ZW Wind Power has developed a new gearbox platform for the largest offshore wind turbines in the market. After prototype testing in 2013/2014 the product is in serial production since 2015. In May 2017 ZF delivered the 100. Gearbox to the customer.
The gearbox is designed for a mechanical gearbox torque of around 8,000 kNm, whereas the platform-concept allows a torque increase to more than 9,000 kNm by implementing minor design modifications only. ZF has chosen the highly reliable Differential Gearbox. The weight of the gearbox is around 72 tons, and is designed to an outer diameter of approximately 3 m (Figure 1).
Figure 1: ZF Differential Gearbox for large offshore wind turbines
The ZF development team has cooperated closely with the turbine manufacturer and the component suppliers. With the consequential application of the FMEA (Failure Mode Effect Analysis) more than 1000 possible failure modes and mitigation activities have been identified. Additionally, a method for a precise reliability prediction has been developed in a sup-project. Last but not least the whole drivetrain has been simulated with respect to the dynamic and static loads, using a complete system model as well as several sub-models (Figure 2).
Figure 2: Finite Element model of the drivetrain and the sup-components
The gearbox’s reliability was demonstrated through an extensive validation comprising tests on component and system test rigs. Extended measurements have been done. The gearbox was fully disassembled and inspected after being tested. The gearbox condition and the load sharing occurred as predicted by simulation (Figure 3).
Figure 3: Gearbox inspection after test (left), measurement and calculation of load distribution (right)
Right after prototype testing ZF put a focus on stabilizing the serial production, which turned out to be a challenge for the complete supply chain due to the big sized components and weights (Figure 4). In Mai 2017 the 100. Gearbox has been produced successfully and on time.
Figure 4: Stabilization of serial production
The differential gearbox with the principle of power split into two epicyclic gear stages has the potential to serve the next generation of 9 to 10 MW wind turbines. The future use of multiple planets allows keeping an unchanged envelope diameter and same gear and bearing sizes for a torque up to 12,000 kNm and more (Figure 5).
Figure 5: Power split concept and torque-Density increase by means of multi Planet Technology
For more details, see the ZF presentation on the International Conference on Gears 2017.