ENGINEERING IS IN DEMAND
Bearings selection and calculations for wind power plants
The demands placed on bearings for wind power plants are extremely high. Therefore,
the procedure for bearing selection and calculations are fundamentally different from
selecting bearings in other application areas.
When selecting bearings for industrial use, design engineers can choose bearings for
a defined bearing location relatively easily, working together with their suppliers if
necessary, and basing their decisions on parameters such as rotary speed and torque,
radial and axial load, and the required rigidity and preload or clearance. The wide
range offered by complete providers such as NSK means that the right bearing is
always available, even for unusual applications or special operating conditions. If this
is not the case, special customised bearings can also be used.
Exacting requirements
In wind power technology, the procedures leading up to the selection of a bearing are
fundamentally different, essentially because of a single parameter: the bearing lifetime.
The manufacturers of wind power plants and the gears for those plants require a
bearing lifetime of 20 years, i.e. 175,000 hours, for their bearings. This is already an
extreme value, but it does not cover all requirements. In addition, it must be taken into
account that external loads acting on the bearing are highly dynamic (that is, irregular)
because of the wind. The unfavourable environmental conditions for offshore facilities,
used ever more frequently, are another factor: sea air is highly corrosive. For generator
plants, electrical corrosion must also be taken into account. Finally, the service options
available to wind farm operators are extremely limited, and any unscheduled downtime
for a bearing will result in significant costs.
The importance of calculating lifetime
Just calculating the lifetime of this type of bearing requires a large degree of expertise.
This is why selecting bearings for wind power plants is more complex than in other
areas of use. Numerous parameters must be taken into account. Besides the bearing
loads and the rotational speeds in the application context, design engineers must
consider the construction in which the bearing will be placed, i.e. the configuration of
the shaft and housing, their materials, and their tolerances.
Based on this data, first the conventional, standardised calculations are made in
accordance with DIN ISO 281, also known as the catalogue method. The parameters
are bearing load, rotational speed, load capacity, and bearing type. In further
calculations, parameters such as temperature, lubrication, and purity of the oil are also
taken into account.
Greater precision than required by the standard
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Appendix 4 of DIN ISO 281 assumes a simplified bearing geometry as the basis for
calculating the modified reference lifetime. In most cases, this sort of calculation is
adequate. However, in the wind power industry, more precise values are preferred.
NSK has developed its STIFF software for this purpose. In calculating the bearing
lifetime, STIFF also takes into account the exact internal geometry, the running
clearance and preload, deformation of the shaft-bearing system, and the load zone
and load distribution between rolling elements and bearing raceway.
In this model, the rolling elements are divided into slices. An individual, modified
reference bearing lifetime is determined for every slice. A more realistic indication of
the total bearing lifetime is obtained based on the share of time for each load case.
Targeted design optimisation
Apart from fast and precise calculation of the bearing lifetime, the STIFF software also
allows short-term version analyses and thus optimisation of existing designs. In
addition, special adaptations of the bearing can be tested quickly. This particular
advantage is frequently exploited: customised bearings are often used in wind power
technology precisely because of the exacting requirements. Furthermore, predictions
of rigidity, estimation of efficiency loss, and investigation of operational safety are part
of the calculation objectives pursued by the NSK Wind Energy unit when selecting
bearings.
Engineering to the fore: Intensive cooperation with customers
Given the amount of data that must be taken into account in this area and the
demanding bearing lifetime requirements, it is not surprising that such projects are
always executed in close cooperation with the customer. Along with engineering,
testing also plays an important role. NSK has designed bearing test rigs that allow
simulation of dynamic loads and motion. These include, for example, a large bearing
test rig capable of applying dynamic forces and moments from the rotor load, and a
test rig for simulating acceleration and deceleration and braking procedures for rapidly
rotating roller bearings.
