The next generation wind gearbox must show great improvements over the last and then be manufacturable around the globe. This article introduces one such design.
The next generation of drivetrains will be large and operate in harsher environments (offshore) than those previous. They will have to be more reliable, easier to service, repair, monitor, and include effective diagnostics and prognostics. These Gen2 units will have to carry greater torque and rotor-bending moments, and show drivetrain dynamics with lower frequencies, for more durable machinery. Operational costs and financial risk must also come down.
The need for improved reliability has driven several efforts in the industry, including:
• A major revision to ISO 61400-4, “Design Requirements for Wind Turbine Gearboxes”
• The long running NREL Gearbox Reliability Collaborative (GRC)
• Stricter wind gearbox requirements in the 2010 edition of the Germanischer Lloyd “Guideline for Certification of Wind Turbines”, and
• Current revision efforts to AGMA 6006-A03, “Design and Specification for Wind Turbine Gearboxes”
Engineering firms have been developing and improving design methods and software since the beginning of the wind industry. Manufacturers have been improving their product by implementing a range of tests such as full load end-of-line testing, off-axis load and dynamic development testing, 100% nital etching, stricter incoming material specifications and inspections, and improved techniques for the heat treatment of large components. Turbine designers have a better understanding of the loads and dynamics in the machinery.
Rotating machinery specialists such as Romax Technology have been involved in these activities along with the development of the next generation of wind-turbine gearboxes. To date the company has developed 14 multi-MW models ranging from 1.5 to 5.5 MW, which are used by 10 different turbine manufacturers, most of which are installing turbines in the Far East. “We won’t design a wind gearbox for a manufacturer unless they go through certification either by GL, DNV, TUV or an equivalent,” says Romax Director of renewables Andy Poon. “This along with our own due diligence on the prototype development and manufacturing helps ensure high-quality manufacturing for our products.”
The company is working on improving the installation and serviceability of the high-speed stage of the wind turbine gearbox. This stage in any wind-turbine gearbox is most susceptible to wear and premature failure, given the high number of cycles and high inertia of roller bearings in larger gearboxes. The company is also developing a diagnostics capability for damage and wear in the gearbox, with condition monitoring equipment and specialized software. If wear in the high-speed stage is detected early enough, then an up-tower replaceable cartridge can be easily fitted by a technician. The high-speed cartridge is easy to assemble. The technician simply removes the whole unit and replaces the high-speed-stage bearings and pinion in the new cartridge. Bearing preload is preset in the factory to ensure good performance. Double helical gears are used in several models on the parallel stages in combination with the high-speed cartridge. This negates the requirement of the axial constraint to resist the load due to gear helix. Then the high-speed bearings need only react to radial loads. It eliminates the problem of double row, taper-roller bearings being unloaded in reverse torque situations.
Another recent significant development is a new way of adapting the high-speed bearing to many common wind turbine gearboxes in service. The concept of this new design was developed after evaluating many offshore machines. Most bearings are manufactured with high quality material, to high tolerances and surface finishes, and then fitted with no real idea of the actual preload,” says comapny Test Team Leader Richard Smith. “Industry falls down on the last 10% of the job. A new approach to retrofitting the high-speed stage takes aim at setting accurate bearing preload, which results in longer bearing life than the previous practice.”
Another feature of the recent gearbox is a split-torque design with rotating ring gears and a dry sump. This removes a common problem of debris accumulating in the 6 o’clock position of the stationary ring gear and the early failure of this gearing. Gravity feeds oil into a sump below the gearbox, so the gear stages need retain no oil. Churning is minimized so less aerated oil returns to the cooling system and drag losses drop.
In addition, the company has an advantage because it has been a long-time developer of commercial engineering software for designing gearboxes as well as being a large wind turbine gearbox design firm. Gears and bearings are designed with microgeometry, and clearance and preload settings after accounting for whole system structural deflections, non-torque loading at the rotor, and the effect of thermal expansions and off-nominal tolerances.
Romax software shows strong validation with extensive gearbox measurement data provided by the NREL GRC, and the software is certified by Germanischer Lloyd for gear contact analysis and rating. The analysis allows for improving function, performance, weight, and cost of gear trains.
The planet carrier provides an example of lessons learned. Generally, manufacturers use cast-steel carriers to meet strength requirements. However the steel has high scrap rate, can be a difficult to work with, and has stringent requirements for nondestructive testing.
Careful analysis has led the company to a suitable lower cost and lower weight (sometime by 1,000 lbs) cast-iron planet carrier. “We worked with GL to approve GJS 400 spherodal graphite cast iron for the planet carrier,” says Design Team Leader Dave Saysell. “By careful analysis and rearrangement of the structure, we can achieve equivalent deflections, strength, and fatigue life with this lower-cost material. Critical attention is paid to strength near the planet-pin fit and the carrier wind-up.” Environmental considerations are important because the material is more suitable for warmer climates. WPE
By: Ashley Crowther, VP Engineering, U.S. Wind Technology Center, Romax Technology Inc., www.romaxtech.com
Filed Under: Gearboxes