Machinery's Handbook, 31st Edition
SCREW THREAD STRESS FORMULAS 1669 If J is less than or equal to 1, the length of engagement determined by Formula (1) is ade quate to prevent stripping of the internal thread; if J is greater than 1, the required length of engagement Q to prevent stripping of the internal thread is obtained by multiplying the length of engagement L e , Formula (1) , by J : (4) In Formula (3) , A s and A n are the shear areas of the external and internal threads, respec tively, given by Formulas (5) and (6): Q JL e =
. 2 1 057735 n +
. 31416
max min
min max K n −
A
nL K
E
(5) (6)
:
D
h
^
=
s
e n
s
. 2 1 057735 +
. 31416
min max n −
A
nL D n e s :
D E s
D
h
^
=
n
In these formulas, n = threads per inch; L e = length of engagement from Formula (1) ; K n max = maximum minor diameter of internal thread; E s min = minimum pitch diameter of the external thread for the class of thread specified; D s min = minimum major diameter of the external thread; and E n max = maximum pitch diameter of internal thread. Load to Break Threaded Portion of Screws and Bolts.— The direct tensile load P to break the threaded portion of a screw or bolt (assuming that no shearing or torsional stresses are acting) can be determined from the following formula: P SA t = where P = load in pounds to break screw; S = ultimate tensile strength of material of screw or bolt in pounds per square inch; and A t = tensile-stress area in square inches from For - mula (2a) , (2b) , or from the screw thread tables. Lock Wire Procedure Detail.— Wire ties are frequently used as a locking device for bolted connections to prevent loosening due to vibration and loading conditions, or tam pering. The use of safety wire ties is illustrated in Fig. 1 and Fig. 2 below. The illustrations assume the use of right-hand threaded fasteners and the following additional rules apply: 1) No more that three bolts may be tied together; 2) bolt heads may be tied as shown only when the female thread receiver is captive; 3) pre-drilled nuts may be tied in a fashion similar to that illustrated with the following conditions: a) Nuts must be heat treated; and b) nuts are factory drilled for use with lock wire; 4) lock wire must fill a minimum of 75 percent of the drilled hole provided for the use of lock wire; and 5) lock wire must be aircraft quality stainless steel of 0.508 mm (0.020 inch) diameter, 0.8128 mm (0.032 inch) diameter, or 1.067 mm (0.042 inch) diameter. Diameter of lock wire is determined by the thread size of the fastener to be safe-tied: a) Thread sizes of 6 mm (0.25 inch) and smaller use 0.508 mm (0.020 inch) wire; b) thread sizes of 6 mm (0.25 inch) to 12 mm (0.5 inch) use 0.8128 mm (0.032 inch) wire; c) thread sizes > 12 mm (0.5 inch) use 1.067 mm (0.042 inch) wire; and d) the larger wire may be used in smaller bolts in cases of conve nience, but smaller wire must not be used in larger fastener sizes.
Fig. 1. Three Bolt Procedure
Fig. 2. Two Bolt Procedure
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