Wind Turbines Under Stress

This planetary gear model gives some sense of the complexity of the gearing required for turbines to handle the transient forces applied to wind generators.

View larger

Wind turbines are impressive structures. But, even the mightiest wind generator can be brought down by something as small as a bearing.  A new, high-fidelity Bearing Simulation Tool (BEAST) provides a clear picture of the behavior of bearings, wind turbine gearboxes, and rotating shafts during simulation. It is helping the National Renewable Energy Laboratory’s (NREL) Gearbox Reliability Collaborative identify gearbox weaknesses and improve designs, and individual companies improve reliability.

Transient forces from the wind put enormous stress on wind turbine main shafts, gearboxes, and other rotating parts. End users and owner-operators report their gearboxes generally last only three to five years, according to Sandy Butterfield, principal engineer at NREL. Simulating the wind’s interaction with mechanical components can identify problems across a broad range of operating conditions very efficiently.

Bearings are key for ensuring critical rotating parts spin freely under all operating conditions, but they exhibit nonlinear behavior and are difficult to model accurately. Until recently, generic bearing models that don’t account for cage motion or variations in materials, finishes, or specifications, have been accepted. Commercial simulation tools have approximated them as stiffness matrices or disregarded them entirely, making many assumptions.

High fidelity models are not new, but applying high-fidelity techniques to bearings is. As bearings are critical components of larger systems, this tool is turning out to be very important.

Bearing models have grown more realistic. They now account for rolling elements, raceways, bearing cages, materials, surface finishes, and lubrication ports.

The SKF Group’s BEAST modeling technology, 40 years in the making, evaluates the dynamic behavior of rolling bearings under real-world conditions. Data from its validated models can be used to analyze the interaction of bearings and rotating elements within a turbine’s housing.

It takes into account the application’s complex boundary conditions and operating environment, such as construction materials and machining tolerances. It simulates complete bearing behavior, including cages. It uses a three-dimensional contact model to account for small-scale geometric variations, such as surface roughness.

Its fully transient models compensate for the flexibility of bearing geometry, yielding more accurate load calculations. This enables studies of internal motions and forces under any given loading condition.  

It couples with CAD, finite element analysis (FEA), and computational fluid dynamics models (CFD) models of wind turbine systems, importing data on its housing, shaft, couplings, assembly, and boundary conditions to calculate bearing behavior based on real designs using actual nominal values, not estimates. It also exports design-specific bearing data back to system models, so modifications and trade-offs can be made as needed.

Here are two cases where the bearing simulation tool helps engineers assess designs to meet critical goals.

Prolonging Service Life

A European turbine manufacturer wanted to determine whether its new wind turbine would last 20 years and to verify its design against a growing body of standards. Turbine designers supplied CAD files and housing structure details to create boundary conditions, as well as thousands of load descriptions as the system adjusted to changing wind speed and direction. These adjustments influenced the housing, producing asymmetric loads and shaft misalignments that affected bearing performance.

Bearings are a key component in industrial gears, but they are rarely modeled with any fidelity. Most simulations rely on simplified bearing models or simply assume rotating parts will spin perfectly.

BEAST provided the following types of outputs to understand where loads were actually applied during operation, making it possible to calculate bearing life accurately:

• Contact forces between all components
• Detailed component contact data such as power loss, lubrication film thickness, and wear
• Animations of all bearing component movements with magnified motions, force, and velocity vectors
• 3-D images of contact pressure and slip velocity distributions

The analysis of the European wind turbine confirmed the main bearing housing design would meet extreme-load and fatigue-load conditions and not lead to premature bearing failure over the required 20-year service life.

Gearbox Reliability Collaboration

BEAST is also helping NREL’s Gearbox Reliability Collaborative investigate wind turbine gearbox reliability issues for the Wind Energy Program. They are modeling more a complete planetary gearbox and helping understand its sensitivities. Six sigma and DOE processes are being used to validate the model and results.

Focusing on how the overall system affects bearing performance and how bearings influence system reliability, BEAST allows the team to:

• Optimize bearing type, mounting condition, and gear combinations
• Analyze how housing or planetary carrier deformation influences bearings
• Identify optimal locations for load stiffeners and lubrication ports

BEAST is also helping generate friction and thermal models that can provide temperature predictions to determine bearing clearance, lubrication intervals and conditions, proper lubricant selection, and ultimate service life.

High Fidelity Future

Detailed and highly specific virtual modeling technologies such as BEAST offer a practical way to test designs against parameters and test conditions too difficult to replicate. Advanced simulations provide insights into how bearings and systems can perform, leading to better wind turbine gearbox designs.

[Adapted from “Tuned to the Forces of Nature,” by Greg Zimmerman, SKF USA, Inc., for Mechanical Engineering, May 2010.]

Transient forces from the wind put enormous stress on wind turbine main shafts, gearboxes, and other rotating parts.  

Tweet

Share

 Email

 Print