Wire rope testing, or bridge cable inspection, is an essential process in fitness-for-serv
ice evaluation of amusement park rides, bridges, cranes, ship loaders and may other load bearing assets. This article briefly introduces the various wire rope testing methods and their respective advantages and disadvantages. The basic wire rope testing methods are visual wire rope testing, magnetic flux leakage (MFL) wire rope testing, long range ultrasonic testing (LRUT) and acoustic emission testing of steel cables.
Visual Inspection of Wire Rope
Visual wire rope is a fast and economical non-destructive testing technique that detects wire breaks on the outside diameter of the wire rope. Wire rope testing is commonly performed using a cloth rag lightly wrapped the rope that catches on protruding wires. Visual wire rope testing is performed routinely across all industries. Visual inspection cannot determine condition under collars, seizing wires, separators, sockets and gatherers since due to accessibility. In addition to wire breaks, this method can detect reductions in diameter, corrosion, birdcage, waviness, kinks and deformations.
Some disadvantages include disadvantages include that it cannot detect corrosion/breaks on the interior strands and under paint. Impossible to size subsurface defects.
Figure 1. Wire rope visual inspection for cross-sectional area changes.
Magnetic Flux Leakage (MFL) Wire Rope Testing
MFL testing of wire rope and steel cables introduces a magnetic field along the primary axis of the wire rope using magnetizing measurement head. Wire breaks cause a disruption in the magnetic field causing it to leak out from the rope. The magnetic flux leakage (MFL) is detected by a Hall sensor in the measuring head. The measuring head is generally equipped with an encoder wheel to accurately track wire break locations.
MFL wire rope testing is most practical and economical on moving ropes since a winching system is not required to pull the measuring head. Additionally, the maximum rope diameter that can be MFL tested is approximately 4”. Lastly, specialized measuring heads, at extra cost, may be required for groups of wire ropes with minimum clearance. Consider that each measuring head has certain size and must small enough to mount in the clearance area between the ropes. Example MFL wire rope testing data is shown in Figure 2. The wire rope testing data are presented as milli-voltage (mV) versus distance correlated to the encoder wheel. The measured mV on the vertical axis is proportional to the magnetic flux leakage caused by wire rope breaks. The upper data (OUT) reports on wire rope testing of the outer strands of the cable. The middle data (INN) reports on wire rope testing of the inner strands of the cable. The bottom (blue) wire rope testing data records loss of metallic area (LMA) during the wire rope inspection.
Figure 2. Magnetic flux leakage (MFL) wire rope testing data.
MFL wire rope testing is performed on ropes in the 0.50 to 4.00” diameter range. Moving rope like those used on amusement park rides, ski-lifts, elevators, and lifting devices are easily tested wit