Rope Technology Basics
All about “wire“
Wires are usually made from specified carbon steel. The carbon content is between 0,4% and 1%, the manganese content between 0,3% and 1%, the silicon content between 0,1% and 0,3% and the contents of phosphorus and sulphur each under 0,45%.
Galvanized wires are zinc coated by going through a bath of liquid zinc. The wire is called “finally galvanized“ if it does not get drawn further after this process. If the wire cross section is further reduced after this process, the wire is called “drawn galvanized“. Bright wires, uncoated, are indicated with the capital letter “U”, whereas zinc coated wires are divided into class “A” and class “B”, depending on the a zinc weigth coating.
Wire rod of 6 to 9mm diameter, the raw material, will be transformed to the desired strength, diameter or shape by rolling or drawing in a cold forming process.
A distinction is made between the wire shapes. A round wire is a wire with a round cross section. Every wire that has a non-round cross section is called a profiled wire. There are oval wires, flat wires, Z- and S-profiled wires, H-shaped wires, trapezoidal or wedge shaped wires and triangular wires. Profiled wires are produced by drawing or rolling processes.
Wire tensile strength
The tensile strength of a wire is defined as the maximum tensile force a wire can stand in longitudinal direction without breaking, divided by the cross section of the wire. The nominal tensile strength of a wire is a theoretical value, the actual tensile strength of the wire should not fall below the nominal tensile strength and should only exceed it in defined limits. Rope wires with the nominal tensile strengths of 1770 N/mm2, 1960 N/mm2 and 2160 N/mm2 are commonly used in modern wire ropes.
All about the strand
A strand consists of one or multiple layers of wires, which are wound in a helical shape around an insert (figure 1)
Lay length of a strand
The lay length of a strand is generally understood as the pitch of the helical lay of the wires, which means the lengths of a strand at which the wire circulates completely one time. By varying the lay length, the contact conditions of adjacent wires, the elastic properties and the breaking strength of a strand can be changed.
Lay direction of a strand
A distinction is made between right hand and left hand lay strands. The lay direction is left hand, when (moving away from the beholder) the wires are rotated counterclockwise (figure 2). The lay direction of a strand is right hand, when its wires (moving away from the beholder) are rotated clockwise (figure 3). The lay direction of a strand is often given by small s for the left hand lay strand and by a small z for the right hand lay strand.
Diameter of a strand
The diameter of a strand is the diameter of the smallest, all wires enclosing enveloped circle. The strand diameter is usually measured with a micrometer caliper and is given accurate to 1/100mm (figure 4).
One understands the formation law by the design of a strand according to which the wires are relatively to each other arranged. So all strands of the design Seale have for example the construction 1 – n – n with n=3, 4, 5, 6, 7, 8, 9….. wire layers, which get stranded parallel to each other in a single operation. According to EN 12385-3 these are connected by a minus „-„ sign in the designation. The name of a Seale strand design Seale 17 is therefore 1-8-8, the designation of a strand design Seale 19 is 1-9-9.
Compacted round strands
A compacted round strand starts as a conventional torsion free round wire strand. The strands are compacted either during stranding or in a separate operation afterwards to form a smaller diameter by rolling or drawing. The originally round wires are heavily deformed by both the compacting tool and the adjacent wires (figure 8).
Fill factor of a strand
The fill factor of a strand is defined as the ratio of the metallic cross section (or as simplified calculation the sum of the single wire cross sections) related to the area of the smallest circle circumscribing the strand. The fill factor specifies the amount of space which the strand takes in the rope meaning the amount of steel. The fill factors of the most common strands are between 0,70 and 0,82. This means, that the amount of steel in the strand is about 70% to 82%. The fill factors of strands can be considerably increased by compacting. Usually the fill factor of a strand increases with an increasing number of wires. A Seale 15 strand (1-7-7) for example has a fill factor of about 0,77 and a Seale 19 strand (1-9-9) has a fill factor of about 0,79.
Figure 8: Uncompacted and compacted round strands