Bucket wheel excavator BG 285

It was the first repair to the steering cylinder since the bucket wheel excavator BG 285 was built in 1975. Wear and enormous impacts had completely destroyed the piston sealing system and the sealing material. The axial retaining rings for the piston seal had completely come loose from the piston and become stuck in the cylinder tube. Due to the large amount of leakage via the piston sealing system that had come apart, the forces necessary to move the steering shaft could no longer be achieved. The excavator was not manoeuvrable.

As Montanhydraulik Reparatur und Service had already successfully undertaken two repairs on such cylinders in the past, the repair company was also commissioned with repairing this steering cylinder. The rod head assembly represented the biggest challenge here. Once shrunk onto the rod through heating and cooling in order to transfer tensile forces of 10,000 kN, it had to be removed again in a similar manner in order to dismantle the 45-tonne cylinder. It actually required three attempts, ten hours of patience and a degree of technical creativity before the six-tonne rod head came loose with the help of hydraulic pressure and heating the rod head to 300 ºC.

Good collaboration with Montanhydraulik in Holzwickede

In the process, it was discovered that the 610 mm-long and 500 mm-large fitting diameter of both parts was highly grooved and displayed many deep seizure marks. However, thanks to the support from Montanhydraulik in Holzwickede, Germany, the repair duration was only extended by one day. For the necessary machining operations, the technicians first produced a steady rest bearing that could be freely adjusted to the piston rod and turned the rod by 4 mm. They then welded the rotated diameter on using the SAW technique and finish-turned it as required. The rod head was machined out to +0.5 mm. This work required the utmost precision, as the force fit according to the original design had to be produced to transfer the tensile forces in full once more without any problems. 

Vertical cylinder assembly

The final hurdle was to shrink-fit the rod eye onto the piston rod again in such a way that the force fit is used for the full length and the repaired steering cylinder guarantees the fault-free operation of the bucket wheel excavator. During the cylinder installation, the piston rod including rod and cylinder head was attached to the hook of a 300-t mobile crane, lowered through the opened hall roof. Underneath this was the vertical rod eye, installed in the steel frame, which was again heated to 300 ºC, while the fitting diameter of the rod was cooled to –150 ºC using liquid nitrogen. The piston rod was then gradually lowered into the rod eye. Following successful functional and compression testing, along with filling with 3,500 litres of hydraulic oil, the steering cylinder was finally transported back to the customer on a special low-loader – on time and restored in mint condition.

See the installation of the steering cylinder in a short video here.

The following requirements were successfully fulfilled:

  • max. 3.5 weeks repair time
  • assembly capacity suitable for cylinder weight and size
  • manufacturing capacity for replacement parts of 1,000 mm diameter
  • renewal and optimisation of the sealing and guiding elements
  • repair of the fitting of piston rod/rod head
  • refurbishment and re-polishing of the piston rod chrome plating
  • new sealing of the cylinder, including with the new dimensions coming about as a result of the repair
  • re-fabrication of six retaining rings for rod seal (ø 1,000 mm), a seal for the guide bushing and the bronze piston (ø 1,000 mm)
  • honing of the cylinder tube
  • inspection and reconditioning of the valves mounted on the cylinder
  • installation of valve connection plates and adaptation of the pipes to implement the mounting position of the valve blocks
  • compression testing and refilling with hydraulic oil
  • renewal of the cylinder paint according to customer specifications for corrosion protection coats including sandblasting

Technical data hydraulic cylinder:

  • weight: 45 t
  • piston diameter: 1,000 mm
  • rod diameter: 500 mm
  • total stroke: 4,400 mm
  • oil filling: approx. 3,500 l

Technical data bucket wheel excavator 285:

  • year built: 1975
  • service weight: 13,500 t
  • nominal transportation capacity: 200,000 m3/day
  • length: 210 m
  • height: 92 m
  • chassis width: 45 m
  • number of caterpillars: 12
  • driving power: 16,560 kW