Wire Rope is an item often found on Wire Rope Cranes. Unfortunately though these wires are not unbreakable and can/will succumb to the pressure of constant use and may potentially snap when in use. Which is why it is important to know what to look out for in an unsafe wire rope, the Government of Canada recommends a visual inspection of the wire before each use, but full inspections should be undertaken by a trained professional periodically. This article will cover what causes wire ropes to break, what your professional inspector will do in order to ensure your rope is safe and what you can look out for when completing your frequent inspection in order to ensure the rope is safe to work with.
What Can Cause a Wire Rope to Break?
When you hear the term wire rope you may picture in your mind a metal, and seemingly unbreakable rope, and though wire ropes can and will stand up better than many other rope types it is unfortunately not unbreakable. Some things that can cause a wire rope to break include:
- Wear in areas that have contact with hoist sheaves and drums
- Corrosion which is caused from lack of lubrication and exposure to heat or moisture
- After too much repeated bending even under normal operating conditions the wire may become fatigued (Internal and External)
- If you do not follow the manufacturers’ weight charts and overload the safe working limit
- Mechanical abuse because of crushing, cutting or dragging the rope
- Using the rope when it is frozen or too stiff
- Kinks caused by improper installation of a rope, sudden release of a load or knots that were made to shorten a rope can cause the rope to become compromised
- Crushing, which his caused by spooling onto multi-layer drums
Many of these causes can be minimized by the use of a proper crane design and rope maintenance procedures, most of these causes though are unavoidable and are considered to be part of a normal rope life. The two main causes that are considered unavoidable are crushing and internal and external fatigue.
Unavoidable Wire Breakage Causes
Let’s go into more detail about the two main and unavoidable wire breakage causes.
Many wire ropes are subject to a lot of repetitive bending over a sheave which causes the wire to develop cracks in its individual wires. These broken wires often develop in the sections that move over sheaves. This process will become escalated if a rope travels on and off of a grooved single layer drum, which causes this to go through a bending cycle. Tests in the past have shown that winding on a single layer drum is equal to bending over a sheave because it causes similar damage.
Fatigue breaks often develop in segments like stated before, these segments are usually the part of the rope surface that comes into direct contact with a sheave or drum. Because this is caused from external elements rubbing, often times these breakages are external and visible for the eye to see. Once broken wires start to appear, it creates a domino effect and quickly many more will appear. Square ends of wires are common for fatigue breaks. These breaks are considered a long term condition and are to be considered part of the normal to the operating process.
Internal Breaks, these breaks can develop over time based upon the loading of the hoist. Many ropes are made of a torque-balanced multi-strand design which comprises of two or more layers of strands. A torque balance is created in multi strand ropes, by layering the outside and inside ropes in opposite directions. Multi-strand ropes offer much more flexibility and have a more wear resistant profile. Though the wires in these ropes touch locally and at an angle, which causes them to be subject to both the effect of radial load, relative motion between wires and bending stresses when bent on sheaves or drums.
Which is why multi strand ropes are more prone to develop internal broken wires.
Wire Rope Cores:
Nicking and fatigue patterns such as the ones discussed before occur in Independent Wire Rope Cores or IWRC ropes. IWRC ropes have outer wires of the outer strands which have a larger diameter than the outer core strands. This helps to minimize inner strand nicking between the outer strands the IWRC. The outer strands and the IWRC strands are approximately parallel. Often these outer strands are supported by their neighbors while the outer IWRC wires are relatively unsupported.
With these geometrical features it allows for the wire to fluctuate under tensile loads, the outer IWRC wires are continuously forced into valleys in between the outer strand wires and then released. This system results in secondary bending stresses which leads to a large number of core wires with fatigue breaks. These breaks are often close together and form in groups. This eventually leads to the IWRC breaking or completely disintegrating into short pieces of wire that lay half a length long. This condition is often called complete rope core failure.
It is as the IWRC fails, and the outer strands lose their radial support then valley breaks form. Valley breaks occur when the outer strand wires bear against each other tangentially. Which results in inter strand nicking which restricts the movement of strands within the rope. Without the freedom to move, secondary fatigue breaks occur in the outer strands which will develop a stand tangent points. These breaks occur in the valleys between the outer strands hence why they are called valley breaks.
