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 MIL-HDBK-1038
5.3.21
Threaded Fasteners. Many standard commercial components such as gear
reducers, brakes, and pillow blocks include mounting holes or slots which may
not be large enough for fasteners required to provide the mechanical design
factors. NCC policy is to accept SAE J429 Grade 5 fasteners or ASTM A325 bolts of
the intended diameter, with appropriate nuts, for mounting such components.
Higher strength SAE J429 Grade 8 fasteners and ASTM A490 bolts, with appropriate
nuts, may also be used but the installation torque for all mounting fasteners and
bolts is required to correspond to 67 percent of the SAE J429 Grade 5 material
yield strength. SAE J429 Grade 8 and ASTM A490 fasteners may only be pre-loaded
to their full installation torques when so prescribed by the component
manufacturer. In cases where a tapped hole is used because installation of a nut
is impractical, the tapped material must have yield and ultimate strengths at
least equal to those of the corresponding nuts. A hardened steel washer must be
installed over each mounting hole or slot.
All custom designed joints and connections are required to have fasteners
and bolts sized for design factors of 4.00 and 5.00 based on material yield and
ultimate strengths, respectively. Tapped threads must be analyzed for the
required length of engagement.
5.3.22
Surface Finishes. Surface finish roughness height ratings in micro-
inches for custom designed and machined components are required to be as follows:
Items or Locations
Rating
Shaft seats for bearing inner races
63
Bearing housing bores for fixed outer race
125
Bearing housing bores for floating outer race
63
Bushing bores, rubbing surfaces of flanges
63
Rubbing surfaces of bushing flanges and thrust washers
63
Pins in bushings
32
Bores of gears, couplings, and wheels
63
Seats for gears, couplings, and wheels
125
Wire rope grooves (on drums and sheaves)
125
Keys, key seats, and keyways
125
Surfaces in contact with synthetic seal lips
16
Surfaces in contact with felt strip seals
63
Gears (working surfaces and root fillets)
63
Brake wheels (working surfaces)
63
All other shaft, axle, pin, bore, and mounting surfaces should be
finished to a rating of 125. Any finer surface finish is satisfactory.
5.3.23
Fits. All components that transmit drive or braking torque pinions,
gears, brakes, and wheels are required to be interference fitted (pressed) on
their shafts or axles, in addition to being keyed. Interference fits must comply
with the tolerance ranges of ANSI B4.1 for medium drive fits (FN2) unless the
material, length of engagement, or imposed loading justify another fit.
Locational interference fits (LN2 or LN3) may be used for installation of large
components, such as wire rope drum gears on drum barrels. For design purposes,
all drive and braking torque is assumed to be transmitted by the keys, regardless
of the degree of press fit.
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