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|  MIL-F- 14252 ( AR )
3.6.19 Hardware and fastenings. - All screw type fasteners shall
be tight enough to cover t he purpose Intended by the design.. Al 1
screw-thread assemblies shall be made vibration-proof. Lock
washers shall be provided under all nuts, except those of' the
self-locking and castellated types. Bolts or machine screws,
when not intended for use with nuts and when not suitably locked
by other means, shall be of the self-locking type, shall have
drilled heads for locking wires, or shall be secured with lock
(see 3.13.2).
washers.
3.6.19.1 Material and treatment. - Fastening devices (screws,
bolts, nuts, washers, etc.) shall be of alloy steel or corrosion-
resistant steel, unless electrical considerations require use
of other materials (see 3.11.14.11), and shall be heat treated
as required to produce the necessary physical properties.
3.6.19.2 Thread form and dimensions. - Threaded parts shall con-
2.4. Unless otherwise speci-
form to the requirements or
, peened, staked, or otherwise
fied, threads shall not be swa
permanently fixed.
.
extension.- Bolts and screws shall extend not
3.6.19.3 Thread
less than one full thread beyond the nut and shall extend not
more than two full threads where excessive projectiion of thread
may be hazardous to personnel or may be a jeopardy to the opera-
tion of the equipment.
3.6.19.4 Thread engagement. - Thread engagement of screws and
tapped parts oth er than nuts shall be a minimum effect-
bolts
In
ive thread length equal to 1-1/2 times the diameter of the bolt
or screw.
restrictions.- The use of thread sizes No. 3 and
3.6. 19.5 Screw
12 screw fasteners in.mechanical application is prohibited.
Screw and bolt sizes 5/16, 7/16 and 9/16 Inch diameters are non-
preferred sizes but may be used where necessary. All sizes of
commercial standard screw threads are acceptable on commercial
items (switches, receptacles, etc.). Standard length screws
and bolts, In accordance with Ordnance Corps standards (see 3.7),
shall be used wherever practicable.
3.6.19.6 Set
screws.- Set screws shall be used only where uni-
quely required by design of the equipment. Cone-point set screws
may be used where the engaging surfaces are suitably countersunk
to receive the point; otherwise cup-point set screws shall be
used. Flat surfaces shall be provided for engagement of cup-
point screws when the per: is not adjustable In angular relation-
ship to the shaft on which It is secured. Where two set screws
are used, the angle between their major axes shall not be less
than 90 degrees nor more than 120 degrees.
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