4.5 General Considerations for Testing 



It is best to start a test on a section of rope which is 

 known to be in good condition. This will give the best 

 calibration for nominal rope weight on the IMA trace and 

 will allow the LF trace to be calibrated for an area 

 where the LF signal will be relatively clean. 



4.5.1 LF Signal Generation 



The local fault (LF) reading is affected by many things . 

 Magnetic dipoles which the Hall sensors will detect are 

 formed by broken wires, nicks, corrosion and wear. 

 Broken wire dipoles and how they show on the chart record 

 is a function of how large the wire is and how large the 

 separation is between the ends of the break. In general 

 the magnitude of a broken wire signal will increase as 

 the separation between the ends increases to the point 

 where the zero crossing becomes a plateau as shown in 

 Fig. 3.6. Broken wires with the ends 'butted' together 

 will not show on the chart record as no dipole is pro- 

 duced. A large broken wire with the same end separation 

 will show a larger signal than a small wire. 



Wire rope, when new, will show a low level local fault 

 signal associated with the lay of the rope. The ampli- 

 tude of this signal is a function of the size and con- 

 struction of the rope. The signal is higher for larger 

 ..^ ropes of the same construction and is higher for stranded 

 ropes than for locked coil ropes . 



The lay noise and a broken wire signal are shown in Fig. 

 4.9 on the left. On the right of Fig. 4.9 is the chart 

 recording of the same rope in an area where the wear is 

 high and corrosion is present. The LF Gain is the same for 

 both sections of the graph. 



As can be seen the LF 'background noise ' increases dram- 

 atically with wear and corrosion on the rope and in this 

 case is assuming the magnitude o-f the broken wire signal 

 on the left part of the chart. As rope wear increases 

 the broken wire signal may become lost in this 'background 

 noise ' . 



The increase in the 'envelope' of the LF trace is an 

 important qualitative indication of rope condition. The 

 increase with time in the signal's 'envelope' is an im- 

 portant piece of rope history to keep track of as its 

 growth will accelerate as the rope nears the end of its 

 life. A-30 



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