Wind turbine components include (from right) the hub (with slewing ring) and the main frame. The main frame holds the main shaft and bearing housing, and the gearbox with its slewing ring. A connecting ring is shown inside the tower. (Source: Sandvik Coromant)

Last month, the United States Departments of Interior and Energy announced a strategic plan to fund offshore wind energy projects, with incentives that will add up to $50.5 million over the next five years. These incentives (see “Details of the Strategic Wind Farm Plan”), part of the U.S. goal to access 80% of its electricity from clean energy sources by 2030, is an ambitious plan that will likely put strain on the wind turbine supply chain to produce sufficient machines in the appropriate timeframe.

Part of the overall high-productivity solution must be productive and efficient machining of the each part of the wind turbine. Recent developments in metal cutting tools allow machine shops making turbine components to reduce cutting times and achieve consistent quality using fewer types of tools.

For instance, the main shaft in a wind turbine is a large, forged component where 40 percent of the material is commonly removed from a forging weighing several tons. A week-long process is not unusual to complete the various roughing, turning and drilling operations in the main shaft (see figure). The machinist’s selection of the best tool holders, insert geometry and grades has a tremendous effect on the total number of hours that the shaft is processed on the machine.

The forged surface of the main shaft demands metal cutting inserts with secure performance and high temperature resistance due to the long contact times. The industry has begun adopting wipers that are adept at performing heavy duty turning operations at high removal rates, from forged skin to close-tolerance finishing.

Machining Inside the Gearbox & Hub

The machine shops that produce wind turbine parts, such as the hub, planetary carrier (part of the gearbox) and main frame, face numerous challenges. For instance, the complex shape of the hub makes work-holding difficult, and as a result, vibration commonly becomes an issue. We recommend a combination of Capto holders and Silent Tools dampening adaptors to increase the application’s stability and avoid vibration when using long tools to access awkward areas.

Inside the gearbox are planetary carriers, which are typically manufactured from nodular cast iron and can be machined in different ways depending on the machine tool and machining strategy. While one machinist prefers turning for metal removal, another chooses milling. Helical interpolation, a flexible three-axis ramping technique, provides rough boring using long-edge indexable insert cutters for high efficiency. For fine-finish boring, a cutter that is capable of micrometer-level adjustments is recommended.

Machining inside the hub can be both challenging and time consuming. One proven, cost efficient solution is back facing with high precision using a multi-purpose side and face cutter.

Similarly, a milling cutter that enables light cutting action and uses eight cutting edges and shims to protect the cutter body, can provide highly efficient face milling of hubs.

Turbine Towers & Connecting Rings

Most large wind turbine towers come with tubular steel sections that are 60-100 feet in length. The tube sections are bolted together using connecting rings – one at each end of the section. Each tower contains 6-8 connecting rings, which are 10-30 feet in diameter. Machining the connecting rings typically involves drilling hundreds of thousands of holes per year, because the rings are produced in large volumes. Bolt hole productivity can be drastically improved by applying the latest indexable drills correctly, potentially halving drill times for many machine shops.

For example, in the drilling of connecting rings, one ring producer was experiencing a problem with vibration. By replacing the existing drill with a CoroDrill 880, the manufacturer was able to balance the cutting forces for vibration control and reduce cycle time by 35 percent.

The slewing ring connects the tower to the nacelle and causing it to rotate in response to the wind’s direction. Three slewing rings connect the turbine blades with the hub to adjust the pitch angle of the blades. They start out as forged rings, up to 30 feet in diameter, and the gear teeth are machined using gear milling tools. One of the most challenging machining operations is the production of ball tracks. Rather than employing expensive grinding operations, round cutting inserts can be used to rough out the grooves, providing optimal chip-breaking and high productivity, while round ceramic inserts can finish the grooves.

Details of the Strategic Wind Farm Plan

On February 7, Secretary of Energy Steven Chu and Secretary of the Interior Ken Salazar announced a new project to speed up offshore wind energy development in the United States. The plan commits up to $50.5 million in research and development funding for projects that will support offshore wind power farms in high-priority areas including several sites off the coasts of Delaware, Maryland, New Jersey and Virginia.

According to Global Wind Energy Council, global installations of wind energy reached 194 GW in 2010, a 22.5% increase over 2009 levels. In 2010, U.S. installation of new wind energy capacity dropped to 5 GW, from the 2009’s level of 10 GW. European new capacity was down slightly from 10.7 GW to 9.9 GW between 2009 and 2010. For the first time, more than half of the new wind power was added outside Europe and North America, primarily driven by China, which installed 16.5 GW of capacity in 2010.

The U.S. Department of Energy set a goal of deploying enough offshore wind farms to generate 54 GW of electricity by 2030, at a cost of energy of 7 cents per kilowatt-hour (kWh), with an interim target of 10 GW of capacity deployed by 2020, at a cost of energy of 10 cents per kWh.

The Strategic Work Plan includes three components: technology development ($25 million of the total budget); studying and removing “market barriers,” including environmental risk reduction and supply chain development (up to $18 million in the next three years); and up to $7.5 million to develop and refine next-generation turbine drive-trains.

This article was authored by Brent Godfrey, Wind Power Application Specialist from Sandvik Coromant and appeared in North American Clean Energy (Volume 5, Issue 2) on April 14, 2011.