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Selecting the right steel rope is crucial for success

21st February 2013


Günther H Gfrerrer provides fundamental information on the use of special steel wire ropes in multi-layer winding configurations.

No matter whether you are faced with choosing a rope for a new project or intend to replace an existing rope, selecting the right rope type is crucial as to whether the rope will meet the desired requirements or not.

Generally, we distinguish between rotation-resistant ropes (Teufelberger TK Series) and non rotation resistance ropes (Teufelberger Q Series). Which rope is the right one for a new project depends on the specific application. Do not hesitate to contact Teufelberger for help with picking the right type of rope.

For existing installations, it will be helpful to start by taking a look at the crane log which contains the most important data concerning the rope (diameter, minimum breaking force, etc).

Apart from what is described in the crane log, when using the rope in a multi-layer winding configuration, customer satisfaction can be enhanced considerably by replacing regular lay ropes with langs lay ropes.

As compared with regular lay ropes, langs lay ropes offer greater flexibility and thus a superior ease of winding.

Furthermore, langs lay ropes provide greater wear resistance due to the bigger area of contact between the various wires. This gives them a longer service life vis-à-vis regular lay ropes.

Exercising caution

Exercise caution when using langs lay ropes on systems with heave compensation, as the greatest stresses occur not in the region of the drum, but in the region of the heave compensation. Therefore, repeated NDT inspections are necessary when using langs lay ropes.

Generally, the following rule of thumb applies to single-layer winding configurations: for a right-handed drum pitch a left-handed rope is required, and vice versa.

Several years of experience and countless examples have shown that with replacement ropes changing the rope's lay direction from right to left or from left to right will considerably improve their life span.

Before carrying out such changes, we recommend that you contact the supplier/manufacturer of the replacement rope. In multi-layer winding configurations, and when using non-rotating ropes, the drum pitch is of secondary importance.

Installing the rope

The life of a steel wire rope depends to a large extent on how the rope is installed. Pulling the rope on under load as opposed to pulling the rope on manually and in an unloaded state helps avoid damage to the rope and reduces winding problems in ongoing operations which occur due to the gap between the various 360 degree turns.

In the case of flat drums, it must be ensured that the gap between the flange and the rope is not greater than 0.5 times the actual rope diameter, as otherwise the rope will be cutting into the layer located underneath (Fig. 1). This causes severe winding problems and may even lead to the rope being damaged, which consequently has to be discarded. A possible remedy is an inserted metal sheet that suitably regulates the utilisable drum width.

The rope length must be selected such that the first layer is partly unwound. This helps prevent issues when changing between the first and the second layer and between the second and the third layer. At the same time, it can be avoided that the first layer is used as auxiliary grooving.

After the rope has been installed, it is essential to break the rope in under various different load cycles. This also includes running through the load quadrangle several times. This helps prolong the service life of the entire Drum - Rope - Sheave system.

Maintenance/inspections

Regular relubrication helps prolong the rope's useful life span considerably. However, not all types of grease are suited for relubrication, as it needs to be compatible with the grease originally used by the manufacturer.

Teufelberger conducts grease tests on a regular basis in order to continually update the list of compatible lubricants by including new grease products. Incompatible grease may cause the remaining original grease to be removed from the rope. As a consequence, the rope will reach its discard condition faster, as its lubrication will be insufficient.

In addition to ongoing maintenance, inspections are another useful measure in order to detect possible signs of wear and, where necessary, initiate countermeasures on time.

Discard criteria

Teufelberger recommends compliance with ISO 4309 which defines discard criteria such as the number of broken wires, the inspection of the actual rope diameter, external and internal corrosion, deformations, etc.

In order to determine the acceptable number of broken wires in the context of either single-layer or multi-layer winding, regular lay or langs lay ropes, we rely on the so-called Rope Category Number (RCN). The RCN as such does not provide any information on the life span of a rope, but it indicates only how many broken wires over a length of 30 times the rope's nominal diameter or six times the rope's nominal diameter, respectively, are acceptable.

