Bearings - 19/06/2009
Rolling bearings meet the increasing challenges of the emerging wind energy industry.
Wind power is a sunrise industry for drive technology in general, and for the manufacturers of rolling bearings in particular. On the one hand, the economic outlook in the medium term and in the long term is excellent. On the other hand, however, the demands on life span, load-bearing capacity and service-friendliness of the rolling bearings are so high that manufacturers are making strenuous efforts to improve the performance levels of the bearings. The new developments generated by these efforts have knock-on benefits for other rolling bearing applications.
Wind power technology is on the rise: the world market grew to an overall capacity of over 120,000 MW in 2008, and growth of 25-30 % was achieved over each of the last three years. According to statements by the VDMA Power Systems Trade Association, annual growth rates of 20 % are predicted for 2009 to 2014.
Up to 2007, Germany was the global leader in the exploitation of wind energy. However in 2008 the US assumed the top position, with an installed power rating of 25,200 MW (compared to Germany’s figure of 23,903 MW). Exceptional growth has also been recorded in China and India. This has proven to be profitable for German’s machine building industry, since wind power manufacturers export over 80 % of their production.
The high rates of growth, which are expected to continue, make this an attractive market to suppliers of drive components, particularly in Germany where there is a high concentration of potential customers. However, entry barriers to the market are high because the requirements placed on the drives for wind power installations are extremely rigorous.
Ever increasing requirements
These requirements are continuously increasing, for several reasons. One reason is that the installations are becoming more powerful. At present, prototypes of wind turbines with power ratings of 5 MW are being tested. Even more powerful turbines are in the development stage. A second reason is that we are now starting – after a long lead time – to exploit offshore technology: the first marine wind parks are already under construction. In this particular application, the designers must bear in mind that the drive components will be exposed to even higher dynamic loading. This is in the context that wind power installations can only be serviced at great expense (the next extreme requirement). Nevertheless, the installations are being developed for a life span of at least 20 years. This means that the specifications concerning availability and long-term durability are extremely exacting.
Multiple bearing locations with various different performance profiles
As early as the 1980s, NSK was actively developing rolling bearings for all the important components of a wind power installation. In addition to the bearings in the main gearbox, these comprise the bearings for the rotor shaft, the yaw and pitch gearbox bearings, the generator bearings and the bearings in the auxiliary units, such as pumps, pump motors and cooling motors.
We have thus been part of the development of the industry for more than 20 years. This is of particular benefit because very different demands are made on the various different bearings in the main drive train. With rotor shaft bearings for example, static and dynamic load bearing capacities and the useful life of the grease and re-greasing intervals are crucial. In addition to the high radial loads, the axial loading must be absorbed without significant axial displacement. With generator bearings, electrical insulation is an important factor.
New concepts for the main gearboxes
In modern wind power installations, the reduction required in the main drive train is provided predominantly by multi-stage planetary gearboxes or differential gearboxes in the megawatt and multi-megawatt classes. Various different types of rolling bearing are used at different locations in the gearboxes in the MW power range. Not only must the designer select the appropriate type of bearing construction, he or she must also specify the type of bearing (for example). Bearings using fixed and non-locating bearings, a fixed bearing combination or a floating bearing combination are available, each of which has specific advantages and disadvantages. In the case of the fixed bearing combination, which permits exact guiding of the shaft, particularly in the axial direction, there is a danger of reciprocal jamming. In this case you need to select bearing construction types that can absorb axial forces, at least in one direction, as well as radial forces. This would include taper roller bearings, for example.
A special material can increase the life span of bearings tenfold
A similar situation applies to floating bearings where the shaft can be displaced axially by a specific distance. Fixed bearing combinations and floating bearings are used for bearing designs for the planetary carrier and the planetary gear.
Current rolling bearing calculations for wind power gearboxes generally consist of standardised calculation instructions, manufacturer-specific calculations and partly of analysis processes adapted to quite specific problems. The aims of the calculations include forecasts of the rigidity, estimation of the power loss, exact calculation of the life span of the bearing and estimation of operational safety. The required life span, considering the operational conditions, such as the temperature, lubrication, purity of the oil and the collective load, is in excess of 175,000 hours. All these calculations are carried out by the engineers at NSK Wind Energy Division in close collaboration with the customer and with their colleagues in the Technology Centres.
