Bureau of Standards Technologic Paper No. 95

Upper Thread Upper Thread


Fig. 6.—Double-locked stitch seam
Fig. 7.—Shuttle stitch seam


4 Technologic Papers of the Bureau of Standards
An examination of the test specimens having No. 80 under
thread showed that this finer and weaker thread had been pulled
up into the hole formed by the needle. For this reason it is very
probable that with the freedom of movement, which is a charac-
teristic of the under thread of the double-locked stitch, the ten-
sions on the upper and under threads of the seam are more uni-
formly distributed, thereby causing a higher breaking strength
of the seam than when a stronger under thread is used.
The relative values of the two types of stitching may be largely
dependent upon the extent to which the strength of a seam is
affected by the breaking of a thread. Table 1 shows that cut-
ting one stitching thread at the middle of the seam weakened
the double-locked stitch less than it weakened the shuttle stitch.
Previous results of tests have shown, all other things being equal,
that the resulting tensile strength of the seam is the same when
either the upper or the under thread is cut. The weakening
appeared to depend somewhat upon the kind of fabric used, which
may have been due to the extreme narrowness of the test speci-
mens. It is interesting to note that even though the double-
locked seam was cut or broken it was still stronger than the uncut
seam of the shuttle stitch.
Another series of tests was made on cotton bunting and awning
cloth, using somewhat wider specimens than before and both
single and double rows of stitching. Preliminary experiments had
shown that equal test strengths were obtained when specimens
1 ]/2 inches and wider were used in the testing machine with
1-inch jaws. For these experiments specimens if£ inches wide
were chosen. The results of these tests are given in Tables 3
and 4, which show as before a greater tensile strength for the seams
sewed by the double-locked stitch. It was observed that when the
tension was applied several stitches pulled out in the seams
sewed by the .shuttle stitch, but this did not happen in the case
of the double-locked stitch. Each result is the mean of three
observations.
Comparative tests were also made with specimens in which
one stitching thread of each row was cut at the middle. It was
found that the shuttle-stitch seam was weakened much more than
the double-locked stitch and that the pulling out of the threads
in the former stitch caused the seam to give way before the fabric
was torn. Photographs of two of these test specimens are shown,
Figs. 6 and 7. These illustrate how the threads of the shuttle
stitch pulled out while the double-locked stitches remain intact