Effect of debris trapped in wire rope on the results of its magnetic inspections
It is a common belief that products of rope wear, and in particular of its corrosion, present in wire ropes as trapped debris do influence results of magnetic inspection of ropes. Debris is believed to decrease the measured value of LMA (Lass of Magnetic Area) and may, as a result, lead to over estimation of the actual rope strength (Kawecki & Hansel1994).
The relation between rope debris and the results of magnetic inspections of ropes was investigated by Geiler & Rousseau (1990). Theoretical analysis led these researchers to believe that magnetisation of corrosion products cannot exceed 8.5% of that in comparable steel specimens. Consequently, the effect of rope debris on the magnetic instrument LMA indications may be, at most, 1/10th that of the solid iron. They followed with an experiment in which a length of rope fibre core was replaced by a plastic tube filled in with debris and the rope was tested for LMA with several magnetic rope testers. The experiment indicated that "the corrosion product signals could hardly be detected, if at all, on either the LMA or the LF (Local Fault) trace". The researchers concluded that rope debris is weakly ferromagnetic and while trapped in a wire rope it does not result in a significant error of the measured value of LMA.
However, Geiler and Rousseau placed the debris in the rope center replacing its fibre core. ln this location the debris may not have been brought to full saturation. The strength of instrument magnets may not have been sufficient to magnetize the rope (Hamelin et al. 1993) while at the same time debris requires very strength magnetic field to become saturated (Geiler & Rousseau 1990). It follows that debris trapped on rope surface, or close to it, may have a significant impact on the LMA readings.
Author(s): T.S. Golosinski