So to go over what we just learned, internal broken wires occur often in ropes that are operated with large diameter sheaves and high factors of safety. These breakages can occur when a reeving system incorporates sheaves lined with plastic or all plastic sheaves; these sheave units offer more elastic support than their steel counter parts. Which causes the pressure between outer wires and sheave grooves to be reduced to the point where the first wire breaks will occur internally.
Spooling on Multi-Layer Drums:
If a section of a rope travels on and off of a grooved multi-layer drum, then it goes through what is called a bending cycle. The bending cycle occurs by a section of rope spooling in the first layer and are bent around the smooth drum surface, but when the second layer rolls around the rope section in the first layer will be spooled over. This causes the first layer to become compressed and damaged on the upper side by the second rope layer. With continued spooling the rope layers in the second and higher layers will in turn be damaged on both sides during contact with their neighbouring rope layers. This damage is caused both by the compression of the rope and by the rope laying on a rough surface.
Accelerated wear occurs where the point of the rope is squeezed between the drum flange and the previous layer. Often times the slap of rope at the crossover points causes peening, martensitic embrittlement and/or wire plucking, further associated rope damage is caused when the rope crosses over from layer to layer on a drum.
In addition, if the lower wire rope areas where not spooled under sufficiently high tension the lower wraps can become displaced by the additional rope sections which would allow for these new rope section to slide down in between them, which will lead to severe rope damage.
How to Inspect Your Wire Rope and When You Should Retire It
Two different ideologies have been formed:
- Statutory Life Policy: this mandates that a rope be retired at a certain predetermined time
- Retirement for Clause: this means a rope is retired only when a rope has deteriorated to a certain point.
Many regulators have decided that the Statutory Life Policy to be overly wasteful and they tend to use the Retirement for Clause Policy. A wire rope deteriorates slowly over its entire service, but in order to be aware of the state of deterioration, a wire rope must be periodically inspected. Moderate deterioration is normally present, and low levels of deterioration does not justify retirement. Which is why you have wire rope inspections in order to monitor the normal process of deterioration. This ensures that the rope can be retired before it can become dangerous. In addition these inspections can detect unexpected damage or corrosion on the wire rope which will allow for you to take corrective actions in order to ensure the longevity of the wire rope.
Two Major Non-Destructive Inspection Methods:
- Visual Inspection
This system is useful for detecting external rope deterioration. In order to use this approach, the inspector will lightly grab the rope with a rag. The inspector then glides the cloth over the rope. Often times external broken wires will porcupine (stick up). When the rope moves along the wire it will be snagged on the broken wire. The inspector will then stops dragging the cloth along the wire and visually inspects the condition of the wire.
Frequently broken wires often do no porcupine, which is why a different test procedure must be utilized. This test involves moving along the rope two or three feet at a time and visually examining the rope at area. This method though can become tiresome because often times the rope is covered in grease and many internal and external defects will avoid detection through this method.
Another method involves measuring the wire ropes diameter. This involves comparing the current ropes diameter to the original ropes diameter. Changes in the ropes diameter indicates external and/or internal rope damage. Through this method is not perfect because many different wire breakages damages do not change the ropes diameter.
You can also check for several visible signs of distributed losses of metallic cross-sectional area. This is often caused by corrosion, abrasion and wear. In order to internally check for damage you can insert a marlin spike under two strands and rotate it to lift the strands and open the rope.
Visual inspections are often not well suited for the detection of internal rope damage. Which means that they have limited value as the only means of wire rope inspection. Though visual inspections do not require special machines. When completed by a knowledgeable and experienced rope examiner though visual inspections can be valuable tools for evaluating rope degeneration.
- Electromagnetic Inspections:
Electromagnetic Inspections or EM gives a detailed insight to the exact condition of a rope. EM is a very reliable inspection method and is a universally accepted method for inspecting wire ropes for mining, ski lifts and other similar industries. There are two distinct EM inspection methods which have been developed in order to classify defects called Localized-Flaw (LF Inspection) and Loss-of-Metallic-Area Inspection (LMA Inspection type)
LF Inspection is similar to the rag-and-visual method. This inspection method is suited primarily for finding localized flaws, such as broken wires. Which is why, small hand-held LF instruments are called electronic rags.
LMA Inspections detects and measures changes of the metallic cross-sectional area which is caused by wear and corrosion. This method is much more reliable than visual diameter checks, because it can replace diameter measurements made with a caliper.
Electromagnetic and visual wire rope inspection methods are like peanut butter and jelly or cookies and milk they are the perfect combination, and both are essential for safe rope operation. Which is why both methods are often used in order to ensure maximum safety.
