Experiences with Ropes
Case studies – good and bad experiences with ropes described and explained
1st – 3rd September 2003
IFT, University of Stuttgart, Germany
Lifetime and discard for multilayer spooling in cranes
Owing to the multitude of parameters acting on a rope during service, it is impossible to determine the lifetime of a rope solely from the calculated values of stress in its wires. To facilitate rope endurance predictions and, thereby, predictions on the reliability of ropes, experimental research is the only method.
In the past 75 years, over ten thousand tests have been conducted at the IFT in Stuttgart whilst investigating the various parameters which affect the endurance and discard of ropes running over sheaves. Unfortunately, there are still extensive areas in the fields of rope technology and rope applications which have not been systematically investigated.
On the other hand it is crucial, especially in a sensitive and important area such as rope technology, to continually verify existing and novel calculative approaches with the help of serial testing. Only by means of statistically proven tests it is possible to meet and improve the required safety and reliability standards.
The author’s paper is based on the latest investigations by the IFT, and highlights the necessity of undertaking experimental research. It covers the following areas of current concern:
- influence of multi-layer spooling on the lifetime of ropes
- influence of the fleet angle on the lifetime of ropes running over sheaves
- ropes under fluctuation tension
Due to the time limit imposed on the speakers, the lecture will exclusively centre around the subject of “Influence of Multi-Layer Spooling on the Lifetime of Ropes”.
IFT, University of Stuttgart, Germany
Endurance and discard of ropes in lock gate drives
The Main-Danube Canal is the principal section of the “Europakanal”, situated between the river areas of the Danube and the Main. The topographical terraces are negotiated with the help of locks whose gates are operated with the aid of ropes. These ropes, running over sheaves, are exposed to strongly corrosive influences which – in the combination of steel sheave with steel wire rope – result in numerable rope discards. This paper describes an investigation to provide a solution to this problem, testing as alternatives: a round stranded rope of carbon steel wires and one of stainless steel wires both running over a plastic grooved sheave in bending fatigue. The test assessed both endurance and discard criteria.
University of Mining and Metallurgy, Krakow, Poland
Wire rope endurance on multi-rope friction winders
In installations where wire ropes are used in parallel, great care must be taken to ensure that they share the load evenly. One such application where ropes are used in this way is on a multi-rope friction winder. In practice, owing to different rope stiffnesses and different diameters of the friction wheels the loads may be very different (the actual difference in diameters may be quite small and caused by wear of the sheave groove inserts). In service, load variations of as much as 100% between ropes have been observed and measured – one rope may carry no load whilst another carries twice the normal service load. Periodic inspection of the load on multi-rope friction winders measures the tension in the rope, and if necessary the groove of the friction wheel is modified to balance the loads. Subsequent service shows the rope which has experienced an over load to have a superior endurance to one which was previously slack and unworked. This paper describes this kind of installation and presents a model to predict the difference in service endurance. The model gives good agreement between theory and practice.
Makeyevka Safety in Mines Research Institute (MakNII), Makeyevka, Ukraine
Dangerous deterioration of steel wire ropes occurring over a local length
The author reports on some cases of local deteriorations in steel wire ropes, selected from over 150 rope failure case studies with which he has been involved with over 30 years experience. Rope deterioration or damage over a local length is brought about by lateral loads affecting instantaneously or repeatedly, bending- or fatigue-related wire breaks concentrated over a short length and creating abnormal (as compared with the remaining length) loss of metallic area. The causes behind the severe damage occurring over very short lengths arise from particular operation conditions which, though not always alleviated, should be accounted for by the inspection personnel. Sometimes damage occurs through minor, hardly visible residual strain effects originating during installation or operation after non-standard interaction with solid objects.
The author highlights how mandatory rope checks should be carried out along the whole length rather than by extrapolation of the results from a length to the remaining unchecked length.
V.Yu. Volokhovsky*, A.N. Vorontsov*, A.Ya. Kagan*& V.V.Sukhorukov**
*Moscow Power Engineering Institute (Technical University), Moscow, Russia
**Intron Plus Ltd., Moscow, Russia
Stochastic assessment of steel rope strength using magnetic NDT results
A mechanical model for the strength assessment of damaged steel wire ropes is proposed. The authors consider straight ordinary lay stranded rope under static pure tensile loading. Both the loss of cross sectional metallic area (LMA) and wire breaks – local faults (LF) measured by magnetic flux detector are considered as input parameters for the model. The cumulative effect of grouped wire breaks on the total share of the load is taken into account by an empirical influence function of the frictional forces in the rope. The Monte-Carlo technique is performed for the simulation of LMA and LF distributions across the rope section and along the inspected length. The estimates of damaged rope strength for different initial failure probabilities are obtained. Deterioration patterns related to lower, expected and upper strength estimates are presented.
