The accompanying short table shows the way in which the stress changes in two-part slings, for various inclinations of the ends to the horizontal. The use of the table will be evident, from what has been said above and from the illustrative example that is given below.

Effect Of The Obliquity Of A Sling

Example: Total load, 2,000 pounds. Two-part sling, with ends making an angle of 5° with the horizontal. Solution: Opposite 5° in the first column, we find 5.737 in the second column; and multiplying this by the total load to be supported, we have 5.737 X 2,000= 11,474 lbs., which, as indicated in Fig. 44, is the stress on each end of the sling.

When the load to be lifted has sharp corners or edges, as is often the case with structural steel and other similar objects, pads or wooden protective pieces should be applied at these corners, to prevent the slings from being abraded or otherwise damaged where they come in contact with the load. This is specially important when the slings consist of wire cable or fiber rope, though it should also be done even when they are made of chain.

To show the importance of protecting sharp corners where slings run over them, reference may be made to Fig. 45. The total load is here supposed to be 2,000 pounds, and as the ends of the two-part sling make an angle of 30 degrees with the horizontal, each of these ends is also subject to a tension of 2,000 pounds. This tension is exerted partly to hold up the load, but it also tends to draw the sling horizontally against the load, at the points indicated by the arrows; and if the sling is pliable, and is placed about the load symmetrically as shown in the illustration, it will press against the load, under the assumed conditions, with a horizontal force of 1,732 pounds at each of the upper corners. This pressure may damage either the sling or the load, or both of them; and it is to prevent damage from this source, as well as from the direct weight, that the pads are recommended. Wooden corner-pieces are often provided, as shown in Fig. 46, for use in hoisting loads with sharp angles. If pads of burlap or other soft material are used, they should be thick and heavy enough to sustain the pressure well, and to distribute it over a considerable area, instead of allowing it to be concentrated directly at the edges of the object to be lifted. Precautions of this kind are often neglected in the United States, but they receive attention in European practice, and it is to be hoped that their importance will soon be more widely admitted in this country.

So far as weight is concerned, loads might often be safely hoisted by the use of single-part wire-cable slings; but in handling structural steel it is customary to use two-part, or "bridle" slings, so that the loads may be properly balanced, and hoisted in a horizontal position. Bridle toggle-slings are also used for similar work, and toggle-column slings are often used for hoisting and setting steel columns. When toggle slings are employed it is not necessary for a man to climb up a column or go into other dangerous elevated positions to release the slings, because they can be detached by a man standing upon the ground or the floor.

Fig. 45. Illustrating the Pressure of the Sling against the Load.

Fig. 46. Illustrating the Use of Wooden Corner-pieces.

It is advisable to have two complete sets of slings of various sizes constantly on hand, and all slings that are not in use should be stored in a place specially provided for them, and locked up. They should be in charge of an experienced man, who should be held responsible for their condition. The man charged with the care of the slings should give them out as they are needed, and always with due regard to the use to which they are to be put. In this way it is possible to guard effectively against the use of slings of inadequate strength. All slings should be promptly returned to the official custodian, when they are no longer needed for the work for which they were given out. As an additional precaution, every sling should be provided with a small identification tag, which should be firmly fastened to it. The tag should give the maximum stress that the sling can safely withstand in use, and in the case of a chain sling it should also give the date of the latest annealing.