The constitutive element of the steel wire rope is the wire. It is obtained form the wire rod, by the operation of drawing. The assembly of several wires arranged helically around a central core composed of a metal wire or textile fiber, constitutes the strand. The rope is the set of a number of strands that are arranged around a textile or metal core through the stranding operation. The strands, before this step, are subjected to the preformation. This operation allows to give them the helical shape that they will assume in the final formation of the rope.
The main features of the ropes are:
- Lay direction
- Breaking load
- Metallic cross section
- Unit weight
The diameter of the rope is the diameter of the circle circumscribed to the normal section of the rope. The figure below shows how to make a correctly measure:
The effective diameter is the average of four values taken in two points distant at least one meter, in each of them are carried two measurements distant 90° form each other.
The construction of a rope indicates its composition. The stranded ropes are identified, pointing out the following:
- The number of strands composing the rope
- The number of wires composing each strand
- The type of the core
The lay direction refers to:
- The outer wires with respect to the strands
- The strands with respect to the rope
In the stranded ropes we can check the following conditions:
- REGULAR lay (right or left): the outer wires in the strand have opposite lay direction to the lay direction of the strands in the rope.
- LANG lay (right or left): the outer wires in the strand have the same lay direction of the lay direction of the strands in the rope.
The breaking load is the maximum force or stress that a rope can support before arriving at break.
- Minimum breaking load guaranteed: it defines the scope of the rope and it applies to effects of the accident prevention regulations.
- Effective breaking load: is obtained in the breaking test traction of a length of rope.
- Calculated breaking load : it is a superior value than the previous but it is purely theoretical and it is not valid for determining the scope of the rope
Metallic cross section
It is given by the sum of the sections of all the wires composing the rope.
It is the weight of a unit of length of the rope ; in the tabs products is indicated in correspondence of each rope and is expressed in kg per meter.
ROPES WITH COMPACTED STRANDS
The compacting of strands is a cold deformation process, which consists in reducing the diameter of strand and its wires by passing through a die or a rollers pairs. This process generates profound changes in the shape of the wires:
- It increases the metallic cross section of the strand;
- It extends the areas of contact between the wires;
- It makes the surface of the strand smoother and more regular, therefore less permeable;
- It distributes more uniformly the tensions on the wires;
- It makes the strand more stable with respect to the transversal forces
The advantages resulting from the compaction allow the use of ropes with compacted strands in all sectors and in particular in those applications where high stresses are found and where they requires a high load capacity.
NO ROTATING ROPES
The no rotating ropes are composed of two or more layers of strands that are stranded with alternated lay direction so as to reduce the rotation.
They are particularly indicated in hoisting equipment in which it’s necessary the stability of the suspended and non-guided loads and in cases where the distance between the higher pulleys and the pulley on the hook is high, as for example in the tower crane.
The following illustrations show the main examples of deterioration suffered by the ropes and their causes..
|Considerable number of broken wires with a serious wear caused
by a rubbing under tension on a sharp edge.
|Basket deformation caused by a forced rotation around small
grooves or excessive deflection angle.
|Expulsion of the metal core, generally associated with a basket
deformation in the adjacent area.
|Expulsion of internal wires of the strands. Defect found in a
rope subjected to repeated shock loads.
|Wear with relaxation of the strands, twisted
during the installation but also assembled.