Planetary gears provide high power density compared with parallel axis gears by splitting the input torque into multiple par- allel sun–planet and ring–planet load paths, which are commonly used in wind turbine drivetrains. However, planet gear loads are not usually equal.6,7 Unequal loads increase planet bearing forces, which is a potential for premature failure. Planetary gear load sharing factor is an important design parameter for drivetrain reliability, defined as the ratio of the maximum torque that the single planet gear carries to the average torque among all planet gears. The degree of unequal load sharing has implications for tolerance schemes. Among various manufacturing tolerances, carrier pin positions have a greater impact than others.8

Non-torque loads are one of the major sources for gearbox reliability issues that contribute to premature gearbox failures or internal component damage.4,9 Many gearbox failures are initiated at the bearings rather than the gears.2 In the original GRC drivetrain, non-torque loads led to unequal load sharing among planets and between planet bearing rows, potentially resulting in bearing skidding or fatigue. Non-torque loads also cause gear misalignment and abnormal gear tooth contact— tooth edge loading, partial contact loss and reversing contact—potentially resulting in tooth pitting.9 The sensitivity to non- torque loads is an inherent reliability issue of three-point suspension drivetrains.

This work investigates two distinct drivetrain design solutions to minimize the impact of non-torque loads on the gear- box. The redesigned GRC gearbox intends to solve the reliability issues associated with non-torque loads inside the gear- box, and Alstom’s Pure Torque drivetrain solves the reliability issues outside the gearbox by altering the non-torque load transfer path. The redesigned GRC gearbox uses tapered roller bearings in the planetary gear stage to replace conventional cylindrical roller element bearings, perting non-torque loads to the gearbox housing rather than the gear meshes. Alstom’s Pure Torque drivetrain has a hub support configuration that perts the non-torque loads directly into the tower through drivetrain frames, rather than through the gearbox as in other design approaches. The main shaft behaves similarly as a torque tube. The non-torque load paths of these designs transmitted from the rotor to the gearbox are illustrated in Figure  4.

Besides these two studied drivetrains, Hick’s flexpin technique10 can also improve planetary load sharing, and it has

been used in planetary gears with more than three planets, resulting in higher torque density and reduced gearbox size and mass. In addition, flexpins can alleviate planet bearing misalignment caused by the pin deformation of a cantilevered carrier. The misalignment caused by a cantilevered pin of the carrier is offset by the misalignment of a sleeve cantilevered from the opposite end of the pin. The planet bearings, therefore, stay parallel to the carrier rotational axis while the carrier itself tilts, which improves the load sharing between planet bearing rows.

Multiple approaches were used jointly to evaluate Alstom’s Pure Torque design, including an experiment conducted in the field, a high-fidelity computational model established in SAMCEF11 and a reduced order analytical formulation. Field testing of Alstom’s ECO100 turbine was conducted at the National Wind Technology Center (NWTC) at NREL, and ex- tensive data on turbine and drivetrain loads were collected. Experimental data were correlated with the modeling results and help justify the modeling assumptions and boundaries. The computational model includes all wind turbine components, thus accounting for the interaction between the drivetrain and the rest of the turbine. The analytical formulation uses a static approach to study the load path of the non-torque loads between the rotor and the  drivetrain.

The major objectives of this study are to

1. Discuss the gearbox reliability issues caused by non-torque loads in the original GRC drivetrain;