Wire rope designation
A wire rope is identified not only by its components but also by its lay and construction. When referring to the lay, it is important to distinguish between the direction of lay of the strands in the rope and the direction of the wires in the strands. A wire rope is known as "right lay" if the strands are laid in a right helix, similar to a right hand thread and a "left lay" if the strands are laid to the left.
There are two main types of lay:
Ordinary lay (regular lay) – a rope in which the direction of lay of the wires in the strands is opposite to the direction of lay of the strands in the rope. In this type of ropes, the outer wires of the strands are laid approximately parallel to the rope's axis. These ropes are designated sZ for a right ordinary lay rope and zS for a left ordinary lay rope.
Lang lay – in this type of lay the direction of lay of the wires in the strands is the same as the direction of lay of the strands in the rope. The outer wires in the strands are at an angle relative to the rope's axis. Lang lay ropes are designated zZ for a right lang lay rope and sS for left lang lay rope.
A wire rope under load develops an internal torque in a direction opposite the closing direction. In lang lay ropes this torque is larger than ordinary lay ropes because the opposing directions of lay counteract this tendency to some extent. However, a lang lay rope has better wear resistance when running over sheaves because a longer part of the wires is in contact with the sheave. The load is distributed over a larger area and the stresses are lower.
Wire rope constructions are also divided into two additional classes:
Cross lay – when the strands are composed of layers of wires of the same diameter, each layer is stranded in a separate operation with a different lay length. The wires of different layer cross each other at an angle making point contact between them.
Parallel lay – these strands are manufactured in one operation. The strand is composed of a number of layers of wires of different diameters. The relative position of each wire is constant so the wires are parallel and make linear contact.
Parallel lay has the advantage that the contact between layers is along a line, not in a few points, resulting in a larger contact area which reduces the stresses and improves the ropes resistance to fatigue and radial stresses.
Cross lay constructions have the advantage that they are easier to design and manufacture. The machines required to make them are smaller and simpler. This type of ropes was very common in the past but now is used mostly for very small diameter ropes or very large ones. Most running ropes nowadays are parallel lay.
In a pre-formed wire rope during the closing stage the strands are given a helical shape. This process reduces almost completely the tendency of the rope to unravel and reduces the elastic stress in the wires forming the strands.
This process has a few advantages:
Except for a few special constructions and some special applications, all the ropes made by Wire Rope Works Messilot are pre-formed.
The core is the central component of the rope around which the strands are closed in one or more layers. The core can be made in a number of forms:
For general applications the fiber core is very common because these ropes are more flexible. A steel core makes the rope somewhat less flexible but it increases the breaking strength and the rope's resistance to crushing and radial pressure.
Steel core ropes are also more suitable for high temperature applications. Fiber core ropes have a maximum working temperature of 100 °C (210 °F) while steel core ropes can be used in temperatures up to 150 °C (300 °F).
Natural Fiber Core – NFC or Synthetic Fiber Core – SFC: Natural fiber cores are used primarily for elevator ropes. They are made of sisal yarn. Synthetic fiber cores are mostly made of polypropylene and are used for most other purposes and in some cases also for elevator ropes. For wire ropes up to 8 mm in diameter (5/16") the core is made of a single yarn or a single strand made of yarns. For ropes above 8 mm the cores are made of 3 or 4 strands which are made of yarns. Four strand cores also have a center strand.
Wire Strand Core – WSC: small diameter wire ropes and many rotation resistant ropes normally have a core that is a single strand in a construction similar to that of the rope's strands.
Independent Wire Rope Core – IWRC: Single layer wire ropes of diameter above 12 mm (½" and higher) normally have an IWRC. This core is made in a separate operation. It is by itself composed of strands and a core.
Parallel Closed Wire Rope Core – PWRC: this type of core is also composed of a wire strand core and strands but it is not closed in a separate operation. Instead, it is closed together with the rope strands in one operation with the same lay length in parallel lay. This type of core has the benefits of linear contact between the core strands and the rope's outer strands and also a higher metallic cross section so it has better fatigue resistance and higher breaking force than a similar diameter rope with IWRC. It does however have a higher torque factor than IWRC ropes. Another disadvantage is that a much larger closing machine is required to make it so the diameter range we can offer is limited.
This term refers to the number of strands that form the rope, number of wires in each strand, the arrangement of the wires in the strands and the arrangement of strands in the rope. In general ropes constructions are designated by two groups of digits separated by a multiplication sign "´". The second group may have additional prefix and/or suffix letters. The first group is the number of strands in the rope. The second group is the number of wires in the strands, their arrangement and other properties relating to the strand. As an example, 18x7 is a wire rope composed of 18 strands of 7 wires each. In North America, if the core is a WSC similar to the rope's strands, it is counted as an additional strand so that 7x19 is actually the same as 6x19-WSC. European and ISO designations do not include the core in the number of strands.
There are many different rope constructions, each with its different properties, advantages and disadvantages. Stranded ropes are divided to two main groups:
Rope constructions are further divided into classes according to the number of wires in the strands. For example: a rope of class 6x19 may actually be 6x26 Warrington Seale. Ropes with different constructions within the same class have similar properties.