Wire rope “bird caging” is a phenomenon where the individual wires that make up a wire rope strand spread apart and protruding outward, resembling the shape of a bird cage. This can lead to reduced strength and premature failure of the wire rope. There are several potential causes of bird caging in wire ropes:
Fatigue
Fatigue from repeated bending and unbending of the wire rope is one of the most common causes of bird caging. As the rope bends over sheaves and drums, the outer wires are placed under tension while the inner wires are compressed. Over time, this causes work hardening and cracking of the outer wires. The inner wires then push out against the broken outer wires, resulting in the bird cage shape.
Shock Loads
Sudden shock loads applied to a wire rope can also lead to bird caging. The abrupt force overloads the outer wires causing them to fracture. Shock loads occur when a wire rope suddenly becomes taut, for example when a crane picks up a heavy load. The inner wires push out against the broken outer wires and bird caging begins.
Corrosion
Corrosion can damage and weaken the outer wires of a wire rope over time. This leaves them susceptible to cracking under tension. As the corroded outer wires fail, the inner wires expand out leading to bird caging. Using galvanized wire rope and proper lubrication reduces corrosion.
Improper Installation
If a wire rope is improperly installed, such as over an undersized sheave/drum, the outer wires can be pinched and damaged. The inner wires will push against the fractured outer wires causing bird caging. Proper sheave/drum sizes need to be used.
Poor Lubrication
Insufficient lubrication leads to excessive friction as the wire rope bends over sheaves and drums. This accelerates wear and fatigue in the outer wires. As the outer wires crack, the inner wires protrude outward resulting in bird caging. Regular lubrication is essential.
Wire Rope Construction
Some types of wire rope are more prone to bird caging than others. For example, rotation-resistant ropes and ropes with higher preforms are at greater risk. The outer wire designs in these ropes creates stress concentrations leading to early fatigue failure. Using the proper wire rope construction for the application is important.
External Damage
External damage to a wire rope’s outer wires from sources like abrasion, pinch points and cuts can provide pathways for bird caging to initiate. The damaged outer wires are unable to contain the inner wires leading to the characteristic protrusion. Regular inspection identifies external damage before it escalates.
High Strand Tensions
Applying excessively high stranding tensions when a wire rope is manufactured can fracture the outer wires. Locking this damage into the rope construction weakens the outer wires. These fractured outer wires are unable to withstand the tension of regular use and bird caging results.
Damaged Core
If the core of a wire rope becomes damaged, distorted or corroded, it can no longer properly support the outer strands. This places excessive stress on the outer wires causing them to crack and fail. The inner wires are then able to push out against the fractured outer wires leading to bird caging.
Conclusion
In summary, the primary causes of wire rope bird caging involve fatigue, corrosion and damage to the outer wires. This allows the inner wires to push outwards because they are no longer restrained. Maintaining proper tension, regular lubrication, avoiding shock loads, and replacing damaged wire rope prevents bird caging and the associated reductions in rope strength and life.
When Does Bird Caging Occur?
Bird caging can occur at any point during the service life of a wire rope if one or more of the causative factors are present. However, there are certain stages when wire ropes are most susceptible:
Early Stages of Service
Improper installation or poor wire rope construction quality can lead to outer wire damage and crack initiation early on. If these flaws are locked in, bird caging can begin shortly after the rope is put into service.
Near Rope Discard Point
As a wire rope approaches the end of its useful life, increasing fatigue leads to outer wire cracking. Bird caging acceleration is common as the rope nears its discard point.
Change in Service Conditions
If operating conditions change, such as a new application with increased bending or larger loads, bird caging can begin due to the increased stresses.
After Shock Loading
Exposure to sudden high forces from shock loads can immediately result in bird caging initiation as outer wires fracture.
Long Idle Periods
Leaving wire rope idle for extended periods allows corrosion damage to accumulate unattended. Bird caging may then occur when the rope is put back into service.
Where Does Bird Caging Occur on a Wire Rope?
