boom hoist lines are reeved. The height of the gantryprovides an angle between the boom pendant lines andboom that reduces the compression forces placed on theboom during raising and lifting operations. On somemodels of cranes, the gantry is adjustable, allowing it tobe lowered so the crane can travelbridges.WARNINGRefer to the operator’sunder wires andmanual forinstructions on how to raise and lower thegantry. A trial-and-error method of lowering orraising the gantry can cause serious injury ordeath.NOTE: Raising the boom while the gantry is inthe lowered position lowers the angle between thependants lines and boom. This places unseencompression stresses on the boom; therefore, alwaysraise the gantry before raising the boom or lifting aload.Boom MastSome models of cranes are equipped with a boommast instead of a gantry. The boom mast, sometimescalled live mast, consists of a structural frame hingedat or near the bottom of the boom butt (fig. 12-19).Figure 12-19.—Boom mast.12-10The tip of the boom mast supports the boom hoistsheaves and boom pendant lines. The boom mast workslike the gantry, as it increases the angle between theboom pendants and boom, decreasing the compressionforces placed on the boom.Bridle AssemblyThe bridle assembly is part of the boomsuspension system and is sometimes called a floatingharness. The bridle assembly may be connected tothe boom mast or as a floating harness on a craneequipped with a gantry. The bridle assembly is theconnection point for the boom pendant lines and is anassembly of sheaves in which the boom hoist wirerope reeves through.Boom StopsBoom stops are designed to prevent the boomfrom going over backwards in case a load line breaks.They will not stop the boom if the operator forgets todisengage the boom hoist control lever. However,some models of cranes are equipped with a boomupper limit switch that prevents the operator fromraising the boom past a preset boom angle. Thisswitch also prevents operators from raising the boominto the boom stops. Most cranes that are equippedwith the upper limit switch also have a bypass switchthat allows the operator to raise the boom past thepreset boom angle.Two types of boom stops areshown in figures 12-18 and 12-19.House AssemblyThe house assembly is a revolving superstructurethat sets on top of the carrier frame (fig. 12-20). Itprovides a mount for the hoist mechanisms and engineand is sometimes called the machinery deck. Theoperator’s cab and counterweight are attached to thehome assembly.OPERATOR’S CAB.— The control levers for alattice boom crane are located in the operator’s cab. Thecontrol levers that are shown in figure 12-21 are typicalof most cranes.Typical crane controls areas follows:1. The swing lever, when pulled towards you,rotates the house assembly in one direction, and whenpushed, the house assembly rotates in the oppositedirection.
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