The platform has a first turntable with its vertical axis (20) approximately concentric on the platform (12) and supports a horizontal beam (24) which pivots around the vertical axis. The horizontal beam supports an axially offset second turntable (26) with an upper carrying chassis. For double motion of the horizontal beam the second turntable is connected via a gear acting between the boom (38) and beam. The gear may be formed via two intermeshing gears between both rotation axes (20,28). The first axis of the first turntable may be driven whilst the axis (28) of the second turntable may be firmly connected to the rotary tower (32).
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