Rocking Rollers. Further scientific news by TU Delft Colophon DO-Archive. Rocking Rollers. by Joost van Kasteren

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1 Rocking Rollers Rocking Rollers Further scientific news by TU Delft Colophon DO-Archive by Joost van Kasteren The weight of people and luggage makes the conveyor belts at Schiphol Airport sag a little, causing extra wear to both the plastic guide rollers and the rubber conveyor belt itself. Together with Karel Drenth, Gabriel Lodewijks, Professor of Transport Engineering & Logistics at Delft University of Technology, has devised a suspension system in which the guide wheel positions itself to match the weight of people and luggage. The system is simplicity itself, so the Delft engineers could hardly believe it had not been patented before. Now they patented it. Ore from the manganese mines near Moanda in Gabon is carried to the storage and transfer yards by a 6.5 kilometre long belt conveyor system. The system cuts straight through the mountainous jungle of the Gabon interior before. Upon arrival, the ore is moved by train to the port of Libreville. The belt conveyor has been with us for about a century and a half, Lodewijks explains. The system consists of an endless belt supported by rollers or wheels that loops around a pair of drums and which can maintain a constant flow of people or materials. Belt conveyors are being used for a wide variety of applications. They are found in mines where they are used to bring ore to the file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (1 van 8) [ :04:36]

2 surface, but also at power stations to move coal to the burners. The longest belt conveyor in the Netherlands, measuring about 2.5 kilometres, is near Rotterdam, where it carries coal from a bulk terminal to a nearby coal-fired power station. Australia has a belt over nine kilometres long carrying coal straight from an open pit mine to a power station. And some are even longer, according to Lodewijks. A conveyor belt at the Henderson mine near Denver, Colorado (usa), carries molybdenum ore over a distance of 18 kilometres. Any longer than that, and you would run into the limit of what is technically feasible, Lodewijks says. But over shorter distances, the possibilities for belt conveyors are endless. An increasing number of airports use them to carry passengers from the departure lounges to the boarding gates and vice versa. These «moving sidewalks» are made either of steel plates that slot together, like a horizontal escalator, or of a rubber belt running over guide wheels, rather like the conveyor systems using in the mining industry. The advantage of the latter variety is that it operates a lot more quietly and is simpler to construct than the flat escalator. Both types can be found at Schiphol Amsterdam Airport. Stiletto heels Lodewijks shows a piece of the type of belt used at Schiphol Airport. It is made of rubber and is at least five centimetres thick. Grooves along the length of the upper surface make sure nothing can get caught in the belt. Lodewijks: At the end of the moving sidewalk, the grooves meet a rake plate that prevents stiletto heels or children s feet getting caught between the moving belt and the stationary floor. The rake plate also collects any debris from the belt so it cannot end up inside the mechanism. The five-centimetre thick rubber belt body is reinforced with steel wire configured to ensure maximum transverse rigidity. The belt is not as stiff along its longitudinal axis as in the transverse direction, Lodewijks explains, or it would be impossible to loop it around the drive drums. Since the belt is as stiff as a board along its full 1.20 metre width, it needs no support. The only support is at the sides, where it is carried by polymer wheels fixed to the belt frame at fifteen centimetre intervals. Even so, the belt barely flexes at all, not even with people standing close together. The wheels cause the problem, Lodewijks says. Although the belt barely flexes at all, it still bends a little. The guide wheel shafts at the sides are fixed in place, they cannot swivel, so the wheels stay upright whatever happens. As the belt flexes, it no longer runs flat along the top of the wheel, but touches it at an angle. The result is that the wheels wear down at the sides, and that a groove gets worn in the underside of the belt, so regular maintenance is required to replace the worn parts. Airport Although the extra wear is not very significant at the current belt speed, about 0.7 m/s (approx. 2.5 km/h), it would be a different matter if the belt speed were to be increased to, say, 2.8 m/s (10 km/h). This is now being considered, since the capacity of belt conveyor systems At Hendrina (South Africa), a system of five long belt conveyors was installed in It is used to carry coal from an open pit mine straight to the Hendrina power station, a distance of 35 kilometres. Not only do the belts have to cross roads, fields, and rivers, they must also steer clear of small villages as much as possible. The belt conveyor systems used at airports to move passengers differ from those used for ore and coal, in that their main design targets are safety, speed, and capacity. In large-scale belt conveyor systems, logistics plays as much a role as transport technology. Belt conveyors in large-scale systems are modular in construction. A single module consists of a steel frame carrying triple roller sets for the outgoing belt section, and twin roller sets for the return section. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (2 van 8) [ :04:36]

