The coupling needs for wind turbines pose problems that are beyond the performance specifications of standard offerings. And no wonder. Wind generators are big — really big. They operate at speeds from 12 to 22.5 rpm driving a generator that turns at 1,000 to 1,800 rpm, and they are subject to constantly varying wind forces and loads.

Blades for these behemoths are more than 115 ft in length and weigh more than 7-1/2 tons each. The distance from blade tip to blade tip is 204 ft — about the length of a 415-passenger 747 airplane’s wing span. The turbine generator system is positioned as high as 262 ft in the air, completely isolated from routine observation and maintenance. The entire system of propeller, shaft, gears, coupling, and generator is designed and built to function continuously without failure for a projected 20+ years; all this, while being subjected to changing climate conditions from sub-zero cold to extreme tropical heat.

An exploding technology

As wind power has become an important energy source — renewable, nonpolluting, and price competitive — it has become quite successful as an emerging technology. Wind power is the fastest growing source of electricity generation and supplies enough megawatts to meet the annual needs of two million U.S. households.

Originating 20 years ago in Denmark, large wind farms are today operated on five continents. In the U.S., concentrations of these farms have developed in Iowa, Minnesota, Texas, California, Wyoming, and Colorado, and claim a 15 percent global market share that continues to grow. Denmark-based NEG Micon is one of the world leaders in wind turbine manufacturing, with wind farms being designed and built throughout the world.

Energy needs drive coupling requirements

The problem with wind power in the 1980s when it became a serious energy alternative was that it cost twice that of conventional electric power. Now, thanks to favorable legislation encouraging its development and technological advances, wind energy is on a comparable price level with coal and nuclear sources. This competitive price feature, plus the zero-emission, non-polluting, and renewable advantages, is behind wind energy’s rapid growth.

To reach a price competitive level, NEG Micon invested heavily in wind turbine designs that lowered cost and maximized energy production. Turbine size was increased dramatically with recently installed turbines delivering up to 2,000 kW. One of these giant wind turbines generates enough energy to service the needs of 300 average homes for a year. And as turbine designs have grown in size along with their energy output, the drive systems have become larger, more sophisticated, and more efficient.

Oscillating loads tough on couplings

A key component in wind generator drive systems is the coupling, which connects a high speed gearbox shaft to an asynchronous generator. The gearbox mounts on rubber bushings to absorb the changing aerodynamic forces of the wind. Gearboxes still move around, however, producing heavy oscillating loads transmitted and absorbed by the couplings. NEG Micon required a coupling that would not only handle these loads, but also would do so over years of continuous service with little or no maintenance.

“NEG Micon originally focused on elastomeric coupling designs because of the flexibility to absorb oscillating loads,” says Brian Onkst Sr., design engineer for coupling manufacturer Zero- Max Inc., Plymouth, Minn. “The problem was that elastomeric couplings didn’t have the long-term durability needed for years of operation without component replacement.”

“Then the company examined a new type of composite-disc coupling, which at the time wasn’t available in large enough sizes to accommodate wind turbines,” says Onkst. “They liked the fact that the coupling had high misalignment capability and also substantial vibration and shock control features along with a material design that could stand up to all weather elements. We partnered with their engineers and developed an appropriate design in the correct size. Then we tested prototypes, which led to the models now in use.”

Design details

The coupling used in the NM2000 turbines is a larger version of a standard product. Using two disc packs in a spacer configuration, the hubs mount on the 150-mm diameter shafts. The overall disk diameter is 460 mm with a coupling length of 578.5 mm. Couplings for the NM 900 turbine have a 110 mm shaft diameter at the gearbox end and a 100 mm diameter at the generator end. Overall disk diameter is 348 mm, while the overall length is 440 mm.

The disc pack’s open arm design, together with its composite material, are the keys to handling the oscillating loads. The open arm design provides the high misalignment capacity found in many elastomeric couplings, but with higher torsional stiffness because of the composite discs. In most applications, the open arm disc allows up to 1.0 mm parallel misalignment and up to 3° angular misalignment. The composite material has a ply or fiber orientation to it, which gives it rigidity and also has a damping effect on shock and noise.

Early in the turbine design process, NEG Micon engineers had to modify the distance between gearbox and turbine generator by 100 mm. This necessitated reducing overall length of the coupling by an equal distance. Using a linear variable displacement transducer (LVDT), they determined misalignment conditions present. However, the required change added approximately 20 percent more misalignment (stress) to the discs in the coupling. Misalignment went from 1 to 1.2°. But because of the coupling’s high misalignment capability, upon testing the modified version, the discs handled the added stress with no signs of fatigue.

Coupling is environmentally tough

One reason composite disc couplings were designed was to provide chemical and moisture resistance in hostile environments too difficult or impossible for elastomeric and steel disc couplings. In the wind turbine application, fluctuating weather conditions including winter and summer extremes seem to have little effect. All couplings have operated continuously for more than one-and-a-half years without maintenance. According to NEG Micon’s engineers, the coupling is visually inspected and bolt-tension checked during routine service.

Fretting corrosion to the discs, especially around bolt locations, often is a problem in steel disc couplings, especially where intermittent torque is experienced. In this application, no sign of fretting corrosion occurred during the test period.

Future wind turbine couplings

If today’s wind turbines seem big, tomorrow’s will seem monstrous as larger designs are in the works, especially for offshore installation on the open seas. These, according to NEG Micon, will operate at similar speeds as current turbines that utilize the asynchronous generators, but at much higher torque. Equipping these larger turbines with the composite disc couplings should prove as successful as current turbines, according to the company’s design engineers, because the disc packs can be scaled up to handle more torque.

In addition to wind turbines, the newer large coupling designs have application in large commercial workboats, off-highway vehicles, paper converting machines, printing presses, and mining equipment.