When selecting a steel wire rope to suit a particular application, the following characteristics should be taken into consideration.
- Rotation Resistance
- Fatigue Resistance
- Resistance to wear and abrasion
- Resistance to crushing
- Resistance to corrosion
- Rope extension
The responsibility for determining the minimum strength of a 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 rope 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 fleet angle, all of which have an effect on rope performance.
Once the strength (referred to as minimum breaking force or minimum breaking load) of the rope has been determined it is then necessary to consider which type of rope will be suitable for the intended duty. It is essential 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 operators
Bridon 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 critical to determine whether there is a requirement to use a Rotation Resistant rope. 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.
When both ends of a rope are fixed, the applied force generates “torque” at the fixing points. 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 recognized 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 diagrambelow serves to illustrate the differences in rotational properties between the three basic types of stranded rope. Specific information including the torque factor and the turn value expressed in degrees per lay length for individual rope constructions can be found on page 79.
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 (see page 76 to 78).
“Endurance” Rotation Resistant ropes ensure that problems associated with cabling and load rotation are minimized. Bridon is pleased to offer advice on any specific problems associated with rope rotation. Bridon can verify the rotational characteristics of individual wire ropes
through testing on its specially designed in-house machine. All Bridon products intended for lifting applications have been subject to this “Twistcheck” testing program.
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.
Resistance to Abrasive Wear
Abrasive wear can take place between rope and sheave and between 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 1” diameter rope. Wire rope on adjacent drum wraps can cause point contact and accelerated wear.
There are some applications where the use of a galvanized wire rope may be beneficial. If corrosion is not a significant issue relating to rope life 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 minimize 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 in service.
If rope extension is critical, refer to technical data on page 79. Non Dyform wire rope on adjacent drum wraps can cause point contact and accelerated wear.
Selection of a Dyform product will reduce abrasion through improved contact conditions. The smooth surface of Dyform rope creates better contact and leads to longer life.
In multi - layer spooling applications where there is more than one layer of rope on the drum it is essential to install the rope with some back tension. Bridon recommends an installation tension of between 2% and 10% of the minimum breaking force of the 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 fiber core will help this situation. Additional resistance to crushing is offered by a Constructex or Dyform rope resulting from its high steel fill-factor.
Applications where severe crushing is the mode of deterioration will benefit from the use of Constructex.