Steel Wire Ropes
Wire- most basic component of ropes which are wound helically to form strands.
Strands - formed by a number of helically wound wires, a number of strands are helically wound together to form the rope/
Lay - the direction of the helical winding
Lay Length - Pitch of helical winding, typically around 6.5 times the diameter.
Ordinary Lay -The wires are wound in the opposite hand to the direction the strands are wound together.The torque created by the wire lay subtracts from the rotational torque created by the strand lay
Langs Lay - Were the wires are wound in the same direction as the strands are wound around together to form the Rope ( offers better wear resistance but turns to hide wire breaks on the inner sharper turn
Steel wire ropes are formed of small diameter wires wound together to form a strands. These strands are then wound together to form the rope.
The properties of the rope is dependent on the components from which it is constructed. These properties include amongst others:
These properties come from the construction of the rope but importantly by the properties of the individual wires . The wires are manufactured in a controlled manner with subsequent cold working and in certain circumstance dynamic strain ageing to ensure that these properties are achieved.
Some Rope manufacturers have their own steel foundry in which steel rods and subsequently steel wires with the correct composition and properties are created.
The sequence is formation, heat treatment, galvanising and cold working resulting in the wire which will be wound together to form the ropes.
Basic steel billets are heated in the foundry to 1000’C and repeatedly rolled to the correct diameter. The resultant steel is fully austenitic with large grains containing pure Iron with areas of FeC hard alloys.
Critical to the operation is the controlled cooling process passing through molten lead at 550’c. This results in a very fine Pearlitic structure ideal for wire drawing, this achieves the best balance of High tensile strength with good ductility
The Wires are hot dip galvanising coming out of the bath vertically to ensure concentric covering. The galvanising process is controlled to ensure a FeC/Al bonding lay of correct thickness to ensure that it remains intact during the diameter reduction process.
Essential to forming the ideal properties in the wire is the cold working process of wire drawing. In this the wires are pulled though a die to reduce the diameter
The lead in angle into the die is critical to ensure the homogenous compressive reduction is consistent through from the surface thorough to the core. The reduction is done by a series of dies ( up to 14) with a diameter reduction of around 19% at each stage.
Heat is generated by the drawing process and this is controlled by water cooling the die..
The wires are pulled through the wire drawing system by a capstan. When pulled off the reel the wire passes through a Calcium and Sodium stearate powder hydrodynamic lubricant.
The capstan is water cooled. The residence time of the wire on the capstan and the wire drawing speed are critical to the final properties of the wire
Strain ageing is an effect well understood and whose resultant effects in the property of the wire can be positive although over the long term has a negative impact.
Steel wire ropes will loose strength over a period of time due to the loss of ductility associated to Strain Ageing. The loss of ductility means that the individual wires can not efficiently share the load evenly through the construction.
The Steel crystalline structure is not homogenous, dislocations exist where the atoms are not properly aligned , relatively movement is allowed between the lattices and bonds can break and re-form.
It is this process that give the ductile property top the material
Free or Interstial Carbon and Nitrogen atoms are diffused within the matrix and may accumulate in the dislocation areas moving to a lower energy state. In doing so they increase the rigidity of this area preventing the free movement.
This diffusion process naturally occurs over time and may be accelerated by increased heat, for this reason the need to control temperatures especially during the cold working process.
The Pearlite microstructure is formed of lamellar's of Ferrite interspersed with thinner layers of hard Cementite, where these cementite layer is broken the surrounding dislocations may become flooded with Carbon Interstial atoms.
This effect may be controlled in low carbon steels using alloying elements to tie up the interstial atoms but it is not possible in High Carbon steels. Thus this effect is more prevalent in wires manufactured with Higher tensile property.
Strain Ageing may be considered to come in two mechanisms:
Dynamic Strain Ageing- occurs as part of the cold working process and when properly controlled enhances the properties of the wire.
Static Strain Ageing- occurs though age and has a deleterious effect of loss of strength due to loss of ductility.
The above shows the effects of reducing the ‘per Die’ cold working percentage by increasing the number of dies in the process. Typical maximum of 14 is employed
The above shows the effect of increasing the draw speed by 10%.
Steel wire ropes used for lifting operations may be classed into two main groups
Non or Low Rotation multilayer
Ordinary ropes are specified by stating the number of strands and the number of elements in the main load carrying strands. Above a 7x7 rope would have 7 strands containing 7 elements. The internal strand performs an important function of supporting the shape of the rope. In high overload as it is not helically wound as in the outer strands it tends to see the overload earlier and will break providing an initial warning of imminent catastrophic failure.
Modern ropes formed with an Independent Wire Rope Core provide better load shearing. in the overload conditions.
When load the helical winding produces a rotational torque which causes the load to rotate if not fitted with a swivel.
The majority of the load carried in these ropes is on the outer strands and therefore wire breaks would be expected to be seen in these rather than in the core.
Non or Low Rotation Multi-strand ropes
Low- rotation ropes are named in the same way as standard topes with the number of strands with the number of elements in the major strands.e.g. 36 x 19
These ropes are so named as the impart a lower rotational torque on the load than the standard ropes. This is achieved by having the inner strands would opposite hand to the inner strands.
Load is shared equally between the inner and outer strands and therefore wire breaks may be evenly distributed.
The ratio between the calculated minimum breaking force of the rope and the calculated minimum aggregate breaking force of the rope.
The aggregate breaking force is calculated by summated the wire diameters and their tensile strength. The reduction is directly as a result of the rope construction and the effect under load.
Rope Rotational Torque - Torque factor
With a section of rope seized at both ends a standard load is applied, the lay of the rope causes a torques to be exerted which is measured.
This torques is a specification for a wire rope design and is called the Torque factor and is determined as follows:
Torque Arm = Torque/Axial Load
Torque factor = Torque Arm/Rope Diameter
Rope Rotation - Turn Factor
A standard load is applied to the free end of a section of rope. The ,ay of the rope causes the load to rotate.
The degree of rotation or Turn factor is determined as follows
Turn Factor = degree/Lay Length at 20% MBL
Standard 6 or 8 strand ropes typically have linear torque characteristics with Torque increasing with applied load.
Multi-strand ropes have a complex response to applied load oftern with a torque reversal at about 20-25% applied load
130t x 3000m x 76mm rope
Low Rotation Multistrand
MBL = 456t
MBL = 534t
Turn Factor = 160o per Lay
Turn Factor 0.8o per lay
Payload 28,5% MBL
1442 complete rotations increasing lay length by 30%