MIL-DTL-917F(SH)
3.2.24.8 Thread locking of mechanical assemblies. Mechanical assemblies where a holding screw, bolt, nut, or
fastened part must maintain a tight connection to work properly and avoid parts falling into the equipment due to
specified operational conditions, including shock, vibration, and heating, shall be secured by detailed fastener
methods, restrictions, and the order of precedence listed in S9086-CJ-STM-010/075 and S9505-AM-GYD-010.
There are four basic techniques used to prevent threaded fasteners from loosening in service: preload, prevailing
torque, mechanical, and chemical. The preferred methods for securing mechanical assemblies are as follows:
a. Reusable self-locking fasteners nut self-locking, bolt self-locking (plastic insert), screw self-locking
(plastic insert), and nut spring beam. The bolt (plastic insert) and screw (plastic insert) self-locking methods may be
used only where removal for maintenance is very infrequent (see 3.2.24.9.2).
b. Pins with castellated nut (safety wire can be used), in holes drilled through the body of the end through
the bolt, and through the bolt at a point beyond the nut. Pin types include cotter, grooved taper, taper, straight solid,
spiral, and roll. Castellated nuts with cotter pinning or safety wiring are not recommended for applications requiring
accurate loads such as bus joints.
c.
Tab locks.
d. Safety cable and locking wire for use to lock bolts when only bolt heads are available to apply a locking
device (see 3.2.24.12).
e.
Lockwasher (see 3.2.24.11.1).
f.
Blind nuts and captive fasteners.
g.
Liquid locking adhesive (see 3.2.24.13).
h. Non-liquid thread locking adhesive may be used on any electrical joint except contact nuts and contact
spacer nuts.
i. Deformation of screw or bolt threads projecting from nut or secured part. This method may be used only in
cases where disassembly is never required for maintenance or repair.
3.2.24.9 Fastening devices. Fastening devices (nuts, bolts, screws, lockwashers, flat washers, and so forth)
shall be made of corrosion-resistant material (see 3.3.2.3) or shall be treated to resist corrosion without paint
(see 3.14.1). Spring type locking devices, such as lockwashers and retaining rings, when made of precipitation
hardened semi-austenitic corrosion-resistant steel, do not require additional protection against corrosion. Aluminum
alloy fasteners are not to be considered corrosion-resistant.
3.2.24.9.1 Nuts, bolts, and screws. Nuts, bolts, and screws shall be in accordance with the following: FF-S-86,
FF-S-92, FF-S-200, FF-S-210 (for guidance only), MIL-DTL-1222, NASM17828, NASM17829, NASM17830,
12-point collar screws with head style as shown on NASM21250, or equal. Where lockwashers are used with
screws and bolts, the lockwasher may be a separate piece or attached as part of an assembled fastener.
3.2.24.9.2 Self-locking screws and bolts. Self-locking screws and bolts shall be in accordance with
MIL-DTL-18240. Screws or bolts threaded into non-metallic inserts shall not be used for electrical connections.
3.2.24.9.3 Flat-head screws. Flat-head screws shall not be used in material of a thickness less than 1½ times
the height of the screw head. Wherever flat-head screws are used, the head shall be properly and completely seated
in the material.
3.2.24.9.4 Blind fasteners. Blind fasteners, when used, shall be in accordance with NASM8975.
3.2.24.9.5 Plastic ring and spring beam nuts. Self-locking plastic ring and spring beam nuts shall be in
accordance with MIL-DTL-32258.
3.2.24.10 Thread-cutting screws. Thread-cutting (self-tapping) screws shall not be used, except for information
and identification plates (see 3.17.4).
3.2.24.11 Washers. Washers shall be commercial types if Government specification sizes are not available.
Refer to FF-W-84 (spring lock) or FF-W-92 (plain flat) for guidance.
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