Crane safety devices: the height limit device. Hook rose from a predetermined height, the weight limit Buying touch heavy cranes, open limit switch, 'had about' indicator lights that light heavy crane company, and cut off the hook lifting, arm extended, the boom volts to the operation of such action to ensure safety. Then just manipulate the hook down boom boom retracted or lifted up (ie safe side operation), etc. When the handle so that the limit weight lifting constraints, normal operation is restored. Crane Development origin: history. Early 20th century, Europe began to use the tower crane. Crane hoisting mechanism mainly run institutions, luffing mechanism, slewing mechanism and metal structure. Hoisting mechanism is the basic working body of the crane, mostly by hanging system and winch composition, but also lift weights through the hydraulic system.