GANESH KUMAR SINGH
HAULING STONES
Many industries use ropeways to transport raw materials to their production lines. In a cement factory a ropeway can be used to bring limestone from the quarry to the processing plant, which is one of the central activities in cement production. The cost of transporting limestone determines the cost of the cement produced, and, as a result, the commercial viability of a plant. Heavy-duty vehicles are also often used to transport stone. Though this practice has the advantage of flexibility, a large number of mechanics are needed because vehicles require regular maintenance to avoid breakdown. Furthermore, transport by trucks is expensive because they run on imported fuel. Shortages of fuels, which occur from time to time due to local, national and international causes, jeopardise their smooth operation.
A ropeway is a reliable system for moving stones. Though a ropeway runs on expensive equipment requiring high initial capital investment, its maintenance and operational costs are less compared to those of trucks. Another advantage of a ropeway is that, except for high winds—which last for a short time anyway—unfavourable weather conditions, in particular the monsoon rains, do not affect its operation. Its effective operation is, however, critically dependent upon regular maintenance and immediate repair. Continuity is essential as even a brief stoppage in the provision of limestone can completely disrupt cement production.
For a cement factory, using an electric ropeway is a viable alternative to relying on trucks. A ropeway ensures a regular supply of stones and, in view of Nepal’s hydroelectricity potential, fits well into the local economy. The following sections describe some technical aspects of the ropeway used at the Hetauda Cement Factory; the technical details are provided in Annex B.
THE ROPEWAY SYSTEM
The Hetauda Cement Factory uses a bi-cable circulating ropeway system to bring stones from a quarry located 11 kilometres away (Figure 7.1). Usha Breco Ltd., an Indian company, constructed the ropeway, which can transport 150 tonnes of stones an hour from the silo at the quarry through the needle gate and the apron feeder and along the conveyor to the unloading station at the factory. Not only is a ropeway reliable, the cost of transporting a tonne of limestone by ropeway is Rs 61, thirty rupees less than transporting the same amount by truck. By using a ropeway the factory saves about two million rupees annually in stone transportation alone. The ropeway presents no major operational problems as long as the supply of electricity is uninterrupted.
The Hetauda Cement Factory Ropeway System has three major components: line equipment, a loading station and an unloading station.
Line equipment
The essential characteristic of this system is the use of two static overhead ropes on which carriers run. Each carrier has a hanger, from which a bucket is suspended. The hanger is attached to the hauling rope, which moves continuously in a loop. The wheels of the carrier enable it to ride over the track rope. The ropes are supported at a convenient height above the ground by means of trestles or towers spread out along the line. Each trestle is provided with oscillating saddles with grooves that support the track ropes and rollers that support the hauling rope. One track rope carries loaded buckets, while a second rope, whose diameter is smaller, carries empty buckets. Because the buckets carry loads from the quarry to the plant but return to the quarry empty, two different sizes of track ropes are used. The two track cables run parallel; the distance between them (the ropeway gauge) is four metres. The carriers grip the moving hauling rope along the entire line, they are detached only at the loading, divide and unloading stations, where they are removed from the track rope and moved onto fixed rails.
If the distance between the supporting structures is great, the tension in the track ropes is high. This situation often occurs when a tower cannot be located at a proper site or the ropeway crosses a wide valley. When spans are long, large and strong supports are provided and pressure-frame-cum-track-rope divide stations are installed. In these stations, a battery of rollers support the hauling rope. The track rope is supported either by the front or rear saddle of the pressure-frame-cum-trackrope divide station. It has to be anchored near the station entrance or exit either by
FIGURE 7.1:
Profile of ropeway from loading to unloading stations
Pressure
Loading frame
dead-weight or tension anchors. A tension of about 76 metric tonnes is provided on a loaded rope, whereas on the unloaded side the tension is maintained at a maximum of 33 metric tonnes. The track rope is divided into small lengths along the ropeway line to avoid high tension and the need for thick ropes. A frame called a pressure frame (PF) is used to take up the load. In addition to the PFs, the ropeway has track-rope divide (TRD) stations where track ropes on one side of the station can be tensioned using weights. There are three PFs (PF-1, PF-2 and PF-4) and three TRDs (TRD-3, TRD-5 and TRD-6) in the Hetauda Cement Factory Ropeway. There are fifty-seven trestles and one divide station. In all, there are nine stations. At each TRD and PF station, buckets move onto rails upon entering the station.
