As wind turbines have grown from the kilowatt class machines of the late 1980’s to the multi-megawatt monsters installed today, it has become necessary for the drivetrain to evolve from off the shelf industrial gearboxes to specially designed, high power units that can handle the variable loads associated with a large rotor diameter as well as meet turbine OEM requirements to be inexpensive and lightweight. Enter the 3-stage gearbox with a low speed planetary stage and two parallel stages which is an economical choice for wind turbines with power ratings between 500 KW and 2.5 MW.
By upgrading to planetary gears, designers created high power density gearboxes that are burly enough to act as the rear support for the main shaft, yet compact enough to maintain a reasonable nacelle size. Compared to parallel stage gearing, planetary gear sets require high precision manufacturing to ensure that the load share between the planets is equalized and that planet gear tilting moments are adequately controlled
During this evolution designers quickly realized that spur gears were not appropriate for the planet stage and began utilizing helical gears. For equal size gears, helical cut teeth have more load carrying capability and run quieter than straight cut teeth. The challenge with helical gearing is handling the axial load generated as the gears mesh. Gearbox designers have experimented with Spherical Roller Bearings and 4-point contact ball bearings before concluding that Tapered Roller Bearings are the best method to react the thrust load generated by helical gears.
The demand for robust but lightweight gearboxes have required that manufactures explore heat treatments that harden the gears surface to improve resistance to surface wear and debris damage. Case carburization is now the most common heat treatment method. The drawbacks to carburization include increased cost of manufacture and part distortion. Profile grinding is required to fix any part distortion introduced in the heat treatment furnace. If grinding is too aggressive the load bearing surface of the gear teeth can become “burned”. Gears affected by grinding burn appear normal to the naked eye but can fail prematurely once placed into operation.
Lubrication systems have been significantly improved as wind turbine gearboxes evolve. The oil itself has progressed from simple mineral oils to manufactured Polyalphaolefin (PAO) synthetics. Additive chemistries are engineered to prevent oil oxidation, foaming and sludge formation as well as to prevent micropitting and wear on the load bearing surfaces of the gears and bearings. A high level of oil filtration is now achieved with 50/10 micron combination filters being the standard. Optional 3-5 micron offline filtration units becoming commonplace as owners are seeing the benefits of keeping the oil as clean as possible. Where early models of gearboxes could get away with a passive splash lubrication scheme, modern gearboxes force oil through a pathway of galley ways, oil rings and sprayers to actively lubricate bearings. This improvement ensures that the proper amount of filtered and cooled oil is delivered to each component at all times.
Lately wind turbine power ratings have exceeded 2.5 MW, pushing the drivetrain requirements further and inspiring new gearbox designs such as multiple planet stages and differential designs. As rotor sizes increase, Wind turbine gearbox manufactures must continue to advance their designs and manufacturing capabilities to meet the turbine OEMs demand for lightweight, cost competitive components along with end user demands for high reliability
Figure 1: Example of 2 parallel stage gearbox design from mid 1990’s
Figure 2: Example of 3 stage (planetary / parallel / parallel) gearbox design from mid 2000’s
To learn more about wind turbine drivetrains please join us for The Second Annual Romax InSight Wind Turbine Technical Symposium this September in Breckenridge, Colorado. Over the two-day symposium, InSight engineers and guest speakers from some of the largest US turbine operators in the US will discuss pitch bearings, main bearing upgrades, gearbox remanufacture, cutting edge diagnostics and many other topics. Network with the industries top consulting engineering firm and with your peers for in depth discussion of common main component issues and how to improve turbine reliability.
Request an invitation for the two-day symposium here: