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ONYX and NREL’s GRC Gearbox 3 – Part 1: Design and Assembly

ONYX InSight has been significantly involved in the NREL GRC (Gearbox Reliability Collaborative) since 2008, with modelling, analysis, design and testing activities.

“Gearbox 3” is a redesign of the gearbox previously tested with modifications to improve reliability and to support specific research goals.

A little history

The GRC program commenced in 2007 and aimed to bring understanding to failures in wind turbine gearboxes and to assist the industry with improvements to reliability. It has certainly been a successful program. Two gearboxes were used for the research program. Both were heavily instrumented and tested, “heavily” has some weight here! Picture over 100 channels with strain gauges, proximity probes and thermocouples all over the planetary stage. After testing the gearboxes were torn down and all the wearing surfaces were thoroughly picked over, the measurement data was carefully analyzed and a list of improvements for the gearbox prepared.

Along the way ONYX validated our gearbox modelling and analysis with RomaxWind to the amazing array of test data.

With a validated analysis model and list of required improvements we set out preparing design upgrades for one of the gearboxes. Our scope: analysis, drawings, parts procurement, manufacturing support and testing support.


The planet bearings are tapered (TRB) and preloaded with the rollers set directly to run on the planet gear inner surface. They were heavily instrumented for temperature and strain measurements to compare to the previous design which featured cylindrical roller bearings (CRB). The image shows the exploded planet assembly (left), instrumented planet (top right) and assembly using liquid nitrogen (bottom right).

Gearbox 3 Objectives

  • Pack in the latest and greatest design practices and technologies into a state of the art wind turbine gearbox. Nearly every component both inside and outside the gearbox has been heavily instrumented
  • Test on NREL’s 2.5MW dynamometer in July and August of 2016
  • Compare the performance of the redesigned gearbox against the original Gearboxes 1 & 2

Gearbox Upgrades for Life Extension

In 3 point mounted drivetrains the gearbox directly supports the turbine’s rotor. The massive non-torque loads from rotor weight (at least 36 tons in this case), aerodynamic wind shear and yaw moments cause the gearbox housing to flex. Innovative design improvements to Gearbox 3 have been made to stiffen the gearbox, thus keeping the gears and bearings in proper alignment. In traditional wind gearbox designs this flexing causes high contact stresses and undesired load distributions (not uniform across the machine elements), which lead to premature failure of the parts.  It’s similar to your car wheels being out of alignment causing the edges of the tire tread wearing off over a period of months, rather than seeing even wear over a period of years.

The life limiting component of the wind turbine gearbox is the planetary section, specifically the planet bearings. The redesigned gearbox utilizes tapered roller bearings to support the input shaft (turbine rotor) and preloaded tapered planet bearings that have been upsized by integrating the outer raceway directly into the planet gear. But don’t stop there. The planetary stage has more improvements: ring gear stiffening, planet pin interference fits, optimized gearing micro-geometry, upgraded bolted joints and forced lubrication to the planet bearings (it was splash lubricated).

The final outcome

Gearbox 3 has 3x L10 life for the planetary stage compared to the original design.

Traditional gearbox lubrication systems are a combination of forced spray and splash into the sump reservoir. In the redesign we changed to dry sump lubrication with forced spray nozzles to 9 gearing locations and 8 bearing locations.  This required an innovative rotatory hydraulic joint to provide fresh oil directly to the planet bearings to extend their life. Gearbox 3 will also have increased efficiency (ultimately raising a turbine’s energy production) and due to the dry sump any debris is better removed by filtration to mitigate the spread of failure.

 

 Model showing the lubrication spray nozzles providing clean oil directly to the gear meshes. The spray is represented by the gold cones.

The gearbox manufacturing, instrumentation and assembly was completed in March 2016 as a collaboration between ONYX InSight and Brad Foote Gear Works in Chicago, IL. As with any prototype gearbox, innovative assembly methods were used to protect the instrumentation and custom tooling needed to be developed to fit this tight gearbox together. Liquid nitrogen was used to shrink the planet pins at a temperature of -320°F. Both the ring gear and high speed shaft have dozens of strain gauges.

 

The delicate instrumentation on the high speed shaft (left) and ring gear (right) was protected during assembly.

The completed gearbox has been shipped to NREL’s test facility in Colorado to be mounted into the original nacelle and mated to the dynamometer. Check back here for more updates as the test data starts to roll in this Fall 2016.

Follow this link to a NREL paper explaining the history of the GRC gearbox program >

Download paper

And read more about how ONYX apply our gearbox knowledge to support owners in upgrading their gearboxes for extended life here >>

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