| 摘要 |
A slewing unit mounted to the upper end of a tower rotates a combination jib and counterjib which extends in a horizontal direction about a vertical axis. A first electrical drive unit powers the slewing unit to rotate the jib to a selected angular position relative to the tower. A trolley is movable longitudinally along the jib. A second electrical drive unit moves the trolley to a selected longitudinal position along the jib. A third electrical drive unit mounted on the trolley extends and retracts a rigid elongate load lifting boom in a vertical direction. A load handler, such as a hook, clam-shell bucket or load lifting platform, is connected to the lower end of the boom.
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