The jacking mechanism is for the insertion of further tower sections, particularly in a tower crane, having a cage of larger cross-section than the tower. A support (16) on the cage (4) moves the upper tower section (2) upwards in the cage so as to leave sufficient room for insertion of a further section (7) above the lower section (1). This support works in conjunction with a lifting devices (13) on the upper section. A guide (8) is mounted between the vertical members of the upper section and those (9) of the cage, and sliding always takes place between these members. Detachable connections are provided between the bottom of the cage and the lower section, and there are other connections between the sections of the tower themselves.
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