Bearings - 2011-05-05
The trend towards wind turbines with ever-greater drive capacities has not just prompted the emergence of bearings capable of handling the all-new dimensions of the multi-megawatt class. In response to the higher outputs and torques, engineers are also developing new concepts, such as planet wheels with an integrated raceway. As well as all this, new limits are constantly being set and surpassed – for example with regards to dynamic loads and resistance to adverse environments. It has become – and remains – crucial to explore new avenues in materials development in order to meet the exacting demands of the wind power industry. Conventional bearing steels cannot deliver satisfactory results in terms of service life: turbine manufacturers generally demand a useful life of 175,000 hours.
Bearings made from materials with fewer non-metallic inclusions
Life tests have shown that the rolling fatigue life of carbon-chromium alloy steel used for bearings is significantly affected by non-metallic inclusions. In response to this, NSK has developed a new material in conjunction with steel producers – Z steel – which contains far fewer non-metallic inclusions. Tests under operating conditions show that the service life of bearings made from this steel is up to 1.8 times longer than that of bearings made from conventional, vacuum-degassed steel. If the bearings need to be even more durable, Super Tough (Super-TF) is the material to use. Thanks to the different alloying elements in the bearing steel and modified heat treatment, Super-TF bearings boast service lives up to ten times longer than those of conventional bearing steels. This material is often used for bearings in wind turbines as a result. It is suitable for a wide range of bearing designs, such as cylindrical roller bearings, tapered roller bearings, spherical roller bearings, deep-groove ball bearings and angular-contact ball bearings.
Hybrid bearings with ceramic are particularly robust
Ceramics are the ideal material in other areas of mechanical engineering when excellent resistance to wear is needed. NSK has already gained extensive experience with its ROBUST bearings, which have heavy-duty steel raceways and the option of ceramic balls. They are used in the machine-tool industry.
Drawing on this experience, NSK has developed electrically insulated bearings with ceramic coated outer rings specifically for the requirements of the wind power industry. They boast a long service life – turbine manufacturers demand 175,000 operating hours under extremely dynamic loads and the new bearings fulfil this requirement. At the same time, the use of ceramics prevents electricity from passing through the power train and minimises the associated damage, e.g. when current peaks or circulating currents (electrical corrosion) occur. This bearing design is therefore ideal for use in generators.
Normally ceramic bearings have a permanent ceramic coating, which is applied to the steel surface using plasma spraying. As an alternative, NSK also offers, hybrid bearings with full ceramic balls - NSK built on extensive experience when developing these components. Due in part to the high demands placed on service life (twinned with minimal servicing) for the wind power industry Lubricant producers also face major challenges when it comes to bearings. Lubrication demands a very high level of expertise as much of the damage registered within the anticipated operating life is attributable to lubrication errors.
Integrating lubrication into the application-specific design
This bearing damage usually takes the form of wear, fatigue damage, seizing marks or overheating. Whatever the damage profile, it is caused by faulty lubrication, which also includes over lubrication. Damage can be prevented by integrating lubrication into the application-specific design. Important aspects of suitable bearing lubrication include maintaining an optimum oil film barrier between the working surfaces, proactively reducing friction, repelling and/or eliminating contamination, providing rust protection and, above all, safeguarding heat transfer.
To fulfil the requirement of separating working surfaces, it is possible to draw on a description of the lubrication status derived from the theory of elastohydrodynamic lubrication. According to this, the ratio between the existing and the necessary operating oil viscosity must be greater than 1. This ratio is generally greater than 1 for bearings in the high-speed shaft, high-speed intermediate shaft and low-speed intermediate shaft.
The ratio is usually less than 1 for bearings in planet wheels and pinion cages. However, it can be raised to 1 in such cases by using lubricants with proven EP additives. The crucial heat transfer in the bearing depends on the load, the design-specific coefficient of friction, the bearing size, the speed and the ambient temperature. Once these parameters have been identified, the necessary oil volume flow can be reliably assessed. When determining the oil supply method, the bearing design including the cage construction must be taken into account to ensure that the oil can flow through the bearing.
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