MIL-HDBK-1038
Bearing outer races in non-rotating components/housings may be press
fitted or installed with an intentional loose fit. Such installations have one
fixed bearing which is selected and sized to accept the axial load on one end of a
shaft or axle, and a loosely fitted floating bearing, with axial float, on the
other end.
Tapered roller bearing installations require that the inner and outer
races be advanced into each other to obtain the desired preload with their tapered
rollers. One of the races must therefore have a loose fit and be positioned
axially and locked by means of a nut on the shaft or axle, or by shimming the
outer race at the face of the bearing cap flange.
5.3.7
Mounted Bearings. In selecting mounted bearings (pillow block and flange
cartridge type) the adequacy of the housing must also be considered. In the case
of pillow blocks, the direction of radial load with respect to the base may govern
the capacity of the assembly. The full capacity of the bearing itself is often
obtainable only when the radial load is directed towards the base of the pillow
block. Flange cartridge bearings should be fitted into the support structure so
that the radial load is transferred only through the wall of the cylindrical
section without imposing any bending on the flange. Manufacturers' published
ratings for the mounted bearings, with consideration of the load direction and
mounting details, must be followed.
5.3.8
Bushings. For bushings subjected to rocker (dithering) motion, the
maximum allowable bearing stress in the bore is limited to 20 percent of the
material yield strength in compression, based on the projected area. For bushings
subjected to full rotation and sliding, as those in fleeting sheaves, the maximum
bearing stress in the bore is further restricted to 1000 psi or the bushing
manufacturer's maximum recommended product of bearing stress and rubbing velocity,
whichever is less. (Bearing stress on bushings is always based on the projected
area; that is, the product of the length and bore of the bushing. The area of the
grease grooves need not be subtracted.) Bushing wall thickness must be at least
1/16 of the bushing bore.
Bushing flanges and thrust washers which are intended for specific and
significant loads, are designed to the same criteria as the bushings. Flange
thickness and face width should be at least equal to, and three times the bushing
wall thickness, respectively.
5.3.9
Keys, Key Seats, and Keyways. Keys are designed with minimum design
factors of 4.00 and 5.00 in shear based on the yield and ultimate shear strengths
of the material, respectively. (The shear yield and ultimate strengths may be
taken as 60 percent and 70 percent of the yield and ultimate tensile strengths of
the common key materials.) Key seats (milled in the shaft) and keyways (broached
in hubs) are designed with the same design factors for bearing stress but based on
the material compressive strengths and the net surface areas in contact.
customary to take the material tensile strengths as the compressive strength
values.)
5.3.10
Sheaves. Running sheave pitch diameters for 6x37 class of wire rope
construction must be at least 30 times the wire rope diameter. For non-rotating
and spin-resistant wire ropes of various constructions, running sheave pitch
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