Each steel-latticed trestle has a cast saddle with grooves for supporting the track rope and providing for oscillation. When the line is empty of buckets, the hauling rope sags excessively; thus, two rollers are provided on each trestle to support it. V-shaped rods guide the hauling rope onto the rollers. A cathead is provided at the top of each trestle to allow for the lifting of the track ropes with suitable tackle while removing a saddle or hoisting a rope. Each tower supports seven wires at the top; they are used for telephoning, issuing commands, emergency control, standby operations, and to indicate faults at the unloading station. These wires help control and monitor ropeway operations. There are 164 bottom-discharge buckets with catch levers and chains for automatic tripping and self-latching. Each carriage has a bucket made of a steel frame, four wheels with nylon liners, and an automatic jaw-gripping device equipped with an operating lever to clamp it to the hauling rope.
Loading station
Once limestone is mined, it is crushed using heavy machines at the quarry. Then it is fed to a silo, from which it flows through a needle gate onto an apron feeder. The
FIGURE 7.2: Loading station at Hetauda Cement Factory (schematic)
Needle gateSilo
feeder, in turn, discharges the material onto a belt conveyor, which carries it to the loading station, where it is fed to the rotary distributor for loading into buckets (Figure 7.2).
The apron feeder and the belt conveyor are designed to transfer 360 tonnes of limestone per hour at peak capacity. The apron feeder has a variable speed drive arrangement to regulate the flow of limestone. The centre distance of the apron feeder is 3,658 millimetres. A 15-kW motor with a controlled eddy current, variable speed drive and a speed rating between 120 and 1200 rpm is used to drive the feeder. The belt conveyor has an independent drive provided by a 10-HP squirrel cage motor. The conveyor is equipped with a weighing arrangement to indicate the flow rate of the material.
A speed-sensing device controls the start and stop of the apron feeder motor. Unless the ropeway attains full speed, the apron feeder will not start. The ropeway takes nearly 90 seconds to accelerate to full speed, whereas the feeder motor accelerates instantly. This speed-sensing device is used to prevent the spillage of material on the floor of the station due to any inaccuracy in timing its transfer to the ropeway from the feeder.
The rotary distributor is divided into four compartments, each with an attached chute. The complete unit rotates in such a way that material is delivered to each compartment as it passes under the four discharge chutes. Each chute, in turn, feeds the material into a ropeway bucket, which is pushed around the station rail loop by an arm attached to the rotating distributor chute. The speed of the distributor is adjusted so that it will continuously deliver full buckets of material at the correct time interval. The distributor is driven by the driving sheave, so its speed is controlled by the speed of the ropeway.
A steel structure forms the rail support, which carries the buckets after they arrive at the station and are detached from the hauling rope. The speed of the buckets is reduced and finally stopped by an automatic spacer, which allows each bucket to enter the rotary distributor at the correct time interval. The spacer is guided by a trip mechanism operated by the distributor. The bucket is then loaded by the distributor and pushed automatically around the return loop onto the outgoing side. At this point the rail slopes in order to accelerate the buckets to the speed of the hauling rope under the control of the overhead chain retarder driven from the ropeway sheave. Each bucket is then attached to the hauling rope using a jaw-gripping mechanism before it passes out onto the line. Four rail switches are provided either for transferring buckets from the station rail to the parking rail, where the buckets can be repaired, inspected, or changed, or for loading new buckets from the parking rail to the station rail.
The main drive gear comprises a 240-kW, 1500-rpm DC motor, a drum-type coupling with a manual brake, a gear box, a geared coupling, a driving pinion, a fleeting sheave in a spherical roller bearing plumber blocks, and a thrust bearing. The sheave is driven by spurring fitted to the sheave rim. Also attached to the sheave rim is a brake ring. The hand brake is operated by weights. The drive sheave is mounted on a sheave frame and the complete drive gear unit is mounted on a fabricated bedplate fixed on a foundation.
Also present at the loading station is the hauling rope lubricator, which consists of a tank filled with oil and is equipped with a shut-off cork and rope lubricating brushes. The hauling rope is lubricated using a drip-feed system.
Unloading station components
A loaded bucket arriving at a station is transferred from the rope to the rail after passing over the entrance saddle bar. It continues through the unlocking frame, where its grip is automatically detached from the hauling rope. The bucket then decelerates under the control of the overhead chain haulage. It passes over the hopper, where it is emptied when the bucket-catch strikes the trip gear. Under the control of a chain retarder, the bucket then passes onto an accelerating grade into the locking frame, where it is re-attached to the hauling rope. It then passes over the exit saddle and saddle bar onto the return sidetrack rope. The unloading station has provisions for removing damaged buckets from the line and replacing them with good buckets.
Damaged buckets are kept in the parking rail for maintenance. Rail switches are operated manually and are used to put on or take off buckets. The unloading station also houses the hauling rope tension gear.
INSPECTION, REPAIR AND MAINTENANCE
To ensure its good performance, long life and smooth operation, it is necessary that the various parts of the ropeway system be inspected regularly and repaired if necessary. The track rope, hauling rope, carriers, tension equipment, and towers, as well as other equipment in the loading and unloading stations, should be inspected every three months or after 1,000 hours of operation. They should be cleaned, greased, and adjusted as required.