Thorough inspections must consider all aspects of a rope’s condition including:
- The findings from a visual inspection
- The results of an EM rope inspection
- The rope’s operating conditions and any related damage mechanisms
- The history of a rope which is put under test and the history of its predecessors
A program that involves periodic inspections is extremely effective. In order to establish baseline data for future inspections, a wire rope inspection program should begin with an initial inspection after a break-in-period. Then the inspections should follow within scheduled intervals, with documentation of the ropes deterioration over its entire service life.
Inspecting Ropes That Develop Internal Broken Wires:
For multi strand ropes often times visual inspections are ineffective which is why statutory life policy for a ropes retirement is often adopted. This means that these ropes are often discarded long before they should be, meaning millions of dollars’ worth of perfectly good wire ropes is being thrown away annually.
Some people have suggested that non-rotating ropes should not be used if cranes use single layer winding on a drum. Following this line of thought this would mean multi-strand ropes should be used only when winding on multi-layer drums. Which would cause wires to break on the surface faster than internal wire damage can occur, these non-rotating wire ropes will be replaced long before internal fatigue can set in.
When internal broken wires are the problem electromagnetic rope testing can be the solution. Though there are some factors one needs to take into account such as: certain regulations require rope retirement when a certain number of broken wires per unit of rope length exceed a set limit. This discard number that is specified in retirement standards refers solely to external wire breaks. Which means the condition of a wire rope with internal breaks is therefore left up to the inspector.
Though you also need to take into account detailed detection and quantitative characterization of internal broken wires in ropes with many breaks and cluster breaks could be a problem. These difficulties are caused by the fact that electromagnetic wire rope can be influenced by a number of parameters such as:
- Broken wire cross-sectional area
- Broken wire gap wide
- The position of the broken wire within the cross-section of the rope
- Clusters of broken wires can cause an additional problem because the relative position of broken wires with respect to each other within the rope is not known
- Broken wires with zero or tight gap widths are not detectable by electromagnetic inspection because they do not have a sufficient magnetic leakage flux.
When you consider all of this you can quickly realize that you can only estimate the number of broken wires that have formed on a wire rope. You can use the LF trace for the detection of broken wires, though unfortunately it is not quantitative so it cannot be used to estimate the number of broken wires. Though it is good to note that if many internal broken wires are present a LMA trace will show rapid relatively small variations of cross-section.
Inspection of Ropes That Spool on Multi-Layer Drums:
As we discussed before, wind-on-drum damage usually occurs externally and can be detected by visual inspections. Though despite their simplicity, visual inspections are not easy and require a well-qualified and experienced inspector.
An electromagnetic inspection will help to enhance the accuracy and reliability of the inspection, by combining visual and EM methods they will be able to detect deterioration at the earliest stages. The inspections can be then used as an effective preventive maintenance tool. For example, the inspector early on detects corrosion, and you immediately apply the corrective action of improving the lubrication of the wire rope.
Wire ropes should be inspected by a certified inspector when installing it, and periodically throughout its life cycle. A wire rope should go through a quick, but thorough inspection every day that you use it at the beginning and end of each shift and you should keep records of all inspections. Ensure that your certified wire rope inspector uses a combination of visual inspection methods and electromagnetic inspection methods because this will ensure the optimum safety and longevity of the rope. This is especially true for ropes that are more likely to develop internal broken wires, and inspections completed by a certified inspector is the best way of having a preventive maintenance program and extending the life of your wire rope.
Does your company use a wire rope?
What do you do in order to ensure it is safe to use?
Comment bellow and share!
Thank you to the following books and websites for providing the information required in order to write this blog post:
OSH Answers Fact Sheets. Retrieved February 10, 2016, from https://www.ccohs.ca/oshanswers/safety_haz/materials_handling/hoist.html
Queensland Division of Workplace Health and Safety, “Non-rotating hoist wire ropes, multi fall configurations, Health and Safety Alert,” http://www.whs.qld.gov.au/alerts/97-i-5.pdf
Verreet, R. “Wire rope damage due to bending fatigue and drum crushing,” O.I.P.E.E.C.(International Organization for the Study of the Endurance of Wire Rope) Bulletin 85, June 2003, Reading (UK), ODN 0738, pp. 27-46.
Wire Rope Users Manual, Third Edition. Woodstock, MD: Wire Rope Technical Board (1993).