Super-TF technology for wind power technology
Aside from the selection of the bearing design, the choice of material also plays a
crucial role: a bearing lifetime of 175,000 hours, for example, under unfavourable
conditions and dynamic loads can only be achieved through the use of special
materials. That is why NSK has drawn on their many years of material expertise to
develop special materials suitable for extreme operating conditions. One such
development is the Super-TF technology. This technology deploys a high-purity
bearing steel combined with a special heat treatment technology, guaranteeing an
optimised retained austenite content. This material ensures that any foreign matter that
penetrates the bearing generates less stress in the material than in conventional steel.
As a result, damage from impurities occurs less frequently.
Bearings made from this material and used with a clean lubricant have roughly twice
the lifetime. The Super-TF technology can be used for a large range of bearing types,
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such as cylindrical roller bearings, which are among the most frequently used bearing
types in wind power technology, as well as for tapered and spherical roller bearings.
Different types of bearings
In modern wind power plants of the megawatt and multi-megawatt classes, multi-stage
planetary gears and differential gears primarily provide the necessary speed reduction
of the main power train. In this case, design engineers must select not just the
appropriate bearing type, but also determine the type of bearing arrangement. Bearing
assemblies with fixed and free-end bearings, an adjusted bearing assembly or a
floating bearing assembly, are available. Every bearing type has certain advantages
and disadvantages. An adjusted bearing assembly that allows precise guidance of the
shaft, particularly in the axial direction, therefore runs the risk of reciprocal tensioning.
Therefore, bearing assembly types such as tapered roller bearings, which allow axial
forces to be accommodated in at least one direction along with radial forces, must be
selected here. The same applies to floating bearing assemblies, in which the shaft can
move axially for a certain distance. Fixed and floating bearing assemblies are used, for
example, in bearing designs for planet carrier and planet wheels.
New designs for main gears
In addition to the trend for higher and higher performance and offshore facilities, new
drive and gear designs also present design engineers with new challenges. NSK is
working closely with the manufacturers of wind power plants and their drive
components. The goal is to help develop new designs (that increase the efficiency of
wind turbines or reduce weight in the nacelle, for example) for the market quickly and
give their users a competitive advantage. NSK has the advantage of over twenty years
experience in developing bearings for the wind power industry an advantage that also
benefits users of bearings outside the wind power sector.
About NSK
Established almost 100 years ago, NSK (Nippon Seiko Kabushiki Kaisha) is a
Japanese-listed company that has evolved from a regional ball bearing supplier to a
roller bearing specialist and automotive supplier with a global market presence. Today,
NSK employs more than 30,500 employees in 29 countries. As per March 2014 NSK
achieved a turnover of 6,2 billion Euros. This result has been driven by ever-increasing
investment in research & development, enabling the company to continuously improve
the quality of its products and services. This investment supports NSK’s objective of
“No. 1 in Total Quality”. In addition to a complete rolling bearing portfolio, NSK
develops and manufactures precision components and mechatronic products, as well
as systems and components for the automotive industry, including wheel bearing units
and electric power-steering systems.
In 1963, NSK’s first European subsidiary, Düsseldorf, Germany, was opened and in
1976, the first European production facility in Peterlee, England. Today, NSK Europe
supports pan-European sales with production locations in England, Poland and
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Germany, logistics centres in the Netherlands, Germany and England and technology
centres in Germany, England, France and Poland. In 1990, NSK purchased the UPI
Group including the renowned European bearing manufacturer RHP, with its factory in
Newark (UK). Additionally, NSK has developed a comprehensive network of
authorised sales distributors. NSK Europe is divided into application-based business
divisions: Industry rolling bearing technologies & linear and precision technology
(EIBU) as well as bearing modules and steering systems for the automotive industry
(EABU & ESBU). In this organisation, NSK Europe's 3,500 employees achieved a
turnover of over 900 million Euros as per March 2014.
Editor Contact
DMA Europa Ltd. : Elizabeth Preciado-Achury
Tel: +44 (0)1562 751436
Fax: +44 (0)1562 748315
Web: www.dmaeuropa.com
Email: [email protected]
Reader Contact
NSK UK Ltd : Helena Measham, Customer Service & Marketing Communications
Manager
Tel: 0500 2327464
Web: www.nskeurope.com
Email: [email protected]
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