To lubricate the ropes there is a track-rope oiling car equipped with a special carriage which has an oil tank. As the car traverses the line, oil is fed onto the rope from the tank through pipes. A special car called an inspection car is used to inspect the ropes. The inspector sits inside the car, which is enclosed in wire mesh for his protection. The car is fitted with a collapsible ladder on which the inspector can stand when inspecting the rope.
Routine maintenance is necessary to ensure that bolts are tight and that steel parts missing from the towers, stations and pressure frame are replaced. To prevent corrosion, the structures are periodically painted. The alignment and level of the foundations of the structures are also regularly checked. The foundation material is likely to erode or sink in the monsoon, and the structures may tilt. The level of a trestle is important as its sinking may lead to serious damage or may cause the track rope to lift off the trestle. Insulators and overhead wires on the tower top are checked to ensure that they are in proper working condition. Damage to the wires, such as the fault and emergency wires in the unloading station, will prevent the ropeway from starting. An oiling and cleaning record is maintained for each station. A record of the cars, arranged in numerical order, is also kept to note the date on which each carrier was last oiled and serviced. The major areas requiring repair and maintenance are discussed below.
Track rope
To prevent the track rope from becoming dried out and rusty and to ensure that the rope is lubricated at all times, the oiling carrier must be sent out along the line at regular intervals. The exact interval depends on humidity, temperature, and other weather-related factors. The rope must be lubricated more often during the rainy season and must also be checked to ensure that there is no breakage. Both activities can be done from the inspection carrier. If a wire is broken, immediate remedial action must be taken, either by brazing the wires or by using a sleeve cover over the broken part. While carrying out repairs, care must be taken that there are no protrusions of wire which could cut into the nylon carriage wheels. Also, when the rope is lifted from a saddle onto a trestle, care must be taken so that the rope doesn’t kink.
Track rope tensioning
To replace the entire track rope or a section of it, the following procedure is adopted. The tackle is rigged at the dead end in order to lower the tension weight and the old rope is lowered to the ground. The new rope is pulled out over the ground, protecting it from obstacles using logs and rollers. The actual method of pulling depends on the availability of equipment. At the Hetauda Cement Factory a heavy bulldozer with tracks and a winch drive is usually used. It is necessary to position the reels along the line and to avoid pulling the entire length at once, a process which requires a very powerful winch.
Splicing
When it is necessary to splice the hauling rope, the first step is choosing a location for carrying out the work. Splicing can be done only on the ground. Any point along the line where the trestles are not very high and access is convenient can be chosen.
Carrier
The carriages are inspected every three months. They are cleaned, greased, and adjusted. All worn-out parts are replaced. A spare carrier replaces any damaged carrier on the line so that the ropeway can still operate while the damaged carrier is being repaired. The damaged carrier is repaired immediately and kept in a parking rail at a station as a spare. All castings are carefully examined for cracks, particularly if a carrier has fallen or derailed. Another potential problem is with the carriage unlocking rollers; if they are worn out they may prevent the carrier from detaching from the rope. They must be kept well lubricated so that they can rotate easily and do not wear out. A roller must be replaced as soon as its diameter is worn down by more than 10 millimetres. Likewise, gripping jaws always have to be kept clean so that they work properly; this step is necessary because safety depends heavily on the reliable action of moving parts. The nylon liners in the carriage wheels break or wear out often. When that happens, they have to be replaced with new carriage wheels or the nylon liners have to be changed.
OPERATIONAL PROBLEMS
Ropeway operation sometimes poses problems. Despite routine maintenance and repairs, some mechanical failure is inevitable. Some of the problems that have occurred in the past were due to parts becoming old and worn with use, while others were due to weather or the lack of parts and equipment. Over the years quite a number of problems have occurred, ranging from the simple to the bizarre.
The coupling of a load sidetrack rope failed once. This major snag took oneand-a-half months to rectify. In order to avoid such problems in the future, about 1,000 metres of track rope was replaced while the coupling was being fixed. The track rope has fallen off the towers twice; each time it took two to three months to repair and cost 1.7 million rupees. Of the seven wires on the tower tops, only three are currently working: the emergency wire (for stopping the ropeway) and two telephone wires. There was also a problem with the DC motor winding at one point and four of the coils had to be changed. They were ordered from Kirloskar, an Indian company, at a total cost of one million rupees, but since Kirloskar said it could supply the coils only after six months and it was not desirable to keep the ropeway inactive for such a long time, locally-made coils were used temporarily. The motor started running at a slower speed—1,000 rpm as opposed to 1,400 rpm—but the factory had to make do until the new coils arrived.
