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Bearings - 2015-01-08

Main rotor bearing

One of the goals in the development of rolling bearings for wind turbines is to increase resistance to typical damage patterns and, in so doing, extend bearing life. Progress has now been achieved in this area. In addition, leading providers are expanding their portfolios.

Wind technology is a particularly demanding application area for rolling bearings. Some of the bearings are quite large, they are subjected to high dynamic loads, and the environmental conditions are highly detrimental for the precision components of drive technology.

In these circumstances, where access to the bearings is extremely difficult, achieving a system service life of 175,000 hours or twenty years, poses a great challenge to rolling bearing developers. This situation is aggravated by specific damage patterns, against which preventive measures must be taken.

Expansion of the business area for main rotor bearings
A central project of the international NSK wind business area is the expansion of rolling bearings used in the main rotor bearing. This bearing must meet extremely high requirements. For this reason, intensive research and development work is required. NSK can rely on extensive expertise in the development of rolling bearings for wind turbines and experience in practical operation.

With the product range expansion to include these large rolling bearings, system provider NSK is extending its established portfolio for transmission bearings and can offer solutions for all of the bearing positions of a wind turbine, in which 20 to 30 rolling bearings are typically installed. A heavy (400 kg) spherical roller bearing for a main rotor bearing was presented at the Wind Energy trade fair 2014 in Hamburg, Germany.

Market growth and greater market share
In the area of rolling bearings for wind turbines, NSK is among the leading international manufacturers and expects further increases in production numbers and market share in coming years. Market growth will contribute to this trend: in 2013, systems with a total output of 36 gigawatts (GW) were installed internationally; for 2014, estimates are for about 50 GW – with a total amount of 311 GW in 2013.
In addition, NSK is also planning a significant increase in market share in wind power technology. The recently developed product ranges that NSK introduced at Wind Energy Hamburg 2014 substantiated this expectation.

Integrated rolling bearings for planetary stages
Double-row tapered roller bearings in a back-to-back arrangement are a new development intended for integrated construction designs and can be used in the planetary stages of wind turbine gears. These bearings are produced without an outer ring and are directly integrated in the planet wheels of the gears. Up to now, multiple-row cylindrical roller bearings have been available in this design; now this planetary bearing series will be extended to include tapered roller bearings.

Core competency: material engineering
The main competencies of NSK include material development. Materials are available with compositions and property profiles that have been optimised precisely for specific environmental conditions and demands such as: operating temperatures, high mechanical loads, moisture and particles that penetrate within the bearing. Example materials are the special WTF and STF steels developed by NSK. Rolling bearings manufactured from these materials reach a significantly longer bearing life than bearings made of conventional bearing steels. This also applies for wind power technology that have extreme requirements for bearing life, due to complicated maintenance conditions. . A system service life of twenty years is the goal here. What must also be taken into consideration is the extremely arduous environmental conditions and the extreme dynamic loads in which the bearings are subjected to.

Measures against ‘white structure flaking’
In recent years, NSK has made great efforts towards dealing with ‘white structure flaking’ (WSF), a damage pattern that is the subject of much discussion in the industry. WSF refers to structural changes below raceway surfaces, which weaken the material and can lead to cracks and flaking. Characteristic for this damage pattern is that it can occur relatively early in the lifetime of a wind turbine.
The causes of WSF are not completely known, however, many years ago in the automotive business area, NSK had already observed similar damage to the rolling bearings of passenger vehicle alternators. Here NSK had noticed that electrical currents also played a role and were then able to develop successful preventative measures. Later, significant negative influences were identified for wind power, particularly the introduction of hydrogen via the lubricant and slippage and sliding on the contact surfaces.
For quite some time already, NSK has provided a series of options for bearings in wind turbines in order to reduce this type of damage – for example, black finished raceways (black oxide coating, BOC) and STF materials with carbonitrided surfaces.

New material for wind power technology
The R&D engineers for NSK in Japan have succeeded in simulating WSF damage to a great extent. These findings promoted the development of new materials. The result of these measures was a new material with an extremely high resistance to WSF, which was presented at the Wind Energy trade fair using a cylindrical roller bearing as an example.
This is a new option offered by NSK for countering a cause of damage that leads to many early failures. In trials, it was determined that the resistance of this new material to ‘white structure flaking’ increases by a factor of seven (in comparison with standard rolling bearing steel). In addition, under normal wear conditions, i.e. flaking on the raceway that results from structural changes directly at the surface, the bearings also achieved a considerably longer life. In the simulation of penetrating particles that are introduced into the bearing via the lubricant, a bearing life increase by a factor of three was achieved, in comparison with standard rolling bearing steel.
With this property profile, rolling bearings made of this material are not only suitable for use in wind turbines, but also in the gearboxes of other large systems that are used in the steel and primary industries, as well as in large construction equipment.

Triple-Row Cylindrical Roller Bearings

A current trend in wind power technology is integrated bearings without outer rings for planetary gear units.
The image shows an integrated Triple-Row Cylindrical Roller Bearing

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