Page:EB1911 - Volume 23.djvu/757

 the continuous system is mostly used; here the rope is wound round and round over driver and driven, and, except in rare cases, is joined only at one place. Although the system has the great advantage of the minimum number of joinings, it requires tension pulleys to keep the ropes taut. It is also clear that when the rope breaks at any point the machinery must stand until the repair is completed.

Wire Ropes.—Although the manufacture of ropes is of ancient origin, the practice of making ropes from wire on a large scale is of comparatively recent date. Since 1874, however, great developments have taken place in the manufacture of ropes from different kinds of wire, and the uses to which they can be put have enormously increased. This is owing almost entirely to the introduction of flexible wire ropes which were invented about this time by Messrs Bullivant & Co. Ltd., of 72 Mark Lane, London, E.C. Prior to that date the uses at which wire ropes were put were limited to winding ropes for collieries and hauling, and to cases in which flexibility was not a great desideratum. The introduction of flexibility, however, made possible the use of wire rope for ships' hawsers and rigging, for cranes, derricks and other purposes for which hempen ropes were formerly employed-indeed it has almost entirely superseded hemp for marine uses. The reason is that it is much stronger for the same size than rope made from any other material, whilst for the same strength its size and weight are only about one-third that of hempen rope. Consequently, the required power may be obtained with a wire rope of comparatively small bulk.

Wire rope is specially suited for aerial ropeways which provide a means of conveying ore, metals, merchandise, &c., over ground where it would be difficult to arrange transport by ordinary means. Messrs Bullivant & Co. Ltd., to whom we are indebted for the table of strengths and other particulars, as well as for the sectional illustration of wire ropes, construct seven different systems of aerial ropeways:—

1. The endless running rope, with carriers hanging therefrom and moving with it through frictional contact.

2. An endless rope, with the carriers hanging therefrom and moving with it, being rigidly fixed in position on the rope.

3. The fixed rope, in which the carriers are drawn along and hang from a fixed rope which acts also as a rail, returning on a parallel rope.

4. The single fixed rope, in which one carrier, hanging from a. fixed rope, is drawn to and fro by means of an endless hauling rope.

5. The use of two fixed ropes with an endless hauling rope, in which one carrier travels in one direction, while the other travels on a parallel rope 'in the opposite direction. This is a serviceable type of ropeway, capable of being used over extremely long spans, and of carrying loads up to 5 tons.

6. The use of one fixed rope laced on an incline, on which the carriers (uncontrolled by hauling ropes) with their suspended loads are allowed to run down at a high speed. This is generally called a "shoot."

7. Bullivant's system of aerial ropeway for raising, lowering, and transporting heavy loads, by means of which a load can be hoisted, traversed in either direction and deposited at one operation.

The flexibility of a wire rope depends upon the number of wires of which it is formed; consequently the use to which a rope is to be put will partly determine the number of wires used in its construction. In some cases nearly 400 individual wires are employed in making one rope. Fig. 8 shows in section ten different types of construction, the particulars of which appear below:— 1. Laid rope made of 6 strands of 7 wires each. This is the class of rope most frequently used for hauling ropes where the size of the barrel and sheave will permit; it is also the make of rope in general use for standing rigging, and is such as is required by Lloyd's regulations.

2. Hauling rope made of 6 strands, each strand being of 7 wires covering 7 smaller ones.

3. Hauling rope made of 6 strands, each of 8 wires covering 7 smaller ones.

4. Hauling rope made of 6 strands, each of 10 wires covering 7 smaller ones.

5. Formed rope made of 6 strands of 19 wires each. In larger sizes this make of rope is used for standing rigging on vessels. In smaller sizes it is sometimes used for running rigging, and it is the usual make of rope for trawl warps.

6. Flexible steel wire rope, made of 6 strands each of 12 wires, with hemp heart and hemp centre in each strand. This is the usual make of flexible steel wire rope, 4 in. in circumference and smaller; used for hawsers, running lifts, hoists, &c.

7. Extra flexible steel wire rope made of 6 strands each of 24 wires.

8. Special extra flexible steel wire rope made of 6 strands each of 37 wires.

9. Special extra flexible steel wire rope made of 6 strands each of 61 wires. This is the make of rope usually adopted for large ropes-say over 10 in. in circumference-which are largely used for slipway and salvage purposes.,

10. Cable-laid rope. This is an obsolete form of rope, which is composed of six complete ropes twisted together. . 8.

The following table supplies particulars about wire ropes which are used for general hauling purposes:—

The diameter of drums and sheaves should be about thirty times the circumference of the rope.

For shaft winding at high speed one-tenth of the breaking strain of a rope is sometimes taken as a fair working load. For inclines, the proportion of load to breaking strain varies according to gradient conditions, and friction should be allowed for.

The first requisite in the manufacture of wire ropes is the selection and blending of the different iron ores. The different processes through which the metal passes, and the hammering and drawing into rods, require great experience, and give to it the peculiar properties that are essential for the finished article. The same remarks apply to the annealing and hardening processes, during which the rods are drawn through dies to the required gauge. he wire is now subjected to special processes of galvanizing in order to make it proof against atmospheric and other influences. Afterwards it is wound on bobbins of suitable size, a definite number of which are mounted on the forks or frames of the stranding machine. These forks are swung or pivoted between disks, which are keyed on a hollow main shaft. through which the wires or other material