A ropeway is a mode of transport in which special types of carriers are suspended from, or simply attached to, an overhead rope to facilitate the transfer of materials, goods or passengers, from one point to another. The rope runs the entire length over which the ropeway operates, which typically ranges from a few hundred metres to several kilometres. The rope is stretched between, and supported at, the end-point stations using anchors. When the distance between two stations is large, intermediate towers are used to support the rope. Longer ropeways are comprised of small sections joined together by divide stations. A divide station has the means to transfer carriers from one section to the next.
The capacity of a ropeway is measured by the load it can carry per hour and depends upon factors such as the speed of travel, the number of carriers and their capacity. The number of carriers varies: a simple system may have only a single carrier, while a complex system could have several carriers placed at regular intervals along the line. The basic components of a ropeway are a cable(s), carrier(s), and supporting structures.
COMPONENTS OF A ROPEWAY
Rope: The rope or cable of a ropeway fulfils two functions: it bears the suspended load and hauls it along. Depending upon the design, a cable can do one or both of these functions. A rope is made up of several strands of steel wires spun together. At the centre of the rope, and of each strand in it, is a core which supports the wires and strands respectively, keeping them in the correct position and minimising friction between them. A stand may contain a single ‘king wire’. In a rope the core may be a strand (a wire-strand core) a second smaller rope (an independent wire-rope core), or a fibre core. A steel-core rope is less flexible but stronger than a fibre-core rope of the same size. The greater strength of a steel-core rope allows the rope to retain its shape under high surface pressures, which ensures that the load is evenly distributed among the wires. Rope with a fibre-core is generally used as a hauling cable.
Ropes can also be classified according to the way in which the wires and strands are laid (Figure 4.1). In an ‘equal lay’ rope, all the wires in a strand are wound with the same helix angle. Wires lie either along the crown of an underlying wire or in a valley between two underlying wires. This construction requires that wires of different layers be of different sizes and that the smaller wires be closer to the core. In a ‘crosslay’ rope the wires in all layers are the same size and a different helix angle or length of lay must be used for each layer. This arrangement results in stress being concentrated at the points where the wires of different layers cross each other.
Ropes can be further classified based on the methods used to wind the strands together; there are two common ways: ‘lang-lay’ and ‘ordinary-lay’ (Figure 4.2). In a lang-lay rope, the direction of the lays of the outer layer of wires is the same as the direction of the lays of the strands within the rope. In an ordinary-lay rope the outer layer of wires is laid in the opposite direction of the lay of the inner strands. Because the outer wires are exposed for a longer time in the lang-lay arrangement, a better and more uniform wearing surface is provided. A lang-lay rope is also slightly more flexible than an ordinary lay rope but, at the same time, more susceptible to untwisting and kinking. This makes a lang-lay rope hard to handle and limits its application to situations in which it can be fixed permanently at each end to prevent its untwisting.
Ropes are specified according to their diameter, number of strands, number of wires in each strand, type of core, arrangement of wires in the strand, and direction of rope lay. For example, a rope of a given diameter may be specified by numbers such as 6 x 19 and 9/9/1. The first number (6) denotes the number of strands in the rope. The second number (19) denotes the total number of wires in each strand. The second set of numbers describes the configuration of each strand: the first number indicates the number of wires in the outer layer (9) followed by the number of wires in the inner second layer (9) and the number of wires in the core (1).
Ropeway carrier: A ropeway carrier consists of a carriage and a container (Figure 4.3). The carriage is made up of a box head and a hanger. The box head attaches the container to the hauling rope by means of a clip or wheels that run over the supporting rope in a bi-cable system and/or over the station shunt rail in other ropeway systems. The container may be a bucket, a tray or a passenger cabin and is suspended from the carriage by means of a hanger which, in general, is curved so that the carrier is able to pass by support stations and towers (trestles) without hitting them.
A bi-cable tower has saddles as well as a roller battery, while a mono-cable tower has only a roller battery to support the hauling-carrying rope
Wooden towers can be used in a ropeway which transports materials in a construction project. This type of ropeway is dismantled after construction is complete. Concrete towers are generally used for small ropeways, while steel towers, because they are reusable, are commonly used in all types of ropeways. Steel towers also provide the flexibility to make towers of any size by using steel frames of pole or lattice type. Large towers are built using lattice frames which are transported to the location and assembled.
