KOPE, WIKE, AND BELTING 299 



a single hemp core at the center, and Fig. 169 shows a cross section 

 of a cable in which the center is a wire strand similar to those used 

 on the outside. A cable of the latter type can only be used where 

 little bending is required, as in 

 the case of suspension bridges.' 

 The strands are twisted about the 

 central core either to the right or 

 left. When twisted to the left 

 the rope is designated as left lay, 



FIG. 108 FIG. 160 



and when twisted to the right as 



right lay. The twist is long or short, depending upon the require- 

 in* -iits of service. The shorter the twist the more flexible the rope, 

 and the longer the twist the less flexible. 



249. Testing of rope wire and belting. These materials are usually 



1 in tension. This may be done in an ordinary testing machine, 



providing the proper means are used for holding the specimen. A 



of wire-testing machine is shown in Fig. 170. One end of the 



wire is clamped to the movable head and the other to the stationary 



head, which is provided with a spring balance for registering the pull. 



Many other types of wire-testing machines are in use, some of them 



U-ing arranged to make torsion tests. Many special machines are 



also made for testing rope and belting. 



Since a wire rope is a built-up structure, made of twisted strands, 

 it is not to be expected that it will exhibit such well-defined elastic 

 properties as a single wire tested separately. This is due to the fact 

 that as the tension is increased each strand, which was originally in 

 the form of a helix of a certain pitch, becomes somewhat straightened 

 ami lakes the form of a helix of a greater pitch. On account of the 

 t wis ted condition of the wires in the strands, they do not all carry 

 the same load, and therefore do not all reach their elastic limit at 

 the same time. We find, consequently, upon testing a wire rope, that 

 it has no well-defined elastic limit. 



The individual wires of which the rope is made show a very high 



;ie strength and elastic limit, but exhibit no yield point, as the 



process of drawing seems to destroy the properties of the material 



that nive the yield-point phenomena. The modulus of elasticity is 



ii"t changed appreciably by the process of drawing. 



