changes in the signal size. Figure 4 shows that the signal size changed 

 from a strong signal to a weak signal. The signal size for breaks A and 

 B differ because of their orientation. The amplitude of the spike in 

 the signal trace is related to the loudness of the click. The higher the 

 amplitude of the spike the louder the click. 



Gap Space 



A series of tests were conducted on new wire rope in order to 

 determine broken wire signal size, as a function of longitudinal air-gap 

 space. 



These tests were conducted on 1/2- and 3/4-inch diameter wire 

 ropes. A rope speed of about 200 fpm was maintained for all the tests. 

 Results showed that gaps of 1/ 128th inch or less do not always produce 

 clicks. Gap spaces of greater than 1/128 have a detection rate that is 

 much higher. Test data are shown in Figure 5. 



Results for gap spaces greater than 1/4 inch are not given because 

 the signal size did not increase after the 1/4-inch gap space was 

 reached. It must be realized that the only signal output from the MT 

 75 Rope Tester, as provided by the manufacturer, is clicks and crackles 

 in the headphones. The signal size in Figure 6 represents the loudness 

 of the click, although some clicks in the headphones did not produce a 

 signal on the brush-chart recorder. 



New wire rope is usually clear of background noise. However, it 

 tends to increase when surface corrosion, internal and external wear, 

 peening, etc. begin to develop. As wire ropes approach the end of their 

 useful life this background noise tends to mask the broken wire signals, 

 making detection much harder. If the background noise becomes great it 

 can completely mask the broken wire signal. However, based on field 

 test results using several NDT devices and data supplied by the manufac- 

 turers, background noise is a good indication of the overall condition of 

 the wire rope. When it becomes loud (continuous "thundering") enough to 

 mask the broken wire signals, the wire rope has degraded to a point 

 where it should be considered for replacement. 



Rope Speed 



The signal generated by the search coils, when the broken wires 

 create a magnetic flux leakage field, is normally proportional to rope 

 speed in DC devices. However, the MT 75 rope tester has a preamplifier 

 and speed compensation circuit, which amplifies the search coil signal 

 and equalizes its strength allowing the wire rope to move as slow as 

 5 fpm or as fast as 500 fpm with no significant change in signal 

 strength (Figure 7). 



Wire rope speeds greater than 200 fpm, while allowing good quality 

 signal pickup, are hazardous because the hand-held unit can be pulled 

 from the inspector's hands, or the inspector could be pulled with the 

 rope if it snags the MT 75. The MT 75 can be removed from the moving 

 wire rope by pulling perpendicular to the axis of the wire rope. If the 

 MT 75 is moved to an angle which is not perpendicular from the wire 

 rope, it is difficult to pull free. A lanyard attachment from the 

 equipment to the MT 75 is recommended for the inspector's safety if rope 

 speeds exceed 200 fpm. 



