Romax and NREL have completed testing of the Next Generation Drivetrain on the 2.5MW dynamometer at the National Wind Test Center near Boulder, Colorado. Romax designed, instrumented and assembled the planetary single stage gearbox. The testing began with the typical stepped load approach, followed by progressively aggressive testing sequences. The test culminated with weeks of continuous start-stop cycles and dithering cycles (rocking back and forward), all in an effort to establish the threshold of abuse that planet journal bearings can endure.
The teardown took place in the 5MW dynamometer working space at NREL’s National Wind Test Center. Pictured are the four planet gears in the foreground with the permanent magnet medium-speed generator in the distance. Shown on the right is the integrated drivetrain housing orientated vertically for disassembly.
Prototype and first article gearbox testing is often followed by a disassembly inspection, or ‘teardown’. For this Next Generation Gearbox the teardown inspection had multiple objectives. There was the typical evaluation of gearing and bearing performance during nominal torque and speed test conditions. This is similar to the teardown inspection after a certification bench test to evaluate gear tooth contact patterns for the tooth face load distribution. Additional evaluations were needed for the accelerated life testing and abnormal loading scenarios. Lastly, of great interest was the evaluation of the performance of the new technology in this project: journal bearings in the planetary stage of a wind turbine gearbox. Journal bearings can be thought of as brass bushings with oil lubrication. Key findings were that the bearings ran cool, quiet and produced negligible wear debris, even during the aggressive tests. The planet load sharing, a key design parameter for extending gearbox life, for the four planet flex-pin arrangement was excellent.
The journal bearings had been performing so well during the abusive tests that we sought an additional test to establish the limits of the journals. System design of a wind turbine needs to take account of providing oil feed to the bearings under all conditions: production, idling, grid loss, maintenance and unforeseen events. Of course there are robust ways to mitigate these risks, but a circumstance of a period without oil feed in a turbine’s lifetime is a possibility. One idea was “let’s run it without oil”, with an initial reaction of worry, soon replaced by excitement to be able to test the system under such unfavourable operational conditions. NREL’s engineers agreed with the value and opportunity of such a test and provided an extremely quick turnaround. This final test went ahead with another round of the tough operating condition of stationary dithering this time without the oil pump running. The (hydro-dynamic) journal bearings performed very well during this oil restricted dithering test and returned favourable results upon disassembly.
Let have a look at some of the teardown photos.
The disassembly of the NGD drivetrain was an opportunity to assess the condition of the instrumentation. The planetary stage contained strain gauges, temperature sensors and pressure transducers all subjected to vibrations in this hot oil environment. None of the signals were lost during testing and we found the adhesives, coatings and cable jackets good condition.
Shown here is the internal (ring) gear with the four flex-pin planet shafts that are fixed on one end to the planetary carrier. The evaluation of the ring gear focused on the contact pattern for an indication of the tooth load distribution and related performance of the flex pins to control alignment and tilting moments. Also of interest was any wear indications from the grid fault torsional overload events or the accelerated dithering tests.
The planet gears were removed for inspection of the tooth faces and journal bearings. The teeth were painted with a sacrificial red dye prior to testing to aid in visual inspection after testing. Proper labelling and thorough documentation are critical inputs to the design optimization process that follows a prototype test.
The sun gear pictured here is the most highly, and frequently, stressed gear in the planetary system. It has the least number of teeth and is simultaneously meshing with four planet gears. The teardown found the gear teeth to have no wear or fatigue damage. The only visible surface condition was that of normal ‘run-in’: the knocking down of surf ace asperities, or high spots, remaining from the surface grinding operations at the factory.
One of the journal bearings was cut in half to release it from the planet gear inner bore. The specific locations on the bearing’s surface were identified that corresponded to the different test regimes. The locations were tracked by rotating the main shaft by 45 degrees between dithering tests. The bearing condition was very good and provided valuable feedback on its performance during the abusive tests.
If interested in learning more about the Next Generation Drivetrain then have a read through the previous blogs on this topic or get in touch via email at firstname.lastname@example.org.
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