STANDARD TORQUE LIMIT APPLICATION
This task provides standard torque limits for threaded
fasteners, hose and tube coupling nuts, bulkhead
fittings, and pipe thread fittings, and the minimum
breakaway torque values for determining reusability of
self-locking nuts. These standard torque values apply
only when special torque values are not specified in
procedures. General instructions for installation and fit
of threaded fasteners are as follows:
Thread shall not be in bearing when thickness of
sheet or fitting is 3/32 inch or less. If thickness is
more than 3/32 inch, a maximum of two threads
in bearing is permissible.
In shear applications, thread shall not be in
bearing regardless of material thickness.
Washers are used for the following purposes:
To compensate for differences in bolt
grip length and material thickness due to
manufacturing tolerances, protective coating,
and other surface variances.
To distribute bearing load over a greater area
to prevent damage to material under a bolt
head or nut.
To protect the material surface when a bolt or
nut is tightened.
Prevent galling of aluminum or other soft
material when bolt or nut is tightened.
To insulate dissimilar metals to prevent
corrosion. The washer material should be
similar to the material on which it rests rather
than the bolt or nut material. This insures that
it corrosion occurs, if will be between the bolt
and washer, which can be replaced.
When nut-bolt assemblies are installed, the nut
shall not engage the first incomplete thread next
to the bolt shank.
Nuts are properly installed when all threads are
engaged and the bolt chamfer extends thru the
nut. When flat-end bolts are used, the threaded
end must extend at least 1/32 inch thru the nut.
Threads shall be clean and dry before
installation. If threads are lubricated, torque
limits are reduced by 30 percent.
The tightening sequence in multiple fitting
installation is as follows:
Finger tighten all bolts or nuts.
Snug up opposite bolts or nuts all around.
Tighten opposite bolts or nuts all around to
proper torque. Do not torque adjacent bolts
or nuts in sequence.
All-metal self-locking nuts shall be replaced with
new identical parts at each installation. If new
nuts are not available, all metal self-locking nuts
may be reused. The reused nuts must meet the
required minimum friction torque. See "Friction
Torque in Inch-Pounds for Threaded Fasteners"
in this task.