Elevator Wire Rope
Ropes used in traction drive elevators have a helical structure. This entails that the outer and inner strands of the rope have a fixed contact pressure with the traction sheave.
This leads to the wires of the rope being bent over the sheave and thereby reducing their diameter. In addition, over long service periods the outer wires can be worn away due to abrasion.
Strength
Elevator wire ropes are available in a wide range of strength classes. These are based on the nominal tensile strengths of the outer and inner wires in the strand construction, which are also called rope grades in elevator rope standards EN 12385-5 and ISO 4344 [2].
The outer strands of these ropes are usually made from thicker annealed steel and have a higher breaking strength than the inside strands. During rope manufacture, these wires are subjected to a special heat treatment to increase their tensile and impact strengths.
During a tensile test, the wires are loaded with a fixed load until around 10 % of their nominal breaking strength is reached. The elongation is then measured and recorded.
For 9-strand ropes, the rope elongation curves are determined separately and are shown in Fig. 40. These curves show that the rope elongation module rises when the wire is loaded and, in the case of the DRAKO 300 T, the elongation module increases even after threshold stress application.
Ropes that are pre-stretched can be shortened significantly easier than un-stretched ones because the strands of these ropes compact during operation, which reduces the permanent elongation that accompanies compaction of the rope structure. This can be a significant advantage in traction drive installations because it means that the ropes can be shortened much more quickly during maintenance and remachining of the traction sheaves.
However, a remachined sheave is far more susceptible to abrasion than a new sheave and requires a far longer service life. This is because the grooves in a sheave that has been remachined are abraded at a far faster rate than those of a new sheave which have not undergone this process.
Rigidity
Ropes in elevators are subjected to a high degree of flexural stress. This requires a strand construction which offers the best possible combination of breaking strength and fatigue bending characteristics. In most cases, this is achieved by the use of a strand construction with thick outer wires such as the Seale strand.
The strand construction also ensures a high level of elasticity for the rope, so that the rope remains flexible even under severe flexural stress. In addition to this, the strand structure also prevents wire crossing and thus reduces the risk of internal wire breaks invisible from the outside.
However, the strand construction is not without its own drawbacks, and these are mainly related to the rope’s bending characteristics. The higher breaking strengths in particular mean that the rope diameter must be kept low, as a large radii will result in lower fatigue bending life under load and over time.
This is why the choice of rope construction depends on the type of elevator and the required service life. For example, traction drive elevators require high breaking strengths in order to achieve the highest degree of safety. Therefore, 1770 grade ropes are often used in these installations.
For suspension ropes, the choice of a strand construction is dependent on the type of sheave and deflection sheave. For traction sheaves, the preferred choice is a strand construction with thick outer wires, whereas for deflection sheaves a strand construction with thin wires would be more suitable.
Another consideration for strand construction is the rope’s elongation factor. Elevator ropes are shortened after installation, and their elongation is measured in accordance with EN 81-1/1998 [7].
Permanent elongation is lower for class 6 x 19 + elevator wire rope fibre core ropes than for class 8 x 19+ fibre core ropes (see Fig. 40).
The strand construction of the rope also plays an important role in determining the elongation factor, as well as for maintaining its effective diameter. The elongation of an elevator rope is calculated by dividing the effective rope diameter by its total length, thereby taking into account the volume of the fibre core.
Durability
Durability is a key aspect of elevator wire rope. This is because the rope is exposed to a range of stress factors, involving flexural pressure, tension and compression but also abrasion between the wires and between the rope and sheave.
The rope must also be able to withstand traction sheave wear and damage that can occur when running over the grooves in the sheave material. This wear can be caused by the wires making contact with each other on the groove surface, or by abrasion between the outer strands and the sheave, which is why it is important to choose a sheave material with a good hardness level for the traction sheaves in a rope drive system.
In most cases, a sheave with a hardness of around 200 HB should be used. This is because sheaves with a higher hardness level can cause abrasion between the inner wires and the sheave, which can lead to internal wire breaks. In addition, sheaves with a high hardness level can be subject to severe groove wear that is often difficult to detect even after an entire lifetime of service.
Therefore, it is highly recommended that sheaves be replaced every now and then in traction drive elevators. This can save considerable money and time in the long run as well as avoiding re-machined sheaves which may be prone to abrasions or rope impressions at a faster rate than they were previously.
To protect the sheave from abrasion and rope impressions, a lubricant should be applied to the sheave and the traction sheaves. The amount of lubricant must be carefully calculated to ensure the rope remains firmly fixed to the sheave during operation.
As the lubricant penetrates the fibre core, it reduces the friction between the rope and the sheave. It is important to apply a low amount of lubricant, because excessive amounts can quickly seep out of the fibre core during relubrication and affect the service life of the rope.
The most common strand construction for elevator ropes is the Seale strand, which has thick outer wires to increase resistance against external wear when running over the traction sheave and deflection points. This strand is suited to elevators with a large number of sheaves and a high level of traction capacity. It can be produced from natural fibres or synthetic polypropylene (PP) which is popular in crane and cable car ropes.
Weight
Elevator wire ropes are generally heavier than other suspension ropes due to the higher number of strands and the steel wire core. Compared to an 8 x 19 + FC construction rope, a rope with a steel wire core reduces elastic rope elongation by around 50 %, even if the other installation parameters are kept constant.
The main reason for this is the significantly larger metal cross-section. However, the resulting higher minimum breaking load also exerts an effect. This is particularly noticeable with elevator wire rope elevators that use a lateral arrangement of the traction sheave or are operated at low speeds.
Moreover, a high rope length results in more frequent shortening, which is why it is important to avoid long-term, excessive elongation. In addition, it is essential to regularly check the condition of lubricants in the ropes and relubricate them as required using fluid lubricants with solvents.
In the case of elevators with a steel wire core, an indication of the maximum admissible number of wire breaks is given in accordance with ISO 4344 [3]. By taking this value as a reference, it is possible to determine whether the rope should be discarded immediately or should be monitored normally.
Wire breaks caused by the use of a steel wire core are relatively rare, but can still occur when elevator installations use an atypical strand laying configuration. In particular, this can result in a “wrong-hand lay” rope when the outer strands are placed in the wrong direction under torsional load.
The resulting strand breaks are usually only visible after the rope has undergone extreme bending. Consequently, it is important to ensure that the ropes are not subjected to rotation in order to prevent inner strands from emerging.
The rope should not be repositioned on the traction sheave during operation, as this can lead to a degreasing of the lubricants. Relubrication should be carried out at regular intervals, for example after every round trip or after longer dry periods.