Outstanding materials competence
In addition to selecting the type of bearing construction, the selection of materials is particularly important: it is possible to achieve a life span of 175,000 hours under difficult conditions and dynamic loading, for example, by using special materials. For this reason, NSK has developed materials specifically for extreme operating conditions, drawing on their many years of material competence. One example of such a material is the Super TF Technology. The developers wanted to maximise the life span of the bearing under conditions where normal bearings would show fatigue damage, starting from the surface. Investigations at NSK revealed that this damage can be traced back to foreign particles that are brought into contact with the rollers by the lubricant and cause indentations in the bearing raceways.
The problem was solved by using a high-purity rolling bearing steel in combination with a special heat treatment process that ensures optimised retained austenite content.
Using this Super TF Steel, we can reduce the loading on the bearing in the critical shoulder edge area of the dents. When the projections created following penetration of foreign bodies are rolled over again, the stress created in the material is considerably lower than with conventional steel. As a result, damage caused by contamination occurs much less frequently. In addition, the formation of non-metallic inclusions is monitored throughout the production process and can thus be reduced to a minimum. This material has a particularly homogeneous structure. The Super TF Technology can be applied to a wide range of bearing types, e.g. cylindrical roller bearings, which are some of the most commonly used types of bearings in wind power technology, and also taper roller bearings and self-aligning roller bearings.
In summary, we can say that the high levels of dynamic loading faced by wind power installations pose a particular challenge. Rolling bearings that are used in gearboxes of similar power classifications, e.g. in industrial applications, do not necessarily also prove to be suitable for wind power installations. This is because the external wind forces acting on the drive bring about dynamic loading, causing great stress to the rolling bearings. Individual solutions are required, which are tailored for each application on a case-by-case basis.
Engineering to the fore
Aiming to master this challenge through intensive development, NSK uses complex calculation methods that take into account the precise dynamic loading on the rolling bearings. This approach provides a solid foundation for selecting the correct bearing design. We have also developed rolling bearing test rigs enabling simulation of the dynamic loading and movement conditions. This includes, for example, a large bearing test rig that allows the application of dynamic forces and torques from rotor loading and a test rig for simulating starting and braking forces on cylindrical roller bearings rotating at high speed.
The project engineers in the NSK Wind Energy Division work closely with the developers who use these test rigs. This means that the manufacturers of wind power installations or of gearboxes for these installations receive a reliable grounding in the design or selection of the rolling bearings . NSK has also created a new production facility that exclusively produces rolling bearings for demanding requirements in wind power technology and in the manufacture of large installations. This factory was commissioned in February 2008 and already, it is clear that it will be pushing its capacity limit because of the strong rate of growth and NSK’s increasing market share. We have therefore started planning the second expansion phase for the production facility.
Smaller sizes also catered for
Alongside the trend towards multi-megawatt installations, there is also an interesting development at the other end of the power scale: micro wind turbines are on the increase. These are installations with less than 1 kW power rating for de-centralised energy supply, e.g. on sailing boats, telephone installations and automatic parking machines. In these applications, the rolling bearings are smaller (typical diameters are 17 or 20 mm) and are under considerably less loading. Even in this case, NSK can provide technically advanced and economical solutions for this new field of application, as demonstrated by an application example. One manufacturer of this type of installation intended to use two standard ball bearings for the main drive and the yaw drive that moves the nacelle, but was afraid that the bearings would be too expensive in this price-sensitive market for mini wind turbines. The manufacturer consulted NSK about the problem. In response, the NSK Wind Energy Division suggested a solution using ball bearings with a special “longlife” lubricant, developed especially by NSK . This solution not only increases the life span of the bearing by 10 %, it also saves on cost: the solution they developed is exactly 73 % cheaper. The manufacturer of the micro wind turbine now needs only to factor in 27 % of the costs of the rolling bearing costs originally estimated. This amounts to an annual cost saving of Euro 337,600, based on a planned output of 20,000 units per year. The manufacturer also benefits from improved market opportunities because the turbines can be offered at a lower price.
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