General Guidance on Wire Rope Selection
When selecting a steel wire rope to suit a particular application the following characteristics should be taken into consideration.
• Strength
• Rotation resistance
• Fatigue resistance
• Resistance to wear and abrasion
• Resistance to crushing
• Resistance to corrosion
• Rope extension
Strength
The responsibility for determining the minimum strength of a wire rope for use in a given system rests with the manufacturer of the machine, appliance, or lifting equipment. As part of this process the manufacturer of the machine, appliance or lifting equipment will need to be aware of any local regulations, standards or codes of practice which might govern the design factor of the wire rope (often referred to nowadays as the coefficient of utilisation), and other factors which might influence the design of sheaves and drums, the shape of the groove profiles and corresponding radius, the drum pitch and the angle of fleet, all of which have an effect on rope performance.
Once the strength (referred to as minimum breaking force or minimum breaking load) of the wire rope has been determined it is then necessary to consider which type of wire rope will be suitable for the intended duty. It is important therefore for the designer to be fully aware of the properties, characteristics and limitations on use of the many different kinds of steel wire ropes which are available.
Important note for crane operators Certex recommends that once the machine, appliance or lifting equipment has been taken into service, any replacement rope should possess the required characteristics for the duty in question and should, as a minimum, at least comply with the minimum guaranteed breaking force stated by the original equipment manufacturer.
Resistance to Rotation
It is important to determine whether there is a requirement to use a low rotation or rotation resistant rope. Such ropes are often referred to as multi - strand ropes.
Six or eight strand rope constructions are usually selected unless load rotation on a single part system or “cabling” on a multi - part reeving system are likely to cause operational problems. When loaded, steel wire ropes will generate:
- “Torque” if both ends are fixed.
- “Turn” if one end is unrestrained.
Torque
When both ends of a rope are fixed, the applied force generates “torque” at the fixing points.
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Turn
When one end of a rope is free to rotate, the applied load causes the rope to turn.
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The torque or turn generated will increase as the load applied increases. The degree to which a wire rope generates torque or turn will be influenced by the construction of the rope. Having recognised what can happen when a rope is loaded it is necessary to select the correct type of rope. It should be noted that all ropes will rotate to some degree when loaded.
The diagram serves to illustrate the differences in rotational properties between the four basic types of stranded rope.
The tendency for any rope to turn will increase as the height of lift increases. In a multi - part reeving system the tendency for the rope to cable will increase as the spacing between the parts of rope decreases. Selection of the correct rope will help to prevent “cabling” and rotation of the load.
“Endurance” low rotation and rotation resistant ropes ensure that problems associated with cabling and load rotation are minimised.
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Certex is pleased to offer advice on any specific problems associated with rope rotation.
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Wire Rope Fatigue Resistance
Steel wire ropes are likely to deteriorate due to bend fatigue when subjected to bending around a sheave or drum. The rate of deterioration will be influenced by the number of sheaves in the system, the diameter of the sheaves and drum, and the loading conditions.
When selecting a wire rope for an application where bending fatigue is a principal cause of deterioration it is important to select a rope containing small wires e.g.
6x36 WS (14/7 & 7/7/1) as opposed to a 6x19 S (9/9/1).
Additional resistance to fatigue leading to real cost savings can be achieved by selecting a “Dyform” wire rope.
The smooth surface of the “Dyform” product provides improved rope to sheave contact leading to reduced wear on both rope and sheave.
Increased cross-sectional steel area and improved inter - wire contact ensures that the rope will operate with lower internal stress levels resulting in longer bending fatigue life and lower costs.
This graph illustrates a “doubling” in life when moving from Blue Strand 6x36 to Endurance Dyform 6. This same relationship can be found when moving from any construction into an equivalent Dyform construction
e.g. 18x7 to Endurance Dyform 18 or 35x7 to Endurance Dyform 34LR.
Wire Rope Resistance to Abrasive Wear
Abrasive wear can take place between wire rope and sheave and between wire rope and drum but the greatest cause of abrasion is often through “interference” at the drum.
If abrasion is determined to be a major factor in rope deterioration then a wire rope with relatively large outer wires should be selected.
Comparison of outer wire sizes for single layer 22mm diameter rope.
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6 outer wires
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2.20mm
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9 outer wires
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1.83mm
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12 outer wires
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1.47mm
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14 outer wires
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1.29mm
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16 outer wires
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1.16mm
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Wire rope on adjacent drum laps can cause point contact and accelerated wear .
Non compacted wire rope on adjacent drum laps can cause point contact and accelerated wear. |
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Selection of a compacted product will reduce abrasion through improved contact conditions.
The smooth surface of Compact rope creates better contact and leads to longer life. |
Wire Rope Crush Resistance
In multi-layer coiling applications where there is more than one layer of rope on the drum it is essential to install the wire rope with some back tension. Certex recommends an installation tension of between 2% and 10% of the minimum breaking force of the wire rope. If this is not achieved, or in certain applications where high pressure on underlying rope layers is inevitable e.g. a boom hoist rope raising a boom from the horizontal position, severe crushing damage can be caused to underlying layers.
Selection of a steel core as opposed to a fibre core will help this situation. Additional resistance to crushing is offered by a Comapacted rope resulting from its high steel fill-factor.
Compacted ropes are recommended for multi - layer coiling operations where crushing on lower layers is inevitable.
Wire Rope Corrosion resistance
If the wire rope is to be used in a corrosive environment then a galvanised coating is recommended. If corrosion is not a significant issue then a bright rope can be selected. Where moisture can penetrate the rope and attack the core, plastic impregnation (PI) can be considered.
In order to minimise the effects of corrosion it is important to select a wire rope with a suitable manufacturing lubricant. Further advantages can be gained by lubricating the rope regularly whilst it is in service.
Wire Rope Extension
If wire rope extension is critical please contact Certex.
Ordering a Wire Rope
Communicate as much background information as possible when ordering or enquiring about wire rope. The following is necessary:
Application / intended use.
Rope length and tolerance where applicable.
Nominal Diameter.
Construction, class or brand name.
Core (FC; FFC; WSC; IWRC)
Rope Grade (1770; 1960; 2160)
Wire finish (Bright; Galvanised)
Lay type (Ordinary; Langs)
Lay direction (Left Hand; Right Hand)
Minimum breaking force.
Termination requirements.
Special packaging requirements.
Special identification requirements.
Third party authority (LR; DNV; ABS etc.)