Bucket wheel reclaimers/excavators are among the largest machines ever built, shifting vast amounts of stored solid material – and all at remarkable speed. Operations Engineer gets up close to these Leviathans to find out more
Bucket wheel reclaimers/excavators may not be exactly ‘light on their feet’ – thyssenkrupp Industrial Solutions’ Bagger 288, for example, is heavier than 8,600 cars and as tall as the Statue of Liberty – yet they must combine robustness, speed and high throughputs as they gouge out massive quantities of solid material, such as coal and ores, in ports, power plants, stockyards and steel plants.
With the biggest machines setting a buyer back by around $100 million, economy in action is the lynchpin: something thyssenkrupp Industrial Solution’s excavators have by – well, yes – the bucketload. The Bagger 288 can move as much lignite and overburden in a day as 40,000 workers – 240,000m3 – equivalent to more than 10,000 conventional dump truck loads. The material is picked up by the bucket wheel and transported on conveyors over three metres wide, with the overburden dumped by spreaders.
And efficiency? A single operator controls all excavator movements from the cab, while various safety devices protect all movements of the machine and its components against accidents. The cab can be raised and lowered, just like an elevator, to ensure the operator always has a clear view. Between the undercarriage and superstructure of the Bagger 288 excavator is a huge 20-metre diameter ball-bearing slewing ring. The entire superstructure can be rotated via this bearing to move the bucket wheel into the required position.
Fast forward to the present and thyssenkrupp Industrial Solutions Australia has been chosen to design, supply, construct and commission the largest-ever rail-mounted bucket wheel reclaimer as part of a €150 million contract (main image). Featuring an ore-moving capacity of 20,000 t/h, the reclaimer will be installed at the new BHP South Flank iron ore mine in the Pilbara region of Western Australia. With the first ore production targeted for 2021, the site will have an expected output of roughly 80 million tonnes per year, making it one of the biggest iron ore operations in the world.
THE SHOW MUST GO ON
With so much at stake, any downtime is likely to have a massive impact on costs and productivity, so a vital aspect of keeping bucket wheel reclaimers in optimal condition is scheduled maintenance. thyssenkrupp’s Maintenance Assistant System (MAS) tracks every maintenance activity completed and records this in the system logbook, ensuring a pre-set maintenance rota is adhered to, so interruption is minimised and expensive repairs avoided. Also, as part of the standard equipment, a Local Analyzer Interface (LAI) is installed on each machine. This collects and stores various data, such as measuring signals, useful for early recognition of possible defects or for subsequent analysis of failures, or damage.
Bosch Rexroth is supplying Hägglunds drive systems for the slew function on both the bucket wheel reclaimer and two stacker machines at the same mine site. Each slew drive will comprise four Hägglunds CB motors, with torque arms and brakes. “Bucket wheel reclaimers are among the most popular applications worldwide for our larger Hägglunds motors and direct drive systems,” says Uno Sundelin, manager of mining and materials handling at Hägglunds Products. “It feels especially appropriate that our largest Hägglunds motor to date, the Hägglunds CBm 8000, will be serving one of the largest mining projects ever.”
The hydraulic direct drive systems deliver the high torque needed, but with low weight and a high degree of flexibility. The lightweight hydraulic motor is mounted on the boom to drive the wheel shaft, while the drive unit with electric motors and pumps can be situated in a more central position on the machine. In many cases, this allows equipment manufacturers to eliminate steel bulk and counterweight, which results in a leaner, more agile and more fuel-efficient bucket wheel reclaimer.
Bearings play a massive part in keeping these huge structures rolling, especially where low speeds, in combination with high loads and the frequent starting of machinery, can result in enormous stress for each component. That puts enormous pressure on bearings, where a rating life of 75,000 hours or operating time of 10 years is required, says Schaeffler. “The excavator platform swivels during operation, but the boom does not constantly move up and down. That is why the bearings in the boom must be able to support static loads, as well as vibrations that are due to the digging motion of the bucket wheel. Spherical roller bearings and spherical plain bearings are the best choice for these requirements.”
For the bucket wheel shaft, the bearings must absorb high loads and shock loads, as well as accommodate shaft deflections and misalignments. “Large FAG spherical roller bearings compensate angular misalignment and have a high load-carrying capacity, offering the best characteristics possible for this application,” states Schaeffler. “In many bucket wheel excavators, the transmission for the bucket wheel drive is furnished with bearings on the bucket wheel shaft. Due to the welded-on flanges, mounting can be carried out with split cylindrical roller bearings only. This design allows the bearings to be replaced quickly, reducing downtimes, as well as maintenance costs.”
