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Corrosion-resistant and media lubricated bearings for water power applications

In addition to wind and solar power, water power will make a significant contribution to future supply of renewable energy. This applies to conventional applications such as hydro-storage plants, pumped-storage plants and fresh water river systems, as well as the latest marine water power plants such as tidal and wave energy systems.

For many years, Schaeffler has been a close development partner and supplier of bearings for conventional hydropower plants. Applications in tidal stream and wave power are becoming increasingly important. Depending on the individual design concept, custom bearing solutions are required for water turbines, floating bodies, buoys and oscillating hydrofoils.

The ambient conditions in water and subsea environments place special requirements on the bearings. In these applications, the water itself acts as the lubricating medium. The rolling bearing rings can be made from special, corrosion-resistant steel, with ceramic rolling elements (balls). Bearing cages that guide these rolling elements can also be manufactured from special, water-resistant plastics. Non-hermetic, lightweight seals can be fitted to the bearings, which enable water to enter the bearing, but which prevent small particles from reaching the rolling contacts. In order to develop bearings for these types of applications, Schaeffler is able to draw on decades of experience in materials technology, surface coatings and sealing technology.

Schaeffler does not use heavy seals as these can cause too much friction in the bearing. This enables water power plants to operate in a much more energy-efficient manner. It is also not necessary to use lubricating oils or greases, as the rolling bearings are lubricated by water. This results in savings in lubrication costs, as well as being kinder to the environment.

Reliability and low maintenance requirements also play an important role in all water power systems. Once the plants are installed under the surface of the water, access is clearly very limited. To enable maintenance work to be carried out, for example, the plants would have to be lifted to the surface of the water, which is both time-consuming and costly. The first prototypes of media-lubricated rolling bearings are already in use in pilot projects in North America.

Concepts for tidal stream and wave power plants

Axial turbines are installed underwater to utilise the energy from currents and tides. In a similar way to a stream of air moving the rotors of a wind turbine, in a tidal power plant, the current moves the rotor, which generates electricity. An axial turbine operates on a modified version of this principle by utilising the Venturi effect. During this process, water is fed through a tapered pipe. This increases the velocity of the water, as well as the energy output.

Concepts that use radial turbines are similar to those for axial turbines. Oscillating hydrofoils is another principle for utilising the energy from tidal currents. The inclination angle can be adjusted to enable the current to produce up and down movement of the hydrofoil; similar to the way a dolphin's tail fin moves. This movement is used to generate electricity.

Various concepts are also being developed that utilise wave energy. For example, one system uses several floating structures on the surface of the water that are linked together by joints. The movement of the floating structures relative to each other is utilised to generate electricity. A similar principle is used with buoys, where the vertical movement triggered by wave swell is used to generate electricity. The buoys can operate independently and drive a generator underwater. Another concept involves placing several buoys in a row very close to each other so that they follow the wave swell on the water's surface, similar to a centipede.

In systems similar to these that use horizontal oscillating hydrofoils from tidal energy, wave power systems have been developed that use vertically oscillating shutters. These move with the wave swell by means of joints. Floating tanks 'catch' the waves via a ramp near the shore. A turbine is driven by the water as it flows back. Oscillating water columns have a chamber with one opening above and one opening below the surface of the water. The waves move the resulting column up and down in the chamber. The varying air pressure between the chamber and the ambient air is used to drive an air turbine.

Pelamis Wave Energy Converter

Spherical plain bearings from Schaeffler are already playing an important part in several high profile European wave and tidal energy systems, including the Pelamis P2 wave energy converter. Key to the success of this development is a new joint concept that utilises a new low friction material developed by Schaeffler. This modified PTFE fabric liner has low friction characteristics that effectively eliminate the problem of 'stick-slip'. This has enabled the operating envelope of the P2 machine to be extended beyond the capabilities allowed by standard bearing materials.

Currently, Schaeffler is working closely with Pelamis on improving the design of the ram rod end bearings, by investigating a number of different bearing designs, including rolling element bearings, maintenance-free plain bearings and lubricated plain bearings. The aim is to reduce the weight, size, complexity and cost of the bearing solution, as well as maximising bearing operating life and simplifying assembly.

Maintenance-free plain bearings

Schaeffler's latest development for wave, tidal and other marine applications is the ELGOTEX WA series of plain bearings. Originally developed for use as rudder and stabiliser bearings in marine applications, these bearings provide a high load carrying capacity, are maintenance-free and suitable for dry running in fresh water and seawater. The bearings are also certified to MCM-0112 by Germanischer Lloyd.

The design of ELGOTEX WA plain bearings is optimised for slow swivel movements and high forces that are commonly found in wave and tidal energy applications. The bearing is designed specifically for water lubrication and so does not require seals of the type normally found in conventional rolling and plain bearings. As the bearings do not require oil or grease when used in water, there is no risk of lubricant leakage that may contaminate the environment.

Another benefit of ELGOTEX WA plain bearings over alternative marine bearings is its increased load carrying capacity. While the industry standard to date has been a maximum contact pressure of 10 N/mm2, ELGOTEX WA can support loads of up to 25 N/mm2. Germanischer Lloyd has certified ELGOTEX WA for loads of up to 15 N/mm2 in marine applications. This higher power density ensures that the operating life of the bearings can be significantly increased for the same operating conditions. Alternatively, ELGOTEX WA bearings can support higher loads throughout the same operating life. This opens up opportunities for downsizing of new designs and for reducing the number of bearing variants used in a particular application.

The ELGOTEX WA material comprises two layers that are wound onto each other. The internal sliding layer, which is embedded in a resin matrix with fillers and solid lubricants, is made of continuous synthetic and PTFE fibres. The external layer also comprises continuous glass fibres (glass filament) in epoxy resin. A specific winding angle stabilises the layers, significantly increasing the strength of the bush.

This combination of materials enables ELGOTEX WA plain bearings to resist corrosive media and allows a constantly low friction value with low wear and zero maintenance. The bearings have a high radial load carrying capacity, can be used for axial movements, and are insensitive to shocks and edge pressures. Although the bearings are not sealed, if required, they can be supplied with internal seals for additional protection against abrasive contaminants. As well as marine applications, ELGOTEX WA plain bearings are also suitable for use in ocean engineering, hydraulic steel constructions, hydro-electric power plants, water pumps and water turbines.

Schaeffler (UK) Ltd