of larger wires is more resistant to external abrasion but is less flexible. The CORE, the element around which the strands are laid to form the wire rope, may be of hard fiber, such as manila, hemp, plastic, paper, or sisal. Also, a wire- strand core may be used. Each type of core serves the same  basic  purpose—to  support  the  strands  laid  around it. A FIBER CORE offers the advantage of increased flexibility. Also, it serves as a cushion to reduce the effects of sudden strain and acts as a reservoir for the oil to lubricate the wires and strands to reduce friction between them. Wire rope with a fiber core is used in places where flexibility of the wire rope is important. A  WIRE-STRAND  CORE  not  only  resists  heat better than a fiber core but it also adds about 15 percent to the strength of the wire rope. On the other hand, the wire strand makes the wire rope less flexible than a fiber core would. An INDEPENDENT WIRE-ROPE CORE is a separate wire rope over which the main strands of the wire rope are laid.   It usually consists of six seven-wire strands laid around either a fiber core or a wire-strand core.  This  core  strengthens  the  wire  rope  more, provides  support  against  crushing,  and  supplies maximum resistance to heat. Wire rope may be made by either of two methods. If the strands or wires are shaped to conform to the curvature of the finished wire rope before laying up, the wire rope is termed preformed. If they are not shaped before   fabrication,   the   wire   rope   is   termed nonpreformed. When cut, preformed wire rope tends not to unlay, and it is more flexible than nonpreformed wire rope. With  nonpreformed  wire  rope,  twisting produces a stress in the wires; and, when it is cut or broken,  the  stress  causes  the  strands  to  unlay.  In nonpreformed wire rope, unlaying is rapid and almost instantaneous, which could cause serious injury to someone not familiar with it. The main types of wire rope used by the Navy have 6, 7, 12, 19, 24, or 37 wires in each strand. Usually, the wire rope has six strands laid around a fiber or steel center. Two common types of wire rope, 6 by 19 and 6 by 37 wire rope, are shown in figure 1-4. The 6 by 19 type of wire rope, having six strands with 19 wires in each strand,  is  commonly  used  for  rough  hoisting  and skidding work where abrasion is likely to occur. The 6 by 37 wire rope, having six strands with 37 wires in each strand, is the most flexible of the standard six-strand wire ropes. For that reason, it is particularly suitable when you are going to use small sheaves and drums, such as are used on cranes and similar machinery. Grades of Wire Rope Wire rope is made in a number of different grades, three of which are mild-plow steel, plow steel, and improved-plow steel. MILD-PLOW  STEEL  wire  rope  is  tough  and pliable.  It can stand up under repeated strain and stress, and it has a tensile strength of 200,000 to 220,000 pounds per square inch (psi). PLOW STEEL wire rope is unusually tough and strong. This steel has a tensile strength (resistance to lengthwise stress) of 220,000 to 240,000 psi.  This  wire rope is suitable for hauling, hoisting, and logging. IMPROVED-PLOW  STEEL  wire  rope  is  one  of  the best grades of wire rope available, and most, if not all, of the wire rope you will use in your work will probably be made of this material. It is stronger, tougher, and more resistant to wear than either plow steel or mild-plow steel. Each square inch of improved-plow steel can stand a strain of 240,000 to 260,000 psi. Measuring Wire Rope The size of wire rope is designated by its diameter. The true diameter of a wire rope is considered as being the diameter of the circle that will just enclose all of its strands. Both the correct and incorrect methods of measuring wire rope are shown in figure 1-5. Note, in particular, that the CORRECT WAY is to measure from the top of one strand to the top of the strand directly opposite it. Use calipers to take the measurement; if calipers are not available, an adjustable wrench will do. To  ensure  an  accurate  measurement  of  the  diameter of a wire rope, always measure the wire rope at three Figure 1-4.—Two types of wire rope. 1-6