DIFFERENCE BETWEEN THREADS AND CORDS

The difference between threads and cords is discussed in the following text.

Threads

Filaments (nylon) or staples (cotton) are twisted together to form yarns, and two or more yarns are twisted together to forma thread or ply yarns, as the yarn by itself is too small for practical use. The strength of a thread depends upon the size and number of yarns used to make up the thread. The thread numbers on spools indicate the size of the yarn and the number of yarns that are piled (or twisted) together to give the necessary strength to the thread. For example, a 16-4 thread indicates that the thread was made from a single yarn, size 16, and that four of these single yarns were twisted together to make a thread. The finer the yarn used, the higher its size number. Silk and nylon thread sizes, however, are indicated by letters, such as A, B, etc.; A is finer than B; the farther down the alphabet, the coarser the thread.

Thread is twisted to the left or twisted to the right, depending on its use. Left-twist thread is always used in the sewing machine because the action of the stitch-forming mechanism tends to ravel or break right-twist thread. Left- or righttwist thread may be used for hand sewing. The terms that designate left-twist threads are machine, machine twist, left twist, and Z twist. A cord or thread has left (or Z) twist if, when held in a vertical position, the twist of the yarn follows the slope of the central portion of the letter Z; and right (or S) twist, if it follows the slope of

Figure 10-3.-Thread twist.

the central portion of the letter S, as shown in figure 10-3.

Cords

Unbraided and braided cords are covered in the following paragraphs.

UNBRAIDED CORDS.-Unbraided cord is twisted together in the same manner as thread, as shown in figure 10-3. The difference between threads and cords is that cords are stronger and larger in diameter than threads.

Nylon cords play an important part in the repairing of life support items. To identify nylon cord, you must remember that the larger the number, the larger and stronger the cord.

BRAIDED CORDS.-You know that a braid is three or more strands of material entwined together. Cords also come braided, and in two types: a solid woven cord or a cord with a hollow channel center, as shown in figure 10-4. Solid woven cords are flat. Hollow channel cords sometimes contain several straight, individual

Figure 10-4.-Braided cords..

threads, known as a core. This core increases the strength of the cord and keeps the outer braided cover round. Parachute suspension lines are made from this type of cord.