ROPE

ROPE, a general name for all kinds of cordage, but more correctly applied to such as is above one inch in circumference, the smaller sorts being distinguished by the names of twines, cords, and lines.

The art of twisting into lines and ropes various materials, such as thongs of animal hide, the hairs of animals, tough grasses, and vegetable fibres, is of remote antiquity, and has existed even among the rudest people.

The tarabita or rope-bridge of the Peruvians, and the lasso of the Chilian hunter, are formed by twisting together thongs of ox's hide; and in our own country at the present day ropes for particular purposes are made of horse's hair. The coir-ropes of Ceylon and the Maldive Islands are made from the fibrous husk of the cocoa-nut; the Manila rope from the fibres of a species of the wild banana, the Musa textilis; and the Sunn ropes from those of the Crotolaria juncea.

Many other vegetables have fibres of great tenacity, and fitted for the purposes of the rope-maker; but preference is given to those of the Cannabis sativa, or cultivated hemp, and the Linum usitatissimum, or flax, the fibres of both of which possess in a remarkable degree the essential qualities of flexibility and tenacity. Some idea of the importance of the manufacture under consideration may be obtained from the fact that, in the year ending January 1857, the value of the hemp alone imported into Great Britain and Ireland was £1,953,444, and the value of the cordage exported was £246,925.

The fibres of the hemp are first twisted together to form a thread or yarn. Many yarns are then combined by twisting, and form a strand; three strands are in like manner combined, and form what is properly a rope, and technically termed a shroud-laid rope or hawser-laid rope; and three of these ropes may be again combined, forming what is termed a cable-laid rope. The fibres should be so arranged that each in the finished rope shall offer the greatest resistance to its being torn asunder in the direction of its length.

If we take a bundle of fibres, equal in length and strength, and fasten it at the ends, each fibre will, upon a strain being applied to the bundle, bear its proper share of the stress; and the strength of the bundle will evidently be measured by adding together the strength of the separate fibres. But if we twist this bundle so as to form a thread, the strain will no longer be equally distributed among the fibres; for, by the torsion, the external fibres of the bundle will be wound round those that lie nearest to the centre, and, in proportion to their distance from the heart of the bundle and the amount of twist given, will form spirals more or less inclined from the axis of the thread. The external fibres will in consequence be longer than the internal ones, and the greatest share of the strain will be borne by the latter. Further, by the operation of twisting, the fibres in a thread are strained, and, on account of their position, the external ones the most. It is of importance to consider the proper length of the primary fibres, and the degree of torsion that ought to be given in forming them into a thread.

All threads require the fibres to be so fine, and of such a length, that the quantity of fibres used, and the number of turns each has round its axis, shall be so great as to produce the necessary compression amongst them, to prevent them from sliding upon each other. If the thread be small, the fibres must be fine, and may be short; and if the thread be large, then must the fibres be long. If the fibres be long in proportion to the size of thread to be made from them,

less twisting will obviously be necessary to keep them from sliding; and the finer and softer the fibres are, the more may the twist be diminished; for soft fibres enter into closer combination with each other than those that are hard. Long fibres requiring thus less twist than short ones, it has been a standing rule with all theoretical writers, that fibres should always be spun into the thread endlong, and never by their bight or double. Now, in the practice of hand-spinning, the fibres are always spun into the thread by their bights, and never by their ends.

It is certainly an advantage in threads which are to be used merely as such, to secure as great a length of fibre as possible, as any strain tends directly to pull the fibres asunder; and they are retained in their position merely by the compression among the co-fibres, produced by twisting. But many threads are combined in forming strands; new forces are brought into action, and so at every further combination. This will be better explained by the following diagram. Let aa be the primary fibres, formed into threads b; in each thread the fibres are retained in their position by the compression produced by twisting. Let bb be threads twisted together to form a strand. Here the threads mutually compress each other, and the primary fibres of each thread are compressed by the surrounding threads. Let cc be strands twisted to form a hawser-laid rope. In this the compression on the primary fibres is again increased, and so in the next combination, where the three hawser-laid ropes are twisted into the cable-rope d.

We may from this deduce, that it is not the length so much as the intrinsic strength of the fibre which fits it for the purpose of the rope-maker; and practice perfectly fixes this position, the ropes made from the short waste fibre of the hemp called tow being by no means so weak when compared with those made from the hemp itself, as theory would lead us to suppose, seeing that these fibres are the shortest and weakest of the material. It may therefore be laid down as a rule, that in the making of ropes it is of greater consequence that the fibres should be strong, soft, and finely hackled, than that they should be of great length.

Let us consider a little more how the twist of the thread is affected by its future combination. The fibres are first twisted in a certain direction to form threads. A collection of these threads is then twisted together to form a strand; and this last twist being in a direction contrary to that of the threads, untwists them to a certain extent.

Had the twist in the first instance been no more than would just have kept the fibres from sliding upon each other, it would now be inadequate to produce that effect. Hence one would think it necessary to provide means to put more twist into the threads as they were being formed into strands, or to put as much more twist into the threads while spinning, as the twisting of the strands abstracts from them; but when these strands are combined to form a hawser-laid rope, the direction of the twist is again the same as that of the threads, and restores to them a certain portion of what they had lost. If, however, three of these hawser-laid ropes are formed into a cable-laid rope, the threads are

Rope. again to a certain extent untwisted. The untwisting suffered by the thread in forming the strands is much greater than the subsequent retwisting in forming the rope; and if the thread had been at the first too little twisted, or too soft, as it is termed, it would never make a serviceable rope; and for such ropes as require to be impervious to water, it would be totally unsuitable.

It is, then, on the proper angle of twist in the combined threads, and not in the threads when separate, that the efficiency of the rope depends; and this can be determined by experiment alone.