For its part, Australia-based Slew Bearing Services (SBS) is an ‘old hand’ when it comes to meeting such challenges, specialising as it does in the design and supply of slew bearings and associated services for use in bucket wheel reclaimers across the globe. To accommodate oscillating movements, these bearings are comprised of an inner and outer ring, one of which usually incorporates a gear. The bearing raceways, in conjunction with the rolling elements (and cages or spacers), handle the loads, acting singly or in combination, and in any direction.
“The manufacturing range is up to 8,000mm for solid slew bearings and up to 12,000mm for segmented slew bearings,” says Robert Panovsky (inset), MD and mechanical engineer. One project involving a bucket wheel reclaimer for which SBS was engaged required the manufacture of a 5,280mm ball roller combination slew bearing, pinions and bolts. “We worked with the installation crew to safely replace the old slew bearing and install the new one, as per specification and requirements,” he states. “Ongoing monitoring is taking place and the bearing is performing very well, with wear rate of less than 0.1mm per year.”
SBS is prepared to go the extra mile, too – literally. It manufactured a 5,280mm combi slew bearing for a bucket wheel reclaimer and chartered an Antonov AN-124 cargo plane to deliver the slew bearing, all in under 60 days from start to finish. The company was actively involved at every stage of the design, manufacture, inspection, logistics and installation of the project, while a senior engineer carried out Factory Acceptance Testing.
If bearings are an essential part of the support systems that keep these giant excavators up and running, so, too, are e-chains (energy-chains) and e-chain systems (pictured, above), described by igus as “the umbilical cord of modern machines”. As Frank Schlögel, the company’s head of engineering projects, points out: “The energy chain represents the main power supply and thus the heart of the machine. It ensures data exchange, energy supply and supplies the stacker reclaimer with water via hoses, thus reducing dust emissions from the terminal or mine. Unlike motor cable reels, the energy chain is not completely mounted on the vehicle, but only pulled along with the driver. Here, push forces act on the energy chains, which is why igus has developed stop-dog systems [to prevent movement of individual chain links at a defined radius] for its energy chains. These are particularly light, corrosion-free and maintenance-free,” he says.
While manufacturers of steel motor cable reels have now largely mastered the subject of corrosion, it can still be an issue in extreme environments. “Corrosion can still cause operators to replace their motor cable reels over the years. Bearings and the slip ring body are particularly affected by corrosion and wear,” states Schlögel. The consequence? “In the worst case, long and expensive system failures. In addition, there are maintenance operations for couplings, springs, guide elements, clamp connections, deflection devices, grinding wheels, motors and switches.”
In operation, motor cable reels are associated with time and energy. “Energy chains from igus made of high-performance plastics are different. They are resistant to pellets, dirt, chemicals, salt water and UV radiation. The energy chains are easy to maintain and clean. The cables can be adjusted at any time. If necessary, the individual wires can be replaced, thanks to the modular design of the energy chain.”
What lifespan do they have? “In many applications, energy chains have been in use for almost 10 years and more, without significant maintenance,” he replies. “In a Czech power plant, for example, a 615-metre long energy chain has proven itself for more than 11 years. During this time, only three crossbars had to be replaced.”
Also, igus has started to equip energy chains with intelligent sensors – so-called smart plastics – to monitor the condition of the energy chain. “Our customers can equip their energy chains with breakage detection systems or push/pull force systems,” comments Schlögel. “Here, irregularities in the set parameters, caused, for example, by the breakage of a chain link or blockages in the run of the energy chain, can be used to activate emergency stop systems or initiate maintenance work, even before an expensive system failure occurs.”
As critical pieces of equipment operating in hostile environments, bucket wheel excavator components must equally be able to withstand temperature extremes, water, heavy loads, shock loads, vibration and contaminants. Climates in which they are working may well range from polar cold to desert heat, while altitudes can span 4,300 metres above sea level to 3,800 metres below ground, meaning downtime is a constant threat. SKF’s response to such events is the Cooper split roller bearing. “Its split-to-the-shaft construction means it is easy to assemble and easy to change out in the field, even in the most cramped and inaccessible locations,” says the company. “Split-to-the-shaft bearings offer complete freedom when designing and arranging motors, gearboxes, couplings and framework. Large couplings or gearboxes are simply left in place while the bearing is fitted, saving crucial time and maximising profitability.”
Meanwhile, SKF has manufactured and supplied its largest ever spherical roller bearing (with a 1.25-metre bore diameter) to be used within the mining industry. The bearing is equipped with SKF SensorMount, which measures the actual mounting fit of the bearing onto the shaft, helping to avoid the risk of improper mounting – a major issue for large-size bearings. The bearing weighs 7,780kg, while each roller within it weighs 42kg. “We have worked in close cooperation with the customer, in order to design an optimum 241/1250 bearing that is particularly suitable for applications in the mining industry,” says Daniel Ortega, project manager at SKF’s Gothenburg factory. “These applications have extreme operating conditions and are very demanding, from a bearing service life perspective.”