Straps.—Straps are wrought-iron bands of different designs, and are used to form a connection between timbers. Branched straps (fig. [101]) are used to strengthen angle joints. They are usually fixed in pairs, and being fastened on the surface of the timbers they have an advantage over bolts in that they do not cut into the material. If the timbers settle at all, the straps may become subject to cross strains.
Fig. 101
Wire Ropes.—Wire ropes are now in general use for heavy purposes.
They are stranded and laid similarly to fibre ropes. They should be of mild plough steel wire. The number of wires in a strand varies from 12 to 37, and the number of strands is usually 6.
The following table gives the breaking strains of the ropes according to their circumference, and the least diameter of barrel and sheaves around which they may be worked at slow speeds.
In the [table (p. 110)] the diameters of the pulleys, &c. may be slightly reduced for the more flexible ropes, but better results can always be gained by using pulleys and sheaves of larger diameters.
A few points on the working of these ropes may be useful.
To remove a kink throw a turn out; it cannot be taken out by strain.
The ropes should be ungalvanised, and kept greased with any oil that does not contain acid or alkali.