Tension stations: Tension stations placed at appropriate points along a ropeway line are used to limit the length and maintain the tension of the track cable. The cable is provided with the required tension using track tension equipment or by anchoring it to the ground.
Drive station: A drive station contains the drive system of a ropeway, which includes a motor drive mechanism and a driving sheave. It also has equipment to maintain the tension of the rope. Suspending counter-weights from a tower or in a pit or a combination of both usually achieves this.
Return station: This station at the end of the line contains a return sheave, from which carriers move back towards the drive station. Equipment to maintain the tension of the hauling cable may be placed in this station as well.
Divide stations: These intermediate stations are used in long ropeways to limit the length and tension of the rope. They divide a ropeway into smaller segments, each of which functions as a separate ropeway. These stations are also provided with fixed rails so that carriers can be transferred from one segment to the next, either manually or automatically.
Sheaves: Sheaves are wheels of large diameter found at both ends of the line and in the stations. The hauling rope passes around them. A driving sheave is powered either by an engine or an electric motor connected to the sheave with a suitable drive mechanism. At the other end of the line there is a return sheave which is not powered but is free to rotate. It responds to floating counter-weights, which maintain the tension in the rope. The rotation of the driving sheave and the difference in tension between the lines at the incoming and outgoing ends causes the rope to move.
TYPES OF ROPEWAYS
Two major types of ropeways systems are used: bi-cable and mono-cable systems. A bicable system uses two ropes. The first is a load-carrying rope which is fixed at the stations at both ends and stays stationary. This rope is also called a track rope, track cable, or skyline. Carriers are suspended from the track rope by wheels which glide over it. The second rope is a hauling rope whose ends are joined to make a loop. The loop is mounted on the sheaves at the stations. The carriages, which are suspended from the track rope, are attached to the hauling rope at fixed intervals using grips. At the drive station, a motor powers the drive sheave. When the sheave rotates, the hauling rope moves in a circle and so do the carriages. When a carriage enters the station its grips are detached from the hauling rope and it moves onto the station rail (see Figure 4.5).
Bi-cable ropeway system
A mono-cable system uses a single rope, which serves the dual purpose of carrying and hauling carriers (see Figure 4.6). Carriages used in a mono-cable system do not have wheels; instead, they are attached to the rope by grips and move when the rope moves.
Ropeway systems may be further sub-divided into circulating and non-circulating systems and systems using fixed or detachable clips.
In a non-circulating system, the hauling rope (or hauling-carrying rope in the case of a mono-cable system) moves forwards and backwards between the loading and unloading stations. One important point about a non-circulating system is that, in a system with one track cable, it can move only one carrier. In a non-circulating system, if the hauling rope winds into a drum then the ropeway is called a drum-type non-circulating bi-cable system (Figure 4.7). In this case, when the carrier goes down, it moves with the force of gravity. The hauling rope has two functions: the first is to control the speed of the carrier when it goes down and the second is to pull it up.
Instead of using a drum, the hauling rope can also form a loop but in this case the direction of motion of the rope must be changed in order to move the carrier back and forth. Such a system is called a looped non-circulating bi-able system (Figure 4.8). A looped system may also use two track cables, in which case it can move two carriers, one carrier on each side. The carriers are always on the same side of the line, because they do not pass the rotating sheaves.
If the carriers run on two sides, then the ropeway may be a mono-cable or a bi-cable to-and-fro system with one carrier on each side of the line. This type of system is arranged so that when one carrier is at the start station, the other is at the return station.
In a circulating system, the hauling rope runs continuously in one direction and passes around the sheaves at the ends. The cars move from one side of the rope to the
other as the rope moves from one station to the other and back. The carriers are detached from the hauling cable temporarily at a station and transferred to a fixed rail for loading and unloading; meanwhile the hauling rope keeps moving with other carriers.
