To prevent a jib elevating rope from being overwound at the elevation of a tower boom.
An overwinding cutout device for tower crane is formed so that a pressure fed from a hydraulic pump 34 to a hydraulic pump 31 through a primary main pipe 32A and a secondary main pipe 33B by setting the relief pressure setting of a variable relief valve 38 installed in mid-course of the primary main pipe 32A to a low pressure (a pressure to a degree that the looseness of the jib elevating rope is prevented) when a limit switch 30 is opened by the connection of a tower boom to a tower jib so as to limit the torque of a jib elevating drum 5 to a degree that the looseness of the jib elevating rope is prevented. Thus, even if the operating lever for the jib elevating drum 5 is operated excessively at the time of elevation of the tower boom, it can be prevented securely that the jib elevating drum 5 winds up the jib elevating rope excessively.
MADARAME KOICHI
SUZUKI TAKAYUKI
