Powertrain technology depends on the reliability and robustness of their individual components. These include the gearbox and gear wheels and IDR talked with specialist trans-mission manufacturer C.u. W Keller GmbH & Co. KG (KELLER) about meeting customer demands for larger and stronger dredges with its custom designed gearboxes. The company also recently delivered gearboxes for a large dredge in China.
The company has more than 100 years experience designing and manufacturing in the steel and mining industries. Keller added gear units for marine applications 17 years ago specializing in dredge applications.
In October 2016 Keller delivered three pump gearboxes for one of the largest dredging vessels in the Chinese market. Tianjin Dredging Corporation (TDC) is building the dredge Tian Kun. Keller built the most essential part of the drive train the cutterhead gearbox. In total the project involved gearing systems for two inboard pumps and an underwater pump a cutterhead gearbox with a nominal engine power of 5000 kW.
Leading up to that project Keller had mainly served European dredging customers. Over the last 10 years the company has delivered about 75 percent of all large cutterhead gearboxes in Europe. Keller’s venture into China all began in 2006 with a relatively small cutterhead gear unit (an output of 1000 kW) – small by today’s standards – for Boskalis’ dredge okra.
After the Jokra more requests came quickly. Eventually in 2015 Keller designed and built the first 7000 kW gearbox the 25th cutterhead gearbox in the 1000 kW and larger class.
The trend for larger and larger gearboxes follows the trend for larger and more powerful dredges. “Larger cutters have longer ladders and thus are able to cut deeper than smaller ones” said Marcel Wiegman managing director for Keller.
Cutters with a cutterhead power of more than 2500 kW are considered “rock cutters” which are able to cut faster through the hardest rocks on the seabed. Wiegman said the new Suez Canal was widened and deepened by the majority of the world’s largest cutter dredges. “This job was done in a very very short period. It would not have been such without a powerful fleet available” he said.
TDC gave the approval for the latest project only after lengthy discussions about the gearing specifications. A great many issues which influenced the drive construction had to be thought through. The gearbox for this latest dredge will be just below the water but Wiegman said other gearboxes can be designed to work in as much as 30 meters under water. Those require an absolutely watertight system which requires de-tailed engineering and sealing systems.
The cutter gearbox is designed with a forged output shaft flange which is bolted to the flange of a cutter shaft.
Other considerations Wiegman said include what design collaterals would be required and which electric control would be considered. The actual design layout of the gearbox only got underway after these parameters were sorted out. Here it became clear that the specification factors for the TDC 5000 kW gearbox were comparable with those high-level specifications of a 7000 kW cutterhead gearing system Keller delivered in 2015.
In addition to the classic bearing shaft and interlocking calculations needed for design Wiegman said comprehensive research was also carried out on the deformation and stress behavior of the gear-casing structure using FEM (Finite Elements Method) analysis. “It’s a method to calculate the stresses that can occur in a gearbox under working conditions” he said. The gearbox is designed with plain bearings that Wiegman said are maintenance free. Roller bearings would be more sensitive to the shocks that come with the cutting process and would require more maintenance.
The analysis also helped optimize the design of the casing structure which experiences immense and changing loads during the dredging process. Having designed a large number of cutter gearboxes over the last 10 years Keller has tested thicknesses with respect to the foundation plates material of the gearbox casings and bolt sizes to optimize the design. Experience in the deformation behavior and the stiffness ratios of the conductor structures on which the cutterhead drive is installed was important too. The deformation behavior not only affects the structural load but also impacts the smooth operation of gears and bearings significantly. Wiegman said this requires working closely with the shipyard and/or dredge designer. The final analysis also helped in determining the tooth-contact-stress levels in the tooth engagement.
Keller is long-standing member of the Re-search Association for Drive Technology in Germany (FVA which in German stands for Forschungsverein Antriebstechnik.) The group is a selected research organization of German manufacturers of drive components who pro-vide practice feedback on calculating programs. For Rohr University in Bochem Germany Keller also manufactured gears for test benches and calculation analysis.
The final TDC gears have a final weight of 180000 kg (180 tonnes) and rank among the largest gearboxes that the company has built. In addition to the company’s experience in the manufacturing of large gear wheels Wiegman said its expertise in the impact of heat treatment on the quality of the component parts is important to Keller designs.
Before delivery Keller does extensive quality control tests on the drive and gear mechanisms. The gearboxes run for several hours Wiegman said. Support images of the gear toothing are taken and these are then compared with the calculations; the noises and vibrations are measured and the transmission tightness verified. In addition the bearing temperatures are recorded over a period of several hours.
After the final inspection and acceptance by TDC the complete powertrain was put to-gether in a seaworthy package and shipped to the harbor in Bonn Germany. The powertrain was then transported via Rotterdam The Netherlands on its long sea route and was delivered to the dredge in China in October 2016.