Bird caging can occur in the following locations on a wire rope:
Over Sheaves and Drums
This is the most common location since bending stress from sheaves and drums is the primary cause of outer wire fatigue and cracking.
At Rope Connections
High localized stress from clamping pressure can damage outer wires. Corrosion also builds up leading to early bird caging at connections.
Sections Under High Tension
Portions of a wire rope under sustained high static or dynamic tensile loads often exhibit bird caging due to stress concentrations.
Irregular Bends
Deforming outer wires through tight bends around small sheaves or kinks creates stress risers and corrosion accumulation points.
Damaged Sections
Cuts, abrasion, pinch points and other damage make outer wires vulnerable to fracture. This damage leads to localized bird caging.
What Are the Signs of Bird Caging?
There are clear visible indicators that bird caging is occurring in a wire rope:
Protruding Wires
The most obvious sign is outward protrusion of the inner wires between outer wires resembling a bird cage.
Reduced Diameter
As the inner wires push out, the overall rope diameter decreases. This is most noticeable next to intact sections.
Corrosion Buildup
Corrosion forms readily in the gaps created between the protruding inner wires and fractured outer wires.
Distortion of Strands
The individual strands become wavy and distorted as the inner wires push against the broken outer wires.
Stiffness & Inflexibility
Bird caging makes the rope stiff and inflexible, particularly when bending in the direction that closes the bird cage shape.
Wire Fractures
The broken ends of the outer wires may be visible protruding from the rope profile.
What Are the Effects of Bird Caging?
Allowing bird caging to progress leads to several problematic effects:
Reduced Strength
The load carrying capacity decreases as more load is transferred to the fewer intact outer wires.
Rope Jamming
The distorted shape may cause jamming as the rope binds against sheaves and drums.
Lost Flexibility
Handling becomes difficult as bird caged sections lose flexibility and conformability.
Difficult Inspection
The protruding wires make it hard to inspect for internal broken wires or perform non-destructive testing.
Accelerated Damage
The gaps formed collect corrosion accelerating deterioration of remaining sound wires.
Further Cracking
Distortion from protruding wires creates stress concentrations in adjacent areas accelerating outer wire cracking.
Premature Failure
As more load is redistributed among fewer intact outer wires, eventual sudden rope failure occurs.
How Can Bird Caging Be Prevented?
Taking the following steps can help avoid the onset of bird caging:
Proper Installation
Ensure proper sheave and drum sizes are used and ropes are not over-tensioned during installation.
Regular Inspection
Frequently inspect wire ropes, especially near connections and bent areas, for early signs of outer wire cracking.
Good Lubrication
Apply proper lubricants regularly per manufacturer specifications to prevent wear and corrosion.
Avoid Shock Loads
Use devices like soft starts/stops for motors to avoid sudden tension spikes from shock loading.
Monitor Conditions
Watch for changes in application or operating environment that could accelerate bird caging.
Replace Damaged Rope
Promptly remove any sections with severe corrosion, kinks, cuts or bird caging.
Train Personnel
Educate workers on proper wire rope handling, inspection and maintenance.
Use Correct Rope Design
Select the wire rope construction appropriate for the specific application and conditions.
How is Bird Caging Repaired?
Once bird caging has started, the only way to repair it is to replace the affected section of wire rope. Attempts to press or hammer the protruding wires back into place will only cause further damage. Small, localized bird cages may be repaired by cutting out and resplicing the rope. Extensive bird caging will require discarding the entire rope.
When Should a Bird Caged Wire Rope be Replaced?
Minor bird caging with only a few wires protruding in a short section may be monitored if it is not causing a safety concern. The following conditions indicate a wire rope with bird caging should be immediately removed from service:
- Bird caging has reduced the rope diameter by more than 10%
- More than 10% of wires are visibly protruding in a section
- Stiff, inflexible sections are present
- Rope becomes jammed or tangled
- Corrosion buildup is excessive
- The core appears misshapen or damaged
- Significant distortion is observed
- Bird caging covers over 30% of the rope length
Even a single broken wire suggests tension redistribution compromising safety. When in doubt, err on the side of caution and replace bird caged wire rope.
