Page:The American Cyclopædia (1879) Volume XIV.djvu/446

 ROPE three or more ropes twisted together form a left-handed cable of nine strands. The proper twist to give the yarn averages about one turn and a half to the inch. The degree of twist to the rope may be determined by constructing a right-angled triangle, the base of which is the circumference and the height the length of one turn of the strand measured parallel to the axis. The difference between this height and the hypothenuse is the quantity by which the rope is twisted. The rope maker's rule for a three-strand rope is to have one turn to as many inches as are contained in the circum- ference of the rope. A three-inch rope, for example, should have one turn in three inches, measured on a line paralled to its axis. Three- stranded right-hand fope is commonly called plain laid. Four-stranded rope is made with a smaller rope in its centre, called a heart. If in making a rope the twist of the strands, instead of being reversed, is made the same as the yarn, right-handed, then the rope itself becomes left-handed, commonly called back- handed rope. It is more pliable but not as strong as the plain laid. The continual twist- ing necessary to bind the fibres into a per- manent bundle " shortens in " its length. Plain- laid rope takes up 46 fathoms of the original yarn for every 100 fathoms of rope. It re- quires 2,488'8 Ibs. of hemp to produce' one ton of rope of 20-thread yarn, or about 11 per cent, more hemp than yarn. The size of rope is designated by its circumference; thus a six-inch rope measures six inches in cir- cumference. The Strength of Rope. The ut- most strength of good hemp rope was formerly supposed to be about 6,400 Ibs. to the square inch ; but 9,200 Ibs. is nearer the average strength. Tarred hemp ropes of 3$ and 8 in., made at the government ropewalk, on a trial, required respectively a strain of 14,622 and 10/T25 Ibs. to break them, and therefore their utmost strength per square inch was 15,000 and 14,975 Ibs., considerably more than double the strength ordinarily assigned to good hemp rope. These ropes were not made expressly for the trial. Rope stretches from one seventh to one fifth, and its diameter is diminished from one seventh to one fourth before breaking. A rough but safe rule for finding the breaking strain in tons of plain-laid rope is to square half the circumference. Thus, in a six-inch rope, the square of three is equal to nine tons, the table giving ten. A fore-handed rope is 25 per cent, stronger than one laid back-handed. A plain-laid rope is stronger than a cable-laid by about one sixth, owing to its having less twist. Four-stranded rope is weaker than three, about one thirteenth of its yarns going into heart. The heart forms the centre round which the strands circle. On applying a breaking strain, the heart breaks first, when an unequal strain is brought on the strands, and they part in de- tail. The strength of manila rope is about one third less than that made of hemp. Repeated experiments show that there is a great varia- tion in the strength of rope cut from the same coil, amounting sometimes in large ropes to sev- eral hundred pounds. Hide Rope is made of strips of green oxhide. The hide is stretched on frames, and when partially dry is placed on a revolving table, the ragged edges stripped off, and the entire hide cut into one yarn or strip by a knife placed for the purpose, the table and knife being worked by machinery. Two or more strips are united by a rope yarn. These strips are then reeled upon bobbins placed in the frames on the laying ground, and without giving any twist to the hide yarn they are laid up into strands, &c., just as in making hemp rope. Wire Rope. The best charcoal iron wire or steel is used. The first step in the process of manufacture is to wind the wires on bobbins, the ends of separate pieces of wire being joined by brazing or twisting. Having seven bobbins filled, six are put in a small machine, and one in a reel stand conveniently situated for leading the wire down through a fair leader and thence up through the vertical shaft ; this single wire is for the heart, around which are wrapped the six wires placed on the horizontal disk. As the disk revolves the six bobbins turn on their own centres in an oppo- site direction, so as to avoid twisting the wires. The proper tension on the wire is maintained by friction bands attached to the bobbins. The six wires with the single wire in the centre are for the heart of the strands. Having formed the heart (which is wound up on a bobbin as it is made), it is placed in a reel stand as be- fore. On the machine to form the strand are twelve bobbins filled with wire; the machi- nery is put in motion, the seven-wire heart drawn up the vertical shaft, and the twelve wires wrapped about it. As the strand is formed it is wound up on a bobbin, which at each successive step increases in size. When the required length of strand is on the bobbin, known by a register fitted for the purpose, the machinery is stopped, the strand cut, the full bobbin removed, and an empty one put in its place. When seven bobbins are filled with strands, six are placed on another machine, a bobbin containing the heart being placed in the rear; the machinery is put in motion, the heart drawn through, and the six strands wrapped about it. The six strands of 19 wires each contain 114 wires, to which the 19 wires in the heart must be add- ed, giving 133 wires to a rope. There are six wire-rope machines, made by Jackson and Wat- kins, London, and now in operation in the government ropewalk, known as A, the largest, B, C, D, E, and F, the smallest. A and B Wire Hope with Hemp Heart.