These form an important part of the hoisting equipment in building construction and every precaution should be taken to see that they are kept in first-class condition. Wire cables, chains, and manila ropes are all used for slings, their relative strengths, for similar diameters, varying in the order in which they are here named. For many reasons aside from its strength, a wire-cable sling is to be preferred to a chain, or to a fiber rope of any kind. Ordinarily, deterioration is easily detected in wire cables, because it is commonly indicated by broken strands that are readily discoverable by an experienced and qualified inspector. Chains, on the other hand, may sometimes be used almost up to the moment of failure with no manifest external evidence of weakness other than the existence of a few seemingly unimportant bruises, although a careful microscopic examination will often disclose a multitude of small cracks, showing that the metal has become either "fatigued" or strained beyond its yield-point by the severe stresses to which it has been subjected.

Wire-cable slings, on account of their pliability, are often bent at very sharp angles, not only while being adjusted to their loads, but also when they are under strain. Sharp bends of this kind should always be carefully avoided, not only because they are immediately dangerous, but also because, when taken in connection with the twisting and untwisting to which the strands of the cable are subject while in use, they cause rapid deterioration of the sling. In making a thorough inspection of a wire-cable sling it is advisable to clamp the sling in two places, and partially untwist the intervening section so that the interior wires can be seen and examined.

The method of attaching the slings to the load, and to the hook of the hoisting cable, is of great importance and this part of the work should be intrusted only to experienced and responsible persons. Loads may often be safely hoisted by the use of a single sling, but in other cases two or three slings are required, - the number to be used depending not only upon the weight of the load but also upon its shape.

In placing a sling about a load, it is important to see that the turns of the sling do not lie one over another, because an excessive strain is likely to be thrown upon one of them unless careful attention is given to this point. A sling composed of a single length of wire cable, with spliced eyes, should never be used for hoisting a heavy load by hooking into only one of the eyes, because if this is done there will be a tendency for the load to revolve, thus unwinding the cable and permitting the splice to slip. On the other hand, when using a doubled sling with both ends engaged in the hoisting hook, it is important to adjust the sling so as to equalize the stress as well as possible, and prevent it from becoming unduly concentrated in certain parts.

When placing chain slings about loads, carefully avoid twisting the chains, because if they are twisted an excessive load may be thrown upon some of the links.

All slings should be kept in good condition. To prevent rusting, chain slings should be frequently oiled, and slings made of wire rope should be treated with oil at proper intervals, or preferably with special protective dressings prepared for this purpose. The inner wires often become corroded through exposure to the weather, even when the outer ones remain in comparatively good condition.

The stresses that are thrown upon slings and ropes vary a great deal with conditions, and they are often influenced to a marked degree by circumstances which the casual observer might consider trivial and unimportant. In particular, the inclination or obliquity of the sling, in those parts which lie between the supporting hook and the points at which the sling first touches the load, must be carefully considered, because this is a highly important feature in connection with safety. None of these parts should make an angle of less than 45 degrees with the horizontal. The reason for this will be seen by referring to the accompanying outline sketches.

In order to fix the attention upon the effect that the obliquity of the sling has upon the intensity of the stress, and avoid the necessity of repeatedly qualifying our statements so as to make allowance for the stiffness of wire cables and for other circumstances, we shall assume that the sling is perfectly flexible in all the cases shown in the sketches, and also that the load is symmetrical in shape and symmetrically supported, and that the branches of the sling (between the hook and the load) are equal in length and equally inclined. For simplicity we shall also assume that the total load that is to be supported is 2,000 pounds in each case.

Fig. 40.

Fig. 41.

Fig. 42. Illustrating the Obliquity of Slings.

Under these conditions, if the ends of the sling are exactly vertical, as in Fig. 40, the stress on each one of them will evidently be 1,000 pounds. If the ends are inclined, however, as shown in Figs. 41, 42, 43, and 44, the stress upon each of them will be greater than 1,000 pounds in every case, and it will increase as the obliquity of the ends increases, - that is, as they become more and more inclined toward a horizontal position. We shall not undertake to explain this fact fully, because a proper understanding of it calls for a knowledge of the elements of theoretical mechanics, and readers who possess this knowledge will perceive the reason for the increased stress, without explanation. In brief, however, we may say that the stress on each of the inclined ends must have a vertical component equal to 1,000 pounds; and as there must also be a horizontal component whenever the sling stands obliquely, the total tension in each of the inclined ends must always exceed 1,000 pounds.

Fig. 43.

Fig. 44. Illustrating the Obliquity of Slings.

If the sling is of such a length that its ends, between the hook and the load, are inclined to the horizontal at an angle of 60 degrees, as indicated in Fig. 41, the stress on each end will be 15.5 per cent. greater than it would be if the ends were vertical, as in Fig. 40. That is, in Fig. 41 each end will be subject to a total stress of 1,155 pounds.

If the ends of the sling make an angle of 45 degrees with the horizontal, as shown in Fig. 42, the tension on each of them will be 1,414 pounds; and if they lie at a still greater obliquity, so as to make an angle of 30 degrees with the horizontal as shown in Fig. 43, each of them will be subject to a stress of 2,000 pounds. If the obliquity increases still further, the stress will also continue to increase, and in a yet more rapid ratio; and when the ends of the sling approach the horizontal position quite closely, the stress upon them may become very great indeed. For example, if the sling were so short that its ends made an angle of only 5 degrees with the horizontal, as indicated in Fig. 44, each end would have to support a stress of 11,474 pounds.

These figures show very clearly the importance of giving careful attention to the inclination of the free ends of the sling. Men engaged in hoisting too often take it for granted that the tension on a sling is everywhere the same, and if the sling is strong enough to support the load in safety when the ends are vertical, they assume that it is safe to hook it around the load in any way whatever. It is plain, from what has been said above, that this is far from being the case. The sling should always be long enough to allow the ends to be at least as steep as shown in Fig. 42; or, in other words, the ends should never make an angle of less than 45 degrees with the ground.