A new problem that has occurred after 17 years of operation is that one pressure frame has sunk because of a landslide and is no longer supporting the track rope. Heavy rainfall and the difficulty in accessing the pressure frame have made it necessary to shut down the ropeway temporarily. Because some of the towers and pressure frames have been built in places that are difficult to access by road, it is difficult to rectify problems which occur at these locations, especially during the monsoon season. Even problems that could have been solved easily became problematic and rendering them time-consuming simply because it was hard to reach the towers. To overcome such problems, the Udayapur Cement Factory, which has a similar ropeway system, has built all its supporting structures in easily accessible locations in order to reduce the time and effort required for repair and maintenance.
One bizarre problem occurred when the local municipality started dumping garbage under the ropeway line. A dumpster truck got entangled in the hauling rope and was lifted by it and carried over a mountain before it fell off. Luckily no one was hurt, but it could have been a serious problem endangering lives and damaging the ropeway. This unexpected problem was the result of ignorance. A few other operational problems that have been encountered are itemised below.
1 High voltage can cause surges and even burn ropeway components, both of which are problematical.
2 Breaker trips due to interruptions in the supply of electricity are a common problem faced by the ropeway.
3. Gearbox lubrication pressure needs to be maintained at all times.
4. High wind speeds cause buckets to swing. It is important to stop the system until the wind speed decreases.
5. Scattered pieces of limestone accumulate along the line, causing low clearance and obstructing the passage of the buckets. These accumulations need to be swept away periodically .
6. Nylon tyres should be replaced as soon as they are damaged.
7. Nuts and bolts become loose during operation; their tightness must be checked daily before and after operation.
8. The gearbox is a vital part of the system and when changed should be replaced. A spare must be kept available in the inventory.
9. If the grip of a bucket is loose, it fails to hold onto the rope. It then runs faster due to gravity, striking other buckets on the line and damaging them. To prevent this, all grips have to be checked regularly.
10. It is necessary to employ a trained splicing expert. The factory has trained technicians to splice the hauling rope.
11. The emergency system still uses a naked wire, which causes problems because it breaks down often. If possible, naked wires should not be used.
12. The factory uses a naked wire and an old-style telephone system for communication. It is unreliable and breaks down frequently. As a result, communication among the various stations along the ropeway fails every now and then. A modern communication system would prevent such failures.
13. The side roller should run smoothly. While the clamp is being unlocked, the carriage should not jerk sideways or grind on guard angles. Locking and unlocking must be smooth and free from any jerking or snagging.
14. A minimum of three buckets should always be in the retarder and at the spacer. Fewer than three buckets results in a missed spacing and the material for the missing bucket will be discharged onto the ground. To prevent the material from spilling, whenever damaged buckets are put on the parking rail for servicing, good buckets should replace them.
CONCLUSION
This semi-automatic ropeway system brings limestone from the quarry to the Hetauda Cement Factory as part of the continuous feeding of raw material to the production process. For this reason, it could be thought of as an industrial ropeway. For the plant to remain operational, the ropeway must keep delivering limestone; any delay in supply diminishes the plant’s cement output. Unlike other ropeways used for transporting cargo or passengers, this ropeway is an integral part of the production process and must run to ensure that cement production continues. Time is very important in any production line. Proper coordination between the operation of the ropeway and that of the main cement plant is critical.
Because it carries stones, the parts of this ropeway wear out quickly. Also, since it works automatically at several stations, including the loading and unloading stations, a snag at one point creates problems for the entire cement production system. Continuous monitoring of the system and a very effective repair and maintenance team is an absolute necessity. Though a few problems have hampered the delivery of stones in the past, the maintenance team at the factory has ensured the proper functioning of the ropeway.
The ropeway runs for long hours. The people involved in the operation and maintenance of the ropeway at the Hetauda Cement Factory are able to operate it as required and to deal with challenges successfully. The ropeway helps bring down the cost of cement production to a competitive level, which is important if the industry is to contribute to the national economy.
The use of a ropeway to transport limestone has proved to be effective. The main reason for its success is that once the ropeway begins running it can transport limestone almost continuously with very few stoppages. Fewer employees can operate and maintain it than would be needed to operate a fleet of trucks. Time and again there have been shortages of petrol and diesel in Nepal; if trucks had been used this would have resulted in disruptions in the supply of limestone to the factory.
One problem regarding the ropeway is that when breakdowns occur the time required to repair the system can sometimes be long (measured in months) simply because replacement parts are not available within the country. The stoppage can result in financial losses because production is hampered. Another problem is that some towers and stations are located in isolated places and accessing them to conduct repairs is difficult, especially during the monsoon. A final disadvantage is that a specialised maintenance crew is needed to check the system thoroughly because of the large distances over which inspections and repairs must be carried out.
Source: Ropeway in Nepal
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