When many threads or yarns are combined to form strands, the effects produced on the latter by strains are analogous to those produced on the fibres when formed into threads, and, from the greater size of the component parts, are more apparent.

In the ordinary method of procedure the threads are all stretched to the same length, and then twisted together in a direction contrary to their individual twist. This, by winding the external yarns round those beneath them, shortens the whole mass, and puckers up the yarns nearest the centre, and thus the greatest share of any strain is thrown on the external yarns. It has therefore been considered of primary importance in all inventions intended to improve the making of ropes, to equalize the strain on the yarns in the strands.

Belfour attempted to effect this by shortening the internal yarns in the degree necessary to prevent their puckering up. Now, although the fibres of hemp are not in themselves very extensible, yet the rope formed from them is; and when such an extension takes place in a rope formed by Belfour's method, the strain is thrown entirely upon the internal yarns, which, if the strain be great enough, will break from the centre outwards.

The manner in which the external yarns of a strand lengthen, will be seen at once by fig. 2. Here $\backslash alpha$ is a section of a newly formed strand; $\backslash alpha\text{'}$ a section of the same strand after having been used; while $bc\text{'}$ is a part of the surface of the first, on the stretch out; and $bc\text{'}$ a part of the surface of the other. By the straining produced by use, the yarns in $\backslash alpha$ are brought into closer contact, and the diameter of the strand is reduced. If in the strand $\backslash alpha$, $bo$ represents the angle at which any external yarn is supposed to lie, this, by the reduction of the diameter, will in $\backslash alpha\text{'}$ be changed to $bc\text{'}$; and if in $\backslash alpha$, $de$ represents a heart-yarn, this yarn in $\backslash alpha\text{'}$ must stretch to $e\text{'}$, or break, which, as it is very little extensible, it is likely to do. It is of importance to observe, that the breaking of the heart-yarns of a strand in this manner is attended with much greater danger than the breaking of the external yarns, as the injury not only remains concealed, but water can easily penetrate to the core of the strand, which in consequence speedily decays.

It would appear, then, that a certain degree of puckering in the internal yarns of a strand is necessary to compensate for the extension that the superficial yarns undergo by use; and where ropes are made by machinery, it would be perfectly possible to arrange it so that, from the central yarns outwards, every one could be wrought into the strand in due length to allow for the stretching.

The further operations are the forming the strands into hawser-laid ropes, and these again into cable-laid ropes. The many uses to which ropes are applied preclude the possibility of applying specific rules to their manufacture.

Some require flexibility, others impenetrability to water; strength may in some be of primary importance, or it may be secondary to other qualities which better adapt the rope for its peculiar purpose. The goodness of the rope depends upon the previous operations; but it must be a standing rule in all the processes, never to make use of so much twist as will impair the strength of the fibre.

To prevent the decay of such ropes as are exposed to continual changes from wet to dry, the yarns forming them are soaked in hot tar previously to their being worked up. It would be well if some other substance than tar could be found suited for this purpose, as it unfortunately happens that ropes lose much of their strength in the operation of tarring; and after having been kept for some time, the loss of strength is progressively increased, and this to a greater extent in hot than in cold climates. M. Duhamel made several experiments on this subject in 1741-1746, and from the results obtained he concludes,

1st, That untarred cordage in constant use is one third more durable than the same cordage when tarred;

2dly, That untarred cordage retains its strength for a longer time when kept in store;

3dly, That untarred cordage resists the ordinary influence of the weather one fourth longer than when it is tarred.

Some experiments were made in 1803 by Mr Chapman, civil engineer, to determine the effects of a new process of washing the tar to free it from the soluble substances contained in it, and for which process he had obtained a patent. Yet although the result of these experiments proved beyond a doubt the superiority of Mr Chapman's method, it is singular that it has never come into use. The same gentleman made some other experiments in 1806-1807, confirmatory of those of M. Duhamel. The result of one series is given in the table below.

We shall now offer a few of the rules which have been given by different authors for computing the strength of ropes.

Mr Tredgold says, "that in a hawser-rope, it may be proved that the strength of the straight fibres of hemp is to the strength of the rope as the radius is to the mean between the square and the cube of the cosine of the angle of twist, when the fibres are all equally extended, and the angle of twist measured at the greatest stretch the rope will endure without fracture. The cosine of the angle under these circumstances is in general about 0.87, and therefore the strength is about 0.708 times the strength of the hemp, or very little exceeding two thirds of its strength; but in most cases the loss of strength will be greater than one third, because the stretching of the different parts is unequal. And in a cable-laid rope, that the strength of the hemp is to that of the cable as the radius is to the mean of the third or fourth power of the cosine of the angle of twist under the same circumstances as before; or, that its strength is to that of the ropes which form it simply as the cosine of the angle of twist. This, in usual cases, will be nearly as eighty-seven to a hundred, that is, there are thirteen parts in every hundred of the strength lost in forming cable-laid ropes."

The following rule is given by Dr Robison for finding the strength of ropes: Multiply the circumference of the rope in inches by itself, and the fifth part of the product will be the number of tons which the rope will carry. Thus, in the experiment of Mr Chapman, the weakest tarred rope

Rope-making. broke with two tons, and by our rule we find that it ought to carry with safety one ton eleven hundredweight. The practical rules of the workshop are as follows:—To find the breaking weight of 3-strand hawsers; square the circumference, and divide by 3. To find the breaking weight of 3-strand cables; square the circumference, and divide by 5.

The following rule may also be of use:—To find the weight in pounds of a foot in length of any hempen rope, multiply the square of the circumference in inches by 0.045 for shroud-laid ropes, and by 0.027 for cables. (x. n.)