time, the best preservative is made by adding to one barrel of Stockholm tar, one barrel of fresh slacked lime. Boil well and use while hot for saturating the rope. Wherever wire-rope is to be worked over a sheave, the diameter of the sheave and the speed of running become very important factors. The larger the sheave and the lower the speed, the better. All manufacturers of wire-rope prescribe a minimum diameter for sheaves, and their guaranteed breaking strains and estimated safe-working loads are for these minimum diameters, and for moderate speed. A high speed increases the wear upon the rope, not only by the friction on the pulley, but still more by the friction of the wires upon each other-a point which is often overlooked. The importance of this interior friction will be realized if we consider the "play" which necessarily goes on between the fibres of a rope which is being alternately bent and straightened in running over a pulley. This play of course increases with the speed, and is greater with a small sheave than with a large one. The same consideration enters in where a rope is alternately stretched and relaxed under a straight but varying pull, as for example, in towing. This emphasizes the importance of interior lubrication. A hemp core (Plate 13) holds the lubricating material in the heart of the rope, and gives it out under pressure of a heavy strain. This is one of the most important reasons for preferring hemp to wire for the core, although the primary reason has to do with flexibility. In a comparative test, an unlubricated rope broke after 16,000 bends, while the same rope, well lubricated, stood 38,700 bends. A stili further advantage of the hemp core is that it forms a cushion upon which the strands close in as the rope contracts under a heavy pull, thus acting with the elasticity of the wire and the "give" which results for the spiral lay, to relieve the effect of sudden stresses. In addition to the question of friction in running over a sheave, the distortion of the rope wherever it passes around a relatively sharp bend, whether on a moving pulley or a stationary chock or bollard, is a factor of great importance. Those fibres which lie farthest from the centre of the curve are stretched, while those which hug the round of the bend are more or less compressed. Thus the outer fibres may give way before the inner ones begin to feel the strain. Wire-rope is habitually kept on reels. In receiving a line and transferring it from one reel to another care should be taken to unreel it, instead of slipping off the successive bights over the end of the reel, as is sometimes done. While the strength, lightness and durability of wire-rope are important factors in its favor, its most valuable characteristic, as compared with hemp, manila, and chain, is its reliability. Within its proper working limits, it almost never fails. Hemp and manila may be rotten at the core and show no sign, or they may have been weakened by excessive strains and give no indication of it except, perhaps, that they are a little "long-jawed." A chain may be made of worthless material, or, if of the best material and made with every care, it may have flaws which no inspection can reveal. Hemp and manila ropes are made up of a great number of fibres from a few inches to several feet in length. Wire-rope is made of a small number of wires of the full length of the rope, each of which is manufactured, inspected and tested, individually, throughout its full length, before it goes into the rope. The inspection is so simple that a flaw can hardly be overlooked and it is most improbable that any number of wires can have flaws. which in the end appear at the same point of a strand. Thus it is almost impossible that any serious flaw should exist in a wirerope as manufactured. A flaw due to kinking can always be seen. An accident with wire-rope is almost necessarily due to carelessness. Assuming the rope to have a well-lubricated hemp core, and to be used only over properly proportioned pulleys, the outside strands will be the first to wear out, and the reduction in their diameter becomes the measure of the wear of the rope as a whole. The rule given by manufacturers is that the rope should be discarded when the outside wires are reduced to one-half of their original diameter. The wire-rope used in the United States Navy is made at the Boston Navy Yard. The suitability of a rope for any given purpose depends not only upon its strength but upon several other qualities. For certain purposes, flexibility is more important than strength. In cases where much wear is involved, it is desirable that the individual wires should be fairly large, so that the outside wires shall not wear through too quickly. For standing rigging, a wire core is admissible, while for running rigging of all kinds a hemp core is absolutely necessary. If a rope is to be exposed to the weather it must be galvanized; otherwise, not. It is important, in ordering wire-rope, to specify the purpose to which it is to be applied, and as many of the conditions of its use as practicable. PLATE 12 shows various types of wire-rope for different purposes: As a rule, these are made commercially in the following grades of material, the comparative strength being as indicated, taking the best wrought iron rope as unity. All wire-rope made at the Boston Navy Yard for the Navy is of crucible steel. The following tables give the characteristics of various ropes, beginning with manila of the best quality: NOTE-See Rules for Working Load at end of this section. For plow steel add 20% to safe load. For monitor steel add 30% to safe load. Type 1 is the most inflexible rope manufactured, being made up of a small number of large wires. When used around a drum for the transmission of power, the diameter of the drum should be as large as possible. A minimum diameter of 10 feet is specified for a rope 4 inches in circumference. Type 2 (Type A) is used in the Navy for standing rigging and in some cases for wheel-ropes and running-gear, but this only in small sizes and with one or two parts working over large sheaves. As its individual wires are large, it is well suited to stand abrasive wear. Type 3 (Type AA). This type is much more flexible than Type 2 (A) and has practically the same strength. It is used for crane falls and other running-gear in large sizes, for which Type 2 would be too stiff. It is used a o for special hawsers, where great strength is required, combined with good flexibility, but is only issued for this purpose on special orders, Type 4 being the usual type for hawsers. Being made of smaller wires than Type 3, it is not so durable, as the wires chafe through, weakening the rope and making it rough and hard on the hands. |