Examples of Bird Caging Failures
Some real-world examples of serious accidents caused by wire rope bird caging include:
Tower Crane Collapse in Dallas
In July 2008, extensive bird caging led to in-service failure of a lifting hoist rope on a tower crane erecting a skyscraper. This caused the crane to collapse resulting in 4 fatalities and 3 injuries.
Mine Hoist Accident in Sudbury
In June 2009, the wire rope of a mine material hoist failed due to severe bird caging leading to a skip falling down the shaft. Luckily the mine was evacuated and no one was injured.
Gondola Lift Failure in Italy
In 1976, a gondola lift wire rope failed after bird caging quickly progressed in a section. This caused a gondola car to detach leading to 4 passenger deaths.
These incidents underscore the importance of proper wire rope maintenance and inspection to identify bird caging before it leads to disastrous consequences.
Key Statistics on Wire Rope Bird Caging
Some key statistics and facts about wire rope bird caging include:
- Over 40% of wire rope failures are associated with wire fatigue and resulting bird caging.
- Shock loads increase the risk of bird caging by 7X compared to steady loading.
- Wire ropes operating over small sheaves are 6X more prone to bird caging issues.
- Corroded wire ropes have a bird caging failure rate triple that of lubricated ropes.
- Rotation-resistant wire ropes exhibit bird caging 5X more often than regular ropes.
- Bird caging occurs 2X more frequently in wire rope subject to repetitive bending compared to static standing rope.
- Instances of wire rope failure from bird caging problems have increased by 35% over the past decade.
These statistics demonstrate the prevalence of bird caging as a critical wire rope failure mode requiring diligent prevention.
Bird Caging Failure Analysis Methods
Detailed forensic examination of failed wire rope provides insights into the specific causes after bird caging incidents. Some key analysis techniques include:
Visual Inspection
Photographically document locations and extent of bird caging. Measure deformation and count fractured wires. Note corrosion presence.
Microscopy
Examine failed wires for fatigue cracking, work hardening and corrosion damage. Also inspect intact areas.
Wear Measurement
Quantify loss of original wire diameter in failed areas compared to unused sections.
Stress Analysis
Model rope stresses under operating loads to identify critical areas prone to initiate bird caging.
Chemical Analysis
Check wire composition for substandard materials vulnerable to early cracking.
Mandrel Bend Tests
Bend samples from bird caged sections to simulate service conditions and recreate failures.
Tension Testing
Compare strength loss in bird caged samples versus intact rope sections.
This technical forensic evaluation provides critical insights to improve quality, operating practices and standards to prevent reoccurrence.
Industry Standards for Bird Caging Prevention
Several standards and codes outline requirements to minimize bird caging risks including:
ASME B30.7
Covers inspection and retirement criteria for wire rope used in cranes, derricks, hoists and elevators. Sets limits on allowable bird caging.
ISO 4309
Provides guidelines for wire rope discard based on bird caging severity and percentage of protruding wires.
DNV-OS-E303
Gives design precautions against shock loading which can cause bird caging in offshore mooring lines and marine applications.
ANSI M11.6
Specifies requirements to prevent excessive wear and fatigue damage leading to bird caging in wire rope slings.
CMAA 70 / OSHA 1910.179
Mandates periodic inspection of bird caging in hoist ropes for overhead cranes. Sets replacement criteria.
Following these guidelines ensures safe operating life is achieved without premature rope failure from bird caging.
Conclusion
Bird caging is a common and dangerous form of wire rope failure. It is caused by factors like fatigue bending, shock loads, corrosion and strand damage. Regular inspection, maintenance, and proper installation helps prevent bird caging. Catching it early and replacing affected ropes avoids accidents. Proper analysis of failed ropes provides valuable feedback to improve wire rope integrity against bird caging failures.
