MIL-HDBK-1038
recommended that the tip of the pawl include an alignment guide which continuously
centers the pawl on the ratchet.
4.3.6.28 Threaded Fasteners. All threaded fasteners of custom designed
components, both internal to the assembly and those interfacing with foundations,
must be designed or selected to meet the mechanical design criteria. Threaded
fasteners of commercial assemblies, both internal and mounting hardware, should
comply with the applicable industry standards. Assemblies such as gear reducers,
brakes, pillow blocks, electric motors, and other items, have mounting provisions
(integral feet or flanges with bolt holes or slots) for specific sizes of
fasteners. Unless specified otherwise by the manufacturer, the mounting hardware
should be SAE J429 Grade 5 fasteners or ASTM A325 structural bolts with matching
nuts and hardened steel washers regardless of the resulting design factor. (The
holes or slots may not be enlarged to accept a bigger fastener to obtain the
design factor required for custom designed assemblies.) The installation torque,
with lubricated threads, should correspond to approximately 67 percent of the
fastener yield strength. The mounting fasteners are normally intended to be
subjected only to tensile loads. It is desirable, but not mandatory, to have no
threads in the shear plane of the joint. Rough cast surfaces around the fastener
holes should be spot faced to ensure solid seating of the fastener head or nut.
The contact surfaces of joints must be in full contact prior to torquing the
fasteners; torquing must not be used to force components into alignment or
contact.
For some commercial assemblies, such as hydraulic high torque/low speed
motors, the manufacturers specify the type of mounting fasteners to be used. In
the particular cases, the manufacturers' directions must be followed, regardless
of other design requirements of this handbook.
4.3.6.29 Shims. Accurate shimming of the foot (base) mounted assemblies is
critical for proper shaft alignment and avoiding high initial stresses in the
mounting feet. It is also important that the shims do not deteriorate due to
corrosion and lose their thickness. NCC policy is to require corrosion resistant
shims in all applications and environments outdoor and indoor. Commercially
available kits provide a wide assortment of neatly pre-cut shims slotted for
standard fastener sizes. Hand made shims should only be used when the commercial
shims do not fit the requirements. Shim stacks should be limited to three shims.
(Commercially available shaft alignment equipment, in addition to
providing highly accurate indication of the shaft positions, calculates the
required amount of shimming under each mounting foot or corner of the housing
base.)
4.3.6.30 Shear Bars. Foot (base) mounted components and housings that are
subjected to shear loads which exceed the holding capacity of the mounting
fasteners, require additional restraints (shear bars) to maintain their alignment.
(The most common examples of such components are pillow blocks supporting a gear
pinion, where the gear tooth alignment is critical.) In these cases, the edges of
the feet in line with the applied load should be machined flat and the shear bars
notched and fitted tightly, with full face contact, to the machine's surface. The
fillet welds between the shear bars and the foundation must be oriented parallel
with the applied load. Shear bars that are fitted against unfurnished foot edges
and fillet welded parallel to the edge serve no useful purpose.
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