3 at airports is proving insufficient at current speeds. Lodewijks: A one-terminal concept, as at Schiphol Amsterdam Airport, is feasible only if passengers can move quickly from one side of the airport to the other to catch a connecting flight. The capacity of belt conveyors is reduced even more at low speeds when people who are in a hurry start walking along them. Differences in speed reduce the overall capacity, as we can see every day on our motorways. According to Lodewijks, running a conveyor belt at double speed does require some extra precautions to prevent people tripping as they step on and off the belt. The necessary technology is already available. Lodewijks shows a photograph of an «accelerated walkway» developed by the French company, cnim. At the entry and exit points, the belt moves at walking speed, but in a number of stages in between it reaches a speed of 12 km/h. Its capacity is 13,000 passengers per hour. This type of belt conveyor has been in service at the Montparnasse subway station in Paris since the spring of Polar point Back to the problem of wear. If the speed of the conveyor belt is doubled, the amount of wear increases fourfold. If, as in the Paris example, the speed is increased to five times the original value, the amount of wear increases 25 times over, causing problems not only with the cost, but also with maintenance. Every time the worn support wheels need replacing, the belt conveyor has to be taken out of service. The solution that Lodewijks and Drenth have come up with consists of a guide wheel that tilts whenever the belt flexes under the weight of the passengers. The Polaroll, as the wheel has been dubbed, is in fact a support system that comes in two parts. One is a support block that is attached to the guide frame. It features a pair of curved slots, one at the top, and one at the front. Then there is another block to which the wheel shaft is attached, and which has two pins. The working principle of the system is simplicity itself. The block carrying the wheel is attached to the support block by fitting the two pins into the curved slots, first the one at the top, and then the one at the front. With the bare belt running over the wheel, the wheel remains in the normal, upright position. As the belt flexes under the weight of passengers, the wheel is able to tilt slightly as the pins move along the curved slots. Once the load has moved on, the wheel moves back to its former position. If a dozen people stand close together on the belt, the wheel simply tilts a little more than usual. The secret is in the configuration of the curved slots and pins, Lodewijks says, each set of which follows a circle with its centre point exactly in the middle of the top surface of the wheel, the polar point. As soon as the belt flexes to form an angle with the wheel, this forces the pins to shift and so tilt the block carrying the wheel. At the same time, the belt acts as a counterforce, provided the belt is more or less rigid along its width. If the belt were flexible, the pins would keep moving to and fro, and the wheel would be fluttering all the time. Fenner Dunlop manufactures conveyor belts for passenger transport systems. The belt consists of a steel wire and canvas sandwich cover topped with a rubber tread. Just like metal escalator treads, the belt tread features grooves into which the teeth of the rake plate fit to collect debris safely away from the belt mechanism. The belt is supported on either side by rollers. With a person standing on the belt, the vertical downward force causes the belt to flex, so the sides of the belt move upwards and inwards. As a result, the belt now rests on the edge of the support roller, which causes extra wear to the belt and roller. Current support rollers feature a plastic shell and an internal ball bearing. The wheel is fitted to the frame by means of its threaded shaft. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (3 van 8) [ :04:36]

