Critical lift a lifting operation judged to carry a

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Critical lift. A lifting operation judged to carry a potentially high level of risk due tofactors such as load weight, complex procedures, high value, or presence of hazards.Though no uniform standard exists for identifying a critical lift, there is a consensus thatrisk can be mitigated by engineering, planning, and field controls (Fig. 20.3).Derrick. A lifting device that uses ropes for hoisting, with or without a boom, utilizing amast or equivalent member held at the head by guys or braces. Unlike a crane, a derrickdoes not have an integral winch (Fig. 20.4).Friction crane. A mobile crane that relies on mechanical power transmission; the opera-tor engages clutches and brakes to engage power and stop hoist motion (Fig. 20.2).Hammerhead jib. A tower-crane jib (boom) fixed at a horizontal or near-horizontalangle, equipped with a trolleyto support the hook block and traverse loads radially fromthe tower (Fig. 20.5).Hydraulic crane. The vernacular name for a crane for a telescoping cantilevered boomcrane; the telescoping of the boom and the various other motions of the crane are typi-cally powered by hydrostatic (hydraulic) drives (Fig. 20.6).Jib. (1) In U.S. practice, an extension to the boom mounted at the boom tip, in line withthe boom longitudinal axis or offset to it (Fig. 20.6). (2) On tower cranes, the structural20-Ratay_Ch20_p20.1-20.38.indd21/20/125:28 PM
CRANES20.3Figure 20.1 The telescopic boom on this all-terrain cranecarries both a main load fall and an auxiliarytip for a whip line. The carrier, with a separate driving cab, is suited both for highway driving and off-roadtravel. Note that all wheels are turned to maneuver the crane into operating position, demonstrating the crabsteering. This crane carrier has five axles but machines with heavier road weight have as many as nine.(Paul Yuskevich.)member extending forward from the mast to support the lifting trolley, sheaves, hookblock, and load. (3) In Europe, the term is used for a crane boom. Correspondingly, theextension to the main boom is known as a fly jib.Latticed boom. A boom constructed of four longitudinal corner members, called chords,assembled with transverse and/or diagonal members, called lacings, to form a trusswork in two directions. The chords carry the axial boom forces and bending moments,while the lacings resist the shear forces (Fig. 20.2).Lifting capacity. The maximum gross load weight that a crane manufacturer has deter-mined a crane can safely handle under specified conditions as stipulated in the loadchart.Lift crane. A crane configured for lifting, booming, swinging, or traveling with loadsattached to the crane hook block as contrasted to a crane configured for lifting materialwith a bucket or grapple.Load. The suspended weight applied to the crane, including the weight of lifting hard-ware such as hook block, shackles, and slings.

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