By Amethyst Cavallaro
Online Editor, Power Engineering
All wind turbines are large but the Repower 5M wind turbine is huge.
Manufactured by Repower Systems AG from Hamburg, the 5M stands 120 meters above the ground and produces 5 MW of electrical power. With its three-bladed 126-meter diameter blade span, the 5M is the largest and one of the most efficient wind turbines in the world.
During early design stages and later, the 5M presented a significant technical challenge in several different fields. One of the most crucial was the design, manufacture and installation of a bearing system for the 1.5 meter diameter rotor shaft that supports the 130 tones three bladed rotor.
New bearing system on hollow rotor shaft
Even though it has to support the heavy rotor, the cast iron rotor shaft is hollow. This is to meet weight, cost and performance requirements and it was just one the factors that had to be considered by SKF when they were given the challenge of designing, manufacturing and installing a bearing system for the shaft.
To eliminate the influence of positional and deflection related errors, the SKF specialists in cooperation with the Repower designers developed a system with the shaft supported by two bearings. In the non-locating position is an SKF CARB toroidal roller bearing. The other bearing, in the locating position, is an SKF axially locating spherical roller bearing. This arrangement offers high load carrying capacity and the lowest possible bearing plus housing weight for a two bearing arrangement.
Simulation and calculation tool solve problems
To give Repower a bearing system that would meet all performance requirements, SKF made use an in-house simulation tool, called the "Beast."
"Beast" is a dynamic simulation program that can be regarded as a virtual test rig in the computer. Traditional methods of long and costly experiments on a test rig are replaced by faster, and more detailed computer simulations. The program was first used in the development of the SKF range of CARB bearings, so naturally it was used again in smaller sized CARB bearings to evaluate how the inclined shaft in the 5M would influence bearing behavior. This bearing with its 1.5 meter inside diameter weighs 2,700 kg and is the largest of its type ever manufactured by SKF. So its behavior on an inclined shaft was an unknown in the early stages of the design process. The finished product represents a milestone in bearing design.
Further work involving FEM calculations was needed to determine the interference fit on the hollow cast iron rotor shaft. In smaller sized bearings, experience has yielded standard recommendations that can found in SKF catalogues.
Both tools proved invaluable because large bearing systems and their mountings have totally different characteristics from those of the more usual size of bearing.
Lubricating a 1500mm diameter bearing
Bearing size also gave rise to extraordinary lubrication considerations. The behavior of grease in a 500mm diameter bearing is well documented. But this is not the case for a bearing with a diameter of 1500mm. Lubrication specialists at SKF set out to discover whether grease distribution in a 1.5 meter bearing would cover all the relevant surfaces or would gravity take it to the bottom of the housing. Questions such as this had to be answered and resulted in the housing being designed to cover oil lubrication as well as grease. Later inspections have shown that the grease lubrication works well and the housing cover design was simplified.
Induction heater for bearing assembly
Another challenge for the bearing specialists was to devise ways to overcome difficulties during assembly and installation of the two huge rotor shaft bearings. There were no commercially available induction heaters that could uniformly heat the 2,700kg CARB bearing and the 3,320 kg spherical roller. This meant that when the bearings and their two 8,000 kg housings came to be heated for correct assembly and installation, SKF had to design and have built a new 1,000 kVA induction heater.
Also because the weight of the roller set in the CARB bearing is almost 800 kg it had to be specially clamped during heat up and assembly. Without induced safe clamping the weight of the roller set would make the rollers clamp loosely between the inner and outer ring, which could damage the raceways. Normal loose clamping also occurs in standard size bearings mounted vertically but because the weight is so much less it would do no harm. However, as an aspect of quality assurance SKF also clamps smaller bearings to ensure a safe mounting process.
Axial fixation of the bearings presented another major challenge. Fixation is usually done by shaft nuts, but the shaft in the 5M turbine has a diameter of 1.5 metres. The frictional moment on a thread of this diameter is enormous. SKF solved the problem by designing a novel, safety split nut; the largest ever made. They call it the HMS lock nut and its most notable feature is that it does not require a keyway in the shaft. This makes the overall design more robust and reduces manufacturing costs. The system is easy to mount and open for maintenance and repair, and there are no problems with fretting corrosion when dismounting.
Destined for a life at sea
The first 5M wind turbine was erected in Germany at the coastal city of Brunsbuttel and came on stream in February 2005 as a pilot project to evaluate the total turbine design. That's because the designers of the 5M believe that wind at sea is a much better source of power than wind on land. They believe that a power provider can get double the electricity from the wind at sea compared to land because wind speed at sea is slightly higher and the wind frequency is much higher. Consequently the Brunsbuttel installation is only a practice run in preparation for the series installation of 5M turbines out at sea.
The first proposed site is 25 kilometers from the Scottish coast in 44 meters of water. The site is part of the Talisman DOWNVIND project supported by the European Union.
Knowing that the 5M was being conceived from the start for operation in an inhospitable environment was an additional reason why SKF took extra care in the design, manufacture and installation of the rotor shaft bearing system. As well as having excellent protection against corrosion, redundancy in many components and continuous electronic monitoring of critical components, the 5M turbine has a rotor shaft bearing system that has been engineered for long-term reliability.