NSK provides a range of bearings for unusual applications or special operating conditions, as well as customised bearings for wind power applications.
When selecting bearings for industrial use, design engineers can choose bearings for a defined bearing location relatively easily, working together with suppliers if necessary, and basing decisions on parameters such as rotary speed and torque, radial and axial load, and the required rigidity and preload or clearance.
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 (approximately 175,000 hours).
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 (irregular) because of the wind.
The unfavourable environmental conditions for offshore facilities, used ever more frequently, are another factor; sea air, for instance, 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.
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; for example, 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 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.
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.
Given the amount of data that must be taken into account in this area and the demanding bearing lifetime requirements, such projects are always executed in close cooperation with the customer.
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.
NSK has developed special materials suitable for extreme operating conditions, such as its 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.
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, 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.
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 users a competitive advantage.