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TB 55-1900-232-10
Screw-pin shackles, other than the special forged shackles for stoppers, must never be used for
connections in towing rigs. The pin could back out due to the constant vibration set up by the
hydrodynamic actions on the towline.
Navy die lock chain characteristics are included in Table D-1. The similar Baldt "DiLok" chain is 11 percent
stronger and 1 percent heavier Table D-2 provides the characteristics of Navy stud link chain. Navy stud link
chain is equivalent to commercial grade 3 as shown in Table D-3 Grade 3 chain is about 3 percent stronger
than Navy Standard die lock, Grade 2 is only about 72 percent as strong as die lock, and Grade 1 is only 51
percent as strong.
Detachable chain connecting links frequently are used in lieu of more traditional shackles, because they will
pass through a smaller space and are less likely to "hang up" during the rigging process Pear- shaped
detachable links fit two chain sizes. The strength of this link is identical to that of the larger chain size which is
designed to accommodate. Figures D-1 through D-3 and Tables D-4 through D-6 describe detachable links and
an improved locking system for use with the tapered link pins. End links are special studless links 1/8 inch to
1/4 inch larger than the chain size. They are larger than the chain size to compensate for the lack of a stud
They have the same strength as the parent chain system.
A safety shackle is characterized by a pin that is secured by a bolt on the outside of the shackle For towing use,
the bolt itself is secured by a small machine bolt with two nuts jammed together to prevent rotation of the large
nut. Screw-pin shackles, which use a threaded pin that screws into the body of the shackle, have no place m
towing, even though TB 55-1900-232-10  rotation of the pin might be prevented for a time by a wire mousing
through the eye on the large end of the pin.
Navy shackles are manufactured according to MIL-S-24214A (SH) (Ref. 14) which covers two types, two
grades and three classes of shackles. Tables D-7 through D-9 provide the physical dimensions and strengths of
safety shackles. Note the significant difference in strength between Grade A and Grade B shackles. The
shackle size and safe working load will be shown in raised or stamped letters. The pins and bolts of Grade A-
Regular Strength-shackles are unmarked, but Grade B pins and bolts are marked "HS."
Calculated or predicted design loads are compared to a baseline strength in computing the safety factor
Conversely, the baseline strength is divided by the recommended safety factor to determine the allow- able
design load. Table 6-4 provides the recommended factors of safety for use in designing towing systems Note
that safety factors, for a given type design and service, are referenced to different baselines such as breaking
strength, yield strength or proof load
For chain, safety factors are referred to as "proof load;' a load demonstrated as part of the manufacturing
process which intentionally introduces a permanent stretch that improves the strength of the chain Proof load
for chain is 66 percent of minimum break strength.
For other forged-type hardware, such as shackles, proof load is a load at which no permanent deformation is
observed after the load is released This is important where the component must mate with other components or
where the component has parts which must fit together In the case of shackles, it is important to be able to
remove the pin after use. Unlike chain, however, there is no consistant relation- ship between proof load and
breaking load.


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