PURPOSE: To prevent beforehand the occurrence of a great accident by rupture of a rope by setting safe and economical replacing time by judging the service life of a jib elevation/depression rope automatically. ;CONSTITUTION: A jib elevation/depression drum rotating position detector and a jib elevation/depression rope tensile force detector are arranged, and signals of the respective detectors are inputted to a control part. A jib elevation/ depression drum rotating position and a sheave passing frequency when a virtual point set on a jib elevation/depression rope passes through a sheave, are stored beforehand in a memory. When a jib is risen up and fallen down, the control part reads in jib elevation/depression rope tensile force and the jib drum rotating position, and finds a service life conversion factor to a legal safety factor, and corrects a sheave passing frequency stored in the memory by the service life conversion factor, and judges that the jib elevation/depression rope completes the service life when a maximum value of the sheave passing frequency reaches a frequency of standard bending fatigue.;COPYRIGHT: (C)1995,JPO
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