V.V. Sukhorukov*, V.S. Kotelnikov**, V.G. Zhukov** & A.A. Khudoshin***
*Intron Plus Ltd., Moscow, **Gosgortekhnadzor, Moscow, ***Tekhkranenergo Ltd., Vladimir, Russia
Importance of ropes NDT for loading cranes safety rising
Russian State Rules for the crane safe exploitation require instrumental testing of ropes by magnetic flaw detectors during periodic crane inspection. Unfortunately, not all inspection centres use the magnetic testing, contenting themselves only with visual inspection, which is subjective and does not provide detection of inner faults. That is why the percentage of rope failures is rather high in the general statistics of crane damage and accidents. Analysis of two accidents with cranes in 2001 in the Moscow region shows that both of them could have been avoided if the inspectors had used a magnetic flaw detector. Statistics of more than 50 crane rope inspections indicates that about 25% of crane ropes in use should be discarded.
R. Eshkenazy, M.P. Weiss & D. Elata
Faculty of Mechanical Engineering, Israel Institute of Technology, Israel
Torsion and bending stresses in non-rotation-resistance tower-crane ropes
The simulated predictions of a specific 18×7 14Ø[mm] rope are presented and compared to a real service failure. In this work, a novel model of the stresses in individual wires of a general rope is shown. This model accounts for the double-helix configuration of each wire in the rope. The model is used to predict the mechanical response of a non-rotation-resistance tower-crane rope. These predictions are in agreement with new experimental data, thus validating the model. The new model is then utilized to simulate fatigue failure of individual wires by considering their stress history and using the two-term fatigue model.
D. Smith & P. McCann
Health and Safety Laboratory, Buxton, UK
Evaluation of instruments for the non-destructive testing (NDT) of wire ropes
NDT is becoming increasingly important for evaluating the condition of wire ropes in service. However, previous work has suggested poor consistency in the gathering and analysis of NDT data.
This work evaluated the performance of seven NDT instruments on a representative selection of ropes. Both the equipment and its operation were assessed. Formal reports provided by the rope inspectors were compared with actual faults found during subsequent dismantling of test ropes.
Performance, operation and analysis varied between instruments. Quantified loss of metallic area data was particularly variable and overall rope condition reports were not consistent. Documented methods of work were rare and often based only on local requirements.
D. Siegert & P. Brevet
LCPC Bouguenais, France
Fatigue of stay cables inside end fittings: high frequencies of wind induced vibrations (Von Karman Vortex)
A twenty year old stay cable was dismantled as it showed lots of wire breaks at one of its terminations. Examination of the wire failures showed evidence of fatigue. Analysis of the dynamic behaviour of the stay cable was carried out. It showed that the combination of flexural bending stresses with fretting stresses between the outside layer and wire serving lead to a stress range beyond the endurance threshold. Recorded measurements of transversal vibration displacements near the sockets show that high modes of wind induced vibrations (Karman Vortex) were involved in the fatigue occurrence. The examination of internal state of three other cables confirms the analysis of displacement records in relation with the fatigue limit.
Nootdorp, The Netherlands
Wire rope fracture by exceptional damage
At first sight a broken wire rope with a lot of broken wires led to the wrong conclusion of fatigue as the cause of the breakage. Although most wire fractures were without necking, the service live of the rope was too short for fatigue. A closer examination of these broken wires revealed that the surface of the fractures were too coarse-grained for fatigue fractures. Microscopic examination showed that the metallic structure and the hardness near the fractures were changed. The core wires were found to be discoloured. These things pointed to electric conduction through the rope.
Wire Rope Consultant, Germany
Quality of Steel Wire Ropes
Rope life is evaluated and compared by the results achieved on fatigue testing machines. The author discusses the many other indicators showing rope quality which should give good performance.
General: Consistency e. g. reproduction of wire rope in all details, achieved by craftsmanship or industrialised manufacturing, controlling all possible given setting values within tolerances. Reasons why there are great differences in rope life/performance.
Rope construction: Design values, lay length, lay length relation between strand/rope and rope/core, crossing angles between outer strand wires and core wires, amount of clearance and clearance differences, relation of core-Ø to strand-Ø in relation to rope diameter or fibre core density factor, are influencing rope service life.
In manufacturing: The type of machines, wire strand guiding angles, type of capstan (tilted), tooling etc are influencing torque and consistency. Criteria for consistency and reproduction.
Inspection during manufacturing: Loop turn test, helix-test, rope imprint test, rope end inspection, pushed out/pulled in strands and/or core.
Statistical evaluation of test data: Spinning loss, rope diameter, rope diameter reduction under force, type of wire breaks, wire nicking after break test, and what it means.
Unirope Ltd., Ontario, Canada
Failure of 19×7 ropes on Tower Cranes
The paper describes the investigation into recent failures of 19×7 wire rope used on a Linden Hammerhead- and on a Kodiac Luffing Jib Tower Crane. The presentation includes graphic pictures of core and strand details and will show that such wires ropes will deteriorate from the inside out and will explain why such ropes are impractical to use on construction cranes. It will further present a breaking strength test done on a actual ‘used’ tower crane rope which failed about 75% below the minimum breaking strength when subjected to a test were one end was allowed to rotate freely. The presented cases are currently cause for a Ministry of Labour action to ban 19×7 ropes from use on Construction Tower Cranes in the Canadian Province of Ontario.
