In the centre of the largest Bulgarian lignite-mining area, the Maritza basin, a large-scale bucket wheel excavator of the type SRs 4000+VR is under construction. The machine will be operating
in overburden.

Similar machines built by MAN TAKRAF are in operation in lignite opencast mines in Greece and Siberia in adverse conditions.

The total machine (excavator plus loading unit) has a service weight of 5,730t and has been designed for a theoretical capacity (bulk material) of 11,000m³/h.

The equipment was supplied by TAKRAF and stored by the Bulgarian mining company on the erection site for about 10 years.

In 2000 it was decided to erect the equipment complex while simultaneously modernising the bucket wheel head and electrical components.

Together with Bulgarian erection firms MAN TAKRAF was awarded the contract for carrying out the technical assistance / erection supervision and quality assurance for complex erection processes.

Parallel to the erection the bucket wheel gearbox (SRG) of 3 x 630kW was dismantled and an intensive inspection and technical analysis by experts was performed.

As a result of the expertise and further feasibility studies, it was decided to develop a new bucket wheel gearbox.

MAN TAKRAF was awarded the contract to plan, design and supply a new large-scale gearbox for the SRs 4000. The aim was to substantially increase the reliability and reduce the cost for
spare parts and maintenance.

The applicable EU/DIN standards for calculation, design and quality management and the current environmental European directives had to be considered.

Taking the technical parameters into account, a two-motor drive with 1,000kW drive power each was developed. The inner parts of the gearbox such as bevel gear stage, differential stage, antifriction bearings and sensors can be used in a one-motor drive with 1,000kW drive power as they are identical in construction.

This solution alone results in substantial cost savings in the spare parts stock since the client has one such 1,000kW drive in operation on smaller MAN TAKRAF excavators.

The mechanical and electrical modules identical in construction are, in connection with patented solutions, for fixing the gearbox

a) to the bucket wheel body

b) to the bucket wheel shaft

the prerequisite for the complex gearbox unification between SRs 4000 and SRs 2000 for the Bulgarian opencast mines in the Maritza basin.

The large-scale gearboxes were transported with special vehicles by road and river boat on the Danube to Bulgaria.

The technical lifetime service for such exposed equipment parts will also be ensured by the manufacturer and supplier MAN TAKRAF.

With these investment projects for mastering the complicated geological and hydrological mining conditions (abrasiveness, sandstone horizons and high stickiness of overburden), decisive steps
were taken to improve efficiency and costs at MINI MARIZA ISTOK EAD.

MAN TAKRAF has completed its range of large-scale gearboxes and is in a position to provide bucket wheel gearboxes from 500kW up to, at present, 2,500kW drive power for all bucket wheel excavators operating in opencast mining. In summer 2005 the 20th bucket wheel gearbox of the “New Generation“ with 800kW drive power was dispatched from the Lauchhammer works to Southeast Europe to modernise bucket wheel excavators of the type SRs 1300.