Reliable, accurate and powerful – Couplings for energy generation
Long service life, difficult access and high performance - factors that place high demands on the components used in energy generation. The couplings used in this field are no exception. They are often used in several places in the individual industries.
An offshore wind farm with 100 wind turbines covering a total area of 35 square kilometres creates a total capacity of 300 megawatts at optimum weather conditions and thus meets the power requirement of approx. 210,000 private households. For the wind power station to be able to produce efficient energy, operation with a constant and optimum rpm is required. This prevents larger fluctuations during feeding into the power grid. If the rotor speed is too slow, not enough power is generated. If the rotor speed is too high, this can damage the system. For this reason, in order to be able to control the blade position of the wind turbines depending on the wind direction and wind intensity, pitch control systems are central elements of the systems. They measure, monitor and control the inclination of the rotor blades, enabling a constant rpm even during changing wind conditions. To achieve a maximum degree of efficiency, the rotatory components of the wind turbines are precisely positioned by means of sensors. The coupling specialist R+W Antriebselemente equips the systems with metal bellows couplings that are backlash- and maintenance-free, which meet the high requirements. Apart from a safe torque transmission, the torsionally stiff metal bellows couplings guarantee precise positioning and a high degree of repeatability.
In addition to the general technical functions, the installed metal bellows couplings must withstand the constant and typical stresses, like large temperature fluctuations, moisture or other environmental factors in the offshore area. Besides the contact with aggressive and contaminated media like salty sea air and lubricants, a temperature application range between -40° degrees and +100° Celsius must also be considered for the coupling design.
With a special version of the metal bellows coupling BK2 / 150, R+W provided a coupling solution which withstands the adverse conditions. Owing to welded hub-bellow connections as well as a special surface treatment of the steel clamping hubs, a certain corrosion protection can be ensured in addition to a safe torque transmission.
Before the wind turbines generate heat under operating conditions, the individual components have to undergo harsh tests. Special test benches test the components in a controlled environment on their loading capacity and stress them to their limit. Test benches must create different load scenarios and be able to provide reproducible results.
The requirements that R+W Kupplungen must meet on a large-scale test bench for wind power generators are accordingly huge. The generator and all drive elements must withstand the heavy load placed on them there to be able to deliver the desired technical and economic success in practice. Lasting security and high precision is required for this during measurements. Here, couplings are required in two places in particular. A safety coupling must securely limit the torque and immediately disconnect the drive and driven side in case of a crash or torque overload. A torsionally stiff connecting shaft must withstand peak torques of up to 840 KNm and also bridge a shaft spacing of just under two metres.
To secure the gearbox against torques that are too high, an overload coupling has been installed. The coupling must separate the connection between engine and gearbox within milliseconds in case of a torque overload. The backlash- and maintenance-free safety couplings of the ST series are predestined for this purpose. If the set torque is exceeded, the safety coupling releases and protects the entire system against machine damage and downtimes. The segments of the coupling only lock back into place through manual intervention and an associated status check, thus continuing the system's operation.
There were defined requirements for the development of the torsionally stiff connecting shaft: Amongst others, the dimensions and maximum load capacity of 840 KNm were clearly specified. The development was based on heavy load metal bellows couplings of the BX series, which are designed for nominal torques up to 100 KNm as standard.
One of the largest challenges for the R+W engineers was the design of the metal bellows. This, amongst others, is intended for balancing axial, lateral and angular shaft misalignments. As the first designs were twice as large and three times as strong as the data requested by the customer, it quickly became clear that a customer-oriented solution could not be developed without an FEM analysis. In connection with the first endurance tests, the analysis finally illustrated the optimisation possibilities with regard to the design. Based on these findings, R+W was finally able to realise the components significantly more compact and ultimately according to customer requirements.
In addition to energy generation through wind power, biogas production for example also benefits from the properties of the couplings. Methane is produced by a biogas plant at the end of the process chain, which is then fed into a gas engine. The engine generates the desired electricity with a reciprocating engine. In preparation for the process, the fragmentation of the substrates accelerates the bacterial hydrolysis and thus the decomposition process. During this fragmentation, unwanted materials can get into the system, like for example pieces of animal hooves, screws or stones, which can cause expensive damage to the plant. To prevent this, vibration-reducing and shock-absorbing elastomer safety couplings are used here.
The primary task of the coupling of type ES2 / 450 used here is to link the input and output shafts and transmit the torque free of play. The coupling compensates for any misalignment occurring between the shafts by an elastomer insert made from high quality TPU. In this application, an elastomer insert with good attenuation (Shore hardness 98 Sh A) produces a balanced ratio between torque transmission and simultaneous attenuation of application-induced vibrations and impacts. Furthermore, the ES2 safety coupling is available with a 64 Sh D elastomer insert. This has higher torsional stiffness compared to the 98 Sh A elastomer insert.
If an unusually hard object gets into the material flow, the grinding system is blocked and the torque can rise exponentially. In case of a torque defined in advance, the coupling separates the power flow between engine and rotor due to the spring-tensioned ball-detent principle within a few milliseconds. This protects the whole system against damage due to torque peaks. Due to the special detent principle in the body of the coupling, this process is absolutely backlash-free, even after several detents. Depending on requirements, the series is available in different functional systems: single-position, load locking, multi-position and with full disengagement.
Engine builders and users benefit from modern coupling technology also for other forms of energy generation. Failures must be avoided in particular in decentralised power generation via micro gas turbines. Micro gas turbines work as rotational mass and thus enable a higher degree of efficiency with lower wear and decreased downtimes compared to conventional piston engines in combined heat and power plants, which work as oscillating mass. In a concrete micro gas turbine project, a coupling had to work continuously with a constant 10 Nm torque and an rpm of 70,000 U/min 24 hours a day and 365 days a year. This special requirement had to be considered specifically when developing the coupling. Furthermore, the restoring forces being generated in the process had to be reduced by the coupling, so that bearing damage would not occur in the first place. For these applications, R+W developed a torsionally stiff solution based on the MK2 / 100, which was realised with specially strengthened metal bellows. It withstands even a four-fold overload, thus providing the operator with sufficient security. This is because if components should be destroyed during operation of the turbine, spare parts are often not available. However, due to the comparatively low installation space available to keep the degree of efficiency as high as possible, the coupling could not be designed too big. R+W thus realised the coupling small and compact.
The different examples from energy generation demonstrate the large variety of solutions. In addition, R+W provides further coupling series like for example multiple disk couplings, which are used in other sectors of energy generation, for example in the petroleum industry. While the basic properties of a coupling are often similar, what is ultimately important are lasting reliability, accurate transmission of partially extremely high torques, and a design that optimally integrates into the existing installation space. Depending on requirements, engineers can use the standard products or the individually developed special solutions.