LMA for 30% Crown Wire Wear 



The Magnograph equipment measures total LMA that results from wear 

 and corrosion. Navy inspection criteria specify that a wire rope with 

 30% or more reduction in the diameter of the crown wires must be removed 

 from service. The question posed is: what is the approximate LMA for 

 30% crown wire wear? 



The crown wires of three different wire rope samples (1/2- , 1-1/8- , 

 and 2-inch-diameter 6x25 right-regular-lay, fiber core) were abraded on 

 a belt sander to simulate various degrees of wear. Physical samples had 

 to be used because, for different degrees of wear, the number of crown 

 wires showing wear at any given cross section varied. For example, for 

 5% crown wire diameter reduction, only one crown wire per strand showed 

 wear; but for 30% crown wire diameter reduction, three crown wires 

 showed wear. Using these data, calculations were made of LMA and are 

 shown in Figure 20. It can be seen that for 30% crown wire diameter 

 reduction, the LMA percentages were about 5.5, 4.0, and 2.0 for wire 

 ropes of 1/2- , 1-1/8- , and 2-inch diameter, respectively. 



To an inspector, significant wear is easily detected. The appear- 

 ance of a wire rope with 30% crown wire diameter reduction is visibly 

 startling in its impact because of the "used" or "poor" condition of the 

 rope. Figure 21 is an example of a wire rope with 12% crown wire reduction 

 and one broken wire. It is doubtful that many wire ropes stay in service 

 until crown wire wear is 30%, solely because of the poor appearance of 

 the rope. 



Canadian mining practices require nondestructive evaluation of wire 

 ropes and a 10% LMA is cause for removal of a wire rope from service. 

 To reach 10% LMA, it is apparent that the effect of corrosion has to be 

 more significant than that of wear. Assuming a rope of 1-1/8-inch- 

 diameter, one can determine that reasonable wear can be about 2% LMA; 

 therefore, corrosion would make up the other 8%. Corrosion is more 

 insidious than wear because it can be hidden from view, and corrosion 

 pitting causes stress risers in the wires. Wear is an easy condition to 

 inspect for, but corrosion is a more significant cause for wire rope 

 failures. The Magnograph equipment is an important tool in determining 

 the degree of corrosion in wire ropes. 



FINDINGS 



1. The air gap between ends of a broken wire, as measured in the longi- 

 tudinal direction of the wire rope, is the predominant factor governing 

 LF signal size. 



2. The orientation of a broken wire with respect to the four Hall- 

 effect devices in the sensor head also influences the LF signal size. 



3. Broken wires having a gap of less than 1/32 inch can be detected in 

 new wire ropes, but with used rope the background noise is greater and 

 can obscure the LF signal. Hence, not all broken wires can be detected 

 by the Magnograph equipment. 



4. LF signal size is very slightly influenced by rope speed (Figure 18) 