Prolonging a rope's life

In addition to the ongoing maintenance of the rope, the so-called slip and cut method is used to substantially prolong a rope's life span. In (and only in) multi-layer winding, the greatest stress acting on the rope occurs at the cross-over point between layers. Hence, this constitutes also the region limiting the life span, as the first broken wires will invariably appear in the region exposed to the greatest loads.

In the slip and cut method, the rope is shortened by about one third of the drum's circumference at the rope's point of attachment on the drum. This way, the cross-over point is shifted and consequently the life span lengthened. In this procedure, it is necessary to devote special attention to the rope severing procedure and to the discard criteria stated in ISO 4309.

It is recommended that you employ the slip and cut method - and this is especially true if you proceed in accordance with the total cost of ownership principle.

Under this principle, it is not the mere costs of replacement for the rope that are compared but all costs incurred in connection with exchanging a rope (costs of the standstill, maintenance costs- and costs of the rope change, etc).

When looking at things in this way, many cases demonstrate that the long-living special ropes may be more expensive in terms of their purchase price, however, in total end up being markedly cheaper than conventional steel wire ropes because of their longer service life.

Troubleshooting

When encountering winding problems during day-to-day operation, it is possible for the user to undertake a number of measures in order to determine the cause.

- On grooved drums, the drum pitch can be measured by means of groove gages (Fig. 2) and compared to the actual rope diameter. In this context, it is important that the pitch of the drum is 0.5 to 1.0 per cent greater than the actual rope diameter, as otherwise there would be permanent contact and friction between adjacent rope turns. This would cause wires to break and consequently reduce the rope's life span.

- Check whether the rope entry at the point of attachment abuts the flange correctly. Check whether the rope entry at the point of attachment abuts the flange correctly. After all, warping at the rope entry and in the first layer affect the winding quality in all layers on the drum and thus also the rope's life.

- Check deflection angles of the rope on the drum and between the sheaves. The deflection angle on the drum varies from one position to the next, however, it must be ensured that the maximum deflection of angle a at the flange is between 0.5 and 1.5° (Fig. 3).

A deflection angle that is too small will cause the rope to move up along the flange until it collapses and strongly affects the winding quality. If the angle a is too large, then this will result in a poor winding quality on a drum without grooves. On a grooved drum, an additional twist will be imparted to the rope, which may damage the rope, as the rope will move up the flange of the sheave.

When determining angle b, the angle of the drum pitch a must also be taken into consideration (Fig. 4).

- Check condition of rope structure: Incessant winding and unwinding of the rope may cause the rope to be either twisted open or closed.

- As a result, the rope structure becomes either very tight or very loose. Using a screwdriver, one can easily check the condition and take corrective action as necessary. To do so, insert the tip between two outer strands and try to twist it. If it is easy to move the strands around, then the strand package is too loose, which in turn would shorten the rope's service life.

About Teufelberger

Teufelberger produces steel wire ropes for ropeways and cranes and is also active in the extrusion of plastic strapping products and baler twines. The third pillar of the Teufelberger Group is its synthetic fiber branch.

The group is made up of the three following divisions catering to various market segments:

- Special steel wire ropes for applications such as cranes, ropeways, building and construction, industry, forestry, and slope winches, as well as personal fall protection equipment.

- High-tech fiber ropes for yachting, arboriculture, climbing, forestry and technical applications, and harnesses and ropes for personal fall protection equipment, metal-composite interconnections and overbraiding components

- Fibres and plastics for use in construction materials, wood and timber, baling, can and bottle, corrugted and printing industry; baler twines for harvest making fibre yachting ropes, high-performance industrial ropes, forestry ropes, and ropes for other technical applications, harnesses and ropes for personal fall protection equipment and arboriculture.

Enter √ at www.engineerlive.com/iog

Günther H Gfrerrer is Head of Product Management & Technical Marketing, Wels, Austria. www.Teufelberger.com







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