Resilient, rugged, low-maintenance

For more than ten years now, Holcim Ltd. has been operating six concrete factories, a ­cement plant and three quarries in Croatia. Right from the start, ­Holcim Croatia has largely employed drive units from Nord Drivesystems for the conveyor belts installed at its facilities. The products of the northern German manufacturer are equal to any challenge – no matter whether the conveying systems is fast- or slow-running, whether the conveying distances are short or long or whether rock, sand, gravel or cement has to be transported. Nord plans the systems and supplies reliable and low-maintenance components for the entire drive train. Especially for Croatia, Holcim and Nord concluded a three-year contract for the replacement of old drive units.

Cement production

In the Holcim cement plant in Koromacno on the Istrian east coast, geared motors from Nord have been in service for more than ten years. On average, the plant produces around 500 000 t of cement per year and it can achieve a maximum annual capacity of 750 000 t. The cement is manufactured continuously by the dry process, using predominantly natural raw materials. These raw materials – normally limestone and sand, occasionally industrial slag and ores – are extracted in local quarries or are delivered. The limestone for Koromacno comes from a mine located about 2 km from the plant. The material reserves ensure that cement can be manufactured here for a further 120 years.
In the cement plant, units from Nord are also used for all conveying systems. A number of drives are designed for operation in potentially explosive areas designated as ATEX zones 21 and 22. Most of the drives make use of medium-sized geared motors. Depending on the application, these are spur gear, flat or bevel-geared motors (Figs. 1 and 2) with power ratings of up to 45 kW, generally with the IP55 type of protection.

Nord’s unicase housing design is a major plus factor for operation in rugged conditions. All the bearing seats and sealing surfaces are manufactured in a single clamping in ultramodern CNC centres. The benefits are: extremely high operational reliability, excellent sealing, safe operation at even the highest torques, high axial and radial loading capability, low-noise running, low maintenance requirement and a particularly long service life.

Extraction of raw materials

In the quarries of Sumber ­(Istria) and Ocura (north of ­Zagreb), rock is extracted by blasting and then transported by belt conveyors from hopper to hopper and finally to the preparation system. There, the rock is comminuted in crushers and then classified by screens into the size ranges up to 3.3, 8.8, 16 and 32 mm. The end products are different building materials such as crushed-stone sand, grit, crushed stone etc. These products are subsequently conveyed to stockpiles.

Various different systems, each presenting their own challenges, are used for transporting the material. The belt conveyors used for transporting the large rocks have to overcome gradients of up to 25 degrees. The ground stone with a size range up to 250 μm is conveyed by bucket elevators comprising a double chain strand or a centre chain strand, a link chain or a belt carrying V-shaped buckets made of steel or plastic that are continuously loaded with material.

In the case of screw conveyors (Fig. 3), the screw installed in a trough transports the materials over a distance of several metres. The force of gravity and the friction between the material and the trough walls ensure that the material does not rotate with the screw but is continuously conveyed along the trough. If the filling ratio of the trough is high and the friction at the wall is insufficient, the material merely rotates transversely to the axis of the screw and is not transported. In vertical screw conveyors the friction at the wall therefore has to be produced by centrifugal force, necessitating rotational speeds of 250 to 400 rpm. The screw conveyor can transport material horizontally, at an inclination or vertically.

All these conveying systems are operated by Nord drive systems. Nenad Mackic, Plant Manager of the Sumber quarry says: “We greatly appreciate Nord drive systems for their great reliability and economy of operation. The company’s service and fast delivery capability are also top-notch. It is not without reason that these products have been installed in our quarry for more than ten years and have never caused us any problems.”

Range of drives for heavy-duty

conveying applications

The drive systems from ­Bargteheide near Hamburg can be installed in all kinds of conveyor and used for practically every angle of inclination, conveying distance and speed. In the case of smaller bucket elevators, geared motors with frequency converters are generally used. Because of the weights to be transported, it is necessary to have soft starting via a coupling while the integral anti run-back device prevents the conveying equipment from reversing. The gear units have an efficiency of approx. 95 % and are available as slip-on, foot-mounting or flange-mounting versions, depending on the application. The maximum power ranges up to 200 kW and the maximum torque is 50 000 Nm.

In Koromacno (Fig. 4), drive units from Nord also meet all expectations. Robert Tencic, Purchasing Manager, and Teodor Gobo, Maintenance Manager of ­Holcim Croatia, place great value on the fast and reliable delivery and the quality, good service and good cost/performance ratio of Nord products. Due to the modular construction of the units and the very well harmonized individual components, it is possible to build an especially large number of variants and achieve a large degree of flexibility, as well as short planning and assembly times, so that quick delivery is assured. Gear units with optimum performance, combined with energy-efficient motors and central or decentral converters result in flexible system solutions with very high efficiency and cost effectiveness.

The recently signed three-year contract provides for the replacement of all older geared motors – regardless of make – with drive units from Nord in all Holcim plants in ­Romania, Bulgaria, Hungary, the Czech Republic, Slovakia and the former Yugoslavian countries. This will enable Holcim to benefit from the international setup of the Nord Drivesystems Group, whose sales and service network covers more than 60 countries.

www.nord.com

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