The selection of NSK spherical rolling bearings in one of the world’s largest bucket-wheel excavators has proven to be an astute decision, says the company. Despite six years of heavy duty operation at RWE Power’s opencast mine in Hambach, Germany, the bearings are still going strong – without any relubrication.
Every day, Excavator 290 at the Hambach mine moves up to 240,000 t of coal or, using its 130 t bucketwheel, excavates large volumes of overburden which is then transported to a spreader on conveyor belts in its upper structure. The four central conveyor belts located here have to work hard, not only due to the volume of material transported, but because they operate 24/7 in an adverse environment heavy with dust and moisture. This is where two NSK spherical rolling bearings are helping the mine operator maximise the operational uptime of Excavator 290.
The idea of using NSK bearings was driven by the need to reduce maintenance costs on the mine’s largest machines. Excavator 290, which is 240 m long, 40 m wide, 100 m high and 14,000 t in weight, has been in service since the end of the 1970s. With this in mind, the RWE Power product managers responsible for the conveyor belt drums set up a pilot project that involved installing a sealed, dual row NSK spherical roller bearing with tapered bore inside the drive drum of belt 2 on Excavator 290.
The bearing, which has an external diameter of 500 mm and a shaft diameter of 300 mm, was then replaced at the scheduled time. Inspection of the removed bearing revealed that it was in excellent condition despite having been subject to continuous, intensive use. This provided RWE Power with the confidence to adopt NSK’s spherical rolling bearing technology, a move that has been rewarded with excellent service – with zero maintenance – for the past six years.
Among the reasons for this high level of performance is the efficiency of the seals, the high quality grease used by NSK, plus the Super-TF (STF) raw material used for the bearing. STF is a carbonitrate steel that is subjected to a specific heat treatment that helps retain optimum levels of austenite content. STF bearings have twice the service life in comparison with conventional roller bearing steels with clean lubricant, and where there are contaminants in the grease, bearing life can be increased as much as ten-fold.
