Global Inspections - NDT Inc.
Wire Rope Deterioration
WEAK MAGNETIC TECHNOLOGY
Steel wire ropes are employed to support conveyances that transport people and material. The steel wire rope is the lifeline of these conveyances. Therefore, the structural integrity of the steel wire rope is of paramount importance because if it breaks, the consequences could be very serious. All steel wire ropes deteriorate over time. Three of the causes of the deterioration are abrasion, corrosion and broken wires.
CAUSES OF WIRE ROPE DETERIORATION
• Abrasion: Can either be external, and it would be caused by rubbing, or internal, and it would be caused by inter-strand nicking. (Inter-strand nicking is caused when two adjacent strands rub against each other while the rope is in motion.)
• Corrosion: is caused by the environment in which the rope is located.
• Broken wires: are caused by metal fatigue, mechanical damage, or abuse. The rate of deterioration of a steel wire rope changes over time. Initially, the rate is gradual. However, as the rope approaches the end of its safe working life, the rate increases. Because the rate of deterioration increases over time, it is important that the rope be inspected on a regular basis so that rate can be continually monitored and the rope can be replaced before it breaks. The two most common methods of inspecting steel wire ropes for deterioration are visual inspections, which would include diameter measurements, and non-destructive testing using a dual-function, electro-magnetic, steel wire rope testing instrument (an "EM Instrument"). A visual inspection allows an operator to identify the external deterioration of a steel wire rope. Depending on the circumstances, a visual inspection should be able to identify deterioration in the form of abrasion, corrosion and broken wires. The information that is obtained from a visual inspection can then be used to determine if there are any abnormalities in the rope, abuse or preventable deterioration. Corrective measures can then be instituted through a preventative maintenance program and in this way, the rate of deterioration of the rope may be able to be reduced and thus the life of the rope prolonged.
PROBLEMS WITH VISUAL INSPECTION ONLY
• Abrasion can either be external, and it would be caused by rubbing, or internal, and it would be caused by inter-strand nicking. Inter-strand nicking is caused when two adjacent strands rub against each other while the rope is in motion.
• Internal deterioration may not be identified.
• The results obtained are not objective.
• The results cannot be quantified.
BENEFITS OF AN EM INSTRUMENT
An EM Instrument can provide accurate, objective and quantifiable data on the deterioration, both internal and external, of a steel wire rope. EM Instruments non-destructively test ferromagnetic steel wire ropes for deterioration caused by corrosion, wear, broken wires, etc.
These instruments can test for deterioration that is located both on the outside layer of a rope and on the inside layers of a rope. They can also test for this deterioration in all types of steel wire ropes, including locked coil ropes and plastic coated ropes. However, they can only test ropes that are ferromagnetic.
EM Instruments simultaneously indicate Local Faults (localized deterioration) in the form of broken wires, lay distortion, inter-strand nicking, external wear, or corrosion pitting and measure Loss of Metallic cross-sectional Area caused by corrosion, broken wires, external wear, and inter-strand nicking.
The measured Loss of Metallic cross-sectional Area (LMA), combined with the extent and nature of the pattern of the Local Fault (LF) Channel trace, can be used to assist with the determination of when a steel wire rope has reached the end of its safe working life and should be removed.
The measured LMA, combined with the extent and nature of the pattern of the LF Channel trace, can also be used to estimate the Loss of Breaking Strength (LBS) of a steel wire rope. EM Instruments are portable instruments so that steel wire ropes are tested in situ. EM Instruments consist of 2 components: a Console, which provides a means to display the test results, and a Test Head, through which the rope travels. (The Console and the Test Head are connected by an Inter-connecting Cable.) The Test Head is similar to an elongated donut ring that opens. This allows the Test Head to be placed around the rope that is to be tested. In order to test the rope, either the rope must be pulled through the Test Head or the Test Head must be pulled along the rope.
GOVERNMENT REGULATIONS DICTATE REMOVAL CRITERIA OF A ROPE DEPENDS ON A NUMBER OF FACTORS
GOVERNMENT REGULATIONS
In many jurisdictions, government regulations require that steel wire ropes that are utilized in underground mining operations and aerial transportation systems, for example ski lifts and aerial tramways, be regularly non-destructively tested using an EM Instrument. For steel wire ropes that that are utilized in underground mining operations.
GOVERNMENT REGULATIONS DICTATE REMOVAL CRITERIA OF A ROPE DEPENDS ON A NUMBER OF FACTORS
If the rope travels over a sheave wheel and is carrying its own weight plus the weight of the conveyance for personnel and/or materials, the following is the removal criteria: a) if the estimated LBS reaches 10% OR b) if the number of broken wires in 1 lay length is equal to 5% of the total number of wires, excluding filler wires, used in the construction of the rope.
If the rope is a static rope that does not travel over a sheave wheel and is essentially carrying its own weight, i.e. a guide rope, the following is the removal criteria: a) if the estimated LBS reaches 25% OR b) if the number of broken wires in 1 lay length is equal to 5% of the total number of wires, excluding filler wires, used in the construction of the rope.
For aerial transportation systems, the government regulations provide that the removal criteria be based upon the following: a) the reduction in the metallic cross-sectional area (the LMA) OR b) the number and distribution of broken wires (LF). The service life of a steel wire rope is also determined by the rate of progression of the deterioration in the rope. In order to make this determination, there must be 2 or more comparative tests completed over a period of time, which period of time should not be too long.
However, the visual inspection is also a very important, complementary tool to the results that are obtained from the EM Instrument. It is important that those sections of the steel wire rope where the EM Instrument indicates there is significant deterioration be visually inspected. Only by combining a visual inspection with the results obtained from an EM Instrument, can an accurate evaluation of the rope be made to determine whether or not the steel wire rope should be replaced.
SUMMARY
It is important that a steel wire rope be regularly non-destructively tested using an EM Instrument and that the information that is obtained be augmented by a visual inspection of those sections of the steel wire rope where the EM Instrument indicates there is deterioration that is significant. The information that is then obtained from both of these inspections can be used to accurately determine the complete condition of the steel wire rope and thus track its rate of deterioration so that a valid assessment of the steel wire rope can be made regarding any preventative measures that should be taken to slow down the rate of deterioration or whether it should be replaced because it has reached the end of its safe working life.
Source: http://www.global-ndt.ca/wirerope-inspections/wirerope-deterioration.html
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