4 Cashier In spite of its simplicity, no mention of the invention could be found in literature, Lodewijks says, so a patent was applied for. While the patent application is being processed, a prototype is being constructed at TU Delft of what the patent application somewhat lengthily describes as a roll rotating around an axis for the purpose of supporting a body moving axially, and at least a first support in which the shaft of the roll has been mounted to support the roll. Lodewijks is still waiting for signs of interest from the industry, but he reckons the first ones cannot be far off. Whether they will include companies manufacturing belt conveyors for airports remains to be seen. Lodewijks: Many of these companies also supply the flat metal escalatortype systems, so their need for innovation may not be very urgent. Other possible applications include belt conveyors for ore and coal and for transporting semi finished products in factories, but candidates also include the small belt conveyors that move shopping along to the cashier at the till. In a previous life as a mining consultant for South African mines among others, Lodewijks established many useful contacts among manufacturers of this type of conveyor belts, and he is confident that his invention will find a use. Distributed drive systems In his inaugural speech in late 2001, Lodewijks argued that we are now capable of designing belt conveyor systems that border on what is technically possible. In an attempt to reach across the technical limits, distributed drive systems are increasingly being considered as alternatives. The distinguishing feature of these systems is that the sole remaining task of the conveyor belt itself is to carry the load. The drive power is transmitted by different means. One option is to use more than one drive system. In other words, instead of having a single drive drum at the end of the belt, a number of smaller drive units are used at regular intervals along the belt. Another option is to use two belts, one of which transmits the driving power, with the other carrying the load, the booster belt concept. The advantage of separating the load-carrying capacity from the drive system is that it enables the belt tension to be reduced, which in turn means that a lighter type of construction can be used. A lighter construction can also be achieved by adding «intelligence» to the belt conveyor drive system. Current systems tend to be rather simple, offering only three or our functions: start, stop, steady state, and sometimes an emergency stop. The emergency stop function in particular causes components to be over designed. Lodewijks: Try to imagine a two-kilometre long, end-driven belt carrying ore. Its inertial mass is enormous, so it may take half a minute or longer to come to a halt. In some cases there simply isn t that much time. As an example, Lodewijks recalls a mine in South Africa where the belt conveyor was used not only to bring up ore from below ground, but also to move miners up and down. The men would mount the belt from a moving platform, lie down on the belt, and so be carried down into the mine, often for thousands of metres. The same happened on the way Current types of roller are cylindrical. If the belt flexes under the weight of passengers, the contact area moves to the edge of the roller, causing excessive wear to both the roller and the underside of the belt. The effect is compounded if production methods cause the diameter of the roller to be slightly larger at the edges than at the centre of the running surface. An alternative to the cylindrical roller is to use a curved roller. This somewhat reduces the wear caused by flexing, but it does not remove the problem altogether. Where the support roller meets the belt, the belt will be practically straight, as a result of which the contact area between the belt and the curved roller surface will be minimal. In addition the curvature of the running surface means that the diameter of the roller in the contact area varies, causing the roller to slip on the belt surface, which results in additional wear. In this example, the right-hand edge has even been worn down flat. The laboratory of the Transport Engineering & Logistics Section at Delft University of Technology has a small test unit that is used to test new support rollers. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (4 van 8) [ :04:36]

5 back, in some cases with the miners lying among the ore. Lodewijks: After a heavy day below ground they tended to be rather tired, and often fell asleep, missing the exit platform. If they did, they risked being dropped ten metres or more onto the ore. If they were to stand any chance at all, the conveyor belt had to be capable of being stopped within six seconds, which is the time between passing the platform and the dumping point. Using a distributed drive and intelligent control system, this can be done without the need for grossly over designing the system. In addition the combination offers energy-saving possibilities, by limiting the drive systems to the part of the belt that actually carry loads, for example. The belt is supported by small plastic rollers mounted only 15 centimetres apart. For more information, please contact Prof. Dr. Ir. Gabriel Lodewijks, phone , mailto_g. lodewijks_wbmt.tudelft.html, or Ir. Karel Drenth, phone , k.f. drenth@wbmt.tudelft.nl. Other than a not unpleasant «springy» effect of the belt, passengers cannot feel the difference between a «flat escalator» and a conveyor belt because the rollers are so close together and the belt is extremely stiff along its width. Fitting a new roller of the current type of involves considerable effort on the part of the maintenance engineers. The belt has to be lifted and the roller secured in place with bolts and nuts. To do so, the conveyor belt system has to be partly dismantled. Fitting a new Polaroll is very simple and consists of two actions. First, the top pin of the wheel block is engaged with the upper slot of the sup-port block. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (5 van 8) [ :04:36]

6 Then the lower pin of the wheel block is engaged with the lower slot of the support block, and the roller is rotated into position. The support block is fitted permanently to the conveyor frame, and the pins form part of the wheel block. There is no longer any need to dismantle the con-veyor system. When the belt supports the weight of passengers, it flexes. This causes a force to act on the inner edge of the roll-er s running surface, just like it would on a normal roller. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (6 van 8) [ :04:36]

7 The force acting on the inner edge of the roller results in a torque around the centre line of the roller, causing the roller to rotate around the polar point, so the belt again fits snugly against the roller, minimizing wear. Thanks to this simple mechanism the Polaroll is able to continuously adapt itself to the loads imposed on the belt. In order to enable the belt to be looped around the drive drum at the end of the conveyor system, its rigidity along its length is considerably less than it is along its width. As the illustration shows, the belt is easy to bend by hand. The belt receives its only support from rollers along its edges. In order to minimize flexing along its width, the belt has been made extremely rigid in that direction. It is practically impossible to bend it by hand. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (7 van 8) [ :04:36]

8 Belt conveyor at the Montparnasse underground station in Paris (France). In the future, the speeds of conveyor belts for passenger transport purposes will increase to four to six times their present values. To prevent excessive wear to the belts and support rollers, smart wheels like the Delft Polaroll will be needed. file:///r /BTUD/Teams/permanente%20teams/Team%20TU%.../Projecten/ProjectOutlook(retro)/rocking_ htm (8 van 8) [ :04:36]