School of Construction Management and Engineering, University of Reading, UK
Rope tension in traction winches
Double drum traction winches can develop high tension ratios between rope held on low tension storage reels and rope deployed at high tension. Such winches are used for a variety of applications. The tension profile applicable to the rope passing through such a winch is controlled by the Eytelwein equation and involves local slippage (or creep) at the sliding friction coefficient. In an attempt to reduce the magnitude of creep, winch designs have been developed in which successive wraps have different diameters, to correspond to the rope stretch. This changes the tension profile with potentially serious consequences especially when retrieving rope at low or moderate tension. Furthermore if the two drums on such a winch are not mechanically coupled but hydraulically driven at equal torques, further complications can arise.
Central Mining Institute, Katowice, Poland
Integrated methods for testing and assessment of safety condition of steel wire ropes
Steel wire ropes are important load carrying elements of hoisting equipment, ropeways, ski – and other types of lifts. They usually operate in severe conditions of cyclically varying loads, in the corrosive environment, etc. Therefore, they wear faster when compared with other elements of machinery and equipment. Their technical condition has a vital influence on their continuous, failure – free operation, and, most important of all, on the safety of people using or servicing this equipment. Therefore, of major importance is the problem of various methods of testing and assessment of safety of operating steel wire ropes. The study presents an integrated visual and magnetic method of testing the hoisting ropes in operational conditions. More than 100 tests with the same hoisting rope having surface – contact wires were performed. At present, there are no criteria for the admissible wear of such ropes. For the collected sets of tests results, different methods of analysis and reliable assessment were developed using computer software. Owing to this, the ropes have been successfully operated for eight years.
Even relief of stress in a rope may have disastrous consequences: Two examples
In a study of the recovery length, all wires of sections in a stranded rope were broken on purpose. The sections were examined in static tensile tests. During one of these the rope was to be visually examined before it broke. The tensile force was not increased further, on the contrary it was decreased for “reasons of security”. Shortly after this decrease in force was started, the rope disintegrated explosively.
A number of weights had to be lowered into a pit. A regular lay rope with steel core running over a deflection sheave was used for the purpose. It experienced both strong axial tensile stress and heavy alternating bending at the deflection sheave. The intention was to use the rope for a limited number of cycles only. Unfortunately the rope broke at the deflection sheave after going through less than half the number of planned cycles.
Investigations after the event showed that the rope could in fact withstand almost ten times the number cycles than was envisaged. The rope broke because it was unloaded a number of times during the work process and each time its end was freed. This caused the rope to unwind and kinks arose, a process that distorted the structure of the rope. As one of the kinks rode over the deflection sheave the wires were under uneven tensile stress and experienced increased pressure. This caused them to break one after the other, just like a zip fastener when it opens.
Associated Wire Rope Fabricators, Bethlehem, Pennsylvania, USA
The advent of wire rope “constructions”
Prior to the introduction of the thermal treatment known as “patenting” in 1854, the most important technological innovation of the early wire rope industry was the trial-and-error process whereby ropemakers came to recognize that certain specific arrangements of the wires in a rope provided better service life. This historical paper will present several short case studies describing both successes and failures in Europe and America from which our modern methods of combining wires in ropes have emerged.
Brugg Wire Rope Inc., Switzerland
Extreme Rope Rotation
The problem of rope rotation on a special installation shall be demonstrated and explained. The city transportation authorities of Fribourg, Switzerland, operate a cable car. It is a water powered public transportation system connecting the lower with the upper part of this historical city.
The causes and consequences of a small deviation of a main sheave were very unkind and severe. A real rope experience.
Rope and Ropeways Consulting, Zurich, Switzerland
Examples of vibrations due to a stranded rope running over pulleys and the chosen solution
According to the current state of art, in order to reduce the amplitude of acceleration induced into the structure (like tower or elements of the terminal stations) when a stranded rope is running over a sheave-battery, adjustments have to be made to the geometrical parameters of the sheave-battery as well as to the relation between the rope lay length and the distance between two adjacent sheaves. Yet, the initial cause of the vibration (the helical rope-surface) is accepted without question. It is important to point out here, that adjustments made as described above, will be valid for only one specific wire rope (one certain lay length). Any change in lay length occurring during service for whatever reason will destroy the condition achieved.
This paper presents a reliable method to prevent vibration induced in adjacent structures by means of a stranded rope with an almost cylindrical surface; – the source of the vibration is hereby virtually eliminated. During the development of this product care has been taken, that all the properties necessary in a stranded rope such as splicability, non-destructive-testability, durability etc. either improve or are at least maintained at the current levels. This new stranded rope design is also suitable in applications where single fixed sheaves are used in a vibration-sensitive structure.