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Derrick

mast, derricks, legs, heavy, top and feet

DERRICK, a mechanical device for lifting and shifting heavy weights. The simplest form of derrick is the gin, a tripod of three legs, with a compound pulley at the apex and another at the hoisting hook and a windlass to operate the pulley rope. These simple derricks are easily portable and are used very commonly for lift ing stones, pulling stumps, lowering iron pipes into trenches and similar work. The builders' derrick has two legs framed together like a broad ladder and set wider apart at the bottom than at the top. The foot piece at the bottom is fitted with wheels which run sideways along a plank parallel to the face of the building. The top of the derrick is inclined toward the build ing and is held in position by guy ropes. This form of derrick is used by stone masons to lift and set in place stone facing, sills, lintels, col umns, etc. It is sometimes footed on a beam of an upper story, hanging outward over the street and used also to hoist lumber, window frames and the like.

For large and heavy work, as in excavations and the erections of steel-frame buildings, the derrick in common use is the guy derrick, com prising a mast held upright by four to seven guys widely extended and securely anchored and a boom attached to the mast near the bot tom by a hinged joint, so that it may rise and fall in a vertical plane; the whole having the outline of the letter V. The foot of the mast is a pivot pin set in an opening in a heavy base plate, or, in very large derricks, a ball and socket joint. The guys at the top are attached to a thick circular plate which slips down on a gudgeon pin. The mast is thus free to revolve and if the derrick is small, it is swung by means of a horizontal pushing bar ; if large, the base of the mast is fitted with a bull wheel, from five to eight feet in diameter, which is operated by cable from a winding engine. Derricks are usually equipped with two separate hoists, one lifting the load and the other drawing in the

.guy ropes which control the sag of the boom away from the mast. The winding drums which do this work are generally detached from the derrick itself, but in some cases the winding machinery is built on a platform attached to the base of the mast and revolves with it upon a cogged circular track. In confined situations, as between buildings in cities, where there lacks room to stretch guys to support the mast, the stiff-leg derrick is used. In this type the mast is held at the top by two heavy beams or legs which spread apart at the ground to the ends of an arc of 90 degrees. The bases of these legs are heavily anchored with stone, bags of sand and gravel or other weighty material.

Small derricks are of wood; the masts being from 20 to 45 feet in height, and respectively from 10 to 22 inches square, and with booms relatively from 8 to 16 inches square. A five ton derrick will have a mast 25 feet long, and a boom of 30 feet. The guys will be wire cables of one inch diameter, and the hoisting cable from five-eighths to three-quarters inch in diam eter. If the derrick is upheld with stiff legs, they will have the same cross section as the mast, and be as long as the working area will permit. In stiff-leg derricks the mast is usually much shorter than the boom. The masts and booms of timber derricks are sometimes trussed with iron rods to strengthen them, but for extra heavy work the derrick is often all metal, of latticed structural steel. (See CRANE). Consult Adams, H., 'The Mechanics of Build ing Construction> (New York 1912) ; Hess, H. D., 'Machine Design; Hoists, Derricks, Cranes> (Philadelphia 1912) ; Thomson, T. K, 'Hoisting Machines for the Handling of Ma terials> (New York 1908).