The influence of rope tension on the LMA sensor output of magnetic NDT equipment for wire ropes

Abstract

There are many different wire rope constructions in service which are used in a diverse range of applications. Ropes will be discarded, not only because of faults (typically broken wires) and/or owing to other damage such as wear, corrosion and structural deformations. Almost every rope must be and is inspected in service. As a support to visual inspection, magnetic testing is accepted on a world wide basis, and is probably the most effective way for locating degradation. However, debate is ongoing about the quantitative interpretation of the measurements so obtained. The authors have previously published work assessing the reliability of a range of sets of NDT equipment on different ropes and forms of rope degradation. This paper is based on further research work which was sponsored by the British Council and the Polish State Committee for Scientific Research (KBN). It focuses on the problems associated with the accuracy of non-destructive testing of wire ropes by equipment which incorporates a loss of metallic area (LMA) sensor. (The LMA sensor is probably the most popular for quantifying the level of metallic area loss). The new issue discussed in this paper is the practical study of the effect of tension in the wire rope under test on the LMA signal output. The analysis considers the influence of wire rope tension on: equipment calibration; the position of the null base line of the Hall effect (LMA) sensor; and, readings from the LMA trace. The MD120 was used during these tests. The results provide practical information for the user about the accuracy of NDT inspection using NDT equipment with Hall effect LMA sensors.