After 25 years of operation in the Polish lignite plant of Belchatow, the bucket wheels and drives of the five SRs2000 bucket wheel excavators have been refurbished. The implementation of state-of-the-art components which have proven themselves in mining improves the availability and reliability of the machines. Operating costs are effectively reduced by low-maintenance solutions.
The core element of this refurbishment is the new gearbox based on the well-proven TAKRAF series. In view of the more difficult geological conditions which will be encountered in the future Szczercow mine, a newly developed active overload protection system has been realised. Two years of operating experience has to date met expectations. Beyond realisation of the contract, TAKRAF supports Lignite Plant Belchatow with a specific gearbox service.
Five type SRs2000 bucket wheel excavators were commissioned at the Belchatow lignite plant in Poland between 1980 and 1984. The excavators were supplied by TAKRAF in Lauchhammer. The bucket wheel heads of all five units were refurbished in the period between 2008 and 2010 after approximately 25 years of operation.
Engineers at the Belchatow lignite plant identified the weak points of the original drive solution in a comprehensive analysis of operational experience. A new solution was conceived on the basis of this in cooperation with the manufacturer. This enabled engineers from TAKRAF to demonstrate the experience they had gained in numerous modernisation projects realised in the past. The new components are based on solutions which have proven their worth over many years.
Bucket wheel gearbox
The core element of this solution is the new 2x500kW bucket wheel gearbox from the TAKRAF series, a series which has been subjected to continuous further development. The compact gearboxes in this series were specially designed for open-cast mining and encompass the power range from 500kW – 2500kW. Approximately 35 of these gearbox are to date in use around the world.
The gear teeth are hardened and ground. In addition to this, power splitting is achieved via a planetary gear differential. This makes the new gearbox compacter and considerably lighter than the old model. Wear on the teeth is drastically reduced. Noise emissions have been considerably decreased through targeted optimising of the gear teeth and housing.
The design with differential has several advantages over the standard planetary gearbox in which the output stages are planetary stages:
- The planetary stage is relatively small; this reduces the costs for stocking spare parts
- The output stage is a simple planetary gear stage with a solid, forged hollow shaft
- The modular housing is resistant to torsion; the shaft bearings are integrated directly into the housing walls; intermediate bearings in the housing are dispensed with
optimum lubrication by an efficient and maintenance-friendly oil supply system also contributes to operational reliability.
- Solutions suitable for mining ensure reliable, low-maintenance sealing of the shafts and housing. Important functions are monitored by sensors.
- An auxiliary drive which can be manually switched is integrated in the gearbox for repair work.
All TAKRAF gearboxes are subjected to testing lasting several days under an appropriate partial load at the manufacturer’s facility prior to delivery.
Bucket wheel bearings and gearbox support
The torque is, as was previously the case, transferred from the gearbox to the bucket wheel via a membrane disk. The support on the bucket wheel shaft provided by a second membrane disk is new.
This patented solution replaces the previous support bush (which was susceptible to wear) and has proven its reliability on numerous excavators.
Optimised lubrication and the newly designed multistage seal create important pre-conditions for an extension of the service life of bucket wheel bearings.
The support of the drive in the steel structure was also redesigned. The previous slide bearing was replaced by a coupling rod with articulated bearings. This solution does not require any later adjustment.
A multistage overload protection system consisting of passive and active components was designed in view of the more difficult mining conditions at the new Szczercow open-cast mine.
Basic protection consists of electrical current limiting on the drive motors and mechanical limiting using a friction coupling (double taper friction coupling) between the motor and gearbox. The elements of this basic protection system react if overloading occurs on the coupling or motor. Overload situations with a relatively slow increase in load are effectively countered in this manner.
Surge-like loads occur on the bucket wheel drive under difficult geological conditions which can lead to situations resembling blockages. The excessive inertia of the motor and coupling smoothen load peaks in the motor, similar to with a flywheel. The initial effects of bucket wheel overloading of this kind are therefore experienced through lengthier delays and a certain degree of attenuation on the motor. The bucket wheel and gearbox can only be inadequately protected in these overload situations through the basic protection system.
The bucket wheel was also replaced along with the drive. The new single bar configuration facilitates changing of wear elements and improves the accessibility of the design.
The bucket wheel feeding chute has been fitted with a hydraulically actuated gate at the front. This makes the removal of oversized lumps or stones from the belt easier through reversing.
Engineering for modernisation was realised by TAKRAF in close cooperation with the lignite plant. Production of the modules needed was achieved in cooperation with TAKRAF workshops in Lauchhammer and the lignite plant in Belchatow. Mounting was realised by the lignite plant with the support of a TAKRAF supervisor. Performance specifications were achieved safely and reliably in subsequent tests.
The provision of services was also agreed upon in the context of the supply agreement for the bucket wheel gearbox. TAKRAF specialists conduct on-site inspections in fixed cycles. Important status information is recorded systematically on the basis of checklists and archived in a logbook, beginning with the test run at the manufacturer’s facility. Trend curves aid in the earlier detection of deviations and minimise subsequent damage.
In addition to these inspections, important operating parameter trends are monitored with the aid of an online diagnosis system firmly installed on the gearbox. These parameters include oil and bearing temperatures, oil pressures, load trends, overloading occurrences and characteristic vibration values.
The refurbishment project was successfully concluded in October 2010 with the commissioning of the fifth excavator.
Judging by the operational experience of the first two years, the targets of the modernisation project have all been achieved.