| 摘要 |
A lattice boom crane is provided with multiple boom sections which telescope one from the other to provide a variable length boom. A cable system is connected between the boom sections such that the boom sections are telescoped as the cable is drawn in by a winch mechanism. Cables drawn in by the winch mechanism is directed to a block system which is connected between a gantry structure and the point end of the outermost boom section. The winch mechanism includes two drums about which the cable is alternately wrapped. The block system expands at substantially the same rate as the boom sections extend. Thus, the boom angle, which is determined by the effective cable length between the gantry and the point of the boom, is not significantly effected. In one embodiment of the invention, the cable taken in by the block system is received on a boom hoisting drum which may be controlled to change the effective cable length between the gantry and the point of the boom to vary the boom angle. In a separate embodiment, a separate cable system connected in series with the block system is used to vary the effective length of the cable between the gantry and the point of the boom to vary the boom angle.
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