Text 7. Sheaves, blocks and fittings

Sheaves are used to change the direction of travel of wire ropes. Assembled in the form of blocks, they provide almost any required mechanical advantage. Sheaves should be mounted in exact alignment with each other, but in practice the grooves provide some tolerance for misalignment. A constant misalignment causes the rope to rub one side of the groove, resulting in wear on the rope and sheave and shortening the lives of both.

There is no minimum sheave or drum diameter that would prevent a hoisting mechanism from operating, but increasing sheave and drum diameters have a direct correlation with improved rope life. U.S. practice requires that the winding-drum barrel diameter and the upper block diameter should be no less than 18 rope diameters, and for the lower block the ratio may not be less than 16.

A sheave is described by the rope diameter for which it is grooved and by four other diameters: the outside diameter of its flanges; the diameter to the base of the groove, or tread diameter; its shaft diameter; and its pitch diameter. The latter is the diameter to the center of the rope on the sheave, in other words, the tread-plus-rope diameter.

Sheave blocks are manufactured in several styles to satisfy the varying needs of hoisting service. As a general, they can be of oval or diamond pattern or of the snatch-block type, also known as a gate block. Blocks can be provided with fixed or swiveling single or double hooks as well as with fixed or swiveling shackles or with bails. Snatch blocks are used when there is a need to permit reeving when the end of the rope is not free.

(a) Diamond pattern (b) Oval pattern (c) Snatch blocks

The fittings used to attach wire ropes to structures and to each other are of several kinds and serve different purposes. Rope-end fittings provide the rope end with a loop, eye, pinhole, or hook for attachment. Permanent fittings, which require cutting the rope for removal, are the most efficient, they often have the same strength as that of the rope. The removable fitting types, wedge sockets, clip and thimble loops, and variations, usually have about 80% of rope strength.

Permanent rope fittings should be used wherever possible because of their superior strength and reliability. There are, however, many situations in which permanent fittings are impractical, for example the dead end attachment of the hoist rope.

Shackles are used to connect a rope-end loop or eye at a structural pinhole, lug, or bail. To meet varied conditions of use, the mouth opening is made wide compared with the thickness of a typical connecting plate.

Turnbuckles are used to remove slack from standing ropes such as guy lines or pendants. They are available with hook, loop, or shackle end fittings.

(a) Spelter socket (b) Swedged socket (c) Wedge socket (d) Turnbuckle. (e) Shackle. (f) Crosby-type clips and thimble (g) Hand-spliced eye and thimble

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