Ropeways are also classified according to whether they have fixed or detachable clips. In a fixed-clip system, the cars are permanently attached to the hauling rope and remain so even when travelling around the sheaves. In a detachable-clip system, each car is manually or automatically detached from the continuously running rope when it enters the station. The car then moves along the station rail on a set of wheels provided in its box head to the other side of the line, where it is clipped back onto the moving rope (shown in Figures 4.5 and 4.6). This feature allows for flexibility in the loading and unloading of cars as well as ease of maintenance and easy removal or addition of cars without affecting the operation of the ropeway.
The ropeways mentioned above are powered either by electric motors or engines, but there is also another class of ropeway, the gravity-operated or self-driven ropeway, which has no external power service. This type of ropeway, whose points of loading and unloading are at different elevations is simple to construct, and thereby often used as a temporary setup (Figure 4.9). The type of ropeways currently used in Nepal are compared in Table 4.1.
There are also mobile ropeways called winch ropeways or cable cranes, which consist of the engine-powered drum of a winch and a specially-designed gripping device that travels on a temporarily-installed skyline cable (Figure 4.10). The cable from the winch passes through the gripping device and is used to lift or deliver loads. Unlike other ropeways, which have fixed stations for loading and unloading, a winch can be stopped at any point to drop or pick up a load. This flexibility makes a winch useful for transporting timber out of forests without damaging the environment. It is also used to transport construction materials across valleys and mountains. Laying
the pipes for the penstock of a hydropower plant or for a drinking water system, for example, can be done using a cable crane.
In Nepal, a cable crane was used to haul the concrete used to manufacture the tetra poles which stabilised the Charnawati River in Dolakha District, which had deepened by five metres during a flood in 1987 and, as a result, threatened the Lamosangu-Jiri Road. Tetra poles cast at the site were used to prevent the river from deepening further. Though transporting concrete by truck from the road to the site in the river channel about a kilometre downstream would have been less expensive, the idea of building an access road was rejected as it would have made the riverbanks more unstable and further jeopardised the road. In any case, the cable crane delivered the material right where the tetra poles were placed and thus provided flexibility in their construction.
MAINTENANCE
How well a ropeway functions—and ultimately its viability—depends on how diligently it is maintained. Even when no particular problem has manifested itself, the system must be religiously inspected at regular intervals. In the Barpak case described in this book (see chapters 9 and 10), the snapping of the hauling cable of the ropeway was attributed to the lack of timely lubricating and tensioning of its ropes.1
Usually the manufacturer provides maintenance guidelines that must be followed very carefully, especially if the system is used for more hours a day than it is designed for. The wear and tear of the various moving parts of any ropeway reduces the life of that system. Just because a ropeway is operating does not mean that its overall performance will be free of problems. Tower bolts and parts loosen with time; pulleys are displaced from their original alignment; and the tension provided to the rope will decrease over time. Decreased tension could reduce the system’s efficiency, causing the rope to sag excessively and the carriers to hit the ground. In extreme cases, the rope itself might touch the ground, damaging to it and the carriers. Too much sag also causes carriers to swing when it is windy and can result in collisions with towers. These problems need constant correcting.
Most accidents and ropeway failures occur due to slackness in maintenance as the following example of the 2.5-kilometre-long Darjeeling-Rangeet passenger ropeway demonstrates. Established in 1960, this ropeway was supposedly checked daily and overhauling was done annually. In October 2003, however, it snapped killing four persons and injuring eight others. The failure was attributed to the fact that routine maintenance procedures that were not alert enough to diagnose the brewing problem.2 High quality work is crucial to the successful operation of a ropeway, but is, in itself not sufficient. In order to ensure safe and efficient service, the regular inspection and maintenance of a system is equally essential. Timely and proper maintenance not only enhances the life of a ropeway but also ensures its efficient, smooth and safe operation. Lack of maintenance results in poor operation and can be fatal for operators, users and people living along the ropeway alignment.
Maintaining a ropeway system comprises two main activities: maintaining the tension of the rope and lubricating its moving parts. The rope must be kept at the recommended tension in order to limit its sag and to retain its load-bearing capacity and life. Tension equipment and anchors are used for this purpose. Tensioning must be carried out as soon as any shift in the alignment is noticed or the wire starts to sag more than is recommended.
