Mining company MBR Brazil has implemented a hydraulic drive solution on three of its four bucket wheel stacker/reclaimers and is satisfied with the advantages achieved.
MBR has its terminal for shipping iron ore at Ilha da Guaiba, an island close to the village of Mangaratiba just south of Rio de Janeiro. The iron ore arrives via a specially built railway from its mines which are north in the state of Minas Gerais, around its capital Belo Horizonte. Three huge locomotives pull the 132-wagon iron ore trains up to seven times a day.
The iron ore is used to make products such as cars, aeroplanes, ships, household appliances, tools, surgical instruments or agricultural equipment.
"MBR has always been interested in trying new solutions, testing new ideas to address a certain situation or problem," says Hélio Carvalho, mechanical engineering manager at the terminal.
"When Bardella, one of the main OEM suppliers here in Brazil, introduced a new set-up with a direct hydraulic drive system for the bucket wheel, we became quite interested. We understood that there were many benefits compared with the existing electromechanical system with gearboxes and rather complex control systems to adjust bucket wheel speed. Since we run 24 hours a day, all year round, some other factors were significant, such as service support, assistance, maintenance and spare parts."
Recent advances in electrohydraulic direct drives have caused previous design concepts to be challenged with regard to drive systems for many materials handling applications.
Bucket wheel stacker reclaimers are simpler and more efficient with direct drive hydraulic motors being mounted on the bucket wheel drive shaft. This eliminates the need for gearboxes, bedplates and couplings.
The advantages of this system are reduced weight on the boom, a reduction in shock loading, trouble-free stalling and overload protection, which significantly reduced down time and increased machine availability with less maintenance.
The increased flexibility of the variable speed has further advantages by increasing machine throughput and by giving the ability to use different bucket wheel speeds which vary according to the material conditions or its density.
In addition, these drive systems use standard components that are easily retrofitted to existing machines which give operator flexibility, increased productivity and lower operating costs leading to a new lease of life to existing installations.
Until comparatively recently, most engineers have considered two main methods of providing high torque for low speed drive applications. The most universally accepted solution is the electromechanical drive which uses either AC or DC motors, a fluid coupling and a reduction gearbox. When variable speed is required either thyristor or frequency controllers are used.
The traditional alternative to an electromechanical system is a hydraulic drive. These come as either high or medium speed motors, but they rely on gearboxes for the final speed reduction. Where variable speed is required, variable displacement hydraulic pumps driven by standard AC induction motors are a standard alternative.
The latest generation of high torque, low speed motors, however, has forced a reassessment of previous design constraints. Standard hydraulic motors are now offered with displacements up to 250 L/rev, torque capabilities of 1400 kNm, continuous power capabilities in excess of 1000 kW and a power to weight ratio some two to three times higher than that of conventional solutions. These advances in hydraulic motor design have, in many applications, enabled the elimination of gearboxes from the drive train allowing the simplicity of a direct drive to be incorporated in machine designs and providing designers with a new-found freedom in the application of drive systems.
If we consider that a Hägglunds direct drive weighs between 35 to 50% of an equivalent electro-mechanical solution and then translate that weight reduction to the end of a 20, 30 or 40 m boom, it can be seen how the weight reduction can have a significant influence on the overall machine design.
As a rule of thumb, for every one tonne saved on the boom, double this amount is saved on the counterweight before we consider the slew decks and bogeys, etc. If we then compound this weight reduction with the low inertia of the drive and the subsequent reduction in vibration, tortional effects and shock loads, there are many arguments for fitting a direct hydraulic drive. One of the main benefits of fitting a direct hydraulic drive on a bucket wheel is the versatility and controllability offered by a variable speed drive.
The MBR motors have been supplied by Hägglunds. "They have improved our operations and have given us flexibility, an even production and reduced maintenance. Good operator training will enhance their performance. This is very important, especially since the direct hydraulic system is a new solution for us, at least when it comes to the rotating 'drive area'," says Hélio Carvalho.
"Having operated hydraulic systems we already knew that it is easier to adjust and control forces and limits, so we were quite confident that this system would also benefit the operation of the bucket wheel," says Hélio Carvalho.
"When an overload situation occurs, it is very easy to return to normal operation. The operator just has to move the bucket wheel a little bit out of the pile and then the direct hydraulic system makes it possible to restart again and continue operation, all done in just a couple of seconds. Previously, this type of situation became a nightmare. So the direct hydraulic drive system has decreased down time on the machine and lowered maintenance costs.
"It has become a habit to run the bucket wheel with constant speed, simply because in the past speed changes demanded a great deal of the operators and it was also quite complicated. We have, however, been advised by Hägglunds," continues Hélio Carvalho, "to learn to use different speeds to increase profitability when, for example, we are loading a ship with blended material. This is something we have not done yet, but will do since speed adjustment is now easier using the new hydraulic drive system. Also, when we need to stop the wheel exactly at a certain spot, we can, which means a safer operation. Even the day-to-day maintenance will be reduced and faster," concluded a satisfied Hélio.
