
coolants, etc., from the point at which such liquids may enter the
spindle. A drip or run-off groove adjacent to the open lip of the end
cover is highly desirable and practical.
The axial clearances of the internal faces between slinger and
end cover should be about 1.600 mm (0.0629 in.). The first radial
clearance opening on any design through which liquid may pass
should be made very close, about 0.089 mm (0.0035 in.) on a side. The
inner radial clearances should be between 0.380 mm (0.0149 in.) and
0.190 mm (0.0075 in.).
92 TIMKEN MACHINE TOOL CATALOG