Ropes must always be well lubricated. The exact frequency of lubrication needed depends upon factors such as weather, the duration and type of operation and the type of rope used. Since a fibre-core rope stores lubricant and supplies it gradually to the outer wires, less lubricant needs to be applied to it manually. However, one must still ensure that the core has adequate lubricant. Lubrications should be done more often during the monsoon when falling rain washes away the oil. Long duration of use, and/ or continuous operation also requires that increasing frequency of lubrication to keep the rope clean and to protect it from the weathering effects of air, wind, storm abrasive dust particles. Lubrication also saves the ropes from chemical reactions associated with rains and the chemicals (especially acids) dissolved in it. By minimising friction between moving parts and supports, lubrication ensures that operation is smooth and quiet and as a result, the mechanical elements perform better and last longer.
The misalignment and wearing of pulleys can result in the displacement of the rope, which is heavy and under high tension. A slight shift in its alignment can transfer large stresses to pulleys and rope strands and thereby cause severe damage within a short period. This shift could occur for various reasons, the most likely of which is loose nuts and bolts. To keep the rope properly aligned requires that nuts and bolts be kept tight at all times. If pulleys are provided with rubber linings to minimise the wearing of the rope, they must be replaced as soon as they wear out. Worn-out and non-functional pulleys must also be quickly replaced.
In a bi-cable ropeway, the track rope remains stationary and the pulleys of the carrier roll over it. This action wears the upper side of the rope where the pulleys come in contact with it. Although the spiral construction of a rope causes it to rotate a little when a load passes along it, the other three sides of the rope wear less rapidly. To avoid the excessive wearing of only the top, the rope must be periodically rotated 90 degrees to ensure uniform wear around the rope and to prolong its life.
Maintenance also involves moving the track rope a few metres from its existing position. Though the rope is fixed and remains stationary, it moves slightly from its position at the saddle when a loaded carrier passes along it. After the loaded carrier has passed, the rope returns to its original position. In addition, changes in temperature cause the contraction and/or expansion of the rope relative to the saddle. This movement results in the lower surface to wear faster than the upper side of the rope caused by the movement of carriers because the additional load of the rope adds to the cargo load. Wear is also higher because the friction in sliding in higher than that in rolling. Since the rope comes back to its original place when the ropeway is at rest, such wear is difficult to observe. To prevent excessive wear, the track rope must be moved along its length by a few metres so that a new section rests on the saddle. This operation can be performed when rotating the rope.
The fraying of ropes causes most ropeway accidents. A rope does not snap at once; instead, fraying commences gradually. As each individual wire of a strand breaks, the strength of the rope is considerably reduced. Each successive wire breaks more easily and until the rope snaps. To prevent this from happening, the condition of the rope must be carefully checked. If any individual wire is broken, it indicates that the rope’s overall strength has declined. Broken wires can be spliced together by a skilled technician, but repeated splicing can reduce the rope’s integrity and it must be replaced to improve safety. These maintenance activities help prevent accidents.
Ensuring effective maintenance essentially means inculcating a technological culture and a scientific outlook, which attributes are not prevalent to the required degree in any rural setting in developing countries. Since the ropeway industry is in its infancy in Nepal there are very few local experts that can provide maintenance support. Unlike operation training, which is provided by ropeway manufacturers at the time of installation, and for which the operators develop skill over time, such support, even if provided rigorously in the beginning, is likely to be weak when the actual maintenance activities are undertaken. A sound maintenance culture will come about only if those engaged in various aspects of ropeway industry—those in civil society concerned with safety as well as those in the government crafting and enforcing laws—engage constructively with each other. It is a long, hard and continuous, but ultimately beneficial process into which industries, owners and communities need to build a fair number of self-help, self-regulating and policing mechanisms.
NOTES
1 Earth Consult, 2000: A Case Study of Rangrung-Barpak Ropeway, A report submitted to ITDG Nepal, Earth Consult (Pvt.) Ltd., Kathmandu.
2 Times News Network, October 20, 2003
-MADHUKAR UPADHYA AND KIRTAN RAM BHANDARY
Source: Ropeway in Nepal
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